Writing User Subroutines with ABAQUS

Writing User Subroutines with ABAQUS
ABAQUS
CONTENTS
Lecture 1
Introduction
Overview of Some User Subroutines . . . . . . . . . . . . . . . . . . . . . . . L1.2
Where User Subroutines Fit into ABAQUS/Standard . . . . . . . . . . L1.6
User Subroutine Calls in the First Iteration . . . . . . . . . . . . . . L1.10
Including User Subroutines in a Model . . . . . . . . . . . . . . . . . . . . L1.11
Using Multiple User Subroutines in a Model . . . . . . . . . . . . . L1.12
Restart Analyses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L1.12
Writing Output from User Subroutines . . . . . . . . . . . . . . . . . . . . L1.13
Path Names for External Files . . . . . . . . . . . . . . . . . . . . . . . . L1.14
Compiling and Linking User Subroutines . . . . . . . . . . . . . . . . . . L1.15
FORTRAN Compiler Levels . . . . . . . . . . . . . . . . . . . . . . . . . L1.17
Debugging Techniques and Proper Programming Habits . . . . . . L1.18
Required FORTRAN Statements . . . . . . . . . . . . . . . . . . . . . . L1.18
Naming Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L1.20
Subroutine Argument Lists . . . . . . . . . . . . . . . . . . . . . . . . . . . L1.20
Solution-Dependent State Variables . . . . . . . . . . . . . . . . . . . . L1.21
Testing Suggestions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L1.24
Lecture 2
User Subroutine: DLOAD
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ABAQUS Usage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
DLOAD vs. UEL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
DLOAD Subroutine Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Variables to be Defined . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Variables for Information Only . . . . . . . . . . . . . . . . . . . . . . . .
Example: Transient Internal Pressure Loading . . . . . . . . . . . . . . .
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Writing User Subroutines with ABAQUS
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L2.3
L2.4
L2.5
L2.6
L2.6
L2.8
TOC.1
ABAQUS
Partial Input Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L2.9
User Subroutine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L2.10
Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L2.11
Example: Asymmetric Pressure Loads . . . . . . . . . . . . . . . . . . . . . L2.12
Partial Input Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L2.14
User Subroutine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L2.15
Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L2.16
Lecture 3
User Subroutine: FILM
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L3.2
ABAQUS Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L3.3
FILM Subroutine Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L3.4
Variables to be Defined . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L3.5
Variables for Information Only . . . . . . . . . . . . . . . . . . . . . . . . . L3.6
Example: Radiation in Finned Surface . . . . . . . . . . . . . . . . . . . . . . L3.8
Partial Input Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L3.12
User Subroutine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L3.13
Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L3.15
Workshop: User Subroutine FILM . . . . . . . . . . . . . . . . . . . . . . . . L3.16
Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L3.16
Problem Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L3.16
Lecture 4
User Subroutine: USDFLD
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L4.2
ABAQUS Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L4.3
Defining Field-Variable-Dependent Material Properties . . . . . L4.5
Defining Field Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L4.9
Accessing Solution Data at Material Points . . . . . . . . . . . . . . L4.11
Explicit vs. Implicit Solution . . . . . . . . . . . . . . . . . . . . . . . . . . L4.12
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ABAQUS
Using Solution-Dependent State Variables . . . . . . . . . . . . . .
User Subroutine GETVRM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
GETVRM Subroutine Interface . . . . . . . . . . . . . . . . . . . . . . . .
Variables Provided to GETVRM . . . . . . . . . . . . . . . . . . . . . . .
Variables Returned by GETVRM . . . . . . . . . . . . . . . . . . . . . . .
Elements Supported by GETVRM . . . . . . . . . . . . . . . . . . . . . .
USDFLD Subroutine Interface . . . . . . . . . . . . . . . . . . . . . . . . . . .
Variables to be Defined . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Variables that may be Defined . . . . . . . . . . . . . . . . . . . . . . . .
Variables for Information Only . . . . . . . . . . . . . . . . . . . . . . .
USDFLD and Automatic Time Incrementation . . . . . . . . . . . .
Example: Laminated Composite Plate Failure. . . . . . . . . . . . . . .
Material Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Partial Input Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
User Subroutine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
L4.13
L4.15
L4.15
L4.15
L4.16
L4.18
L4.19
L4.20
L4.21
L4.22
L4.24
L4.27
L4.29
L4.38
L4.41
L4.45
L4.48
Lecture 5
User Subroutine: URDFIL
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L5.2
ABAQUS Usage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L5.4
Utility Routine POSFIL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L5.5
Utility Routine DBFILE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L5.7
URDFIL Subroutine Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . L5.9
Variables to be Defined . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L5.10
Variables for Information Only . . . . . . . . . . . . . . . . . . . . . . . L5.11
Example: Using URDFIL to Terminate an Analysis . . . . . . . . . . L5.12
Input Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L5.13
User Subroutine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L5.16
Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L5.21
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ABAQUS
Lecture 6
Writing a UMAT or VUMAT
Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L6.2
Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L6.4
Steps Required in Writing a UMAT or VUMAT . . . . . . . . . . . . . . . L6.12
UMAT Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L6.20
UMAT Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L6.25
UMAT Utilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L6.28
UMAT Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L6.29
UMAT Formulation Aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . L6.30
Usage Hints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L6.32
Example 1: Isotropic Isothermal Elasticity . . . . . . . . . . . . . . . . . . L6.33
Governing Equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L6.33
Coding for Isotropic Isothermal Elasticity . . . . . . . . . . . . . . . L6.34
Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L6.36
Example 2: Non-Isothermal Elasticity . . . . . . . . . . . . . . . . . . . . . L6.38
Governing Equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L6.38
Coding for Non-Isothermal Elasticity . . . . . . . . . . . . . . . . . . . L6.39
Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L6.43
Example 3: Neo-Hookean Hyperelasticity . . . . . . . . . . . . . . . . . . L6.44
Governing Equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L6.44
Coding for Neo-Hookean Hyperelasticity . . . . . . . . . . . . . . . . L6.47
Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L6.53
Example 4: Kinematic Hardening Plasticity . . . . . . . . . . . . . . . . . L6.54
Governing Equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L6.54
Integration Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L6.56
Coding for Kinematic Hardening Plasticity . . . . . . . . . . . . . . L6.58
Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L6.66
Example 5: Isotropic Hardening Plasticity . . . . . . . . . . . . . . . . . . L6.69
Governing Equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L6.69
Integration Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L6.71
Coding for Isotropic Mises Plasticity . . . . . . . . . . . . . . . . . . . L6.73
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ABAQUS
Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L6.83
VUMAT Interface. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L6.85
VUMAT Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L6.90
Comparison of VUMAT and UMAT Interfaces . . . . . . . . . . . . . L6.92
VUMAT Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L6.94
VUMAT Formulation Aspects . . . . . . . . . . . . . . . . . . . . . . . . . L6.96
Example 6: VUMAT for Kinematic Hardening . . . . . . . . . . . . . . . L6.98
Coding for Kinematic Hardening Plasticity VUMAT . . . . . . . . L6.99
Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L6.105
Example 7: VUMAT for Isotropic Hardening . . . . . . . . . . . . . . . L6.107
Coding for Isotropic Hardening Plasticity VUMAT . . . . . . . . L6.108
Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L6.117
Lecture 7
Creating a Nonlinear User Element
Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L7.2
Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L7.3
Defining a User Element . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L7.8
Key Characteristics of a User Element . . . . . . . . . . . . . . . . . . . L7.8
Other Important Element Properties . . . . . . . . . . . . . . . . . . . . . L7.9
Defining the User Element Behavior . . . . . . . . . . . . . . . . . . . L7.10
UEL Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L7.13
ABAQUS Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L7.13
Parameter Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L7.14
Data Lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L7.15
More on Keywords and Parameters . . . . . . . . . . . . . . . . . . . . L7.19
User Element Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L7.22
UEL Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L7.23
UEL Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L7.24
UEL Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L7.27
UEL Formulation Aspects and Usage Hints . . . . . . . . . . . . . . L7.28
Coding and Testing the UEL . . . . . . . . . . . . . . . . . . . . . . . . . . L7.30
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ABAQUS
Example 1: Planar Beam Element with Nonlinear
Section Behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L7.32
Objective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L7.32
Coding Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L7.33
Element Formulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L7.35
Element Definition in the Input File . . . . . . . . . . . . . . . . . . . . L7.39
Coding for Planar Beam Example . . . . . . . . . . . . . . . . . . . . . . L7.40
Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L7.45
Generalized Constitutive Behavior . . . . . . . . . . . . . . . . . . . . . L7.46
Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L7.50
Example 2: Force Control Element. . . . . . . . . . . . . . . . . . . . . . . . L7.51
Objective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L7.51
Element Formulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L7.54
Element Definition in the Input File . . . . . . . . . . . . . . . . . . . . L7.56
Coding for Force Control Element Example . . . . . . . . . . . . . . L7.57
Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L7.61
Using Nonlinear User Elements in Various Analysis Procedures L7.63
Overview of Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L7.63
Perturbation Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L7.66
Transient Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L7.69
Transient Heat Transfer Analysis . . . . . . . . . . . . . . . . . . . . . . L7.70
Dynamic Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L7.73
Workshops
Workshop Preliminaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . WP.1
User Subroutine FILM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . W1.1
User Subroutine UMAT: Tangent Stiffness . . . . . . . . . . . . . . . . . . W2.1
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ABAQUS
Lecture 1
Introduction
Overview
• Overview of Some User Subroutines
• Where User Subroutines Fit into ABAQUS/Standard
• Including User Subroutines in a Model
• Writing Output from User Subroutines
• Compiling and Linking User Subroutines
• Debugging Techniques and Proper Programming Habits
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ABAQUS
Overview of Some User Subroutines
Overview of Some User Subroutines
• ABAQUS/Standard provides users with an extensive array of user
subroutines that allow them to adapt ABAQUS to their particular
analysis requirements.
• Chapter 24 of the ABAQUS/Standard Users’ Manual details all 40+
user subroutines available in ABAQUS/Standard.
Some popular user subroutines include
CREEP: Use this subroutine to define time-dependent, viscoplastic
deformation in a material. The deformation is divided into
deviatoric behavior (creep) and volumetric behavior (swelling).
DLOAD: Use this subroutine to define nonuniform, distributed
mechanical loads (pressures and body forces).
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ABAQUS
Overview of Some User Subroutines
FILM: Use this subroutine to describe complex film coefficient
behavior (temperature and field variable dependence) and complex
sink temperature behavior.
FRIC: Use this subroutine when more complex models than those
provided with the ∗FRICTION option are needed to describe the
transmission of shear forces between surfaces. The models defined
in this subroutine must be local models (information is provided
only at the node making contact).
HETVAL: Use this subroutine to define complex models for internal
heat generation in a material, such as might occur when the
material undergoes a phase change.
UEL: Use this subroutine when it is necessary to create elements
with an element formulation that is not available in
ABAQUS/Standard.
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ABAQUS
Overview of Some User Subroutines
UEXPAN: Use this subroutine to define incremental thermal strains
when the material’s thermal expansion is too complex to model
with the ∗EXPANSION option.
UEXTERNALDB: Use this subroutine to help manage external
databases that may be used by other user subroutine or other
software programs that are providing ABAQUS data and/or using
data generated by ABAQUS.
UGENS: Use this subroutine to define complex, nonlinear
mechanical behavior for shell elements directly in terms of the
shell element’s section stiffness.
UMAT: Use this subroutine to define any complex, constitutive
models for materials that cannot be modeled with the available
ABAQUS material models.
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ABAQUS
Overview of Some User Subroutines
UPOREP: Use this subroutine to define the initial pore fluid
pressures in a coupled pore fluid diffusion and stress analysis as a
function of node location.
URDFIL: Use this subroutine to read the data from the results
(.fil) file at the end of an increment. The information can be used
to make decisions such as when to terminate the analysis or
whether to overwrite the results of the previous increment.
USDFLD: Use this subroutine to define the values of field variables
directly at the integration points of elements. The field variable
values can be functions of element variables such as stress or
strain.
UWAVE: Use this user subroutine to define complex wave
kinematics in an ABAQUS/Aqua simulation or to determine when
the configuration of the model should be updated in a stochastic
wave analysis.
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ABAQUS
Where User Subroutines Fit into ABAQUS/Standard
Where User Subroutines Fit into
ABAQUS/Standard
While a complete understanding of the structure of ABAQUS is not
required to develop a user subroutine, it helps if the developer has an
understanding of at least the overall structure.
Figure 1–1 shows the basic flow of data and actions from the start of an
ABAQUS/Standard analysis to the end of a step. Figure 1–2 shows a
much more detailed accounting of how ABAQUS/Standard calculates the
element stiffness during an iteration.
In the figures a
signifies a decision point in the code
or a specific state (i.e., beginning of increment) during the analysis.
A
analysis.
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signifies an action that is taken during the
Writing User Subroutines with ABAQUS
L1.6
ABAQUS
Where User Subroutines Fit into ABAQUS/Standard
Beginning of Analysis
UEXTERNALDB
Define Initial Conditions
UPOREP
Start of Step
Start of Increment
UEXTERNALDB
Start of Iteration
Define K
DLOAD,
FILM,
HETVAL,
UWAVE
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CREEP, FRIC, UEL,
UEXPAN, UGENS,
UMAT, USDFLD
el
Define Loads R
α
Writing User Subroutines with ABAQUS
L1.7
ABAQUS
Where User Subroutines Fit into ABAQUS/Standard
To Start of
Iteration
To Start of
Step
To Start of
Increment
el
Solve K c = R
Converged?
α
No
Write Output
UEXTERNALDB
URDFIL
End of Step?
No
Yes
Figure 1–1. Global Flow in ABAQUS/Standard
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Writing User Subroutines with ABAQUS
L1.8
ABAQUS
Where User Subroutines Fit into ABAQUS/Standard
Start of Increment
Calculate Integration Point Field Variable from Nodal Values
CREEP: ∆ε
Start of Iteration
UEL
UMAT
USDFLD
FILM dh ⁄ d θ ,
HETVAL ∂r ⁄ ∂θ ,
cr
UEXPAN: ∆ε
∆ε
sw
th
Calculate ∆ε
FRIC: ∂∆τ ⁄ ∂∆γ
∂∆σ
Calculate σ , ---------∂∆ε
UGENS: ∂N ⁄ ∂E
Define Loads ∂P ⁄ ∂ x
DLOAD,
UWAVE
Figure 1–2. A More Detailed Flow of ABAQUS/Standard
7/01
Writing User Subroutines with ABAQUS
L1.9
ABAQUS
Where User Subroutines Fit into ABAQUS/Standard
User Subroutine Calls in the First Iteration
• The flow chart in Figure 1–2 is idealized. In the first iteration of an
increment all of the user subroutines shown in the figure are called
twice.
– During the first call the initial stiffness matrix is being formed
using the configuration of the model at the start of the increment.
– During the second call a new stiffness, based on the updated
configuration of the model, is created.
• In subsequent iterations the subroutines are called only once.
– In these subsequent iterations the corrections to the model’s
configuration are calculated using the stiffness from the end of the
previous iteration.
7/01
Writing User Subroutines with ABAQUS
L1.10
ABAQUS
Including User Subroutines in a Model
Including User Subroutines in a Model
• To include user subroutines in an analysis, specify the name of a file
with the user parameter on the ABAQUS execution command.
abaqus job=my_analysis user=my_subroutine
The file should be either source code (my_subroutine.f) or an
object file (my_analysis.o). The file extension can be included
(user=my_analysis.f); otherwise, ABAQUS will determine
automatically which type of file is specified.
7/01
Writing User Subroutines with ABAQUS
L1.11
ABAQUS
Including User Subroutines in a Model
Using Multiple User Subroutines in a Model
• When multiple user subroutines are needed in the analysis, the
individual routines can be combined into a single file.
• A given user subroutine (such as UMAT or FILM) should appear only
once in the specified user subroutine source or object code.
Restart Analyses
• When an analysis that includes a user subroutine is restarted, the user
subroutine must be specified again because the subroutine object or
source code is not stored on the restart (.res) file.
7/01
Writing User Subroutines with ABAQUS
L1.12
ABAQUS
Writing Output from User Subroutines
Writing Output from User Subroutines
• The following unit numbers can be used within a user subroutine to
read and write data from files:
15–18
100+
• In ABAQUS/Standard user subroutines can write debug output to the
message (.msg) file (unit 7) or to the printed output (.dat) file
(unit 6).
– These units do not have to be opened within the user subroutine—
they are opened by ABAQUS.
– These unit numbers cannot be used by user subroutines in
ABAQUS/Explicit.
7/01
Writing User Subroutines with ABAQUS
L1.13
ABAQUS
Writing Output from User Subroutines
Path Names for External Files
• When a file is opened in a user subroutine, ABAQUS assumes that it is
located in the scratch directory created for the simulation; therefore,
full path names must be used in the OPEN statements in the subroutine
to specify the location of the files.
7/01
Writing User Subroutines with ABAQUS
L1.14
ABAQUS
Compiling and Linking User Subroutines
Compiling and Linking User Subroutines
• When a model that contains user subroutines is submitted to ABAQUS,
the correct compile and link commands should be used automatically.
– HKS includes the correct compile and link commands for every
platform on which ABAQUS is supported in the default
environment file (abaqus.env) located in the abaqus_dir/site
directory.
abaqus_dir is the directory in which ABAQUS was installed.
7/01
Writing User Subroutines with ABAQUS
L1.15
ABAQUS
Compiling and Linking User Subroutines
– For example on the Windows NT release, the following commands
are defined in the abaqus_dir\site\abaqus.env file:
compile=”fl32 /c”
link=”link /defaultlib:libf.lib libc.lib user32.lib
netapi32.lib advapi32.lib mpr.lib libelm.lib
/subsystem:console /out:”
• If you encounter compile or link errors, check that the
abaqus_dir\site\abaqus.env file defines the compile and link
commands. If it does not, please contact HKS. We will provide you
with the correct commands for your system.
7/01
Writing User Subroutines with ABAQUS
L1.16
ABAQUS
Compiling and Linking User Subroutines
FORTRAN Compiler Levels
• The FORTRAN compiler levels used to create ABAQUS are shown at
www.abaqus.com/support on the World Wide Web.
– Digital Visual Fortran and Microsoft Visual C++ must be installed
to run user subroutines on computers running Windows NT 4.0.
• If the version of your FORTRAN compiler does not correspond to that
specified on the HKS web site, incompatibilities may occur.
7/01
Writing User Subroutines with ABAQUS
L1.17
ABAQUS
Debugging Techniques and Proper Programming Habits
Debugging Techniques and Proper
Programming Habits
Some programming habits and suggested debugging techniques are
discussed in this section.
Required FORTRAN Statements
• Every user subroutine in ABAQUS/Standard must include the
statement
INCLUDE ‘ABA_PARAM.INC’
as the first statement after the argument list.
7/01
Writing User Subroutines with ABAQUS
L1.18
ABAQUS
Debugging Techniques and Proper Programming Habits
• The file ABA_PARAM.INC is installed on the computer system by the
ABAQUS installation procedure.
– The file specifies either IMPLICIT REAL*8 (A-H, O-Z) for
double precision machines or IMPLICIT REAL (A-H, O-Z) for
single precision machines.
– The ABAQUS execution procedure, which compiles and links the
user subroutine with the rest of ABAQUS, will include the
ABA_PARAM.INC file automatically.
– It is not necessary to find this file and copy it to any particular
directory: ABAQUS will know where to find it.
• Every user subroutine in ABAQUS/Explicit must include the statement
include ‘vaba_param.inc’
7/01
Writing User Subroutines with ABAQUS
L1.19
ABAQUS
Debugging Techniques and Proper Programming Habits
Naming Conventions
• If user subroutines call other subroutines or use COMMON blocks to
pass information, such subroutines or COMMON blocks should begin
with the letter K since this letter is never used to start the name of any
subroutine or COMMON block in ABAQUS.
Subroutine Argument Lists
• The variables passed into a user subroutine via the argument list are
classified as either variables to be defined, variables that can be defined,
or variables passed in for information.
• The user must not alter the values of the “variables passed in for
information.”
– Doing so will have unpredictable results.
7/01
Writing User Subroutines with ABAQUS
L1.20
ABAQUS
Debugging Techniques and Proper Programming Habits
Solution-Dependent State Variables
• Solution-dependent state variables (SDVs) are values that can be
defined to evolve with the solution of an analysis.
– An example of a solution-dependent state variable for the UEL
subroutine is strain.
• Several user subroutines allow the user to define SDVs. Within these
user subroutines the SDVs can be defined as functions of any variables
passed into the user subroutine.
– It is the user’s responsibility to calculate the evolution of the SDVs
within the subroutine; ABAQUS just stores the variables for the
user subroutine.
7/01
Writing User Subroutines with ABAQUS
L1.21
ABAQUS
Debugging Techniques and Proper Programming Habits
• Space must be allocated to store each of the solution-dependent state
variables defined in a user subroutine.
– For most subroutines the number of such variables required at the
integration points or nodes is entered as the only value on the data
line of the ∗DEPVAR option.
*USER MATERIAL
*DEPVAR
8
– For subroutines UEL and UGENS the VARIABLES parameter must
be used on the ∗USER ELEMENT and ∗SHELL GENERAL
SECTION options, respectively.
*USER ELEMENT, VARIABLES=8
– For subroutine FRIC the number of variables is defined with the
DEPVAR parameter on the ∗FRICTION option.
*FRICTION, USER, DEPVAR=8
7/01
Writing User Subroutines with ABAQUS
L1.22
ABAQUS
Debugging Techniques and Proper Programming Habits
• There are two methods available for defining the initial values of
solution-dependent variables.
– The ∗INITIAL CONDITIONS, TYPE=SOLUTION option can be
used to define the variable field in a tabular format
– For complicated cases user subroutine SDVINI can be used to
define the initial values of the SDVs. Invoke this subroutine by
adding the USER parameter to the ∗INITIAL CONDITIONS,
TYPE=SOLUTION option.
7/01
Writing User Subroutines with ABAQUS
L1.23
ABAQUS
Debugging Techniques and Proper Programming Habits
Testing Suggestions
• Always develop and test user subroutines on the smallest possible
model.
• Do not include other complicated features, such as contact, unless they
are absolutely necessary when testing the subroutine.
• Test the most basic model of the user subroutine before adding
additional complexity to the subroutine.
– Whenever a “new” feature is added to the model in a user
subroutine, test it before adding an additional feature.
• When appropriate, try to test the user subroutine in models where only
values of the nodal degrees of freedom (displacement, rotations,
temperature) are specified. Then test the subroutine in models where
fluxes and nodal degrees of freedom are specified.
7/01
Writing User Subroutines with ABAQUS
L1.24
ABAQUS
Debugging Techniques and Proper Programming Habits
• Ensure that arrays passed into a user subroutine with a given dimension
are not used as if they had a larger dimension.
– For example, if a user subroutine is written such that the number of
SDVs is 10 but only 8 SDVs are specified on the ∗DEPVAR
option, the user subroutine will overwrite data stored by ABAQUS;
the consequences of this accident will be unpredictable.
7/01
Writing User Subroutines with ABAQUS
L1.25
ABAQUS
Lecture 2
User Subroutine: DLOAD
Overview
• Introduction
• ABAQUS Usage
• DLOAD Subroutine Interface
• Example: Transient Internal Pressure Loading
• Example: Asymmetric Pressure Loads
7/01
Writing User Subroutines with ABAQUS
L2.1
ABAQUS
Introduction
Introduction
• User subroutine DLOAD is typically used when a load is a complex
function of time and/or position.
– Loads that are simple functions of time can usually be modeled
with the ∗AMPLITUDE option.
– The subroutine can also be used to define a load that varies with
element number and/or integration point number.
7/01
Writing User Subroutines with ABAQUS
L2.2
ABAQUS
ABAQUS Usage
ABAQUS Usage
• The subroutine is called when the ∗DLOAD or *DSLOAD options
contain a nonuniform load type label.
Load type label
– For example,
*DLOAD
ELTOP,P1NU, 10.0
Load magnitude
specifies that the elements in element set ELTOP will be subject to a
force per area on the 1-face of solid (continuum) elements or a
force per unit length in the beam local 1-direction when used with
beam elements.
The magnitude specified on the data line is passed into the
subroutine as the value of variable F.
7/01
Writing User Subroutines with ABAQUS
L2.3
ABAQUS
ABAQUS Usage
• A list of the nonuniform distributed load types that are available for use
with any particular element is given in the ABAQUS/Standard Users’
Manual.
• The AMPLITUDE parameter cannot be used with the ∗DLOAD or
*DSLOAD options when the user subroutine is used to define the
magnitude of the distributed load.
• The distributed load magnitude cannot be written as output with the
∗EL FILE or ∗EL PRINT options.
DLOAD vs. UEL
• If the distributed load is dependent on the element’s deformation rather
than the position of the element, a stiffness is being defined and a user
element subroutine (UEL), not user subroutine DLOAD, is required.
7/01
Writing User Subroutines with ABAQUS
L2.4
ABAQUS
DLOAD Subroutine Interface
DLOAD Subroutine Interface
The interface to user subroutine DLOAD is
SUBROUTINE DLOAD(F, KSTEP, KINC, TIME, NOEL, NPT,
1 LAYER, KSPT, COORDS, JLTYP, SNAME)
C
INCLUDE ‘ABA_PARAM.INC’
C
DIMENSION TIME(2), COORDS(3)
CHARACTER*80 SNAME
user coding to define F
RETURN
END
7/01
Writing User Subroutines with ABAQUS
L2.5
ABAQUS
DLOAD Subroutine Interface
Variables to be Defined
The user has to define only the variable F. It is the magnitude of the
distributed load and has units that depend on the type of distributed load
applied:
• FL
–1
for line loads along one-dimensional (beam) elements,
• FL
–2
for surface loads (e.g., pressures), and
• FL
–3
for body forces (e.g., gravity, centripetal, acceleration).
Variables for Information Only
The following variables are passed into the subroutine:
• The step (KSTEP) and increment number (KINC) in which the routine
is being called.
• The current value of the step (TIME(1)) and the total time
(TIME(2)).
7/01
Writing User Subroutines with ABAQUS
L2.6
ABAQUS
DLOAD Subroutine Interface
• The element number (NOEL) and integration point number (NPT).
• The layer (LAYER) and section point numbers (KSPT), where
appropriate.
• The coordinates of the integration point (COORDS).
– These are the current coordinates if the ∗STEP, NLGEOM option
is used in this or a previous step.
• The identifier (JLTYP) specifying the type of load to be defined in
this call to the user subroutine.
– If multiple user-defined DLOAD types are specified in a model, the
coding for all load types must appear in the subroutine, and this
variable (JLTYP) must be used to test for which load type is to be
defined when the subroutine is called.
• The surface name (SNAME) for a surface-base load definition
(JLTYP=0).
7/01
Writing User Subroutines with ABAQUS
L2.7
ABAQUS
Example: Transient Internal Pressure Loading
Example: Transient Internal Pressure Loading
• The problem models the viscoelastic response in rocket propellent as
the transient pressure load, due to rocket ignition, is applied to the inner
diameter of the rocket motor.
– The transient pressure load is an exponential function of time.
p = 10 ( 1 – e
– 23.03t
) MPa.
• The rocket motor is modeled with a single row of 21 elements.
• The model is fully described in Problem 2.2.7 of the ABAQUS
Verification Manual.
7/01
Writing User Subroutines with ABAQUS
L2.8
ABAQUS
Example: Transient Internal Pressure Loading
Partial Input Data
*HEADING
:
:
*BOUNDARY
ALL,2
*STEP, INC=50
*VISCO, CETOL=7.E-3
0.01, 0.5
*DLOAD
Apply nonuniform DLOAD to face 4 of element 1
1, P4NU
:
:
*END STEP
7/01
Writing User Subroutines with ABAQUS
L2.9
ABAQUS
Example: Transient Internal Pressure Loading
User Subroutine
SUBROUTINE DLOAD(F, KSTEP, KINC, TIME ,NOEL, NPT,
1 LAYER, KSPT, COORDS, JLTYP, SNAME)
C
C
C
EXPONENTIAL PRESSURE LOAD
INCLUDE ‘ABA_PARAM.INC’
C
DIMENSION COORDS(3),TIME(2)
CHARACTER*80 SNAME
DATA TEN,ONE,CONST /10.,1.,-23.03/
F=TEN*(ONE-(EXP(CONST*TIME(1))))
IF(NPT.EQ.1) WRITE(6,*) ‘ LOAD APPLIED’,F,’AT
TIME=’,TIME(1)
RETURN
END
7/01
Writing User Subroutines with ABAQUS
L2.10
ABAQUS
Example: Transient Internal Pressure Loading
Remarks
• The load in this model is defined as a function of step time, time(1).
• The load is applied only to the one element on the inner diameter of the
rocket motor, element 1.
• The load is monitored by writing output to the printed output (.dat)
file, once per iteration, when the distributed load value is defined at the
first integration point.
7/01
Writing User Subroutines with ABAQUS
L2.11
ABAQUS
Example: Asymmetric Pressure Loads
Example: Asymmetric Pressure Loads
• In this problem asymmetric pressure loads are applied to a cylindrical
structure, which is modeled with the CAXA family of elements.
– These elements have axisymmetric geometry but asymmetric
deformation.
– For CAXA elements the third coordinate, COORD(3), of a point is
its θ position around the circumference of the structure
( 0 ≤ θ ≤ 180 ).
– The radial stress distribution at the outer radius, R o , is
σ rr = – p cos θ ,
and at inner radius, R i , it is
Ro
σ rr = – ------ p cos θ .
Ri
7/01
Writing User Subroutines with ABAQUS
L2.12
ABAQUS
Example: Asymmetric Pressure Loads
– The magnitude of p is 10.E3.
– This model is described fully in Problem 1.3.31 of the ABAQUS
Verification Manual.
7/01
Writing User Subroutines with ABAQUS
L2.13
ABAQUS
Example: Asymmetric Pressure Loads
Partial Input Data
*HEADING
ASYMMETRIC INTERAL AND EXTERNAL PRESSURE LOADS
** 6-IN LONG CYLINDER OF 2-IN & 6-IN INNER AND OUTER RADII
:
*ELSET, ELSET=INWALL
1
Ri
Ro
*ELSET, ELSET=OUTWALL
10
:
*STEP
*STATIC
:
*DLOAD
INWALL, P4NU
OUTWALL, P2NU
*END STEP
7/01
Writing User Subroutines with ABAQUS
L2.14
ABAQUS
Example: Asymmetric Pressure Loads
User Subroutine
SUBROUTINE DLOAD(F, KSTEP, KINC, TIME, NOEL, NPT,
1
LAYER, KSPT, COORDS, JLTYP, SNAME)
INCLUDE ‘ABA_PARAM.INC’
DIMENSION COORDS(3)
CHARACTER*80 SNAME
C NOTE THAT COORDS(3) IS THE ANGULAR COORD IN DEGREES
PI=2.* ASIN(1.D0)
THETA=PI*COORDS(3)/180.D0
P=0.
IF(JLTYP.EQ.22) P=10.D3
Test for load type P4NU
IF(JLTYP.EQ.24) P=30.D3
F=P* COS(THETA)
RETURN
END
Test for load type P2NU
7/01
Writing User Subroutines with ABAQUS
L2.15
ABAQUS
Example: Asymmetric Pressure Loads
Remarks
• In this model subroutine DLOAD is used to define two different
nonuniform distributed pressure loads.
– In this model the load type label clearly determined which pressure
distribution should be used.
– In a different model perhaps the element number (NOEL) or radial
position (COORDS(1)) would have to be used to identify which
distribution to define.
– What are some of the methods that could be used to allow the
subroutine to define pressure distributions with different values of
p on the inner and outer radii?
7/01
Writing User Subroutines with ABAQUS
L2.16
ABAQUS
Lecture 3
User Subroutine: FILM
Overview
• Introduction
• ABAQUS Usage
• FILM Subroutine Interface
• Example: Radiation in Finned Surface
• Workshop: User Subroutine FILM
7/01
Writing User Subroutines with ABAQUS
L3.1
ABAQUS
Introduction
Introduction
• User subroutine FILM is typically used when either the film coefficient,
s
h , or sink temperature, θ , is a complex function of time, position,
and/or surface temperature.
s
– h and θ that are simple functions of time usually can be modeled
with the ∗AMPLITUDE option.
– The subroutine can also be used to define a load that varies with
element number and/or integration point number.
7/01
Writing User Subroutines with ABAQUS
L3.2
ABAQUS
ABAQUS Usage
ABAQUS Usage
• The subroutine is called when the ∗FILM or *SFILM options contain a
nonuniform load type label or when the USER parameter is used with
the *CFILM option.
Load type label
• For example,
*FILM
ELLEFT, F6NU, 10.0, 1500
θ
s
h
specifies that the elements in element set ELLEFT will be subject to a
film load (convection boundary condition) on face six (6) of solid
(continuum), heat transfer elements.
– The variable H(1) will be passed into the routine with the value of
h specified on the data line of the ∗FILM option.
– The variable SINK will be passed into the routine with the value of
s
θ specified on the data line of the ∗FILM option.
7/01
Writing User Subroutines with ABAQUS
L3.3
ABAQUS
FILM Subroutine Interface
FILM Subroutine Interface
The interface to user subroutine FILM is:
SUBROUTINE FILM(H, SINK, TEMP, KSTEP, KINC, TIME,
1 NOEL, NPT, COORDS, JLTYP, FIELD, NFIELD, SNAME,
2 NODE, AREA)
C
INCLUDE ‘ABA_PARAM.INC’
C
DIMENSION H(2), TIME(2), COORDS(3), FIELD(NFIELD)
CHARACTER*80 SNAME
user coding to define H(1), H(2), and SINK
RETURN
END
7/01
Writing User Subroutines with ABAQUS
L3.4
ABAQUS
FILM Subroutine Interface
Variables to be Defined
The user has to define the following variables:
• H(1). The film coefficient, h , at this surface point.
–1 –2 –1
Its units are J T L θ .
• H(2). The rate of change of the film coefficient with respect to the
dh
–1 –2 –2
surface temperature at this point ( ------ ). Its units are J T L θ .
dθ
– The rate of convergence during the solution of the nonlinear
equations in an increment is improved by defining this value,
especially when the film coefficient is a strong function of surface
temperature (TEMP).
s
• SINK, the sink temperature, θ .
7/01
Writing User Subroutines with ABAQUS
L3.5
ABAQUS
FILM Subroutine Interface
Variables for Information Only
• The estimated surface temperature at this time at this point (TEMP).
• The step (KSTEP) and increment number (KINC) in which the routine
is being called.
• The current value of the step (TIME(1)) and the total time
(TIME(2)).
• The element number (NOEL) and integration point number (NPT).
• The coordinates of the integration point (COORDS).
– These are the current coordinates if the ∗STEP, NLGEOM option
is used in this or a previous step.
7/01
Writing User Subroutines with ABAQUS
L3.6
ABAQUS
FILM Subroutine Interface
• The identifier (JLTYP) specifying the type of load to be defined in
this call to the user subroutine.
– If multiple user-defined FILM types are specified in a model, the
coding for all film types must appear in the subroutine, and this
variable (JLTYP) must be used to test for which film type is to be
defined when the subroutine is called.
• The interpolated values of field variables, f i , at this point (FIELD).
s
– Any of the variables, h or θ , can be made functions of f i .
• The number of field variables (NFIELD).
• The surface name (SNAME) for a surface-based film definition
(JLTYP=0).
• The node number (NODE) and area (AREA) for a node-based film
definition. The area is passed in as the value specified on the data line
of the *CFILM option.
7/01
Writing User Subroutines with ABAQUS
L3.7
ABAQUS
Example: Radiation in Finned Surface
Example: Radiation in Finned Surface
• This problem is described in detail in Section 4.1.4 of the ABAQUS
Example Problems Manual.
• This problem models the heat transfer through a finned structure.
– In Step 1 the steady-state conditions are obtained.
– In Step 2 the transient response of a 30-minute fire is obtained. The
finned surface is exposed to the fire.
– In Step 3 the 60-minute transient response of the structure when
the steady-state boundary conditions are returned is obtained.
7/01
Writing User Subroutines with ABAQUS
L3.8
ABAQUS
Example: Radiation in Finned Surface
o
External fluid (38o C to 800 C)
.01m
1
.15m
2
.06m
.1m
Wall
3
o
F. E. model
Internal fluid (100 C)
Figure 3–1. Sketch of Finned Structure
7/01
Writing User Subroutines with ABAQUS
L3.9
ABAQUS
Example: Radiation in Finned Surface
Element set TOPF3
Element set TOPF4
Element set TOPF2
2
3
1
Element set BOTF1
Figure 3–2. Finite Element Mesh and Element Sets That Use FILM
7/01
Writing User Subroutines with ABAQUS
L3.10
ABAQUS
Example: Radiation in Finned Surface
• Along the inner wall (element set BOTF1) natural convection transfers
heat between the structure and surrounding fluid, which is at 100° C.
h ( θ ) = 500 θ w –
s 1⁄3
θi
θ w is the temperature of the wall of the structure.
s
θ i is the internal fluid temperature.
• Along the outer, finned surface, radiation and natural convection
boundary conditions exist.
During Steps 1 and 3,
s 1⁄3
h ( θ ) = 2 θw – θ f
.
s
θ f is the external fluid temperature ( 38° C).
During Step 2 (the fire transient), h = 10 .
7/01
Writing User Subroutines with ABAQUS
L3.11
ABAQUS
Example: Radiation in Finned Surface
Partial Input Data
*HEADING
:
*STEP, INC=500
*HEAT TRANSFER, STEADY STATE
1.0
*BOUNDARY
NAMB, 11, , 38.D0
*FILM
BOTF1, F1NU
*FILM
User subroutine FILM used for all these
TOPF3, F3NU
element sets
TOPF4, F4NU
TOPF2, F2NU
:
*END STEP
7/01
Writing User Subroutines with ABAQUS
L3.12
ABAQUS
Example: Radiation in Finned Surface
User Subroutine
subroutine film(h, sink, temp, jstep, jinc, time,
1 noel, npt, coords, jltyp, field, nfield, sname,
node, area)
c
include ‘aba_param.inc’
dimension h(2), coords(3), time(2), field(nfield)
character*80 sname
parameter (two=2.0d0, third=1.0d0/3.0d0)
c
h(1) = 0.0d0
Test to see if the elements are in
h(2) = 0.0d0
element set BOTF1
sink = 0.0d0
a2
= 1.0d0
if (noel.le.11) then
sink = 100.d0
a1 = sign(a2,temp-sink)
h(1) =
500.0d0*(abs(temp-sink))**third
7/01
Writing User Subroutines with ABAQUS
L3.13
ABAQUS
Example: Radiation in Finned Surface
h(2) = a1*third*500.0d0*
1
(abs(temp-sink))**(-two*third)
else if (jstep.eq.1.or.jstep.eq.3) then
sink = 38.d0
a1 = sign(a2,temp-sink)
h(1) =
2.0d0*(abs(temp-sink))**third
h(2) = a1*third*2.0d0*
1
(abs(temp-sink))**(-two*third)
else
s
h ( θ ) and fluid temperature ( θ f ) for the
sink = 800.d0
fire event
h(1) = 10.0d0
end if
c
return
end
7/01
If the first condition is not satisfied, the element
must be on the finned surface. The h ( θ ) for
these elements varies from step to step.
Writing User Subroutines with ABAQUS
L3.14
ABAQUS
Example: Radiation in Finned Surface
Remarks
• It can be helpful to define constants, in this example 2.0 and 0.333 as
parameters in user subroutines.
• Using element numbers as values for conditional statements can limit a
user subroutine to a specific mesh layout.
– This programming/design technique can make the use of the
subroutine in general production work tedious.
– Can you think of other techniques that can be used with this user
subroutine?
dh
• Defining ------ correctly is extremely important in this application.
dθ
– Analyses will converge very slowly if it is incorrect.
7/01
Writing User Subroutines with ABAQUS
L3.15
ABAQUS
Workshop: User Subroutine FILM
Workshop: User Subroutine FILM
Goals
• To learn how to find the compile and link commands used on your
system.
dh
• To see how sensitive the rate of convergence is on the value of ------ .
dθ
dh
• To see if the results are sensitive to the value of ------ .
dθ
Problem Description
• User subroutine FILM will be used to define
s 1⁄3
h ( θ ) = 500 θ w – θ i
,
s
where θ i = 100° C is the fluid temperature.
7/01
Writing User Subroutines with ABAQUS
L3.16
ABAQUS
Workshop: User Subroutine FILM
• The subroutine is tested on a two-element model.
– All nodes initially have a temperature of 77° C ( θ ( t = 0 ) = 77 ).
– The nodes on one end are set to 277° C. The other end has the film
condition applied.
– The steady-state solution is obtained.
• The rate of convergence and results are compared with the following
dh
values of ------ :
dθ
h(2)
h(2)
h(2)
h(2)
7/01
=
=
=
=
a1*third*500.d0*(abs(temp-sink))**(-two*third)
third*500.d0*(abs(temp-sink))**(-two*third)
a1*third*500.0d0*(abs(temp-sink))**(-third)
a1*third*500.0d0*(abs(temp-sink))**(two*third)
Writing User Subroutines with ABAQUS
L3.17
ABAQUS
Lecture 4
User Subroutine: USDFLD
Overview
• Introduction
• ABAQUS Usage
• User Subroutine GETVRM
• USDFLD Subroutine Interface
• Example: Laminated Composite Plate Failure
7/01
Writing User Subroutines with ABAQUS
L4.1
ABAQUS
Introduction
Introduction
• User subroutine USDFLD is typically used when complex material
behavior needs to be modeled and the user does not want to develop a
UMAT subroutine.
– Most material properties in ABAQUS/Standard can be defined as
functions of field variables, f i .
– Subroutine USDFLD allows the user to define f i at every integration
point of an element.
pl
– The subroutine has access to solution data, so f i ( σ, ε, ε , ε̇, etc. ) ;
therefore, the material properties can be a function of the solution
data.
• Subroutine USDFLD can be used only with elements that have material
behavior defined with a ∗MATERIAL option [see Elements
Supported by GETVRM (p. L4.18) for details].
7/01
Writing User Subroutines with ABAQUS
L4.2
ABAQUS
ABAQUS Usage
ABAQUS Usage
• Including user subroutine USDFLD in a model requires considerably
more effort than what is needed for user subroutines DLOAD or FILM.
• Typically the user must define the dependence of material properties,
such as elastic modulus or yield stress, as functions of field
variables, f i .
– This can be accomplished using either tabular input or additional
user subroutines.
7/01
Writing User Subroutines with ABAQUS
L4.3
ABAQUS
ABAQUS Usage
• The USDFLD routine is then written to define the values of f i on an
integration point by integration point basis.
– The ∗USER DEFINED FIELD option is included in the material
definition to indicate that the USDFLD subroutine will be called for
those elements using that material definition.
– The f i can be defined as functions of solution data, such as stress
or strain, available at the integration points.
7/01
Writing User Subroutines with ABAQUS
L4.4
ABAQUS
ABAQUS Usage
Defining Field-Variable-Dependent Material Properties
There are two methods that can be used to create field-variable-dependent
material properties:
• Using tabular definition for built-in ABAQUS material models.
• Using other user subroutines, such as CREEP, to define the material
behavior as a function of f i .
7/01
Writing User Subroutines with ABAQUS
L4.5
ABAQUS
ABAQUS Usage
Tabular Definition:
• Use the DEPENDENCIES parameter on the material options to
specify how many different field variables exist for a given material
option:
*MATERIAL, NAME=POLYMER
*ELASTIC, DEPENDENCIES=1
2000., 0.3, 0., 0.00
1200., 0.3, 0., 0.02
1000., 0.3, 0., 0.04
*EXPANSION, DEPENDENCIES=2
5E-4, 0., 0.00, 0.0
3E-4, 0., 0.02, 0.0
1E-4, 0., 0.04, 0.0
**
5E-5, 0., 0.00, 1.0
2E-5, 0., 0.03, 1.0
8E-6, 0., 0.04, 1.0
7/01
f1
Writing User Subroutines with ABAQUS
f2
L4.6
ABAQUS
ABAQUS Usage
– The elastic modulus ( E ) is a function of field variable #1, f 1 . As
f 1 increases, E decreases— f 1 might represent damage to the
material.
– The thermal expansion coefficient, α , is a function of both f 1 and
field variable #2, f 2 .
– A change in the value of f 1 will affect both E and α .
– ABAQUS will use linear interpolation between data points in the
tabular input and will use the last available material data if f i is
outside of the range specified—it does not extrapolate the data
provided.
– The range of f i does not have to be the same for each material
property.
7/01
Writing User Subroutines with ABAQUS
L4.7
ABAQUS
ABAQUS Usage
Defining Field Variable Dependence within a User
Subroutine:
• The values of f i defined in USDFLD are passed into the following
user subroutines:
CREEP
HETVAL
UEXPAN
UHARD
UHYPEL
UMAT
UMATHT
UTRS
UINTER
• The material properties defined in these subroutines can be made
functions of the f i .
7/01
Writing User Subroutines with ABAQUS
L4.8
ABAQUS
ABAQUS Usage
Defining Field Variables
• Field variables ( f i ) are normally considered nodal data by ABAQUS.
• When ABAQUS begins to calculate the element stresses and stiffness
(i.e., the element loop), it interpolates the nodal values of f i to the
integration (material) points of the elements.
• When subroutine USDFLD is used, these interpolated f i are replaced
with the values defined in the USDFLD subroutine before the material
properties of an element are calculated.
• The values defined by USDFLD are not stored by ABAQUS.
– If you need access to previous values of f i , you must save them as
solution-dependent variables (SDVs) inside USDFLD.
7/01
Writing User Subroutines with ABAQUS
L4.9
ABAQUS
ABAQUS Usage
• If you bypass the USDFLD subroutine (perhaps because the material
properties are not going to change in a given step), the integration
points will use the interpolated values of f i .
– Typically these interpolated f i are the initial values assigned to the
nodes—the default in ABAQUS is to assign a value of 0.0 if no
initial value is given explicitly.
– It is quite possible that using these interpolated values when
defining the material behavior will create incorrect results. Make
sure you understand what ABAQUS is doing.
• The values of f i at the element integration points can be written as
output to the printed output (.dat), results (.fil), and output database
(.odb) files using the output variable FV on the ∗EL PRINT, ∗EL
FILE, and *ELEMENT OUTPUT options, respectively.
– ABAQUS/Viewer can make contour plots of FV#.
7/01
Writing User Subroutines with ABAQUS
L4.10
ABAQUS
ABAQUS Usage
Accessing Solution Data at Material Points
• ABAQUS/Standard allows the f i to be defined as functions of solution
data, such as stress or strain, at the material points.
• The values of the solution data provided are from the beginning of the
current increment.
• Subroutine USDFLD must use the ABAQUS utility routine GETVRM to
access this material point data.
7/01
Writing User Subroutines with ABAQUS
L4.11
ABAQUS
ABAQUS Usage
Explicit vs. Implicit Solution
• Since the USDFLD subroutine has access to material point quantities
only at the start of the increment, the solution dependence introduced in
this way is explicit.
– The material properties for a given increment are not influenced by
the results obtained during the increment.
– Hence, the accuracy of the results depends on the size of the time
increment.
– Therefore, the user can control the time increment in the USDFLD
subroutine by means of the variable PNEWDT.
7/01
Writing User Subroutines with ABAQUS
L4.12
ABAQUS
ABAQUS Usage
• For most nonlinear material behavior (i.e., plasticity)
ABAQUS/Standard uses an implicit integration method to calculate the
material behavior at the end of the current increment.
– Such an implicit integration method allows ABAQUS/Standard to
use any size time increment and yet still have the solution remain
bounded.
Using Solution-Dependent State Variables
• Solution-dependent state variables (SDVs) must be used in USDFLD if
f i have any history dependence.
– ABAQUS/Standard does not store the values of f i calculated in
USDFLD.
• The SDVs updated in USDFLD are then passed into other user
subroutines that can be called at this material point, such as those listed
7/01
Writing User Subroutines with ABAQUS
L4.13
ABAQUS
ABAQUS Usage
in Defining Field-Variable-Dependent Material Properties
(p. L4.5).
• The number of state variables is specified with the ∗DEPVAR option:
*ELASTIC, DEPENDENCIES=1
** Table of modulus values decreasing as a function
** of field variable 1.
2000., 0.3, 0., 0.00
1500., 0.3, 0., 0.01
1200., 0.3, 0., 0.02
1000., 0.3, 0., 0.04
*USER DEFINED FIELD
*DEPVAR
1
7/01
Writing User Subroutines with ABAQUS
L4.14
ABAQUS
User Subroutine GETVRM
User Subroutine GETVRM
The subroutine GETVRM provides USDFLD with access to the solution data
stored in databases during the analysis.
GETVRM Subroutine Interface
CALL GETVRM(‘VAR’, ARRAY, JARRAY, FLGRAY, JRCD,
1 JMAC, JMATYP, MATLAYO, LACCFLA)
Variables Provided to GETVRM
• The variables provided to GETVRM are the output variable key, VAR, for
the desired solution data, and JMAC, JMATYP, MATLAYO, LACCFLA (these
variables are not discussed further in these notes)
• The available output variable keys are listed in the output table in
Section 4.2.1 of the ABAQUS/Standard User’s Manual.
– The variable must be available for results file output at the element
integration points; e.g., S for stress.
7/01
Writing User Subroutines with ABAQUS
L4.15
ABAQUS
User Subroutine GETVRM
Variables Returned by GETVRM
• An array containing individual floating-point components of the output
variable (ARRAY).
• An array containing individual integer value components of the output
variable (JARRAY).
• A character array (FLGRAY) containing flags corresponding to the
individual components.
– Flags will contain either YES, NO, or N/A (not applicable).
• A return code (JRCD). JRCD=0 indicates that GETVRM encountered no
errors, while a value of 1 indicates that there was an output request
error or that all components of the output variable requested are zero.
7/01
Writing User Subroutines with ABAQUS
L4.16
ABAQUS
User Subroutine GETVRM
• The components for a requested variable are written as follows.
– Single index components (and requests without components) are
returned in positions 1, 2, 3, etc.
– Double index components (tensors) are returned in the order 11,
22, 33, 12, 13, 23 for symmetric tensors, followed by 21, 31, 32 for
unsymmetric tensors, such as the deformation gradient.
– Thus, the stresses for a plane stress element are returned as
ARRAY(1) = S11, ARRAY(2) = S22, ARRAY(3) = 0.0, and
ARRAY(4) = S12.
– Three values are always returned for principal value requests, the
minimum value first and maximum value third, regardless of the
dimensionality of the analysis.
7/01
Writing User Subroutines with ABAQUS
L4.17
ABAQUS
User Subroutine GETVRM
Elements Supported by GETVRM
• Since the GETVRM capability pertains to material point quantities, it
cannot be used for most of the element types that do not require a
∗MATERIAL definition.
• The following element types are, therefore, not supported:
DASHPOTx
ROTARYI
SPRINGx
all acoustic elements
JOINTC
all contact elements
JOINTxD
all gasket elements
DRAGxD
all hydrostatic fluid elements
ITSxxx
USA elements
MASS
7/01
Writing User Subroutines with ABAQUS
L4.18
ABAQUS
USDFLD Subroutine Interface
USDFLD Subroutine Interface
The interface to user subroutine USDFLD is:
SUBROUTINE USDFLD(FIELD, STATEV, PNEWDT, DIRECT, T,
1
CELENT, TIME, DTIME, CMNAME, ORNAME, NFIELD,
2
NSTATV, NOEL, NPT, LAYER, KSPT, KSTEP, KINC, NDI,
3
NSHR, COORD, JMAC, JMATYP, MATLAYO, LACCFLA)
C
INCLUDE ‘ABA_PARAM.INC’
C
CHARACTER*80 CMNAME,ORNAME
CHARACTER*8 FLGRAY(15)
DIMENSION FIELD(NFIELD), STATEV(NSTATV), DIRECT(3, 3),
1 T(3, 3), TIME(2), COORD(*), JMAC(*), JMATYP(*)
DIMENSION ARRAY(15), JARRAY(15)
user coding to define FIELD and,
if necessary, STATEV and PNEWDT
7/01
Writing User Subroutines with ABAQUS
L4.19
ABAQUS
USDFLD Subroutine Interface
Variables to be Defined
• The array FIELD(NFIELD) contains the field variables ( f i ) at the
current material (integration) point.
– These are passed in with the values interpolated from the nodes at
the end of the current increment, as specified with the ∗INITIAL
CONDITIONS option or the ∗FIELD option.
– The updated f i are used to calculate the values of material
properties that are a function of field variables. The updated f i are
passed into other user subroutines (CREEP, HETVAL, UEXPAN,
UHARD, UHYPEL, UMAT, UMATHT, and UTRS) that are called at this
material point.
7/01
Writing User Subroutines with ABAQUS
L4.20
ABAQUS
USDFLD Subroutine Interface
Variables that may be Defined
• The array containing the solution-dependent state variables,
STATEV(NSTATV), can be defined in USDFLD.
– These are passed in as the values at the beginning of the increment.
– In all cases STATEV can be updated in this subroutine, and the
updated values are passed into other user subroutines (CREEP,
HETVAL, UEXPAN, UMAT, UMATHT, and UTRS) that are called at this
material point.
– The number of state variables associated with this material point is
defined with the ∗DEPVAR option.
• The ratio, PNEWDT, of suggested new time increment to the time
increment being used (DTIME, see below) can be given.
– This variable allows the user to provide input to the automatic time
incrementation algorithms in ABAQUS.
7/01
Writing User Subroutines with ABAQUS
L4.21
ABAQUS
USDFLD Subroutine Interface
Variables for Information Only
• The number of field variables (NFIELD) that exist at this point.
• The direction cosines in the global coordinate system of the material
directions associated with the current integration point (DIRECT).
– DIRECT(#, 1) defines the first material direction.
• The direction cosines (T) of any rotations of the shell or membrane
material direction about the element’s normal.
• The characteristic element length in the model (CELENT).
• The name (CNAME) of the associated ∗MATERIAL option.
• The name (ORNAME) of the ∗ORIENTATION associated with this
element.
• The number of direct stress components (NDI) and the number of shear
stress components (NSHR).
7/01
Writing User Subroutines with ABAQUS
L4.22
ABAQUS
USDFLD Subroutine Interface
• The step (KSTEP) and increment number (KINC) in which the routine
is being called
• The value of step time (TIME(1)) and the value of total time
(TIME(2)) at the end of the current increment.
• The current time increment (DTIME).
• The element number (NOEL) and integration point number (NPT).
• The layer (LAYER) and section point numbers (KSPT), where
appropriate.
• The coordinates (COORD) at the material point.
• Variables that must be passed into the GETVRM utility routine (JMAC,
JMATYP, MATLAYO, and LACCFLA).
7/01
Writing User Subroutines with ABAQUS
L4.23
ABAQUS
USDFLD Subroutine Interface
USDFLD and Automatic Time Incrementation
• ABAQUS/Standard uses an automatic time incrementation algorithm to
control the size of the time increment used in an analysis.
– This algorithm allows ABAQUS/Standard to reduce the time
increment size when convergence is unlikely or the results are not
accurate enough and to increase the time increment when
convergence is easily obtained.
• Subroutines such as USDFLD can make it impossible for this algorithm
to function properly.
– Therefore, these subroutines are given the variable PNEWDT to
provide information to the incrementation algorithm.
7/01
Writing User Subroutines with ABAQUS
L4.24
ABAQUS
USDFLD Subroutine Interface
• PNEWDT is set to a large value before each call to USDFLD.
• If PNEWDT is redefined to be less than 1.0, ABAQUS must abandon the
time increment and attempt it again with a smaller time increment.
– The suggested new time increment provided to the automatic time
integration algorithms is
PNEWDT∗DTIME,
where the PNEWDT used is the minimum value for all calls to user
subroutines that allow redefinition of PNEWDT for this iteration.
7/01
Writing User Subroutines with ABAQUS
L4.25
ABAQUS
USDFLD Subroutine Interface
• If PNEWDT is given a value that is greater than 1.0 for all calls to user
subroutines for this iteration and the increment converges in this
iteration, ABAQUS may increase the time increment.
– The suggested new time increment provided to the automatic time
integration algorithms is
PNEWDT∗DTIME,
– where the PNEWDT used is the minimum value for all calls to user
subroutines for this iteration.
• If the time increment size should be maintained, set PNEWDT =1.0.
• If automatic time incrementation is not selected for the analysis
procedure, values of PNEWDT that are greater than 1.0 will be ignored
and values of PNEWDT that are less than 1.0 will cause the job to
terminate.
7/01
Writing User Subroutines with ABAQUS
L4.26
ABAQUS
Example: Laminated Composite Plate Failure
Example: Laminated Composite Plate Failure
• This problem is described in detail in the ABAQUS Example Problems
Manual, Section 1.1.14.
• This problem models the damage that occurs in a laminated composite
plate with a hole in the center as it is subjected to in-plane compression.
– The plate consists of graphite-epoxy plies with fiber directions that
are in a ( – 45 / 45 ) layup.
• A quarter-symmetry finite element model (shown in Figure 4–1) was
used in this problem.
– Rather than model the composite plate with shell elements, two
layers of CPS4 elements were used—the thickness of the plate is
large enough that out-of-plane displacements should be minimal.
7/01
Writing User Subroutines with ABAQUS
L4.27
ABAQUS
Example: Laminated Composite Plate Failure
E
4.0 0.25
0.135
2
1.0
3
1
Figure 4–1. Problem Geometry and Finite Element Mesh
7/01
Writing User Subroutines with ABAQUS
L4.28
ABAQUS
Example: Laminated Composite Plate Failure
Material Model
• The material behavior of each ply is described in detail by Chang and
Lessard.
Chang, F-K., and L. B. Lessard, “Damage Tolerance of Laminated
Composites Containing an Open Hole and Subjected to Compressive
Loadings: Part I—Analysis,” Journal of Composite Materials,
vol. 25, pp. 2–43, 1991.
• The initial elastic ply properties are:
longitudinal modulus E 11 = 22700 ksi,
transverse modulus E 22 = 1880 ksi,
shear modulus G = 1010 ksi, and
Poisson’s ratio ν = 0.23 .
7/01
Writing User Subroutines with ABAQUS
L4.29
ABAQUS
Example: Laminated Composite Plate Failure
• The material accumulates damage in shear, leading to a nonlinear
stress-strain relation of the form
3
–1
γ 12 = G 12 σ 12 + ασ 12 ,
(Eq. 4.1)
where G 12 is the (initial) ply shear modulus and the nonlinearity is
– 14
characterized by the factor α = 0.8 ×10 .
• To account for the nonlinearity, the nonlinear stress-strain relation
(Equation 4.1) must be expressed in a different form:
The stress at the end of the increment must be given as a linear
function of the strain.
• The most obvious way to do this is to linearize the nonlinear term,
leading to the relation
(i + 1)
γ 12
7/01
–1
(i) 2
(i + 1)
= ( G 12 + α ( σ 12 ) )σ 12
.
Writing User Subroutines with ABAQUS
(Eq. 4.2)
L4.30
ABAQUS
Example: Laminated Composite Plate Failure
• Inverting the relation given in Equation 4.2 gives
(i + 1)
σ 12
G 12
(i + 1)
= -------------------------------------2- γ 12 ,
(i)
1 + αG 12 ( α 12 )
(Eq. 4.3)
which provides an algorithm to define the effective shear modulus.
– However, this algorithm is not very suitable because it is unstable
at higher strain levels, which is readily demonstrated by stability
analysis (see the Example Problems Manual for details).
• To obtain a more stable algorithm, we write the nonlinear stress-strain
law in the form
3
–1
3
γ 12 + βσ 12 = G 12 σ 12 + ( α + β )σ 12 ,
(Eq. 4.4)
where β is an as yet unknown coefficient.
7/01
Writing User Subroutines with ABAQUS
L4.31
ABAQUS
Example: Laminated Composite Plate Failure
• Using a stability analysis of the expression in Equation 4.4, the optimal
stress-strain algorithm is found to be
(i + 1)
σ 12
(i) 3
(i)
1 + ( 2α ( σ 12 ) ) ⁄ γ 12
(i + 1)
= -----------------------------------------------γ
.
G
12 12
(i) 2
1 + 3αG 12 ( α 12 )
(Eq. 4.5)
Writing this expression in terms of a damage parameter, d , gives
(i + 1)
σ 12
(i + 1)
= ( 1 – d )G 12 γ 12
,
where
(i) 2
(i) 3
(i)
3αG 12 ( α 12 ) – 2α ( σ 12 ) ⁄ γ 12
d = ------------------------------------------------------------------------.
(i) 2
1 + 3αG 12 ( α 12 )
(Eq. 4.6)
• This relation is implemented in user subroutine USDFLD, and the value
of the damage parameter is assigned directly to the third field variable
(FV3) used for definition of the elastic properties.
7/01
Writing User Subroutines with ABAQUS
L4.32
ABAQUS
Example: Laminated Composite Plate Failure
• The following strength properties are used for the composite material:
transverse tensile strength Y t = 14.82 ksi,
ply shear strength S c = 15.5 ksi,
matrix compressive strength Y c = 36.7 ksi, and
fiber buckling strength X c = 392.7 ksi.
7/01
Writing User Subroutines with ABAQUS
L4.33
ABAQUS
Example: Laminated Composite Plate Failure
• The strength parameters can be combined into failure criteria for
multiaxial loading. Three different failure modes are considered in the
model analyzed.
– Fiber bucking failure is not considered in this model because the
primary mode of failure is fiber-matrix shear.
Matrix Tensile Cracking
The failure index for matrix tensile cracking with nonlinear shear
behavior is
4
2
2
em
2
σ
2σ 12 ⁄ G 12 + 3ασ 12
22
=  -------- + ---------------------------------------------.
2
4
 Yt 
2S ⁄ G + 3αS
c
12
c
When the composite fails in this mode, transverse stiffness ( E 22 ) and
Poisson’s ratio become zero.
7/01
Writing User Subroutines with ABAQUS
L4.34
ABAQUS
Example: Laminated Composite Plate Failure
Matrix Compressive Cracking
The form of the failure index is identical to that for the tensile cracking
mode. The same failure index (field variable) is used in USDFLD
because these two modes will not occur simultaneously at the same
point.
Fiber-Matrix Shearing Failure
The failure criterion has essentially the same form as the other two
criteria:
4
2
2
e fs
2
σ
2σ 12 ⁄ G 12 + 3ασ 12
11
=  -------- + ---------------------------------------------.
2
4
 Xc 
2S ⁄ G + 3αS
c
12
c
This failure mechanism can occur simultaneously with the other two
criteria; therefore, a separate failure index is used in USDFLD.
7/01
Writing User Subroutines with ABAQUS
L4.35
ABAQUS
Example: Laminated Composite Plate Failure
• User subroutine USDFLD is used to calculate the values of the active
failure indices:
The value for the matrix tensile cracking/matrix compressive failure
index ( e m ) is stored as SDV(1).
– When the index exceeds a value of 1.0, the value of f 1 is set to 1.0.
The value for the fiber-matrix shear failure index ( e fs ) is stored as
SDV(2).
– When the index exceeds a value of 1.0, the value of f 2 is
set to 1.0.
• Subroutine USDFLD is also used to calculate the value of the damage
parameter, d , in the nonlinear stress-strain relationship.
– The value of d is stored as SDV(3) and f 3 .
• Table 4–1 shows the dependence of material properties on the f i .
7/01
Writing User Subroutines with ABAQUS
L4.36
ABAQUS
Example: Laminated Composite Plate Failure
Table 4–1. Dependence of Elastic Material Properties on f i
Material State
No failure or damage
Matrix failure
Fiber-matrix failure
Shear damage
Matrix & fiber-matrix
Matrix & damage
Fiber-matrix & damage
All failure modes
7/01
Elastic Properties
E 11
E 11
E 11
E 11
E 11
E 11
E 11
E 11
E 22
ν 12
0
E 22
E 22
f1
f2
f3
0
0
0
0
G 12
G 12
1
0
0
0
0
0
1
0
ν 12
0
0
0
1
0
0
0
1
1
0
0
0
0
1
0
1
E 22
0
0
0
1
1
0
0
0
1
1
1
Writing User Subroutines with ABAQUS
L4.37
ABAQUS
Example: Laminated Composite Plate Failure
Partial Input Data
*HEADING
:
**NONLINEAR SHEAR WITH BUILT-IN EXPLICIT FAILURE
**
** FV1: MATRIX COMPRESSIVE/TENSILE FAILURE
** FV2: FIBER-MATRIX SHEAR FAILURE
** FV3: SHEAR NONLINEARITY (DAMAGE) PRIOR TO FAILURE
** TOTAL OF 2^3 = 8 STATES
em
**
*MATERIAL, NAME=T300
*ELASTIC, TYPE=LAMINA, DEPENDENCIES=3
22.7E6, 1.88E6, 0.23, 1.01E6, 1.01E6, 1.01E6, 0., 0,
0, 0
22.7E6, 1.00E0, 0.00, 1.01E6, 1.01E6, 1.01E6, 0., 1,
0, 0
22.7E6, 1.88E6, 0.00, 1.00E0, 1. 01E6 ,1.01E6, 0., 0,
1, 0
= 1.0
e fs = 1.0
7/01
Writing User Subroutines with ABAQUS
L4.38
ABAQUS
Example: Laminated Composite Plate Failure
22.7E6, 1.00E0, 0.00,
1, 0
22.7E6, 1.88E6, 0.23,
0, 1
22.7E6, 1.00E0, 0.00,
0, 1
22.7E6, 1.88E6, 0.00,
1, 1
22.7E6, 1.00E0, 0.00,
1, 1
*DEPVAR
3
*USER DEFINED FIELD
**
7/01
1.00E0, 1.01E6, 1.01E6, 0., 1,
1.00E0, 1.01E6, 1.01E6, 0., 0,
1.00E0, 1.01E6, 1.01E6, 0., 1,
1.00E0, 1.01E6, 1.01E6, 0., 0,
1.00E0, 1.01E6, 1.01E6, 0., 1,
Writing User Subroutines with ABAQUS
Shear damage
L4.39
ABAQUS
Example: Laminated Composite Plate Failure
** ANALYSIS HISTORY
*STEP, INC=200, NLGEOM
*STATIC, DIRECT
0.05, 1.0
*BOUNDARY
XSYMMTRY, XSYMM
YSYMMTRY, YSYMM
1000, 2, , -0.027
:
*END STEP
7/01
Writing User Subroutines with ABAQUS
L4.40
ABAQUS
Example: Laminated Composite Plate Failure
User Subroutine
SUBROUTINE USDFLD (FIELD, STATEV, PNEWDT, DIRECT, T,
1
CELENT, TIME, DTIME, CMNAME, ORNAME, NFIELD, NSTATV,
2
NOEL, NPT, LAYER, KSPT, KSTEP, KINC, NDI, NSHR,
3
COORD, JMAC, JMATYP, MATLAYO, LACCFLA)
C
INCLUDE ‘ABA_PARAM.INC’C MATERIAL AND STRENGTH PARAMETERS
PARAMETER (YT=14.86D3, XC=392.7D3, YC=36.7D3)
PARAMETER (SC=15.5D3, G12=1.01D6, ALPHA=0.8D-14)
C
CHARACTER*80 CMNAME, ORNAME
CHARACTER*8 FLGRAY(15)
DIMENSION FIELD(NFIELD), STATEV(NSTATV), DIRECT(3,3)
DIMENSION T(3, 3), TIME(2), ARRAY(15), JARRAY(15)
DIMENSION COORD(*), JMAC(*), JMATYP(*)
C
C INITIALIZE
EM
EFS
DAMAGE
7/01
FAILURE FLAGS FROM STATEV.
= STATEV(1)
= STATEV(2)
= STATEV(3)
Writing User Subroutines with ABAQUS
L4.41
ABAQUS
Example: Laminated Composite Plate Failure
C GET STRESSES FROM PREVIOUS INCREMENT
CALL GETVRM(‘S’, ARRAY, JARRAY, FLGRAY, JRCD,
1 JMAC, JMATYP, MATLAYO, LACCFLA)
S11 = ARRAY(1)
S22 = ARRAY(2)
S12 = ARRAY(4)
CALL GETVRM(‘E’, ARRAY, JARRAY, FLGRAY, JRCD,
1 JMAC, JMATYP, MATLAYO, LACCFLA)
E12 = ARRAY(4)
C
C DAMAGE INDEX: = 0 IF NO STRAIN TO PREVENT DIVIDE BY ZERO
C
IF (E12.NE.0) THEN
DAMAGE = (3.D0*ALPHA*G12*S12**2 &
2.D0*ALPHA*(S12**3)/E12) /
&
(1.D0 + 3.D0*ALPHA*G12*S12**2)
ELSE
DAMAGE = 0.D0
ENDIF
7/01
Writing User Subroutines with ABAQUS
L4.42
ABAQUS
Example: Laminated Composite Plate Failure
C
F1 = S12**2/(2.D0*G12) + 0.75D0*ALPHA*S12**4
F2 = SC**2 /(2.D0*G12) + 0.75D0*ALPHA*SC**4
C
C MATRIX TENSILE/COMPRESSIVE FAILURE
IF (EM .LT. 1.D0) THEN
IF (S22 .LT. 0.D0) THEN
EM = SQRT((S22/YC)**2 + F1/F2)
ELSE
EM = SQRT((S22/YT)**2 + F1/F2)
ENDIF
STATEV(1) = EM
ENDIF
Value of matrix failure modes is stored as
C
solution-dependent state variable
7/01
Writing User Subroutines with ABAQUS
L4.43
ABAQUS
Example: Laminated Composite Plate Failure
C FIBER-MATRIX SHEAR FAILURE
IF (EFS .LT. 1.D0) THEN
IF (S11 .LT. 0.D0) THEN
EFS = SQRT((S11/XC)**2 + F1/F2)
ELSE
EFS = 0.D0
ENDIF
STATEV(2) = EFS
ENDIF
C
C UPDATE FIELD VARIABLES
FIELD(1) = 0.D0
FIELD(2) = 0.D0
IF (EM .GT. 1.D0) FIELD(1) = 1.D0
IF (EFS .GT. 1.D0) FIELD(2) = 1.D0
FIELD(3) = DAMAGE
STATEV(3) = FIELD(3)
C
RETURN
END
7/01
Writing User Subroutines with ABAQUS
L4.44
ABAQUS
Example: Laminated Composite Plate Failure
Results
• The material model implemented with USDFLD in this example does a
reasonably good job of modeling the experimentally observed behavior
(see Figure 4–2).
• The extent of damage predicted in the composite plate (see Figure 4–3)
during the analysis was very similar to the damage seen in the
experimental specimens.
7/01
Writing User Subroutines with ABAQUS
L4.45
ABAQUS
Example: Laminated Composite Plate Failure
4000
Experiment (Chang et al., 1989)
ABAQUS (CPS4)
ABAQUS (CPS4R)
Applied load P (lb)
3000
2000
1000
0
0.000
0.010
0.020
0.030
Extensometer measurement ∆E (in)
Figure 4–2. Experimental and Numerical Load-Deflection Curves
7/01
Writing User Subroutines with ABAQUS
L4.46
ABAQUS
Example: Laminated Composite Plate Failure
fiber-matrix shear
failure
2
3
1
Figure 4–3. Material Damage in the Plate
7/01
Writing User Subroutines with ABAQUS
L4.47
ABAQUS
Example: Laminated Composite Plate Failure
Remarks
• The values of the failure indices are not assigned directly to the f i :
instead, they are stored as solution-dependent state variables.
– Only if the value of a failure index exceeds 1.0 is the
corresponding user-defined field variable set equal to 1.0.
– After the failure index has exceeded 1.0, the associated f i
continues to have the value 1.0 even though the stresses may
reduce significantly, which ensures that the material does not
“heal” after it has become damaged.
7/01
Writing User Subroutines with ABAQUS
L4.48
ABAQUS
Example: Laminated Composite Plate Failure
• The material model implemented with USDFLD assumes that after
failure occurs the stresses in the failed directions drop to zero
immediately, which corresponds to brittle failure with no energy
absorption.
• This assumption is not very realistic: in reality, the stress-carrying
capacity degrades gradually with increasing strain after failure occurs.
– Hence, the behavior of the composite after onset of failure is not
likely to be captured well by this model.
• Moreover, the instantaneous loss of stress-carrying capacity also makes
the postfailure analysis results strongly dependent on the refinement of
the finite element mesh and the finite element type used.
7/01
Writing User Subroutines with ABAQUS
L4.49
ABAQUS
Example: Laminated Composite Plate Failure
• In this example the only significant nonlinearity in the model is failure
of the composite material. Hence, fixed time incrementation can be
used effectively.
– However, the results of this analysis are highly sensitive to the size
of the time increment. The time increment used in the model
shown, ∆t = 0.05 , is close to the largest allowable value (see
Figure 4–4 and Figure 4–5).
• If other nonlinearities were present in the analysis, the automatic time
incrementation algorithm would likely be needed.
– In those types of analyses the variable PNEWDT would have to be
used in user subroutine USDFLD to help control the size of the time
increment.
7/01
Writing User Subroutines with ABAQUS
L4.50
ABAQUS
Example: Laminated Composite Plate Failure
3.5
3
[ x10 ]
T25_1000
T15_1000
T05_1000
3.0
REACTION FORCE - RF2
2.5
XMIN
XMAX
YMIN
YMAX
0.000E+00
2.409E-02
0.000E+00
3.409E+03
2.0
1.5
1.0
0.5
0.0
0.
4.
8.
12.
DISPLACEMENT - U2
16.
20.
[ x10
24.
-3
]
Figure 4–4. Force Deflection Curves for Analyses with ∆t = 0.05 ,
∆t = 0.15 , and ∆t = 0.25
7/01
Writing User Subroutines with ABAQUS
L4.51
ABAQUS
Example: Laminated Composite Plate Failure
2
3
1
Figure 4–5. Extent of Material Damage in Analysis with ∆t = 0.25
7/01
Writing User Subroutines with ABAQUS
L4.52
ABAQUS
Lecture 5
User Subroutine: URDFIL
Overview
• Introduction
• ABAQUS Usage
• URDFIL Subroutine Interface
• Example: Using URDFIL to Terminate an Analysis
7/01
Writing User Subroutines with ABAQUS
L5.1
ABAQUS
Introduction
Introduction
• Subroutine URDFIL is used to read the results (.fil) file at the end of
an increment.
– Thus, the user can examine the results as the analysis is running.
– This information can be used to make decisions such as whether to
stop the analysis.
– Results can also be extracted from the results file, stored in
COMMON blocks, and passed into other subroutines.
7/01
Writing User Subroutines with ABAQUS
L5.2
ABAQUS
Introduction
• Subroutine URDFIL must call the utility routine DBFILE to read records
from the results file.
– A detailed discussion of this routine is provided in the
ABAQUS/Standard User’s Manual, Section 5.1.4.
• Subroutine URDFIL can call the utility routine POSFIL to begin reading
the results file at a specified step and increment.
– The default behavior is to begin reading the data from the
beginning of the file.
– A detailed discussion of this routine is provided in the
ABAQUS/Standard User’s Manual, Section 5.1.4.
7/01
Writing User Subroutines with ABAQUS
L5.3
ABAQUS
ABAQUS Usage
ABAQUS Usage
• If an analysis requests that data be written to the results (.fil) file
using the ∗EL FILE, ∗NODE FILE, ∗CONTACT FILE, or ∗ENERGY
FILE options, subroutine URDFIL will be called at the end of any
increment in which new information is written to the results file.
– The coding for the subroutine must also be provided via the user
parameter on the command line.
7/01
Writing User Subroutines with ABAQUS
L5.4
ABAQUS
ABAQUS Usage
Utility Routine POSFIL
• The utility routine POSFIL is used to locate the position of a specific
increment of results data stored on the results (.fil) file.
– Once this position is found, the data for that increment can be read.
• The interface for this utility is:
CALL POSFIL(NSTEP, NINC, ARRAY, JRCD)
Variables to be provided to the utility routine:
– The desired step (NSTEP). If this variable is set to 0, the first
available step will be read.
– The desired increment (NINC). If this variable is set to 0, the first
available increment of the specified step will be read.
7/01
Writing User Subroutines with ABAQUS
L5.5
ABAQUS
ABAQUS Usage
Variables returned from the utility routine:
– A real array (ARRAY) containing the values of record 2000 from the
results file for the requested step and increment.
– A return code (JRCD). If it has a value of 0, the specified increment
was found; if it is has a value of 1, the specified increment was not
found.
• If the step and increment requested are not found on the results file,
POSFIL will return an error and leave the user positioned at the end of
the results file.
• POSFIL cannot be used to move backward in the results file.
– The user cannot use POSFIL to find a given increment in the file
and then make a second call to POSFIL later to read an increment
earlier than the first one found.
7/01
Writing User Subroutines with ABAQUS
L5.6
ABAQUS
ABAQUS Usage
Utility Routine DBFILE
• The utility routine DBFILE is used to extract data from the results file.
• The interface for this utility is:
CALL DBFILE(LOP,ARRAY,JRCD)
The only variable to be provided to the utility routine is:
– A flag, LOP, which must be set to 0 when this utility is used in
URDFIL.
Variables returned from the utility routine are:
– The array, ARRAY, containing one record from the results file.
– The flag JRCD is returned as nonzero if an end-of-file marker is
read when DBFILE is called with LOP=0.
7/01
Writing User Subroutines with ABAQUS
L5.7
ABAQUS
ABAQUS Usage
• The formats of the data records for the results file are described in the
ABAQUS/Standard User’s Manual, Section 5.1.2.
– ARRAY must be dimensioned adequately in the user’s routines to
contain the largest record on the file.
– For almost all cases 500 words is sufficient.
– The exceptions arise if the problem definition includes user
elements or user materials that use more than this many state
variables.
– When the results file has been written on a system on which
ABAQUS runs in double precision, ARRAY must be declared
double precision in the user’s routine.
7/01
Writing User Subroutines with ABAQUS
L5.8
ABAQUS
URDFIL Subroutine Interface
URDFIL Subroutine Interface
The interface to user subroutine URDFIL is:
SUBROUTINE URDFIL(LSTOP, LOVRWRT, KSTEP, KINC,
1
DTIME, TIME)
C
INCLUDE ‘ABA_PARAM.INC’
C
DIMENSION ARRAY(513), JRRAY(NPRECD, 513), TIME(2)
EQUIVALENCE (ARRAY(1), JRRAY(1 ,1))
user coding to read the results file
RETURN
END
7/01
Writing User Subroutines with ABAQUS
L5.9
ABAQUS
URDFIL Subroutine Interface
Variables to be Defined
• The flag (LSTOP) to indicate whether an analysis should continue.
– The analysis will be terminated if LSTOP is set to 1.
– Otherwise, the analysis will continue.
• The flag (LOVRWRT) to indicate that the information written to the
results file for the current increment can be overwritten.
– If LOVRWRT is set to 1, information for the current increment will
be overwritten by information written to the results file in a
subsequent increment unless the current increment is the final
increment written to the results file.
– The purpose of this flag is to reduce the size of the results file by
allowing information for an increment to be overwritten by
information for a subsequent increment.
7/01
Writing User Subroutines with ABAQUS
L5.10
ABAQUS
URDFIL Subroutine Interface
• The variable DTIME allows the user to provide input to the automatic
time incrementation algorithms in ABAQUS (if automatic time
incrementation is chosen).
– It is passed in as the size of the next time increment to be taken and
can be updated to increase or reduce the time increment.
– If automatic time incrementation is not selected in the analysis
procedure, updated values of DTIME are ignored.
Variables for Information Only
• The step (KSTEP) and increment number (KINC) in which the routine
is being called.
• The value of step time (TIME(1)) and the value of total time
(TIME(2)) at the end of the current increment.
7/01
Writing User Subroutines with ABAQUS
L5.11
ABAQUS
Example: Using URDFIL to Terminate an Analysis
Example: Using URDFIL to Terminate
an Analysis
• In this example the values of Mises stress and the displacement in the
2-direction of a specific node, 63, are monitored, and if they exceed a
given value, the analysis is stopped.
• The structure is a cantilever beam subjected to a tip load in the
2-direction.
Node 63
2
3
7/01
1
Figure 5–1. Model Geometry
Writing User Subroutines with ABAQUS
L5.12
ABAQUS
Example: Using URDFIL to Terminate an Analysis
Input Data
*HEADING
Demonstration of URDFIL user subroutine
*RESTART,WRITE
*NODE
1, 0, 0
21, 1, 0
85, 0, .1
105 ,1, .1
*NGEN, NSET=BOTTOM
1, 21, 1
*NGEN, NSET=TOP
85, 105, 1
*NFILL, NSET=ALL
BOTTOM, TOP, 4, 21
*NSET, NSET=LEFT, GEN
1, 85, 21
7/01
Writing User Subroutines with ABAQUS
L5.13
ABAQUS
Example: Using URDFIL to Terminate an Analysis
*ELEMENT, TYPE=CPS4R
1, 1, 2, 23, 22
*ELGEN, ELSET=ALL
1, 20, 1, 1, 4, 21, 20
*SOLID SECTION, ELSET=ALL, MATERIAL=STEEL, ORIENTATION=BEAM
0.1
*ORIENTATION, NAME=BEAM
1, 0, 0, 0, 1, 0
1, 0
*MATERIAL, NAME=STEEL
*ELASTIC
2E11, 0.3
*PLASTIC
2E8, 0
2E9, 0.1
*BOUNDARY
LEFT, 1, 2, 0
7/01
Writing User Subroutines with ABAQUS
L5.14
ABAQUS
Example: Using URDFIL to Terminate an Analysis
*STEP, NLGEOM, INC=30
*STATIC
0.1, 1
*CLOAD
63, 2, -1E6
*NODE PRINT, FREQ=0
*EL PRINT, FREQ=0
*EL FILE, FREQUENCY=1
SINV
*NODE FILE, FREQUENCY=1
U
*END STEP
7/01
Write data to the results file
frequently to ensure that you
can monitor the progress of
the analysis.
Writing User Subroutines with ABAQUS
L5.15
ABAQUS
Example: Using URDFIL to Terminate an Analysis
User Subroutine
SUBROUTINE URDFIL(LSTOP, LOVRWRT, KSTEP, KINC,
1
DTIME,TIME)
C
INCLUDE ‘ABA_PARAM.INC’
DIMENSION ARRAY(513), JRRAY(NPRECD, 513), TIME(2)
EQUIVALENCE (ARRAY(1),JRRAY(1,1))
PARAMETER(TOL=5.0D8)
PARAMETER(DEFL=1.5D-1)
LMISES=0
LDEFL=0
C
C Assume that we do not mind if .fil file results are
C overwritten.
C
LOVRWRT=1
C
C Find current increment
7/01
Writing User Subroutines with ABAQUS
L5.16
ABAQUS
Example: Using URDFIL to Terminate an Analysis
C
CALL POSFIL(KSTEP, KINC, ARRAY, JRCD)
C
C Loop over all of the records
Loop over large number
C
to ensure that all records
DO K1=1,999999
are read.
CALL DBFILE(0,ARRAY,JRCD)
IF (JRCD .NE. 0) GO TO 110
KEY=JRRAY(1,2)
Check for end-of-file.
C
C Record 1 contains element information for subsequent
C records
C
IF (KEY .EQ. 1) THEN
IELM = JRRAY(1, 3)
IMATPT = JRRAY(1, 4)
END IF
C
7/01
Writing User Subroutines with ABAQUS
L5.17
ABAQUS
Example: Using URDFIL to Terminate an Analysis
C Record 12 contains values for SINV
C
IF (KEY .EQ. 12) THEN
IF (ARRAY(3) .GT. TOL) THEN
LMISES=1
GOTO 210
END IF
END IF
C
C Record 101 contains U
C
IF (KEY .EQ. 101) THEN
IF (JRRAY(1, 3) .EQ. 63) THEN
IF (ABS(ARRAY(5)) .GT. DEFL) THEN
LDEFL=1
GOTO 210
END IF
Write why analysis is
END IF
terminated to message file
7/01
Writing User Subroutines with ABAQUS
L5.18
ABAQUS
Example: Using URDFIL to Terminate an Analysis
END IF
END DO
110 CONTINUE
C
210 IF (LMISES .EQ. 1) THEN
WRITE(7, *)
WRITE(7, 1023) TOL, IELEM, IMATPT
WRITE(7, *)
LSTOP=1
END IF
IF (LDEFL .EQ. 1) THEN
WRITE(7, *)
WRITE(7, 1024) DEFL
WRITE(7, *)
LSTOP=1
END IF
RETURN
7/01
Writing User Subroutines with ABAQUS
L5.19
ABAQUS
Example: Using URDFIL to Terminate an Analysis
C
C
1023 FORMAT (‘***NOTE: ANALYSIS TERMINATES MISES
&STRESS EXCEEDS’, 2X, E9.3, 1X, ‘IN ELEMENT’, 1X, I6,
&1X, ‘AT INT. PT.’, 1X, I6)
1024 FORMAT (‘***NOTE: ANALYSIS TERMINATES AS DEFLECTION
&OF NODE 63 EXCEEDS’, 2X, E9.3)
END
7/01
Writing User Subroutines with ABAQUS
L5.20
ABAQUS
Example: Using URDFIL to Terminate an Analysis
Remarks
• The analysis terminates when only 12.3% of the defined load is applied
to the model because the tolerance for Mises stress was exceeded in
element 1.
***NOTE: ANALYSIS TERMINATES MISES STRESS EXCEEDS
0.500E+09 IN ELEMENT
1 AT INT. PT.
1
7/01
Writing User Subroutines with ABAQUS
L5.21
ABAQUS
Lecture 6
Writing a UMAT or VUMAT
Overview
• Motivation
• Steps Required in Writing a UMAT or VUMAT
• UMAT Interface
• Examples
• VUMAT Interface
• Examples
7/01
Writing User Subroutines with ABAQUS
L6.1
ABAQUS
Overview
Overview
• ABAQUS/Standard and ABAQUS/Explicit have interfaces that allow
the user to implement general constitutive equations.
– In ABAQUS/Standard the user-defined material model is
implemented in user subroutine UMAT.
– In ABAQUS/Explicit the user-defined material model is
implemented in user subroutine VUMAT.
• Use UMAT and VUMAT when none of the existing material models
included in the ABAQUS material library accurately represents the
behavior of the material to be modeled.
7/01
Writing User Subroutines with ABAQUS
L6.2
ABAQUS
Overview
• These interfaces make it possible to define any (proprietary)
constitutive model of arbitrary complexity.
• User-defined material models can be used with any ABAQUS
structural element type.
• Multiple user materials can be implemented in a single UMAT or VUMAT
routine and can be used together.
In this lecture the implementation of material models in UMAT or VUMAT
will be discussed and illustrated with a number of examples.
7/01
Writing User Subroutines with ABAQUS
L6.3
ABAQUS
Motivation
Motivation
• Proper testing of advanced constitutive models to simulate
experimental results often requires complex finite element models.
– Advanced structural elements
– Complex loading conditions
– Thermomechanical loading
– Contact and friction conditions
– Static and dynamic analysis
7/01
Writing User Subroutines with ABAQUS
L6.4
ABAQUS
Motivation
• Special analysis problems occur if the constitutive model simulates
material instabilities and localization phenomena.
– Special solution techniques are required for quasi-static analysis.
– Robust element formulations should be available.
– Explicit dynamic solution algorithms with robust, vectorized
contact algorithms are desired.
• In addition, robust features are required to present and visualize the
results.
– Contour and path plots of state variables.
– X–Y plots.
– Tabulated results.
7/01
Writing User Subroutines with ABAQUS
L6.5
ABAQUS
Motivation
• The material model developer should be concerned only with the
development of the material model and not the development and
maintenance of the FE software.
– Developments unrelated to material modeling
– Porting problems with new systems
– Long-term program maintenance of user-developed code
7/01
Writing User Subroutines with ABAQUS
L6.6
ABAQUS
Motivation
• “Finite Element Modelling of the Damage Process in Ice,”
R. F. McKenna, I. J. Jordaan, and J. Xiao, ABAQUS Users’ Conference
Proceedings, 1990.
7/01
Writing User Subroutines with ABAQUS
L6.7
ABAQUS
Motivation
• “The Numerical Simulation of Excavations in Deep Level Mining,”
M. F. Snyman, G. P. Mitchell, and J. B. Martin, ABAQUS Users’
Conference Proceedings, 1991.
7/01
Writing User Subroutines with ABAQUS
L6.8
ABAQUS
Motivation
• “Combined Micromechanical and Structural Finite Element Analysis of
Laminated Composites,” R. M. HajAli, D. A. Pecknold, and M. F.
Ahmad, ABAQUS Users’ Conference Proceedings, 1993.
7/01
Writing User Subroutines with ABAQUS
L6.9
ABAQUS
Motivation
• “Deformation Processing of Metal Powders: Cold and Hot Isostatic
Pressing,” R. M. Govindarajan and N. Aravas, private communication,
1993.
7/01
Writing User Subroutines with ABAQUS
L6.10
ABAQUS
Motivation
• “Macroscopic Shape Change and Evolution of Crystallographic
Texture in Pre-textured FCC Metals,” S. R. Kalidindi and Anand, Acta
Metallurgica, 1993.
7/01
Writing User Subroutines with ABAQUS
L6.11
ABAQUS
Steps Required in Writing a UMAT or VUMAT
Steps Required in Writing a UMAT or VUMAT
• Proper definition of the constitutive equation, which requires one of the
following:
– Explicit definition of stress (Cauchy stress for large-strain
applications)
– Definition of the stress rate only (in corotational framework)
• Furthermore, it is likely to require:
– Definition of dependence on time, temperature, or field variables
– Definition of internal state variables, either explicitly or in rate
form
7/01
Writing User Subroutines with ABAQUS
L6.12
ABAQUS
Steps Required in Writing a UMAT or VUMAT
• Transformation of the constitutive rate equation into an incremental
equation using a suitable integration procedure:
– Forward Euler (explicit integration)
– Backward Euler (implicit integration)
– Midpoint method
7/01
Writing User Subroutines with ABAQUS
L6.13
ABAQUS
Steps Required in Writing a UMAT or VUMAT
This is the hard part! Forward Euler (explicit) integration methods are
simple but have a stability limit,
∆ε < ∆ε stab,
where ∆ε stab is usually less than the elastic strain magnitude.
– For explicit integration the time increment must be controlled.
– For implicit or midpoint integration the algorithm is more
complicated and often requires local iteration. However, there is
usually no stability limit.
– An incremental expression for the internal state variables must also
be obtained.
7/01
Writing User Subroutines with ABAQUS
L6.14
ABAQUS
Steps Required in Writing a UMAT or VUMAT
• Calculation of the (consistent) Jacobian (required for
ABAQUS/Standard UMAT only).
• For small-deformation problems (e.g., linear elasticity) or
large-deformation problems with small volume changes (e.g., metal
plasticity), the consistent Jacobian is
∂∆σ
C = ---------- ,
∂∆ε
where ∆σ is the increment in (Cauchy) stress and ∆ε is the
increment in strain. (In finite-strain problems, ε is an
approximation to the logarithmic strain.)
– This matrix may be nonsymmetric as a result of the constitutive
equation or integration procedure.
– The Jacobian is often approximated such that a loss of quadratic
convergence may occur.
7/01
Writing User Subroutines with ABAQUS
L6.15
ABAQUS
Steps Required in Writing a UMAT or VUMAT
– It is easily calculated for forward integration methods (usually the
elasticity matrix).
– If large deformations with large volume changes are considered
(e.g., pressure-dependent plasticity), the exact form of the
consistent Jacobian
1 ∂∆ ( Jσ )
C = --- -----------------J ∂∆ε
should be used to ensure rapid convergence. Here, J is the
determinant of the deformation gradient.
7/01
Writing User Subroutines with ABAQUS
L6.16
ABAQUS
Steps Required in Writing a UMAT or VUMAT
• Hyperelastic constitutive equations
– Total-form constitutive equations relating the Cauchy stress σ and
the deformation gradient F are commonly used to model, for
example, rubber elasticity.
– In this case, the consistent Jacobian is defined through:
δ ( Jσ ) = JC : δD ,
where J = F , C is the material Jacobian, and δD is the virtual
rate of deformation, defined as
δD = sym ( δF ⋅ F – 1 ) .
7/01
Writing User Subroutines with ABAQUS
L6.17
ABAQUS
Steps Required in Writing a UMAT or VUMAT
• Coding the UMAT or VUMAT:
– Follow FORTRAN 77 or C conventions.
– Make sure that the code can be vectorized (for VUMAT only, to be
discussed later).
– Make sure that all variables are defined and initialized properly.
– Use ABAQUS utility routines as required.
– Assign enough storage space for state variables with the
∗DEPVAR option.
7/01
Writing User Subroutines with ABAQUS
L6.18
ABAQUS
Steps Required in Writing a UMAT or VUMAT
• Verifying the UMAT or VUMAT with a small (one element) input file.
1. Run tests with all displacements prescribed to verify the
integration algorithm for stresses and state variables. Suggested
tests include:
– Uniaxial
– Uniaxial in oblique direction
– Uniaxial with finite rotation
– Finite shear
2. Run similar tests with load prescribed to verify the accuracy of the
Jacobian.
3. Compare test results with analytical solutions or standard
ABAQUS material models, if possible. If the above verification is
successful, apply to more complicated problems.
7/01
Writing User Subroutines with ABAQUS
L6.19
ABAQUS
UMAT Interface
UMAT Interface
• These input lines act as the interface to a UMAT in which isotropic
hardening plasticity is defined.
*MATERIAL, NAME=ISOPLAS
*USER MATERIAL, CONSTANTS=8, (UNSYMM)
30.E6, 0.3, 30.E3, 0., 40.E3, 0.1, 50.E3, 0.5
*DEPVAR
13
*INITIAL CONDITIONS, TYPE=SOLUTION
Data line to specify initial solution-dependent variables
*USER SUBROUTINES,(INPUT=file_name)
• The ∗USER MATERIAL option is used to input material constants for
the UMAT. The unsymmetric equation solution technique will be used if
the UNSYMM parameter is used.
7/01
Writing User Subroutines with ABAQUS
L6.20
ABAQUS
UMAT Interface
• The ∗DEPVAR option is used to allocate space at each material point
for solution-dependent state variables (SDVs).
• The ∗INITIAL CONDITIONS, TYPE=SOLUTION option is used to
initialize SDVs if they are starting at a nonzero value.
• Coding for the UMAT is supplied in a separate file. The UMAT is invoked
with the ABAQUS execution procedure, as follows:
abaqus job=... user=....
– The user subroutine must be invoked in a restarted analysis
because user subroutines are not saved on the restart file.
7/01
Writing User Subroutines with ABAQUS
L6.21
ABAQUS
UMAT Interface
• Additional notes:
– If a constant material Jacobian is used and no other nonlinearity is
present, reassembly can be avoided by invoking the quasi-Newton
method with the input line
*SOLUTION TECHNIQUE, REFORM KERNEL=n
– n is the number of iterations done without reassembly.
– This does not offer advantages if other nonlinearities (such as
contact changes) are present.
7/01
Writing User Subroutines with ABAQUS
L6.22
ABAQUS
UMAT Interface
• Solution-dependent state variables can be output with identifiers SDV1,
SDV2, etc. Contour, path, and X–Y plots of SDVs can be plotted in
ABAQUS/Viewer.
• Include only a single UMAT subroutine in the analysis. If more than one
material must be defined, test on the material name in UMAT and
branch.
7/01
Writing User Subroutines with ABAQUS
L6.23
ABAQUS
UMAT Interface
• The UMAT subroutine header is shown below:
SUBROUTINE UMAT(STRESS, STATEV, DDSDDE, SSE, SPD, SCD, RPL,
1 DDSDDT, DRPLDE, DRPLDT, STRAN, DSTRAN, TIME, DTIME, TEMP, DTEMP,
2 PREDEF, DPRED, CMNAME, NDI, NSHR, NTENS, NSTATV, PROPS, NPROPS,
3 COORDS, DROT, PNEWDT, CELENT, DFGRD0, DFGRD1, NOEL, NPT, LAYER,
4 KSPT, KSTEP, KINC)
C
INCLUDE ’ABA_PARAM.INC’
C
CHARACTER*8 CMNAME
C
DIMENSION STRESS(NTENS), STATEV(NSTATV), DDSDDE(NTENS, NTENS),
1 DDSDDT(NTENS), DRPLDE(NTENS), STRAN(NTENS), DSTRAN(NTENS),
2 PREDEF(1), DPRED(1), PROPS(NPROPS), COORDS(3), DROT(3, 3),
3 DFGRD0(3, 3), DFGRD1(3, 3)
– The include statement sets the proper precision for floating point
variables (REAL*8 on most machines).
– The material name, CMNAME, is an 8-byte character variable.
7/01
Writing User Subroutines with ABAQUS
L6.24
ABAQUS
UMAT Interface
UMAT Variables
• The following quantities are available in UMAT:
– Stress, strain, and SDVs at the start of the increment
– Strain increment, rotation increment, and deformation gradient at
the start and end of the increment
– Total and incremental values of time, temperature, and
user-defined field variables
– Material constants, material point position, and a characteristic
element length
– Element, integration point, and composite layer number (for shells
and layered solids)
– Current step and increment numbers
7/01
Writing User Subroutines with ABAQUS
L6.25
ABAQUS
UMAT Interface
• The following quantities must be defined:
– Stress, SDVs, and material Jacobian
• The following variables may be defined:
– Strain energy, plastic dissipation, and “creep” dissipation
– Suggested new (reduced) time increment
Complete descriptions of all parameters are provided in the UMAT section
in Chapter 24 of the ABAQUS/Standard User’s Manual.
7/01
Writing User Subroutines with ABAQUS
L6.26
ABAQUS
UMAT Interface
• The header is usually followed by dimensioning of local arrays. It is
good practice to define constants via parameters and to include
comments.
DIMENSION EELAS(6), EPLAS(6), FLOW(6)
C
PARAMETER(ZERO=0.D0, ONE=1.D0, TWO=2.D0, THREE=3.D0, SIX=6.D0,
1
ENUMAX=.4999D0, NEWTON=10, TOLER=1.0D-6)
C
C ---------------------------------------------------------------C
UMAT FOR ISOTROPIC ELASTICITY AND ISOTROPIC MISES PLASTICITY
C
CANNOT BE USED FOR PLANE STRESS
C ---------------------------------------------------------------C
PROPS(1) - E
C
PROPS(2) - NU
C
PROPS(3..) - YIELD AND HARDENING DATA
C
CALLS UHARD FOR CURVE OF YIELD STRESS VS. PLASTIC STRAIN
C ----------------------------------------------------------------
– The PARAMETER assignments yield accurate floating point constant
definitions on any platform.
7/01
Writing User Subroutines with ABAQUS
L6.27
ABAQUS
UMAT Interface
UMAT Utilities
• Utility routines SINV, SPRINC, SPRIND, and ROTSIG can be called to
assist in coding UMAT.
– SINV will return the first and second invariants of a tensor.
– SPRINC will return the principal values of a tensor.
– SPRIND will return the principal values and directions of a tensor.
– ROTSIG will rotate a tensor with an orientation matrix.
– XIT will terminate an analysis and close all files associated with
the analysis properly.
• For details regarding the arguments required in making these calls,
refer to the UMAT section in Chapter 24 of the ABAQUS/Standard
User’s Manual and the examples in this lecture.
7/01
Writing User Subroutines with ABAQUS
L6.28
ABAQUS
UMAT Interface
UMAT Conventions
• Stresses and strains are stored as vectors.
– For plane stress elements: σ 11, σ 22, σ 12 .
– For (generalized) plane strain and axisymmetric
elements: σ 11, σ 22, σ 33, σ 12.
– For three-dimensional elements: σ 11, σ 22, σ 33, σ 12, σ 13 , σ 23 .
• The shear strain is stored as engineering shear strain,
γ 12 = 2ε 12 .
• The deformation gradient, F ij , is always stored as a three-dimensional
matrix.
7/01
Writing User Subroutines with ABAQUS
L6.29
ABAQUS
UMAT Interface
UMAT Formulation Aspects
• For geometrically nonlinear analysis the strain increment and
incremental rotation passed into the routine are based on the
Hughes-Winget formulae.
– Linearized strain and rotation increments are calculated in the
mid-increment configuration.
– Approximations are made, particularly if rotation increments are
large: more accurate measures can be obtained from the
deformation gradient if desired.
• The user must define the Cauchy stress: when this stress reappears
during the next increment, it will have been rotated with the
incremental rotation, DROT, passed into the subroutine.
– The stress tensor can be rotated back using the utility routine
ROTSIG if this is not desired.
7/01
Writing User Subroutines with ABAQUS
L6.30
ABAQUS
UMAT Interface
• If the ∗ORIENTATION option is used in conjunction with UMAT, stress
and strain components will be in the local system (again, this basis
system rotates with the material in finite-strain analysis).
• Tensor state variables must be rotated in the subroutine (use ROTSIG).
• If UMAT is used with reduced-integration elements or shear flexible
shell or beam elements, the hourglass stiffness and the transverse shear
stiffness must be specified with the ∗HOURGLASS STIFFNESS and
∗TRANSVERSE SHEAR STIFFNESS options, respectively.
7/01
Writing User Subroutines with ABAQUS
L6.31
ABAQUS
UMAT Interface
Usage Hints
• At the start of a new increment, the strain increment is extrapolated
from the previous increment.
– This extrapolation, which may sometimes cause trouble, is turned
off with ∗STEP, EXTRAPOLATION=NO.
• If the strain increment is too large, the variable PNEWDT can be used to
suggest a reduced time increment.
– The code will abandon the current time increment in favor of a
time increment given by PNEWDT*DTIME.
• The characteristic element length can be used to define softening
behavior based on fracture energy concepts.
7/01
Writing User Subroutines with ABAQUS
L6.32
ABAQUS
Example 1: Isotropic Isothermal Elasticity
Example 1: Isotropic Isothermal Elasticity
Governing Equations
• Isothermal elasticity equation (with Lamé’s constants):
σ ij = λδ ij ε kk + 2µε ij ,
or in a Jaumann (corotational) rate form:
σ̇ J ij = λδ ij ε̇ kk + 2µε̇ ij.
• The Jaumann rate equation is integrated in a corotational framework:
J
∆σ ij = λδ ij ∆ε kk + 2µ∆ε ij .
The appropriate coding is shown on the following pages.
7/01
Writing User Subroutines with ABAQUS
L6.33
ABAQUS
Example 1: Isotropic Isothermal Elasticity
Coding for Isotropic Isothermal Elasticity
C ---------------------------------------------------------------C
UMAT FOR ISOTROPIC ELASTICITY
C
CANNOT BE USED FOR PLANE STRESS
C ---------------------------------------------------------------C
PROPS(1) - E
C
PROPS(2) - NU
C ---------------------------------------------------------------C
IF (NDI.NE.3) THEN
WRITE (7, *) ’THIS UMAT MAY ONLY BE USED FOR ELEMENTS
1
WITH THREE DIRECT STRESS COMPONENTS’
CALL XIT
ENDIF
C
C
ELASTIC PROPERTIES
EMOD=PROPS(1)
ENU=PROPS(2)
EBULK3=EMOD/(ONE-TWO*ENU)
EG2=EMOD/(ONE+ENU)
EG=EG2/TWO
EG3=THREE*EG
ELAM=(EBULK3-EG2)/THREE
7/01
Writing User Subroutines with ABAQUS
L6.34
ABAQUS
C
C
C
Example 1: Isotropic Isothermal Elasticity
ELASTIC STIFFNESS
DO K1=1, NDI
DO K2=1, NDI
DDSDDE(K2, K1)=ELAM
END DO
DDSDDE(K1, K1)=EG2+ELAM
END DO
DO K1=NDI+1, NTENS
DDSDDE(K1 ,K1)=EG
END DO
C
C
C
CALCULATE STRESS
DO K1=1, NTENS
DO K2=1, NTENS
STRESS(K2)=STRESS(K2)+DDSDDE(K2, K1)*DSTRAN(K1)
END DO
END DO
C
RETURN
END
7/01
Writing User Subroutines with ABAQUS
L6.35
ABAQUS
Example 1: Isotropic Isothermal Elasticity
Remarks
• This very simple UMAT yields exactly the same results as the ABAQUS
∗ELASTIC option.
– This is true even for large-strain calculations: all necessary
large-strain contributions are generated by ABAQUS.
• The routine can be used with and without the ∗ORIENTATION option.
• It is usually straightforward to write a single routine that handles
(generalized) plane strain, axisymmetric, and three-dimensional
geometries.
– Generally, plane stress must be treated as a separate case because
the stiffness coefficients are different.
• The routine is written in incremental form as a preparation for
subsequent elastic-plastic examples.
7/01
Writing User Subroutines with ABAQUS
L6.36
ABAQUS
Example 1: Isotropic Isothermal Elasticity
• Even for linear analysis, UMAT is called twice for the first iteration of
each increment: once for assembly and once for recovery.
Subsequently, it is called once per iteration: assembly and recovery are
combined.
• A check is performed on the number of direct stress components, and
the analysis is terminated by calling the subroutine, XIT.
– A message is written to the message file (unit=7).
7/01
Writing User Subroutines with ABAQUS
L6.37
ABAQUS
Example 2: Non-Isothermal Elasticity
Example 2: Non-Isothermal Elasticity
Governing Equations
• Non-isothermal elasticity equation:
el
el
σ ij = λ ( T )δ ij ε kk + 2µ ( T )ε ij ,
el
ε ij = ε ij – αT δ ij ,
or in a Jaumann (corotational) rate form:
J
el + 2µε̇ el + λ̇δ ε el + 2µ̇ε el,
σ̇ ij = λδ ij ε̇ kk
ij
ij kk
ij
ε̇ ijel = ε̇ ij – αT˙ δ ij.
• The Jaumann rate equation is integrated in a corotational framework:
el + 2µ∆ε el + ∆λδ ε el + 2∆µε el, ∆ε el = ∆ε – α∆T δ .
∆σ ijJ = λδ ij ∆ε kk
ij
ij kk
ij
ij
ij
ij
The appropriate coding is shown on the following pages.
7/01
Writing User Subroutines with ABAQUS
L6.38
ABAQUS
Example 2: Non-Isothermal Elasticity
Coding for Non-Isothermal Elasticity
C
LOCAL ARRAYS
C ---------------------------------------------------------------C
EELAS - ELASTIC STRAINS
C
ETHERM - THERMAL STRAINS
C
DTHERM - INCREMENTAL THERMAL STRAINS
C
DELDSE - CHANGE IN STIFFNESS DUE TO TEMPERATURE CHANGE
C ---------------------------------------------------------------DIMENSION EELAS(6), ETHERM(6), DTHERM(6), DELDSE(6,6)
C
PARAMETER(ZERO=0.D0, ONE=1.D0, TWO=2.D0, THREE=3.D0, SIX=6.D0)
C ---------------------------------------------------------------C
UMAT FOR ISOTROPIC THERMO-ELASTICITY WITH LINEARLY VARYING
C
MODULI - CANNOT BE USED FOR PLANE STRESS
C ---------------------------------------------------------------C
PROPS(1) - E(T0)
C
PROPS(2) - NU(T0)
C
PROPS(3) - T0
C
PROPS(4) - E(T1)
C
PROPS(5) - NU(T1)
C
PROPS(6) - T1
C
PROPS(7) - ALPHA
C
PROPS(8) - T_INITIAL
7/01
Writing User Subroutines with ABAQUS
L6.39
ABAQUS
C
C
Example 2: Non-Isothermal Elasticity
ELASTIC PROPERTIES AT START OF INCREMENT
FAC1=(TEMP-PROPS(3))/(PROPS(6)-PROPS(3))
IF (FAC1 .LT. ZERO) FAC1=ZERO
IF (FAC1 .GT. ONE) FAC1=ONE
FAC0=ONE-FAC1
EMOD=FAC0*PROPS(1)+FAC1*PROPS(4)
ENU=FAC0*PROPS(2)+FAC1*PROPS(5)
EBULK3=EMOD/(ONE-TWO*ENU)
EG20=EMOD/(ONE+ENU)
EG0=EG20/TWO
ELAM0=(EBULK3-EG20)/THREE
C
C
C
ELASTIC PROPERTIES AT END OF INCREMENT
FAC1=(TEMP+DTEMP-PROPS(3))/(PROPS(6)-PROPS(3))
IF (FAC1 .LT. ZERO) FAC1=ZERO
IF (FAC1 .GT. ONE) FAC1=ONE
FAC0=ONE-FAC1
EMOD=FAC0*PROPS(1)+FAC1*PROPS(4)
ENU=FAC0*PROPS(2)+FAC1*PROPS(5)
EBULK3=EMOD/(ONE-TWO*ENU)
EG2=EMOD/(ONE+ENU)
EG=EG2/TWO
ELAM=(EBULK3-EG2)/THREE
7/01
Writing User Subroutines with ABAQUS
L6.40
ABAQUS
C
C
C
Example 2: Non-Isothermal Elasticity
ELASTIC STIFFNESS AT END OF INCREMENT AND STIFFNESS CHANGE
DO K1=1,NDI
DO K2=1,NDI
DDSDDE(K2,K1)=ELAM
DELDSE(K2,K1)=ELAM-ELAM0
END DO
DDSDDE(K1,K1)=EG2+ELAM
DELDSE(K1,K1)=EG2+ELAM-EG20-ELAM0
END DO
DO K1=NDI+1,NTENS
DDSDDE(K1,K1)=EG
DELDSE(K1,K1)=EG-EG0
END DO
C
C
C
CALCULATE THERMAL EXPANSION
DO K1=1,NDI
ETHERM(K1)=PROPS(7)*(TEMP-PROPS(8))
DTHERM(K1)=PROPS(7)*DTEMP
END DO
7/01
Writing User Subroutines with ABAQUS
L6.41
ABAQUS
Example 2: Non-Isothermal Elasticity
DO K1=NDI+1,NTENS
ETHERM(K1)=ZERO
DTHERM(K1)=ZERO
END DO
C
C
C
CALCULATE STRESS, ELASTIC STRAIN AND THERMAL STRAIN
DO K1=1, NTENS
DO K2=1, NTENS
STRESS(K2)=STRESS(K2)+DDSDDE(K2,K1)*(DSTRAN(K1)-DTHERM(K1))
1
+DELDSE(K2,K1)*( STRAN(K1)-ETHERM(K1))
END DO
ETHERM(K1)=ETHERM(K1)+DTHERM(K1)
EELAS(K1)=STRAN(K1)+DSTRAN(K1)-ETHERM(K1)
END DO
C
C
C
STORE ELASTIC AND THERMAL STRAINS IN STATE VARIABLE ARRAY
DO K1=1, NTENS
STATEV(K1)=EELAS(K1)
STATEV(K1+NTENS)=ETHERM(K1)
END DO
RETURN
END
7/01
Writing User Subroutines with ABAQUS
L6.42
ABAQUS
Example 2: Non-Isothermal Elasticity
Remarks
• This UMAT yields exactly the same results as the ∗ELASTIC option
with temperature dependence.
• The routine is written in incremental form, which allows generalization
to more complex temperature dependence.
7/01
Writing User Subroutines with ABAQUS
L6.43
ABAQUS
Example 3: Neo-Hookean Hyperelasticity
Example 3: Neo-Hookean Hyperelasticity
Governing Equations
• The ∗ELASTIC option does not work well for finite elastic strains
because a proper finite-strain energy function is not defined.
• Hence, we define a proper strain energy density function:
1
2
U = U ( I 1, I 2, J ) = C 10 ( I 1 – 3 ) + ------ ( J – 1 ) .
D1
– Here I 1 , I 2 , and J are the three strain invariants, expressed in terms
of the left Cauchy-Green tensor, B :
I 1 = tr ( B ) ,
7/01
1 2
I 2 = --- ( I 1 – t r ( B ⋅ B ) ) ,
2
B = F ⋅ F T,
Writing User Subroutines with ABAQUS
J = det ( F ) .
L6.44
ABAQUS
Example 3: Neo-Hookean Hyperelasticity
• In actuality, we use the deviatoric invariants I1 and I 2 (see Section 4.6.1
of the ABAQUS Theory Manual for more information).
– The constitutive equation can be written directly in terms of the
deformation gradient:
2
1
2
σ ij = --- C 10  B ij – --- δ ij B kk + ------ ( J – 1 )δ ij ,

 D1
J
3
B ij = B ij ⁄ J 2 / 3 .
• We define the virtual rate of deformation as
1
–1
–1
δD ij = --- ( δF im F mj + F mi δF jm ) .
2
• The Kirchhoff stress is defined through
τ ij = J σ ij .
7/01
Writing User Subroutines with ABAQUS
L6.45
ABAQUS
Example 3: Neo-Hookean Hyperelasticity
• The material Jacobian derives from the variation in Kirchhoff stress:
δτ ij = J C ijkl δD kl ,
where C ijkl are the components of the Jacobian. Using the
Neo-Hookean model,
2
1
C ijkl = --- C 10  --- ( δ ik B jl + B ik δ jl + δ il B jk + B il δ jk )
2
J
2
2
2
2
– --- δ ij B kl – --- B ij δ kl + --- δ ij δ kl B mm  + ------ ( 2J – 1 )δ ij δ kl
 D1
3
9
3
.
– The expression is fairly complex, but it is straightforward to
implement.
– For details of the derivation see Section 4.6.1 of the ABAQUS
Theory Manual.
The appropriate coding is shown on the following pages.
7/01
Writing User Subroutines with ABAQUS
L6.46
ABAQUS
Example 3: Neo-Hookean Hyperelasticity
Coding for Neo-Hookean Hyperelasticity
C
LOCAL ARRAYS
C ---------------------------------------------------------------C
EELAS - LOGARITHMIC ELASTIC STRAINS
C
EELASP - PRINCIPAL ELASTIC STRAINS
C
BBAR
- DEVIATORIC RIGHT CAUCHY-GREEN TENSOR
C
BBARP - PRINCIPAL VALUES OF BBAR
C
BBARN - PRINCIPAL DIRECTION OF BBAR (AND EELAS)
C
DISTGR - DEVIATORIC DEFORMATION GRADIENT (DISTORTION TENSOR)
C ---------------------------------------------------------------C
DIMENSION EELAS(6), EELASP(3), BBAR(6), BBARP(3), BBARN(3, 3),
1
DISTGR(3,3)
C
PARAMETER(ZERO=0.D0, ONE=1.D0, TWO=2.D0, THREE=3.D0, FOUR=4.D0,
1
SIX=6.D0)
C
C ---------------------------------------------------------------C
UMAT FOR COMPRESSIBLE NEO-HOOKEAN HYPERELASTICITY
C
CANNOT BE USED FOR PLANE STRESS
C ---------------------------------------------------------------C
PROPS(1) - E
C
PROPS(2) - NU
7/01
Writing User Subroutines with ABAQUS
L6.47
ABAQUS
Example 3: Neo-Hookean Hyperelasticity
C ---------------------------------------------------------------C
C
ELASTIC PROPERTIES
C
EMOD=PROPS(1)
ENU=PROPS(2)
C10=EMOD/(FOUR*(ONE+ENU))
D1=SIX*(ONE-TWO*ENU)/EMOD
C
C
JACOBIAN AND DISTORTION TENSOR
C
DET=DFGRD1(1, 1)*DFGRD1(2, 2)*DFGRD1(3, 3)
1
-DFGRD1(1, 2)*DFGRD1(2, 1)*DFGRD1(3, 3)
IF(NSHR.EQ.3) THEN
DET=DET+DFGRD1(1, 2)*DFGRD1(2, 3)*DFGRD1(3, 1)
1
+DFGRD1(1, 3)*DFGRD1(3, 2)*DFGRD1(2, 1)
2
-DFGRD1(1, 3)*DFGRD1(3,1)*DFGRD1(2, 2)
3
-DFGRD1(2, 3)*DFGRD1(3, 2)*DFGRD1(1, 1)
END IF
SCALE=DET**(-ONE/THREE)
DO K1=1, 3
DO K2=1, 3
DISTGR(K2, K1)=SCALE*DFGRD1(K2, K1)
END DO
7/01
Writing User Subroutines with ABAQUS
L6.48
ABAQUS
C
C
Example 3: Neo-Hookean Hyperelasticity
END DO
CALCULATE DEVIATORIC LEFT CAUCHY-GREEN DEFORMATION TENSOR
BBAR(1)=DISTGR(1, 1)**2+DISTGR(1, 2)**2+DISTGR(1, 3)**2
BBAR(2)=DISTGR(2, 1)**2+DISTGR(2, 2)**2+DISTGR(2, 3)**2
BBAR(3)=DISTGR(3, 3)**2+DISTGR(3, 1)**2+DISTGR(3, 2)**2
BBAR(4)=DISTGR(1, 1)*DISTGR(2, 1)+DISTGR(1, 2)*DISTGR(2, 2)
1
+DISTGR(1, 3)*DISTGR(2, 3)
IF(NSHR.EQ.3) THEN
BBAR(5)=DISTGR(1, 1)*DISTGR(3, 1)+DISTGR(1, 2)*DISTGR(3, 2)
1
+DISTGR(1, 3)*DISTGR(3, 3)
BBAR(6)=DISTGR(2, 1)*DISTGR(3, 1)+DISTGR(2, 2)*DISTGR(3, 2)
1
+DISTGR(2, 3)*DISTGR(3, 3)
END IF
C
C
C
CALCULATE THE STRESS
TRBBAR=(BBAR(1)+BBAR(2)+BBAR(3))/THREE
EG=TWO*C10/DET
EK=TWO/D1*(TWO*DET-ONE)
PR=TWO/D1*(DET-ONE)
DO K1=1,NDI
STRESS(K1)=EG*(BBAR(K1)-TRBBAR)+PR
END DO
7/01
Writing User Subroutines with ABAQUS
L6.49
ABAQUS
C
C
Example 3: Neo-Hookean Hyperelasticity
DO K1=NDI+1,NDI+NSHR
STRESS(K1)=EG*BBAR(K1)
END DO
CALCULATE THE STIFFNESS
EG23=EG*TWO/THREE
DDSDDE(1, 1)= EG23*(BBAR(1)+TRBBAR)+EK
DDSDDE(2, 2)= EG23*(BBAR(2)+TRBBAR)+EK
DDSDDE(3, 3)= EG23*(BBAR(3)+TRBBAR)+EK
DDSDDE(1, 2)=-EG23*(BBAR(1)+BBAR(2)-TRBBAR)+EK
DDSDDE(1, 3)=-EG23*(BBAR(1)+BBAR(3)-TRBBAR)+EK
DDSDDE(2, 3)=-EG23*(BBAR(2)+BBAR(3)-TRBBAR)+EK
DDSDDE(1, 4)= EG23*BBAR(4)/TWO
DDSDDE(2, 4)= EG23*BBAR(4)/TWO
DDSDDE(3, 4)=-EG23*BBAR(4)
DDSDDE(4, 4)= EG*(BBAR(1)+BBAR(2))/TWO
IF(NSHR.EQ.3) THEN
DDSDDE(1, 5)= EG23*BBAR(5)/TWO
DDSDDE(2, 5)=-EG23*BBAR(5)
DDSDDE(3, 5)= EG23*BBAR(5)/TWO
DDSDDE(1, 6)=-EG23*BBAR(6)
DDSDDE(2, 6)= EG23*BBAR(6)/TWO
DDSDDE(3, 6)= EG23*BBAR(6)/TWO
DDSDDE(5, 5)= EG*(BBAR(1)+BBAR(3))/TWO
7/01
Writing User Subroutines with ABAQUS
L6.50
ABAQUS
Example 3: Neo-Hookean Hyperelasticity
DDSDDE(6, 6)= EG*(BBAR(2)+BBAR(3))/TWO
DDSDDE(4,5)= EG*BBAR(6)/TWO
DDSDDE(4,6)= EG*BBAR(5)/TWO
DDSDDE(5,6)= EG*BBAR(4)/TWO
END IF
DO K1=1, NTENS
DO K2=1, K1-1
DDSDDE(K1, K2)=DDSDDE(K2, K1)
END DO
END DO
C
C
C
CALCULATE LOGARITHMIC ELASTIC STRAINS (OPTIONAL)
CALL SPRIND(BBAR, BBARP, BBARN, 1, NDI, NSHR)
EELASP(1)=LOG(SQRT(BBARP(1))/SCALE)
Call to SPRIND
EELASP(2)=LOG(SQRT(BBARP(2))/SCALE)
EELASP(3)=LOG(SQRT(BBARP(3))/SCALE)
EELAS(1)=EELASP(1)*BBARN(1,1)**2+EELASP(2)*BBARN(2, 1)**2
1
+EELASP(3)*BBARN(3, 1)**2
EELAS(2)=EELASP(1)*BBARN(1, 2)**2+EELASP(2)*BBARN(2, 2)**2
1
+EELASP(3)*BBARN(3, 2)**2
EELAS(3)=EELASP(1)*BBARN(1, 3)**2+EELASP(2)*BBARN(2, 3)**2
1
+EELASP(3)*BBARN(3, 3)**2
EELAS(4)=TWO*(EELASP(1)*BBARN(1, 1)*BBARN(1, 2)
7/01
Writing User Subroutines with ABAQUS
L6.51
ABAQUS
Example 3: Neo-Hookean Hyperelasticity
1
2
+EELASP(2)*BBARN(2, 1)*BBARN(2, 2)
+EELASP(3)*BBARN(3, 1)*BBARN(3, 2))
IF(NSHR.EQ.3) THEN
EELAS(5)=TWO*(EELASP(1)*BBARN(1, 1)*BBARN(1, 3)
1
+EELASP(2)*BBARN(2, 1)*BBARN(2, 3)
2
+EELASP(3)*BBARN(3, 1)*BBARN(3, 3))
EELAS(6)=TWO*(EELASP(1)*BBARN(1, 2)*BBARN(1, 3)
1
+EELASP(2)*BBARN(2, 2)*BBARN(2, 3)
2
+EELASP(3)*BBARN(3, 2)*BBARN(3, 3))
END IF
C
C
C
STORE ELASTIC STRAINS IN STATE VARIABLE ARRAY
DO K1=1, NTENS
STATEV(K1)=EELAS(K1)
END DO
C
RETURN
END
7/01
Writing User Subroutines with ABAQUS
L6.52
ABAQUS
Example 3: Neo-Hookean Hyperelasticity
Remarks
• This UMAT yields exactly the same results as the ∗HYPERELASTIC
option with N = 1 and C 01 = 0 .
• Note the use of the utility SPRIND.
CALL SPRIND(BBAR, BBARP, BBARN, 1, NDI, NSHR)
– Tensor BBAR consists of NDI direct components and NSHR shear
components.
– SPRIND returns the principal values and direction cosines of the
principal directions of BBAR in BBARP and BBARN, respectively.
– A value of 1 is used as the fourth argument to indicate that BBAR
contains stresses. (A value of 2 is used for strains.)
• Hyperelastic materials are often implemented more easily in user
subroutine UHYPER.
7/01
Writing User Subroutines with ABAQUS
L6.53
ABAQUS
Example 4: Kinematic Hardening Plasticity
Example 4: Kinematic Hardening Plasticity
Governing Equations
• Elasticity:
el
el
σ ij = λδ ij ε kk + 2µε ij ,
or in a Jaumann (corotational) rate form:
J
el
el
σ̇ ij = λδ ij ε̇ kk + 2µε̇ ij .
– The Jaumann rate equation is integrated in a corotational
framework:
el
el
∆σ ijJ = λδ ij ∆ε kk + 2µ∆ε ij .
7/01
Writing User Subroutines with ABAQUS
L6.54
ABAQUS
Example 4: Kinematic Hardening Plasticity
• Plasticity:
– Yield function:
3
--- ( S ij – α ij ) ( S ij – α ij ) – σ y = 0 .
2
– Equivalent plastic strain rate:
ε̇ pl =
2 pl pl
--- ε̇ ij ε̇ ij .
3
– Plastic flow law:
3
pl
ε̇ ijpl = --- ( S ij – α ij )ε̇ ⁄ σ y .
2
– Prager-Ziegler (linear) kinematic hardening:
2
α̇ ij = --- hε̇ ijpl .
3
7/01
Writing User Subroutines with ABAQUS
L6.55
ABAQUS
Example 4: Kinematic Hardening Plasticity
Integration Procedure
• We first calculate the equivalent stress based on purely elastic behavior
(elastic predictor):
σ pr =
3 pr
o
o
--- ( S ij – α ij ) ( S ijpr – α ij ) ,
2
S ijpr = S ijo + 2µ∆e ij .
• Plastic flow occurs if the elastic predictor is larger than the yield stress.
The backward Euler method is used to integrate the equations:
3
o
∆ε ijpl = --- ( S ijpr – α ij )∆ε pl ⁄ σ pr .
2
• After some manipulation we obtain a closed form expression for the
equivalent plastic strain increment:
∆ε pl = ( σ pr – σ y ) ⁄ ( h + 3µ ) .
7/01
Writing User Subroutines with ABAQUS
L6.56
ABAQUS
Example 4: Kinematic Hardening Plasticity
• This leads to the following update equations for the shift tensor, the
stress, and the plastic strain:
∆α ij = η ij h∆ε pl,
3
∆ε ijpl = --- η ij ∆ε pl
2
1
pr ,
σ ij = α ijo + ∆α ij + η ij σ y + --- δ ij σ kk
3
η ij = ( S ijpr – α ijo ) ⁄ σ pr .
• In addition, you can readily obtain the consistent Jacobian:
h
∆σ̇ ij = λ * δ ij ∆ε̇ kk + 2µ * ∆ε̇ ij +  ----------------------- – 3µ * η ij η kl ∆ε̇ kl
 1 + h ⁄ 3µ

µ * = µ ( σ y + h∆ε pl ) ⁄ σ pr ,
2
λ * = k – --- µ * .
3
– The integration procedure for kinematic hardening is described in
Section 21 of the ABAQUS/Explicit User’s Manual.
The appropriate coding is shown on the following pages.
7/01
Writing User Subroutines with ABAQUS
L6.57
ABAQUS
Example 4: Kinematic Hardening Plasticity
Coding for Kinematic Hardening Plasticity
C
LOCAL ARRAYS
C ---------------------------------------------------------------C
EELAS - ELASTIC STRAINS
C
EPLAS - PLASTIC STRAINS
C
ALPHA - SHIFT TENSOR
C
FLOW
- PLASTIC FLOW DIRECTIONS
C
OLDS
- STRESS AT START OF INCREMENT
C
OLDPL - PLASTIC STRAINS AT START OF INCREMENT
C
DIMENSION EELAS(6), EPLAS(6), ALPHA(6), FLOW(6), OLDS(6), OLDPL(6)
C
PARAMETER(ZERO=0.D0, ONE=1.D0, TWO=2.D0, THREE=3.D0, SIX=6.D0,
1
ENUMAX=.4999D0, TOLER=1.0D-6)
C
C ---------------------------------------------------------------C
UMAT FOR ISOTROPIC ELASTICITY AND MISES PLASTICITY
C
WITH KINEMATIC HARDENING - CANNOT BE USED FOR PLANE STRESS
C ---------------------------------------------------------------C
PROPS(1) - E
C
PROPS(2) - NU
C
PROPS(3) - SYIELD
C
PROPS(4) - HARD
7/01
Writing User Subroutines with ABAQUS
L6.58
ABAQUS
Example 4: Kinematic Hardening Plasticity
C ---------------------------------------------------------------C
C
ELASTIC PROPERTIES
C
EMOD=PROPS(1)
ENU=MIN(PROPS(2), ENUMAX)
EBULK3=EMOD/(ONE-TWO*ENU)
EG2=EMOD/(ONE+ENU)
EG=EG2/TWO
EG3=THREE*EG
ELAM=(EBULK3-EG2)/THREE
C
C
ELASTIC STIFFNESS
C
DO K1=1, NDI
DO K2=1, NDI
DDSDDE(K2, K1)=ELAM
END DO
DDSDDE(K1, K1)=EG2+ELAM
END DO
DO K1=NDI+1, NTENS
DDSDDE(K1, K1)=EG
END DO
7/01
Writing User Subroutines with ABAQUS
L6.59
ABAQUS
C
C
C
C
Example 4: Kinematic Hardening Plasticity
RECOVER ELASTIC STRAIN, PLASTIC STRAIN AND SHIFT TENSOR AND ROTATE
NOTE: USE CODE 1 FOR (TENSOR) STRESS, CODE 2 FOR (ENGINEERING) STRAIN
CALL ROTSIG(STATEV(
1), DROT, EELAS, 2, NDI, NSHR)
CALL ROTSIG(STATEV( NTENS+1), DROT, EPLAS, 2, NDI, NSHR)
CALL ROTSIG(STATEV(2*NTENS+1), DROT, ALPHA, 1, NDI, NSHR)
C
C
C
C
SAVE STRESS AND PLASTIC STRAINS AND
CALCULATE PREDICTOR STRESS AND ELASTIC STRAIN
Calls to ROTSIG
DO K1=1, NTENS
OLDS(K1)=STRESS(K1)
OLDPL(K1)=EPLAS(K1)
EELAS(K1)=EELAS(K1)+DSTRAN(K1)
DO K2=1, NTENS
STRESS(K2)=STRESS(K2)+DDSDDE(K2, K1)*DSTRAN(K1)
END DO
END DO
7/01
Writing User Subroutines with ABAQUS
L6.60
ABAQUS
C
C
C
Example 4: Kinematic Hardening Plasticity
CALCULATE EQUIVALENT VON MISES STRESS
SMISES=(STRESS(1)-ALPHA(1)-STRESS(2)+ALPHA(2))**2
1
+(STRESS(2)-ALPHA(2)-STRESS(3)+ALPHA(3))**2
2
+(STRESS(3)-ALPHA(3)-STRESS(1)+ALPHA(1))**2
DO K1=NDI+1,NTENS
SMISES=SMISES+SIX*(STRESS(K1)-ALPHA(K1))**2
END DO
SMISES=SQRT(SMISES/TWO)
C
C
C
GET YIELD STRESS AND HARDENING MODULUS
SYIELD=PROPS(3)
HARD=PROPS(4)
C
C
C
DETERMINE IF ACTIVELY YIELDING
IF(SMISES.GT.(ONE+TOLER)*SYIELD) THEN
C
C
7/01
ACTIVELY YIELDING
Writing User Subroutines with ABAQUS
L6.61
ABAQUS
C
C
C
Example 4: Kinematic Hardening Plasticity
SEPARATE THE HYDROSTATIC FROM THE DEVIATORIC STRESS
CALCULATE THE FLOW DIRECTION
SHYDRO=(STRESS(1)+STRESS(2)+STRESS(3))/THREE
DO K1=1,NDI
FLOW(K1)=(STRESS(K1)-ALPHA(K1)-SHYDRO)/SMISES
END DO
DO K1=NDI+1,NTENS
FLOW(K1)=(STRESS(K1)-ALPHA(K1))/SMISES
END DO
C
C
C
SOLVE FOR EQUIVALENT PLASTIC STRAIN INCREMENT
DEQPL=(SMISES-SYIELD)/(EG3+HARD)
7/01
Writing User Subroutines with ABAQUS
L6.62
ABAQUS
C
C
C
C
C
Example 4: Kinematic Hardening Plasticity
UPDATE SHIFT TENSOR, ELASTIC AND PLASTIC STRAINS AND STRESS
DO K1=1,NDI
ALPHA(K1)=ALPHA(K1)+HARD*FLOW(K1)*DEQPL
EPLAS(K1)=EPLAS(K1)+THREE/TWO*FLOW(K1)*DEQPL
EELAS(K1)=EELAS(K1)-THREE/TWO*FLOW(K1)*DEQPL
STRESS(K1)=ALPHA(K1)+FLOW(K1)*SYIELD+SHYDRO
END DO
DO K1=NDI+1,NTENS
ALPHA(K1)=ALPHA(K1)+HARD*FLOW(K1)*DEQPL
EPLAS(K1)=EPLAS(K1)+THREE*FLOW(K1)*DEQPL
EELAS(K1)=EELAS(K1)-THREE*FLOW(K1)*DEQPL
STRESS(K1)=ALPHA(K1)+FLOW(K1)*SYIELD
END DO
CALCULATE PLASTIC DISSIPATION
SPD=ZERO
DO K1=1,NTENS
SPD=SPD+(STRESS(K1)+OLDS(K1))*(EPLAS(K1)-OLDPL(K1))/TWO
END DO
7/01
Writing User Subroutines with ABAQUS
L6.63
ABAQUS
C
C
C
C
Example 4: Kinematic Hardening Plasticity
FORMULATE THE JACOBIAN (MATERIAL TANGENT)
FIRST CALCULATE EFFECTIVE MODULI
EFFG=EG*(SYIELD+HARD*DEQPL)/SMISES
EFFG2=TWO*EFFG
EFFG3=THREE*EFFG
EFFLAM=(EBULK3-EFFG2)/THREE
EFFHRD=EG3*HARD/(EG3+HARD)-EFFG3
DO K1=1, NDI
DO K2=1, NDI
DDSDDE(K2, K1)=EFFLAM
END DO
DDSDDE(K1, K1)=EFFG2+EFFLAM
END DO
DO K1=NDI+1, NTENS
DDSDDE(K1, K1)=EFFG
END DO
DO K1=1, NTENS
DO K2=1, NTENS
DDSDDE(K2, K1)=DDSDDE(K2, K1)+EFFHRD*FLOW(K2)*FLOW(K1)
END DO
END DO
ENDIF
7/01
Writing User Subroutines with ABAQUS
L6.64
ABAQUS
C
C
C
C
Example 4: Kinematic Hardening Plasticity
STORE ELASTIC STRAINS, PLASTIC STRAINS AND SHIFT TENSOR
IN STATE VARIABLE ARRAY
DO K1=1,NTENS
STATEV(K1)=EELAS(K1)
STATEV(K1+NTENS)=EPLAS(K1)
STATEV(K1+2*NTENS)=ALPHA(K1)
END DO
C
RETURN
END
7/01
Writing User Subroutines with ABAQUS
L6.65
ABAQUS
Example 4: Kinematic Hardening Plasticity
Remarks
• This UMAT yields exactly the same results as the ∗PLASTIC option
with KINEMATIC hardening.
– This is also true for large-strain calculations. The necessary
rotations of stress and strain are taken care of by ABAQUS.
7/01
Writing User Subroutines with ABAQUS
L6.66
ABAQUS
Example 4: Kinematic Hardening Plasticity
• Rotation of the shift tensor and the elastic and plastic strains is
accomplished by the calls to ROTSIG. The call
CALL ROTSIG(STATEV(1), DROT, EELAS, 2, NDI, NSHR)
applies the incremental rotation, DROT, to STATEV and stores the result
in ELAS.
– STATEV consists of NDI direct components and NSHR shear
components.
– A value of 1 is used as the fourth argument to indicate that the
transformed array contains tensor shear components such as α ij . A
value of 2 indicates that the array contains engineering shear
pl
components, such as ε ij .
• The rotation should be applied prior to the integration procedure.
7/01
Writing User Subroutines with ABAQUS
L6.67
ABAQUS
Example 4: Kinematic Hardening Plasticity
• The routine is written for linear hardening because the classical
Prager-Ziegler theory is limited to this case.
– More complex nonlinear kinematic hardening models are much
more difficult to integrate.
– However, once a suitable integration procedure is obtained, the
implementation in UMAT is straightforward and follows the
examples discussed here.
7/01
Writing User Subroutines with ABAQUS
L6.68
ABAQUS
Example 5: Isotropic Hardening Plasticity
Example 5: Isotropic Hardening Plasticity
Governing Equations
• Elasticity:
el
el
σ ij = λδ ij ε kk + 2µε ij ,
or in a Jaumann (corotational) rate form:
J
el
el
σ̇ ij = λδ ij ε̇ kk + 2µε̇ ij .
– The Jaumann rate equation is integrated in a corotational
framework:
el
el
∆σ ijJ = λδ ij ∆ε kk + 2µ∆ε ij .
7/01
Writing User Subroutines with ABAQUS
L6.69
ABAQUS
Example 5: Isotropic Hardening Plasticity
• Plasticity:
– Yield function:
1
S ij = σ ij – --- δ ij σ kk .
3
3
--- S ij S ij – σ y ( ε pl ) = 0 ,
2
– Equivalent plastic strain:
t
ε pl =
∫
pl
ε̇ dt,
ε̇
pl
=
2 pl pl
--- ε̇ ij ε̇ ij .
3
0
– Plastic flow law:
3 S ij pl
ε̇ ijpl = --- -----ε̇ .
2 σy
7/01
Writing User Subroutines with ABAQUS
L6.70
ABAQUS
Example 5: Isotropic Hardening Plasticity
Integration Procedure
• We first calculate the von Mises stress based on purely elastic behavior
(elastic predictor):
σ pr =
3 pr pr
--- S ij S ij ,
2
S ijpr = S ijo + 2µ∆e ij .
• If the elastic predictor is larger than the current yield stress, plastic flow
occurs. The backward Euler method is used to integrate the equations.
– After some manipulation we can reduce the problem to a single
equation in terms of the incremental equivalent plastic strain:
σ pr – 3µ∆ε pl = σ y ( ε pl ).
– This equation is solved with Newton’s method.
7/01
Writing User Subroutines with ABAQUS
L6.71
ABAQUS
Example 5: Isotropic Hardening Plasticity
• After the equation is solved, the following update equations for the
stress and the plastic strain can be used:
1
pr ,
σ ij = η ij σ y + --- δ ij σ kk
3
3
∆ε ijpl = --- η ij ∆ε pl
2
η ij = S ijpr ⁄ σ pr.
• In addition, you can readily obtain the consistent Jacobian:
h
∆σ̇ ij = λ * δ ij ∆ε̇ kk + 2µ * ∆ε̇ ij +  ----------------------- – 3µ * η ij η kl ∆ε̇ kl
 1 + h ⁄ 3µ

2
µ * = µσ y ⁄ σ pr, λ * = k – --- µ * ,
3
h = dσ y ⁄ dε pl.
– A detailed discussion about the isotropic plasticity integration
algorithm can be found in Section 4.2.2 of the ABAQUS Theory
Manual.
The appropriate coding is shown on the following pages.
7/01
Writing User Subroutines with ABAQUS
L6.72
ABAQUS
Example 5: Isotropic Hardening Plasticity
Coding for Isotropic Mises Plasticity
C
LOCAL ARRAYS
C ---------------------------------------------------------------C
EELAS - ELASTIC STRAINS
C
EPLAS - PLASTIC STRAINS
C
FLOW
- DIRECTION OF PLASTIC FLOW
C ---------------------------------------------------------------C
DIMENSION EELAS(6),EPLAS(6),FLOW(6), HARD(3)
C
PARAMETER(ZERO=0.D0, ONE=1.D0, TWO=2.D0, THREE=3.D0, SIX=6.D0,
1
ENUMAX=.4999D0, NEWTON=10, TOLER=1.0D-6)
C
C ---------------------------------------------------------------C
UMAT FOR ISOTROPIC ELASTICITY AND ISOTROPIC MISES PLASTICITY
C
CANNOT BE USED FOR PLANE STRESS
C ---------------------------------------------------------------C
PROPS(1) - E
C
PROPS(2) - NU
C
PROPS(3..) - SYIELD AN HARDENING DATA
C
CALLS UHARD FOR CURVE OF YIELD STRESS VS. PLASTIC STRAIN
C ----------------------------------------------------------------
7/01
Writing User Subroutines with ABAQUS
L6.73
ABAQUS
C
C
C
Example 5: Isotropic Hardening Plasticity
ELASTIC PROPERTIES
EMOD=PROPS(1)
ENU=MIN(PROPS(2), ENUMAX)
EBULK3=EMOD/(ONE-TWO*ENU)
EG2=EMOD/(ONE+ENU)
EG=EG2/TWO
EG3=THREE*EG
ELAM=(EBULK3-EG2)/THREE
C
C
C
ELASTIC STIFFNESS
DO K1=1, NDI
DO K2=1, NDI
DDSDDE(K2, K1)=ELAM
END DO
DDSDDE(K1, K1)=EG2+ELAM
END DO
DO K1=NDI+1, NTENS
DDSDDE(K1, K1)=EG
END DO
7/01
Writing User Subroutines with ABAQUS
L6.74
ABAQUS
C
C
C
Example 5: Isotropic Hardening Plasticity
RECOVER ELASTIC AND PLASTIC STRAINS AND ROTATE FORWARD
ALSO RECOVER EQUIVALENT PLASTIC STRAIN
CALL ROTSIG(STATEV(
1), DROT, EELAS, 2, NDI, NSHR)
CALL ROTSIG(STATEV(NTENS+1), DROT, EPLAS, 2, NDI, NSHR)
EQPLAS=STATEV(1+2*NTENS)
C
C
C
CALCULATE PREDICTOR STRESS AND ELASTIC STRAIN
DO K1=1, NTENS
DO K2=1, NTENS
STRESS(K2)=STRESS(K2)+DDSDDE(K2, K1)*DSTRAN(K1)
END DO
EELAS(K1)=EELAS(K1)+DSTRAN(K1)
END DO
C
C
C
CALCULATE EQUIVALENT VON MISES STRESS
SMISES=(STRESS(1)-STRESS(2))**2+(STRESS(2)-STRESS(3))**2
1
+(STRESS(3)-STRESS(1))**2
DO K1=NDI+1,NTENS
SMISES=SMISES+SIX*STRESS(K1)**2
END DO
SMISES=SQRT(SMISES/TWO)
7/01
Writing User Subroutines with ABAQUS
L6.75
ABAQUS
C
C
C
Example 5: Isotropic Hardening Plasticity
GET YIELD STRESS FROM THE SPECIFIED HARDENING CURVE
NVALUE=NPROPS/2-1
CALL UHARD(SYIEL0, HARD, EQPLAS, EQPLASRT,TIME,DTIME,TEMP,
1
DTEMP,NOEL,NPT,LAYER,KSPT,KSTEP,KINC,CMNAME,NSTATV,
2
STATEV,NUMFIELDV,PREDEF,DPRED,NVALUE,PROPS(3))
C
C
C
DETERMINE IF ACTIVELY YIELDING
IF (SMISES.GT.(ONE+TOLER)*SYIEL0) THEN
C
C
C
C
C
ACTIVELY YIELDING
SEPARATE THE HYDROSTATIC FROM THE DEVIATORIC STRESS
CALCULATE THE FLOW DIRECTION
SHYDRO=(STRESS(1)+STRESS(2)+STRESS(3))/THREE
DO K1=1,NDI
FLOW(K1)=(STRESS(K1)-SHYDRO)/SMISES
END DO
DO K1=NDI+1, NTENS
FLOW(K1)=STRESS(K1)/SMISES
END DO
7/01
Writing User Subroutines with ABAQUS
L6.76
ABAQUS
C
C
C
C
SOLVE FOR EQUIVALENT VON MISES STRESS
AND EQUIVALENT PLASTIC STRAIN INCREMENT USING NEWTON ITERATION
1
2
C
C
C
SYIELD=SYIEL0
DEQPL=ZERO
DO KEWTON=1, NEWTON
RHS=SMISES-EG3*DEQPL-SYIELD
DEQPL=DEQPL+RHS/(EG3+HARD(1))
CALL UHARD(SYIELD,HARD,EQPLAS+DEQPL,EQPLASRT,TIME,DTIME,TEMP,
DTEMP,NOEL,NPT,LAYER,KSPT,KSTEP,KINC,CMNAME,NSTATV,
STATEV,NUMFIELDV,PREDEF,DPRED,NVALUE,PROPS(3))
IF(ABS(RHS).LT.TOLER*SYIEL0) GOTO 10
END DO
WRITE WARNING MESSAGE TO THE .MSG FILE
2
1
10
7/01
Example 5: Isotropic Hardening Plasticity
WRITE(7,2) NEWTON
FORMAT(//,30X,’***WARNING - PLASTICITY ALGORITHM DID NOT ’,
’CONVERGE AFTER ’,I3,’ ITERATIONS’)
CONTINUE
Writing User Subroutines with ABAQUS
L6.77
ABAQUS
C
C
C
C
Example 5: Isotropic Hardening Plasticity
UPDATE STRESS, ELASTIC AND PLASTIC STRAINS AND
EQUIVALENT PLASTIC STRAIN
DO K1=1,NDI
STRESS(K1)=FLOW(K1)*SYIELD+SHYDRO
EPLAS(K1)=EPLAS(K1)+THREE/TWO*FLOW(K1)*DEQPL
EELAS(K1)=EELAS(K1)-THREE/TWO*FLOW(K1)*DEQPL
END DO
DO K1=NDI+1,NTENS
STRESS(K1)=FLOW(K1)*SYIELD
EPLAS(K1)=EPLAS(K1)+THREE*FLOW(K1)*DEQPL
EELAS(K1)=EELAS(K1)-THREE*FLOW(K1)*DEQPL
END DO
EQPLAS=EQPLAS+DEQPL
C
C
C
CALCULATE PLASTIC DISSIPATION
SPD=DEQPL*(SYIEL0+SYIELD)/TWO
7/01
Writing User Subroutines with ABAQUS
L6.78
ABAQUS
C
C
C
C
Example 5: Isotropic Hardening Plasticity
FORMULATE THE JACOBIAN (MATERIAL TANGENT)
FIRST CALCULATE EFFECTIVE MODULI
EFFG=EG*SYIELD/SMISES
EFFG2=TWO*EFFG
EFFG3=THREE/TWO*EFFG2
EFFLAM=(EBULK3-EFFG2)/THREE
EFFHRD=EG3*HARD(1)/(EG3+HARD(1))-EFFG3
DO K1=1, NDI
DO K2=1, NDI
DDSDDE(K2, K1)=EFFLAM
END DO
DDSDDE(K1, K1)=EFFG2+EFFLAM
END DO
DO K1=NDI+1, NTENS
DDSDDE(K1, K1)=EFFG
END DO
DO K1=1, NTENS
DO K2=1, NTENS
DDSDDE(K2, K1)=DDSDDE(K2, K1)+EFFHRD*FLOW(K2)*FLOW(K1)
END DO
END DO
ENDIF
7/01
Writing User Subroutines with ABAQUS
L6.79
ABAQUS
C
C
C
C
Example 5: Isotropic Hardening Plasticity
STORE ELASTIC AND (EQUIVALENT) PLASTIC STRAINS
IN STATE VARIABLE ARRAY
DO K1=1, NTENS
STATEV(K1)=EELAS(K1)
STATEV(K1+NTENS)=EPLAS(K1)
END DO
STATEV(1+2*NTENS)=EQPLAS
C
RETURN
END
SUBROUTINE UHARD(SYIELD,HARD,EQPLAS,EQPLASRT,TIME,DTIME,TEMP,
1
DTEMP,NOEL,NPT,LAYER,KSPT,KSTEP,KINC,
2
CMNAME,NSTATV,STATEV,NUMFIELDV,
3
PREDEF,DPRED,NVALUE,TABLE)
INCLUDE ’ABA_PARAM.INC’
CHARACTER*80 CMNAME
DIMENSION HARD(3),STATEV(NSTATV),TIME(*),
1
PREDEF(NUMFIELDV),DPRED(*)
7/01
Writing User Subroutines with ABAQUS
L6.80
ABAQUS
Example 5: Isotropic Hardening Plasticity
C
DIMENSION TABLE(2, NVALUE)
C
PARAMETER(ZERO=0.D0)
C
C
C
C
C
SET YIELD STRESS TO LAST VALUE OF TABLE, HARDENING TO ZERO
SYIELD=TABLE(1, NVALUE)
HARD(1)=ZERO
IF MORE THAN ONE ENTRY, SEARCH TABLE
IF(NVALUE.GT.1) THEN
DO K1=1, NVALUE-1
EQPL1=TABLE(2,K1+1)
IF(EQPLAS.LT.EQPL1) THEN
EQPL0=TABLE(2, K1)
IF(EQPL1.LE.EQPL0) THEN
WRITE(7, 1)
1
FORMAT(//, 30X, ’***ERROR - PLASTIC STRAIN MUST BE ‘,
1
‘ENTERED IN ASCENDING ORDER’)
CALL XIT
ENDIF
7/01
Writing User Subroutines with ABAQUS
L6.81
ABAQUS
C
C
C
Example 5: Isotropic Hardening Plasticity
CURRENT YIELD STRESS AND HARDENING
DEQPL=EQPL1-EQPL0
SYIEL0=TABLE(1, K1)
SYIEL1=TABLE(1, K1+1)
DSYIEL=SYIEL1-SYIEL0
HARD(1)=DSYIEL/DEQPL
SYIELD=SYIEL0+(EQPLAS-EQPL0)*HARD(1)
GOTO 10
ENDIF
END DO
10
CONTINUE
ENDIF
RETURN
END
7/01
Writing User Subroutines with ABAQUS
L6.82
ABAQUS
Example 5: Isotropic Hardening Plasticity
Remarks
• This UMAT yields exactly the same results as the ∗PLASTIC option
with ISOTROPIC hardening.
– This result is also true for large-strain calculations. The necessary
rotations of stress and strain are taken care of by ABAQUS.
– The rotation of elastic and plastic strain, prior to integration, is
accomplished by the calls to ROTSIG.
7/01
Writing User Subroutines with ABAQUS
L6.83
ABAQUS
Example 5: Isotropic Hardening Plasticity
• The routine calls user subroutine UHARD to recover a piecewise linear
hardening curve.
– It is straightforward to replace the piecewise linear curve by an
analytic description.
– A local Newton iteration is used to determine the current yield
stress and hardening modulus.
– If the data are not given in ascending order of strain, the routine
XIT is called, which closes all files and terminates execution.
7/01
Writing User Subroutines with ABAQUS
L6.84
ABAQUS
VUMAT Interface
VUMAT Interface
• These input lines act as the interface to a VUMAT in which kinematic
hardening plasticity is defined.
*MATERIAL, NAME=KINPLAS
*USER MATERIAL, CONSTANTS=4
30.E6, 0.3, 30.E3, 40.E3
*DEPVAR
5
*INITIAL CONDITIONS, TYPE=SOLUTION
Data line to specify initial solution-dependent variables
7/01
Writing User Subroutines with ABAQUS
L6.85
ABAQUS
VUMAT Interface
• The input lines are identical to those for the UMAT interface.
– The user subroutine must be kept in a separate file, and is invoked
with the ABAQUS execution procedure, as follows:
abaqus job=... user=....
– The user subroutine must be invoked in a restarted analysis
because user subroutines are not saved in the restart file.
7/01
Writing User Subroutines with ABAQUS
L6.86
ABAQUS
VUMAT Interface
• Additional notes:
– Solution-dependent state variables can be output with identifiers
SDV1, SDV2, etc. Contour, path, and X–Y plots of SDVs can be
plotted in ABAQUS/Viewer.
– Include only a single VUMAT subroutine in the analysis. If more
than one material must be defined, test on the material name in the
VUMAT routine and branch.
7/01
Writing User Subroutines with ABAQUS
L6.87
ABAQUS
VUMAT Interface
• The VUMAT subroutine header is shown below:
SUBROUTINE VUMAT(
C Read only 1
NBLOCK, NDIR, NSHR, NSTATEV, NFIELDV, NPROPS, LANNEAL,
2
STEPTIME, TOTALTIME, DT, CMNAME, COORDMP, CHARLENGTH,
3
PROPS, DENSITY, STRAININC, RELSPININC,
4
TEMPOLD, STRETCHOLD, DEFGRADOLD, FIELDOLD,
5
STRESSOLD, STATEOLD, ENERINTERNOLD, ENERINELASOLD,
6
TEMPNEW, STRETCHNEW, DEFGRADNEW, FIELDNEW,
C Write only 7
STRESSNEW, STATENEW, ENERINTERNNEW, ENERINELASNEW)
C
INCLUDE ’VABA_PARAM.INC’
C
7/01
Writing User Subroutines with ABAQUS
L6.88
ABAQUS
VUMAT Interface
DIMENSION PROPS(NPROPS), DENSITY(NBLOCK), COORDMP(NBLOCK),
1 CHARLENGTH(NBLOCK), STRAININC(NBLOCK, NDIR+NSHR),
2 RELSPININC(NBLOCK, NSHR), TEMPOLD(NBLOCK),
3 STRETCHOLD(NBLOCK, NDIR+NSHR),DEFGRADOLD(NBLOCK,NDIR+NSHR+NSHR),
4 FIELDOLD(NBLOCK, NFIELDV), STRESSOLD(NBLOCK, NDIR+NSHR),
5 STATEOLD(NBLOCK, NSTATEV), ENERINTERNOLD(NBLOCK),
6 ENERINELASOLD(NBLOCK), TEMPNEW(NBLOCK),
7 STRETCHNEW(NBLOCK, NDIR+NSHR),DEFGRADNEW(NBLOCK,NDIR+NSHR+NSHR),
8 FIELDNEW(NBLOCK, NFIELDV), STRESSNEW(NBLOCK,NDIR+NSHR),
9 STATENEW(NBLOCK, NSTATEV), ENERINTERNNEW(NBLOCK),
1 ENERINELASNEW(NBLOCK)
C
CHARACTER*8 CMNAME
7/01
Writing User Subroutines with ABAQUS
L6.89
ABAQUS
VUMAT Interface
VUMAT Variables
• The following quantities are available in VUMAT, but they cannot be
redefined:
– Stress, stretch, and SDVs at the start of the increment
– Relative rotation vector and deformation gradient at the start and
end of an increment and strain increment
– Total and incremental values of time, temperature, and
user-defined field variables at the start and end of an increment
– Material constants, density, material point position, and a
characteristic element length
– Internal and dissipated energies at the beginning of the increment
– Number of material points to be processed in a call to the routine
(NBLOCK)
7/01
Writing User Subroutines with ABAQUS
L6.90
ABAQUS
VUMAT Interface
– A flag indicating whether the routine is being called during an
annealing process
• The following quantities must be defined:
– Stress and SDVs at the end of an increment
• The following variables may be defined:
– Internal and dissipated energies at the end of the increment
Many of these variables are equivalent or similar to those in UMAT.
Complete descriptions of all parameters are provided in the VUMAT
section in Chapter 21 of the ABAQUS/Explicit User’s Manual.
7/01
Writing User Subroutines with ABAQUS
L6.91
ABAQUS
VUMAT Interface
Comparison of VUMAT and UMAT Interfaces
There are a number of significant differences between the UMAT and
VUMAT interfaces.
• VUMAT uses a two-state architecture: the initial values are in the OLD
arrays, the new values must be put in the NEW arrays.
• The VUMAT interface is written to take advantage of vector
processing.
• The material Jacobian does not need to be defined.
• No information is provided about element numbers.
• The time increment cannot be redefined.
• Utility routines are not available because they would prevent
vectorization.
7/01
Writing User Subroutines with ABAQUS
L6.92
ABAQUS
VUMAT Interface
• The header is usually followed by dimensioning of local arrays. It is
good practice to define constants via parameters and to include
comments.
C
C
C
C
C
C
C
PARAMETER( ZERO = 0.D0, ONE = 1.D0, TWO = 2.D0, THREE = 3.D0,
1
THIRD = 1.D0/3.D0, HALF = .5D0, TWO_THIRDS = 2.D0/3.D0,
2
THREE_HALFS = 1.5D0 )
J2 Mises Plasticity with kinematic hardening for plane strain case.
The state variables are stored as:
STATE(*, 1) = back stress component 11
STATE(*, 2) = back stress component 22
STATE(*, 3) = back stress component 33
STATE(*, 4) = back stress component 12
STATE(*, 5) = equivalent plastic strain
– The PARAMETER assignments yield accurate floating point constant
definitions on any platform.
7/01
Writing User Subroutines with ABAQUS
L6.93
ABAQUS
VUMAT Interface
VUMAT Conventions
• Stresses and strains are stored as vectors.
– For plane stress elements: σ 11, σ 22, σ 12 .
– For plane strain and axisymmetric elements: σ 11, σ 22, σ 33, σ 12.
– For three-dimensional elements: σ 11, σ 22, σ 33, σ 12, σ 23, σ 31 .
For three-dimensional elements, this storage scheme is inconsistent
with that for ABAQUS/Standard.
• The shear strain is stored as tensor shear strains:
1
ε 12 = --- γ 12 .
2
7/01
Writing User Subroutines with ABAQUS
L6.94
ABAQUS
VUMAT Interface
• The deformation gradient is stored similar to the way in which
symmetric tensors are stored.
– For plane stress elements: F 11, F 22, F 12, F 21.
– For plane strain and axisymmetric elements: F 11, F 22, F 33, F 12, F 21.
– For three-dimensional elements:
F 11, F 22, F 33, F 12, F 23, F 31, F 21, F 32, F 13.
7/01
Writing User Subroutines with ABAQUS
L6.95
ABAQUS
VUMAT Interface
VUMAT Formulation Aspects
Vectorized Interface
• In VUMAT the data are passed in and out in large blocks (dimension
NBLOCK). NBLOCK typically is equal to 64 or 128.
– Each entry in an array of length NBLOCK corresponds to a single
material point. All material points in the same block have the same
material name and belong to the same element type.
• This structure allows vectorization of the routine.
– A vectorized VUMAT should make sure that all operations are done
in vector mode with NBLOCK the vector length.
• In vectorized code, branching inside loops should be avoided.
– Element type-based branching should be outside the NBLOCK loop.
7/01
Writing User Subroutines with ABAQUS
L6.96
ABAQUS
VUMAT Interface
Corotational Formulation
• The constitutive equation is formulated in a corotational framework,
based on the Jaumann stress rate.
– The strain increment is obtained with Hughes-Winget.
– Other measures can be obtained from the deformation gradient.
• The user must define the Cauchy stress: this stress reappears during
the next increment as the “old” stress.
• There is no need to rotate tensor state variables.
7/01
Writing User Subroutines with ABAQUS
L6.97
ABAQUS
Example 6: VUMAT for Kinematic Hardening
Example 6: VUMAT for Kinematic Hardening
The governing equations and integration procedure are the same as in
Example 4: Kinematic Hardening Plasticity (p. L6.54).
The Jacobian is not required.
7/01
Writing User Subroutines with ABAQUS
L6.98
ABAQUS
Example 6: VUMAT for Kinematic Hardening
Coding for Kinematic Hardening Plasticity VUMAT
C
E
XNU
YIELD
HARD
C
C
C
=
=
=
=
PROPS(1)
PROPS(2)
PROPS(3)
PROPS(4)
ELASTIC CONSTANTS
TWOMU
THREMU
SIXMU
ALAMDA
TERM
CON1
=
=
=
=
=
=
E / ( ONE + XNU )
THREE_HALFS * TWOMU
THREE * TWOMU
TWOMU * ( E - TWOMU ) / ( SIXMU - TWO * E )
ONE / ( TWOMU * ( ONE + HARD/THREMU ) )
SQRT( TWO_THIRDS )
C
C
7/01
Writing User Subroutines with ABAQUS
L6.99
ABAQUS
Example 6: VUMAT for Kinematic Hardening
C
C If stepTime equals to zero, assume the material pure elastic and use
C initial elastic modulus
C
IF( STEPTIME .EQ. ZERO ) THEN
C
DO I = 1,NBLOCK
C
C Trial Stress
TRACE = STRAININC (I, 1) + STRAININC (I, 2) + STRAININC (I, 3)
STRESSNEW(I, 1)=STRESSOLD(I, 1) + ALAMDA*TRACE
1
+
TWOMU*STRAININC(I,1)
STRESSNEW(I, 2)=STRESSOLD(I, 2) + ALAMDA*TRACE
1
+
TWOMU*STRAININC(I, 2)
STRESSNEW(I, 3)=STRESSOLD(I, 3) + ALAMDA*TRACE
1
+
TWOMU*STRAININC(I,3)
STRESSNEW(I, 4)=STRESSOLD(I, 4)
1
+
TWOMU*STRAININC(I, 4)
END DO
C
ELSE
7/01
Writing User Subroutines with ABAQUS
L6.100
ABAQUS
C
C
C
C
C
C
C
7/01
Example 6: VUMAT for Kinematic Hardening
PLASTICITY CALCULATIONS IN BLOCK FORM
DO I = 1, NBLOCK
Elastic predictor stress
TRACE = STRAININC(I, 1) + STRAININC(I, 2) + STRAININC(I, 3)
SIG1= STRESSOLD(I, 1) + ALAMDA*TRACE + TWOMU*STRAININC(I, 1)
SIG2= STRESSOLD(I, 2) + ALAMDA*TRACE + TWOMU*STRAININC(I, 2)
SIG3= STRESSOLD(I, 3) + ALAMDA*TRACE + TWOMU*STRAININC(I, 3)
SIG4= STRESSOLD(I, 4)
+ TWOMU*STRAININC(I, 4)
Elastic predictor stress measured from the back stress
S1 = SIG1 - STATEOLD(I, 1)
S2 = SIG2 - STATEOLD(I, 2)
S3 = SIG3 - STATEOLD(I, 3)
S4 = SIG4 - STATEOLD(I, 4)
Deviatoric part of predictor stress measured from the back stress
SMEAN = THIRD * ( S1 + S2 + S3 )
DS1 = S1 - SMEAN
DS2 = S2 - SMEAN
DS3 = S3 - SMEAN
Magnitude of the deviatoric predictor stress difference
DSMAG = SQRT( DS1**2 + DS2**2 + DS3**2 + TWO*S4**2 )
Writing User Subroutines with ABAQUS
L6.101
ABAQUS
C
C
C
C
C
C
C
C
C
C
7/01
Example 6: VUMAT for Kinematic Hardening
Check for yield by determining the factor for plasticity, zero for
elastic, one for yield
RADIUS = CON1 * YIELD
FACYLD = ZERO
IF( DSMAG - RADIUS .GE. ZERO ) FACYLD = ONE
Add a protective addition factor to prevent a divide by zero when DSMAG
is zero. If DSMAG is zero, we will not have exceeded the yield stress
and FACYLD will be zero.
DSMAG = DSMAG + ( ONE - FACYLD )
Calculated increment in gamma ( this explicitly includes the time step)
DIFF
= DSMAG - RADIUS
DGAMMA = FACYLD * TERM * DIFF
Update equivalent plastic strain
DEQPS = CON1 * DGAMMA
STATENEW(I, 5) = STATEOLD(I, 5) + DEQPS
Divide DGAMMA by DSMAG so that the deviatoric stresses are explicitly
converted to tensors of unit magnitude in the following calculations
DGAMMA = DGAMMA / DSMAG
Update back stress
FACTOR = HARD * DGAMMA * TWO_THIRDS
STATENEW(I, 1) = STATEOLD(I, 1) + FACTOR * DS1
STATENEW(I, 2) = STATEOLD(I, 2) + FACTOR * DS2
STATENEW(I, 3) = STATEOLD(I, 3) + FACTOR * DS3
STATENEW(I, 4) = STATEOLD(I, 4) + FACTOR * S4
Writing User Subroutines with ABAQUS
L6.102
ABAQUS
C
C
C
Example 6: VUMAT for Kinematic Hardening
Update stress
FACTOR
= TWOMU * DGAMMA
STRESSNEW(I, 1) = SIG1 - FACTOR * DS1
STRESSNEW(I, 2) = SIG2 - FACTOR * DS2
STRESSNEW(I, 3) = SIG3 - FACTOR * DS3
STRESSNEW(I, 4) = SIG4 - FACTOR * S4
Update the specific internal energy STRESS_POWER = HALF * (
1
( STRESSOLD(I, 1)+STRESSNEW(I, 1) )*STRAININC(I,
2
+
( STRESSOLD(I, 2)+STRESSNEW(I, 2) )*STRAININC(I,
3
+
( STRESSOLD(I, 3)+STRESSNEW(I, 3) )*STRAININC(I,
4
+ TWO*( STRESSOLD(I, 4)+STRESSNEW(I, 4) )*STRAININC(I,
ENERINTERNNEW(I) = ENERINTERNOLD(I)
1
+
STRESS_POWER/DENSITY(I)
Update the dissipated inelastic specific energy SMEAN = THIRD* (STRESSNEW(I, 1)+STRESSNEW(I, 2)
1
+
STRESSNEW(I, 3))
EQUIV_STRESS = SQRT( THREE_HALFS
1
*
( (STRESSNEW(I, 1)-SMEAN)**2
2
+
(STRESSNEW(I, 2)-SMEAN)**2
3
+
(STRESSNEW(I, 3)-SMEAN)**2
4
+
TWO * STRESSNEW(I, 4)**2 ) )
1)
2)
3)
4) )
C
7/01
Writing User Subroutines with ABAQUS
L6.103
ABAQUS
Example 6: VUMAT for Kinematic Hardening
1
PLASTIC_WORK_INC = EQUIV_STRESS * DEQPS
ENERINELASNEW(I) = ENERINELASOLD(I)
+
PLASTIC_WORK_INC / DENSITY(I)
C
END DO
C
END IF
RETURN
END
7/01
Writing User Subroutines with ABAQUS
L6.104
ABAQUS
Example 6: VUMAT for Kinematic Hardening
Remarks
• In the datacheck phase, VUMAT is called with a set of fictitious strains
and a TOTALTIME and STEPTIME both equal to 0.0.
– A check is done on the user’s constitutive relation, and an initial
stable time increment is determined based on calculated equivalent
initial material properties.
– Ensure that elastic properties are used in this call to VUMAT;
otherwise, too large an initial time increment may be used, leading
to instability.
– A warning message is printed to the status (.sta) file informing
the user that this check is being performed.
7/01
Writing User Subroutines with ABAQUS
L6.105
ABAQUS
Example 6: VUMAT for Kinematic Hardening
• Special coding techniques are used to obtain vectorized coding.
– All small loops inside the material routine are “unrolled.”
– The same code is executed regardless of whether the behavior is
purely elastic or elastic plastic.
• Special care must be taken to avoid divides by zero.
– No external subroutines are called inside the loop.
– The use of local scalar variables inside the loop is allowed.
– The compiler will automatically expand these local scalar variables
to local vectors.
– Iterations should be avoided.
• If iterations cannot be avoided, use a fixed number of iterations and do
not test on convergence.
7/01
Writing User Subroutines with ABAQUS
L6.106
ABAQUS
Example 7: VUMAT for Isotropic Hardening
Example 7: VUMAT for Isotropic Hardening
The governing equations and integration procedure are the same as in
Example 5: Isotropic Hardening Plasticity (p. L6.69).
The increment of equivalent plastic strain is obtained explicitly through
∆ε pl
σ pr – σ y
= -------------------- ,
3µ + h
where σ y is the yield stress and h = dσ y ⁄ dε pl is the plastic hardening at
the beginning of the increment.
The Jacobian is not required.
7/01
Writing User Subroutines with ABAQUS
L6.107
ABAQUS
Example 7: VUMAT for Isotropic Hardening
Coding for Isotropic Hardening Plasticity VUMAT
C
C
parameter ( zero = 0.d0, one = 1.d0, two = 2.d0,
*
third = 1.d0 / 3.d0, half = 0.5d0, op5 = 1.5d0)
C
C
C
C
C
C
C
C
C
C
C
C
C
C
7/01
For plane strain, axisymmetric, and 3D cases using
the J2 Mises Plasticity with piecewise-linear isotropic hardening.
The state variable is stored as:
STATE(*,1) = equivalent plastic strain
User needs to input
props(1)
Young’s modulus
props(2)
Poisson’s ratio
props(3..)
syield and hardening data
calls vuhard for curve of yield stress vs. plastic strain
Writing User Subroutines with ABAQUS
L6.108
ABAQUS
e
xnu
twomu
alamda
thremu
nvalue
Example 7: VUMAT for Isotropic Hardening
=
=
=
=
=
=
props(1)
props(2)
e / ( one + xnu )
xnu * twomu / ( one - two * xnu )
op5 * twomu
nprops/2-1
C
if ( stepTime .eq. zero ) then
do k = 1, nblock
trace = strainInc(k,1) + strainInc(k,2) + strainInc(k,3)
stressNew(k,1) = stressOld(k,1)
*
+ twomu * strainInc(k,1) + alamda * trace
stressNew(k,2) = stressOld(k,2)
*
+ twomu * strainInc(k,2) + alamda * trace
stressNew(k,3) = stressOld(k,3)
*
+ twomu * strainInc(k,3) + alamda * trace
stressNew(k,4)=stressOld(k,4) + twomu * strainInc(k,4)
if ( nshr .gt. 1 ) then
stressNew(k,5)=stressOld(k,5) + twomu * strainInc(k,5)
stressNew(k,6)=stressOld(k,6) + twomu * strainInc(k,6)
end if
end do
else
7/01
Writing User Subroutines with ABAQUS
L6.109
ABAQUS
Example 7: VUMAT for Isotropic Hardening
do k = 1, nblock
peeqOld=stateOld(k,1)
call vuhard(yieldOld, hard, peeqOld, props(3), nvalue)
trace = strainInc(k,1) + strainInc(k,2) + strainInc(k,3)
s11 = stressOld(k,1) + twomu * strainInc(k,1) + alamda * trace
s22 = stressOld(k,2) + twomu * strainInc(k,2) + alamda * trace
s33 = stressOld(k,3) + twomu * strainInc(k,3) + alamda * trace
s12 = stressOld(k,4) + twomu * strainInc(k,4)
if ( nshr .gt. 1 ) then
s13 = stressOld(k,5) + twomu * strainInc(k,5)
s23 = stressOld(k,6) + twomu * strainInc(k,6)
end if
7/01
Writing User Subroutines with ABAQUS
L6.110
ABAQUS
Example 7: VUMAT for Isotropic Hardening
C
smean = third * ( s11 + s22 + s33 )
s11 = s11 - smean
s22 = s22 - smean
s33 = s33 - smean
if ( nshr .eq. 1 ) then
vmises = sqrt( op5*(s11*s11+s22*s22+s33*s33+two*s12*s12) )
else
*
vmises = sqrt( op5 * ( s11 * s11 + s22 * s22 + s33 * s33 +
two * s12 * s12 + two * s13 * s13 + two * s23 * s23 ) )
end if
C
sigdif = vmises - yieldOld
facyld = zero
if ( sigdif .gt. zero ) facyld = one
deqps = facyld * sigdif / ( thremu + hard )
7/01
Writing User Subroutines with ABAQUS
L6.111
ABAQUS
Example 7: VUMAT for Isotropic Hardening
C
C Update the stress
C
yieldNew = yieldOld + hard * deqps
factor = yieldNew / ( yieldNew + thremu * deqps )
stressNew(k,1)
stressNew(k,2)
stressNew(k,3)
stressNew(k,4)
=
=
=
=
s11
s22
s33
s12
*
*
*
*
factor + smean
factor + smean
factor + smean
factor
if ( nshr .gt. 1 ) then
stressNew(k,5) = s13 * factor
stressNew(k,6) = s23 * factor
end if
C
C Update the state variables
C
stateNew(k,1) = stateOld(k,1) + deqps
7/01
Writing User Subroutines with ABAQUS
L6.112
ABAQUS
Example 7: VUMAT for Isotropic Hardening
C
C Update the specific internal energy C
if ( nshr .eq. 1 ) then
*
*
*
*
stressPower = half * (
( stressOld(k,1) + stressNew(k,1) ) * strainInc(k,1) +
( stressOld(k,2) + stressNew(k,2) ) * strainInc(k,2) +
( stressOld(k,3) + stressNew(k,3) ) * strainInc(k,3) ) +
( stressOld(k,4) + stressNew(k,4) ) * strainInc(k,4)
else
*
*
*
*
*
*
stressPower = half * (
( stressOld(k,1) + stressNew(k,1) ) * strainInc(k,1) +
( stressOld(k,2) + stressNew(k,2) ) * strainInc(k,2) +
( stressOld(k,3) + stressNew(k,3) ) * strainInc(k,3) ) +
( stressOld(k,4) + stressNew(k,4) ) * strainInc(k,4) +
( stressOld(k,5) + stressNew(k,5) ) * strainInc(k,5) +
( stressOld(k,6) + stressNew(k,6) ) * strainInc(k,6)
end if
enerInternNew(k) = enerInternOld(k) + stressPower / density(k)
7/01
Writing User Subroutines with ABAQUS
L6.113
ABAQUS
Example 7: VUMAT for Isotropic Hardening
C
C Update the dissipated inelastic specific energy C
plasticWorkInc = half * ( yieldOld + yieldNew ) * deqps
enerInelasNew(k) = enerInelasOld(k)
*
+ plasticWorkInc / density(k)
end do
end if
C
return
end
7/01
Writing User Subroutines with ABAQUS
L6.114
ABAQUS
Example 7: VUMAT for Isotropic Hardening
subroutine vuhard(syield, hard, eqplas, table, nvalue)
include ’vaba_param.inc’
c
dimension table(2, nvalue)
c
parameter(zero=0.d0)
c
c
c
set yield stress to last value of table, hardening to zero
syield=table(1, nvalue)
hard=zero
c
c
c
if more than one entry, search table
if(nvalue.gt.1) then
do k1=1, nvalue-1
eqpl1=table(2,k1+1)
if(eqplas.lt.eqpl1) then
eqpl0=table(2, k1)
c
c
c
yield stress and hardening
deqpl=eqpl1-eqpl0
syiel0=table(1, k1)
7/01
Writing User Subroutines with ABAQUS
L6.115
ABAQUS
Example 7: VUMAT for Isotropic Hardening
syiel1=table(1, k1+1)
dsyiel=syiel1-syiel0
hard=dsyiel/deqpl
syield=syiel0+(eqplas-eqpl0)*hard
goto 10
endif
end do
10
continue
endif
return
end
7/01
Writing User Subroutines with ABAQUS
L6.116
ABAQUS
Example 7: VUMAT for Isotropic Hardening
Remarks
• This VUMAT yields the same results as the ∗PLASTIC option with
ISOTROPIC hardening.
– This result is also true for large-strain calculations. The necessary
rotations of stress and strain are taken care of by ABAQUS.
• The routine calls user subroutine VUHARD to recover a piecewise linear
hardening curve.
– It is straightforward to replace the piecewise linear curve by an
analytic description.
7/01
Writing User Subroutines with ABAQUS
L6.117
ABAQUS
Lecture 7
Creating a Nonlinear User Element
Overview
• Motivation
• Defining a User Element
• UEL Interface
• Example 1: Planar Beam Element with Nonlinear Section Behavior
• Example 2: Force Control Element
• Using Nonlinear User Elements in Various Analysis Procedures
7/01
Writing User Subroutines with ABAQUS
L7.1
ABAQUS
Overview
Overview
ABAQUS/Standard has an interface that allows users to implement linear
and nonlinear finite elements.
• A nonlinear finite element is implemented in user subroutine UEL.
The interface makes it possible to define any (proprietary) element of
arbitrary complexity.
• If coded properly, user elements can be utilized with most analysis
procedures in ABAQUS/Standard.
• Multiple user elements can be implemented in a single UEL routine
and can be utilized together.
In this lecture the implementation of nonlinear finite elements only will
be discussed and illustrated with examples.
7/01
Writing User Subroutines with ABAQUS
L7.2
ABAQUS
Motivation
Motivation
ABAQUS/Standard is a versatile analysis tool with a large element
library that allows analysis of the most complex structural problems.
However, situations arise in which augmenting the ABAQUS library with
user-defined elements is useful:
• Modeling nonstructural physical processes that are coupled to
structural behavior
• Applying solution-dependent loads
• Modeling active control mechanisms
7/01
Writing User Subroutines with ABAQUS
L7.3
ABAQUS
Motivation
The advantages of implementing user elements in an analysis code such
as ABAQUS, instead of writing a complete analysis code, are obvious:
• ABAQUS offers a large selection of structural elements, analysis
procedures, and modeling tools.
• ABAQUS offers pre- and postprocessing.
– Many third-party vendors offer pre- and postprocessors with
interfaces to ABAQUS.
• Maintaining and porting subroutines is much easier than maintaining
and porting a complete finite element program.
7/01
Writing User Subroutines with ABAQUS
L7.4
ABAQUS
Motivation
• “Finite Element Simulations in Mechanics of Materials and
Deformation Processing Research,” Mary C. Boyce, ABAQUS Users’
Conference Proceedings, 1992.
Figure 7–1. Calculated Binder Force Trajectory Using Active
Global Binder Displacement and Local Strain Control
7/01
Writing User Subroutines with ABAQUS
L7.5
ABAQUS
Motivation
• “User Elements Developed for the Nonlinear Dynamic Analysis of
Reinforced Concrete Structures,” Thomas Wenk, Peter Linde, and
Hugo Bachmann, ABAQUS Users’ Conference Proceedings, 1993.
Figure 7–2. Macro Model Simulating Structural Wall Behavior (left),
Corresponding User Element U3 (right)
Hysterectic Rules For
Flexural Springs K f in
Macro Model (above)
7/01
Writing User Subroutines with ABAQUS
L7.6
ABAQUS
Motivation
• “User Element for Crack Propagation in Concrete-Like Materials,”
R. Vitali and G.L. Zanotelli, ABAQUS Users’ Conference Proceedings,
1994.
Figure 7–3. Configuration at
a General Increment i.
Definition of the User
Element
7/01
Writing User Subroutines with ABAQUS
L7.7
ABAQUS
Defining a User Element
Defining a User Element
Key Characteristics of a User Element
Before a UEL subroutine can be written, the following key characteristics
of the element must be defined:
• The number of nodes on the element
• The number of coordinates present at each node
• The degrees of freedom active at each node
7/01
Writing User Subroutines with ABAQUS
L7.8
ABAQUS
Defining a User Element
Other Important Element Properties
In addition, the following properties must be determined:
• The number of element properties to be defined external to the UEL
• The number of solution-dependent state variables (SDVs) to be
stored per element
• The number of (distributed) load types available for the element
These items need not be determined immediately: they can be added
easily after the basic UEL subroutine is completed.
7/01
Writing User Subroutines with ABAQUS
L7.9
ABAQUS
Defining a User Element
Defining the User Element Behavior
The element’s main contribution to the model during general analysis
N
steps is to provide “fluxes” F at the nodes that depend on the values of
N
the degrees of freedom u at the nodes.
N
• F is defined as a residual quantity: F
N
N
N
= F ext – F int .
N
N
– F ext is the external flux (due to applied distributed loads) and F int
is the internal flux (due to stresses, e.g.) at node N.
• If the degrees of freedom are displacements, the associated fluxes are
the nodal forces. Similarly, rotations correspond to moments and
temperatures to heat fluxes.
• In nonlinear user elements the fluxes/forces will often depend on the
N
increments in the degrees of freedom ∆u and the internal state
α
variables H .
– State variables must be updated in the user subroutine.
7/01
Writing User Subroutines with ABAQUS
L7.10
ABAQUS
Defining a User Element
The solution of the (nonlinear) system of equations in general steps
requires that you define the element Jacobian (stiffness matrix):
K
NM
N
dF
= – ---------.
M
du
• The Jacobian should include all direct and indirect dependencies of
N
N
F on u , which includes terms of the form
N
α
∂F ∂H
.
– ----------α ---------M
∂H ∂u
• A more accurately defined Jacobian improves convergence in general
steps.
• The Jacobian (stiffness) determines the solution for linear
perturbation steps, so it must be exact.
– The Jacobian can be symmetric or nonsymmetric.
7/01
Writing User Subroutines with ABAQUS
L7.11
ABAQUS
Defining a User Element
The complexity of the formulation of a user element can vary greatly.
• Simple elements can be developed to function as “control” and
“feedback” mechanisms in an analysis that consists of regular
elements.
• Complex nonlinear structural elements often require significant effort
in their development.
If the element is built out of a nonlinear material, you should create a
separate subroutine (or series of subroutines) to describe the material
behavior.
• If the material model is implemented in user subroutine UMAT, a call
to UMAT can be included in UEL.
• The integration issues discussed for UMAT also apply to the material
models used in UEL.
7/01
Writing User Subroutines with ABAQUS
L7.12
ABAQUS
UEL Interface
UEL Interface
ABAQUS Options
A user element is defined with the ∗USER ELEMENT option. This
option must appear in the input file before the user element is invoked
with the ∗ELEMENT option.
The syntax for interfacing to UEL is as follows:
*USER ELEMENT, TYPE=Un, NODES=, COORDINATES=,
PROPERTIES=, I PROPERTIES=, VARIABLES=, UNSYMM
Data line(s)
*ELEMENT,TYPE=Un, ELSET=UEL
Data line(s)
*UEL PROPERTY, ELSET=UEL
Data line(s)
*USER SUBROUTINES, (INPUT=file_name)
7/01
Writing User Subroutines with ABAQUS
L7.13
ABAQUS
UEL Interface
Parameter Definition
Parameter
Definition
TYPE
(User-defined) element type of the form Un,
where n is a number
NODES
Number of nodes on the element
COORDINATES Maximum number of coordinates at any node
7/01
PROPERTIES
Number of floating point properties
I PROPERTIES
Number of integer properties
VARIABLES
Number of SDVs
UNSYMM
Flag to indicate that the Jacobian is
unsymmetric
Writing User Subroutines with ABAQUS
L7.14
ABAQUS
UEL Interface
Data Lines
A data line of the form
dof_1, dof_2, …,
where
dof_1 is the first degree of freedom active at the node and
dof_2 is the second degree of freedom active at the node, etc.,
follows the ∗USER ELEMENT option. If all nodes of the user element
have the same active degrees of freedom, no further data are needed.
7/01
Writing User Subroutines with ABAQUS
L7.15
ABAQUS
UEL Interface
However, if some nodes have different active degrees of freedom, enter
subsequent data lines of the form
position, dof_1, dof_2, … ,
where
position is the (local) node number (position) on the element,
dof_1 is the first degree of freedom active at this and following nodes,
and
dof_2 is the second degree of freedom active at this and following
nodes, etc.
The active degrees of freedom can be changed at any node in the element.
7/01
Writing User Subroutines with ABAQUS
L7.16
ABAQUS
UEL Interface
The dimensional units of a degree of freedom for a user element are the
same as those for regular elements in ABAQUS (1–3 are displacements,
4–6 are rotations, etc.).
• This correspondence is important for convergence controls in
nonlinear analysis.
• It is also relevant for three-dimensional rotations in geometric
nonlinear analysis because of the nonlinear nature of finite rotations.
7/01
Writing User Subroutines with ABAQUS
L7.17
ABAQUS
UEL Interface
User elements can have “internal” degrees of freedom in the sense that
they belong to nodes that are not connected to other elements.
• Convergence will be checked for the internal degrees of freedom, so
it is important to choose the internal degrees of freedom
appropriately (i.e., an internal degree of freedom 1 should have the
dimension of displacement).
• For efficiency reasons you should choose internal degree of freedom
numbers that are present at external nodes on the element or
elsewhere in the model.
7/01
Writing User Subroutines with ABAQUS
L7.18
ABAQUS
UEL Interface
More on Keywords and Parameters
The maximum number of coordinates at any node of the element is
specified with the COORDINATES parameter.
• The value of COORDINATES may be increased to match the highest
displacement degree of freedom active on the element.
The total number of SDVs per element is set with the VARIABLES
parameter.
• If the element is integrated numerically, VARIABLES should be set
equal to the number of integration points times the number of SDVs
per point.
• Solution-dependent state variables can be output with the identifiers
SDV1, SDV2, etc. SDVs for any element can be printed only to the
data (.dat), results (.fil), or output database (.odb) files and
plotted as X–Y plots in ABAQUS/Viewer.
7/01
Writing User Subroutines with ABAQUS
L7.19
ABAQUS
UEL Interface
The number of user element properties is given with the PROPERTIES
and I PROPERTIES parameters.
• PROPERTIES determines the number of floating point property
values.
• I PROPERTIES determines the number of integer property values.
Property values are given with the ∗UEL PROPERTY option.
• The properties are assigned on an element set basis; hence, the same
UEL subroutine can be used for user elements with different
properties.
– With this approach “hard-coding” the property values in the user
subroutine is not necessary.
7/01
Writing User Subroutines with ABAQUS
L7.20
ABAQUS
UEL Interface
Coding for the UEL is supplied in a separate file and invoked with the
ABAQUS execution procedure as follows:
abaqus job=... user=....
• The user subroutine must be invoked in a restarted analysis because
user subroutines are not saved on the restart file.
7/01
Writing User Subroutines with ABAQUS
L7.21
ABAQUS
UEL Interface
User Element Loads
Distributed load and flux types can be applied with the ∗DLOAD and
∗DFLUX options by using load type keys Un and UnNU.
• In either case the equivalent nodal load vector for the distributed load
type must be defined in user subroutine UEL.
– If load type key Un is used, the load magnitude is defined on the
data line and can be varied in time with the ∗AMPLITUDE option.
– If load type key UnNU is used, all of the load definition is applied
in user subroutine UEL: a time-dependent load magnitude vector
must be coded.
• If the load depends on the solution variables, the corresponding “load
stiffness” contributions matrix to the Jacobian should be included for
best performance.
7/01
Writing User Subroutines with ABAQUS
L7.22
ABAQUS
UEL Interface
UEL Interface
The interface to user subroutine UEL is:
SUBROUTINE UEL(RHS, AMATRX, SVARS, ENERGY, NDOFEL, NRHS, NSVARS,
1 PROPS, NPROPS, COORDS, MCRD, NNODE, U, DU, V, A, JTYPE, TIME,
2 DTIME, KSTEP, KINC, JELEM, PARAMS, NDLOAD, JDLTYPE, ADLMAG,
3 PREDEF, NPREDF, LFLAGS, MLVARX, DDLMAG, MDLOAD, PNEWDT, JPROPS,
4 NJPRO, PERIOD)
C
INCLUDE ’ABA_PARAM.INC’
C
DIMENSION RHS(MLVARX,*), AMATRX(NDOFEL, NDOFEL), PROPS(*),
1 SVARS(*), ENERGY(*), COORDS(MCRD, NNODE), U(NDOFEL),
2 DU(MLVARX,*), V(NDOFEL), A(NDOFEL), TIME(2), PARAMS(*),
3 JDLTYP(MDLOAD, *), ADLMAG(MDLOAD, *), DDLMAG(MDLOAD, *),
4 PREDEF(2, NPREDF, NNODE), LFLAGS(*), JPROPS(*)
The “include” statement sets the proper precision for floating point
variables (REAL*8 on most machines).
7/01
Writing User Subroutines with ABAQUS
L7.23
ABAQUS
UEL Interface
UEL Variables
The following quantities are available in UEL:
• Coordinates; displacements; incremental displacements; and, for
dynamics, velocities and accelerations
• SDVs at the start of the increment
• Total and incremental values of time, temperature, and user-defined
field variables
• User element properties
• Load types as well as total and incremental load magnitudes
• Element type and user-defined element number
• Procedure type flag and, for dynamics, integration operator values
• Current step and increment numbers
7/01
Writing User Subroutines with ABAQUS
L7.24
ABAQUS
UEL Interface
The following quantities must be defined:
• Right-hand-side vector (residual nodal fluxes or forces)
• Jacobian (stiffness) matrix
• Solution-dependent state variables
The following variables may be defined:
• Energies associated with the element (strain energy, plastic
dissipation, kinetic energy, etc.)
• Suggested new (reduced) time increment
A complete description of all parameters is provided in Section 24.2.19
of the ABAQUS/Standard User’s Manual.
7/01
Writing User Subroutines with ABAQUS
L7.25
ABAQUS
UEL Interface
The header is usually followed by dimensioning of local arrays. It is good
practice to define constants via parameters and to include comments.
dimension b(2, 7), gauss(2)
c
parameter(zero=0.d0, one=1.d0, two=2.d0, three=3.d0, four=4.d0,
1
six=6.d0, eight=8.d0, twelve=12.d0)
data gauss/.211324865d0, .788675135d0/
c
c simple 2-d linear beam element with generalized section properties
c
c get length and direction and cross section dimensions
c
• The PARAMETER assignments yield accurate floating point constant
definitions on any platform.
• Arrays can be initialized with a DATA statement.
7/01
Writing User Subroutines with ABAQUS
L7.26
ABAQUS
UEL Interface
UEL Conventions
The solution variables (displacement, velocity, etc.) are arranged on a
node/degree of freedom basis.
• The degrees of freedom of the first node are first, followed by the
degrees of freedom of the second node, etc.
– Consider a planar beam that uses degrees of freedom 1, 2, and 6
( u x, u y, φ z ) at its first and second node and degrees of freedom 1
and 2 at its third (middle) node. The ordering is:
Element variable
1
2
3
4
5
6
7
8
Node
1
1
1
2
2
2
3
3
Degree of freedom
1
2
6
1
2
6
1
2
• The flux vector and Jacobian matrix must be ordered in the same
way.
7/01
Writing User Subroutines with ABAQUS
L7.27
ABAQUS
UEL Interface
UEL Formulation Aspects and Usage Hints
The displacement, velocities, etc. passed into the UEL are in the global
system, regardless whether the ∗TRANSFORM option is used at any of
the nodes.
• The flux vector and Jacobian matrix must also be formulated in the
global system.
The Jacobian must be formulated as a full matrix, even if it is symmetric.
• If the UNSYMM parameter is not used, ABAQUS will symmetrize
the Jacobian defined by the user.
For transient heat transfer and dynamic analysis, heat capacity and inertia
contributions must be included in the flux vector.
• UELs for these procedures will be discussed later in this lecture.
7/01
Writing User Subroutines with ABAQUS
L7.28
ABAQUS
UEL Interface
At the start of a new increment, the increment in solution variable(s) is
extrapolated from the previous increment.
• The flux vector and the Jacobian must be based on these extrapolated
values.
• If extrapolation is not desired, it can be switched off with
∗STEP, EXTRAPOLATION=NO.
If the increment in solution variable(s) is too large, the variable PNEWDT
can be used to suggest a new time increment.
• ABAQUS will abandon the current time increment and will attempt
the increment again with one that is a factor PNEWDT smaller.
7/01
Writing User Subroutines with ABAQUS
L7.29
ABAQUS
UEL Interface
Coding and Testing the UEL
Follow the basic rules for writing ABAQUS user subroutines.
• Follow FORTRAN 77 or C conventions.
• Make sure that all variables are defined and initialized.
• Assign enough storage space for state variables.
7/01
Writing User Subroutines with ABAQUS
L7.30
ABAQUS
UEL Interface
Complex UELs may have many potential problem areas. Do not use a
large model when trying to debug a UEL.
Verify the UEL with a one-element input file.
1. Run tests using general steps in which all solution variables are
prescribed to verify the resultant fluxes.
2. Run tests using linear perturbation steps in which all loads are
prescribed to verify the element Jacobian (stiffness).
3. Run tests using general steps in which all loads are prescribed to
verify the consistency of the Jacobian and the flux vector.
Gradually increase the complexity of the test problems. Compare the
results with standard ABAQUS elements, if possible.
7/01
Writing User Subroutines with ABAQUS
L7.31
ABAQUS
Example 1: Planar Beam Element with Nonlinear Section Behavior
Example 1: Planar Beam Element with
Nonlinear Section Behavior
Objective
Analyze a planar concrete frame structure.
• The frame is loaded to an extent where significant nonlinearity occurs
in the concrete but the displacements are still small enough that
geometric nonlinearity may be neglected.
• Develop a model that describes the nonlinear section behavior directly
in terms of axial force and bending moment.
– This is similar to the
∗BEAM GENERAL SECTION, SECTION=NONLINEAR
GENERAL option, but allows coupling between the axial and
bending terms.
7/01
Writing User Subroutines with ABAQUS
L7.32
ABAQUS
Example 1: Planar Beam Element with Nonlinear Section Behavior
• The transverse shear deformation can be neglected.
Coding Requirements
The element is integrated numerically; hence, the following quantities
require definition in the UEL:
• The element [B] matrix, which relates the axial strain, ε , and
curvature, κ , to the element displacements, { u e } :
ε
  = [ B]{u } .
e
κ 
 
• A constitutive law [D] relating axial force, F , and moment, M , to
axial strain and curvature:
ε
F
  = [ D]  .
κ 
M 
 
 
7/01
Writing User Subroutines with ABAQUS
L7.33
ABAQUS
Example 1: Planar Beam Element with Nonlinear Section Behavior
• The element stiffness matrix:
[K e] =
∫
l
T
[ B ] [ D ] [ B ]dl .
0
• The element internal force vector:
{Fe} =
∫
0
l
T
F
[ B ]  dl .
M 
• The integration is done numerically:
l
n
∫ Adl = ∑ A l ,
i i
0
i=1
where n is the number of integration points and l i is the length
associated with integration point i .
7/01
Writing User Subroutines with ABAQUS
L7.34
ABAQUS
Example 1: Planar Beam Element with Nonlinear Section Behavior
Element Formulation
The element formulation is based on Euler-Bernoulli beam theory.
• The interpolation is described purely in terms of the displacements,
which are C 1 continuous at the nodes.
• The curvature is obtained as the second derivative of the
displacement normal to the beam.
The simplest two-dimensional beam element has two nodes, with two
displacements and one rotation (u x, u y, φ z ) at each node.
• The active degrees of freedom are 1, 2, and 6.
v loc
u loc
B
A
7/01
Writing User Subroutines with ABAQUS
L7.35
ABAQUS
Example 1: Planar Beam Element with Nonlinear Section Behavior
In its basic form linear interpolation is used for the tangential
displacement, u loc , and cubic interpolation for the normal displacement,
v loc .
• The cubic interpolation for the normal displacement yields a linear
variation for the curvature.
• The linear interpolation for the tangential displacement yields a
constant axial strain.
The constant axial strain and linear curvature variation are inconsistent
and may lead to excessive local axial forces if the axial and bending
behavior are coupled.
• Considering that the intent is to analyze nonlinear concrete behavior,
such coupling will be present.
• The excessive axial forces may lead to overly stiff behavior.
7/01
Writing User Subroutines with ABAQUS
L7.36
ABAQUS
Example 1: Planar Beam Element with Nonlinear Section Behavior
To prevent this problem, an extra “internal” node is added to the element.
The internal node has one degree of freedom: the tangential
displacement.
u
c
B
C
A
Both the axial strain and the curvature now vary linearly. The
interpolation functions are:
A
2
B
2
C
2
u loc = u loc ( 1 – 3ξ + 2ξ ) + u loc ( – ξ + 2ξ ) + u ( 4ξ – 4ξ )
A
2
3
B
3
B
2
2
3
v loc = v loc ( 1 – 3ξ + 2ξ ) + v loc ( 3ξ – 2ξ )
A
2
3
+ φ l ( ξ – 2ξ + ξ ) + φ l ( – ξ + ξ )
where l is the element length and ξ = s ⁄ l is the dimensionless position
along the beam.
7/01
Writing User Subroutines with ABAQUS
L7.37
ABAQUS
Example 1: Planar Beam Element with Nonlinear Section Behavior
• This yields the following expressions for the axial strain and the
curvature:
1 A
B
C
ε = --- [ u loc ( – 3 + 4ξ ) + u loc ( – 1 + 4ξ ) + u ( 4 – 8ξ ) ]
l
1 A
B
A
B
κ = ---2- [ v loc ( – 6 + 12ξ ) + v loc ( 6 – 12ξ ) + φ l ( – 4 + 6ξ ) + φ l ( – 2 + 6ξ ) ]
l
• These linear relations are implemented in the B-matrix of the element.
– The B-matrix also handles the transformation from local to global
displacements at the nodes.
• The element is integrated numerically with a two-point Gauss scheme.
7/01
Writing User Subroutines with ABAQUS
L7.38
ABAQUS
Example 1: Planar Beam Element with Nonlinear Section Behavior
Element Definition in the Input File
The following data lines define the user element in the input file:
*user element, type=u1, nodes=3, coordinates=2, properties=3, variables=8
1, 2, 6
3, 1
*element, type=u1, elset=one
1, 1, 2, 3
*uel property, elset=one
2., 1., 1000.
• The user element name is U1, which is used in the ∗ELEMENT option.
• Eight state variables are allocated, so four variables can be defined at
each integration point.
• Three element properties are allocated: the section height, the section
width, and Young’s modulus.
• The element has three nodes: the third “internal” node is unique to each
element.
7/01
Writing User Subroutines with ABAQUS
L7.39
ABAQUS
Example 1: Planar Beam Element with Nonlinear Section Behavior
Coding for Planar Beam Example
c
c
c
simple 2-d linear beam element with generalized section properties
subroutine uel(rhs, amatrx, svars, energy, ndofel, nrhs, nsvars,
1 props, nprops, coords, mcrd, nnode, u, du, v, a, jtype, time, dtime,
2 kstep, kinc, jelem, params, ndload, jdltyp, adlmag, predef, npredf,
3 lflags, mlvarx, ddlmag, mdload, pnewdt, jprops, njprop, period)
c
include ’aba_param.inc’
c
dimension rhs(mlvarx, *), amatrx(ndofel, ndofel), svars(*), props(*),
1 energy(7), coords(mcrd,nnode), u(ndofel), du(mlvarx, *), v(ndofel),
2 a(ndofel), time(2), params(*), jdltyp(mdload, *), adlmag(mdload, *),
3 ddlmag(mdload, *),predef(2, npredf, nnode), lflags(4), jprops(*)
c
dimension b(2, 7), gauss(2)
c
parameter(zero=0.d0, one=1.d0, two=2.d0, three=3.d0, four=4.d0,
1
six=6.d0, eight=8.d0, twelve=12.d0)
data gauss/.211324865d0, .788675135d0/
c
7/01
Writing User Subroutines with ABAQUS
L7.40
ABAQUS
c
c
Example 1: Planar Beam Element with Nonlinear Section Behavior
calculate length and direction cosines
dx=coords(1, 2)-coords(1, 1)
dy=coords(2, 2)-coords(2, 1)
dl2=dx**2+dy**2
dl=sqrt(dl2)
hdl=dl/two
acos=dx/dl
asin=dy/dl
c
c
c
initialize rhs and lhs
do k1=1, 7
rhs(k1, 1)= zero
do k2=1, 7
amatrx(k1, k2)= zero
end do
end do
c
nsvint=nsvars/2
7/01
Writing User Subroutines with ABAQUS
L7.41
ABAQUS
c
c
c
Example 1: Planar Beam Element with Nonlinear Section Behavior
loop over integration points
do kintk=1, 2
g=gauss(kintk)
c
c
c
make b-matrix
b(1,
b(1,
b(1,
b(1,
b(1,
b(1,
b(1,
b(2,
b(2,
b(2,
b(2,
b(2,
b(2,
b(2,
1)=(-three+four*g)*acos/dl
2)=(-three+four*g)*asin/dl
3)=zero
4)=(-one+four*g)*acos/dl
5)=(-one+four*g)*asin/dl
6)=zero
7)=(four-eight*g)/dl
1)=(-six+twelve*g)*-asin/dl2
2)=(-six+twelve*g)* acos/dl2
3)=(-four+six*g)/dl
4)= (six-twelve*g)*-asin/dl2
5)= (six-twelve*g)* acos/dl2
6)= (-two+six*g)/dl
7)=zero
c
7/01
Writing User Subroutines with ABAQUS
L7.42
ABAQUS
c
c
Example 1: Planar Beam Element with Nonlinear Section Behavior
calculate (incremental) strains and curvatures
eps=zero
deps=zero
cap=zero
dcap=zero
do k=1, 7
eps=eps+b(1, k)*u(k)
deps=deps+b(1, k)*du(k, 1)
cap=cap+b(2, k)*u(k)
dcap=dcap+b(2, k)*du(k, 1)
end do
c
c
c
call constitutive routine ugenb
1
7/01
isvint=1+(kintk-1)*nsvint
bn=zero
bm=zero
daxial=zero
dbend=zero
dcoupl=zero
call ugenb(bn, bm, daxial, dbend, dcoupl, eps, deps, cap, dcap,
svars(isvint), nsvint, props, nprops)
Writing User Subroutines with ABAQUS
L7.43
ABAQUS
c
c
c
Example 1: Planar Beam Element with Nonlinear Section Behavior
assemble rhs and lhs
do k1=1, 7
rhs(k1, 1)=rhs(k1, 1)-hdl*(bn*b(1, k1)+bm*b(2, k1))
bd1=hdl*(daxial*b(1, k1)+dcoupl*b(2, k1))
bd2=hdl*(dcoupl*b(1, k1)+dbend *b(2, k1))
do k2=1, 7
amatrx(k1, k2)=amatrx(k1, k2)+bd1*b(1, k2)+bd2*b(2, k2)
end do
end do
end do
c
return
end
7/01
Writing User Subroutines with ABAQUS
L7.44
ABAQUS
Example 1: Planar Beam Element with Nonlinear Section Behavior
Remarks
• This UEL uses essentially the same formulation as the simple B23
element for geometrically linear analysis.
• The routine can be used with and without the ∗TRANSFORM option.
• It would be relatively straightforward to generalize this routine for
three-dimensional analyses.
– It is much more complicated to extend the routine to geometrically
nonlinear analysis.
• Even for linear analysis this routine is called twice (for each element)
during the first iteration of an increment: once for assembly and once
for recovery. Subsequently, it is called once per iteration: assembly and
recovery are combined.
7/01
Writing User Subroutines with ABAQUS
L7.45
ABAQUS
Example 1: Planar Beam Element with Nonlinear Section Behavior
Generalized Constitutive Behavior
At each integration point the generalized constitutive behavior is
processed in the user-created subroutine UGENB.
• This subroutine is patterned after user subroutine UGENS, which
allows you to model the behavior of a shell.
• The following quantities are passed into UGENB:
– Total and incremental axial strain and curvature
– State variables at the start of the increment
– User element properties
• You must define:
– The axial force and bending moment, as well as the linearized
force/moment-strain/curvature relations
– The solution-dependent state variables
7/01
Writing User Subroutines with ABAQUS
L7.46
ABAQUS
Example 1: Planar Beam Element with Nonlinear Section Behavior
A simple linear elastic subroutine UGENB follows:
subroutine ugenb(bn,bm,daxial,dbend,dcoupl,eps,deps,cap,dcap,
1
svint,nsvint,props,nprops)
c
include ’aba_param.inc’
c
parameter(zero=0.d0,twelve=12.d0)
c
dimension svint(*),props(*)
c
c
c
c
c
c
c
c
c
c
c
c
7/01
variables to be defined by the user
bn
bm
daxial
dbend
dcoupl
-
axial force
bending moment
current tangent axial stiffness
current tangent bending stiffness
tangent coupling term
variables that may be updated
svint
- state variables for this integration point
Writing User Subroutines with ABAQUS
L7.47
ABAQUS
c
c
c
c
c
c
c
c
c
c
c
c
c
c
c
c
c
Example 1: Planar Beam Element with Nonlinear Section Behavior
variables passed in for information
eps
deps
cap
dcap
props
nprops
nsvint
-
axial strain
incremental axial strain
curvature change
incremental curvature change
element properties
# element properties
# state variables
current assumption
props(1) - section height
props(2) - section width
props(3) - Young’s modulus
h=props(1)
w=props(2)
E=props(3)
7/01
Writing User Subroutines with ABAQUS
L7.48
ABAQUS
c
c
c
Example 1: Planar Beam Element with Nonlinear Section Behavior
formulate linear stiffness
daxial=E*h*w
dbend=E*w*h**3/twelve
dcoupl=zero
c
c
c
calculate axial force and moment
bn=svint(1)+daxial*deps
bm=svint(2)+dbend*dcap
c
c
c
store internal variables
svint(1)=bn
svint(2)=bm
svint(3)=eps
svint(4)=cap
c
return
end
7/01
Writing User Subroutines with ABAQUS
L7.49
ABAQUS
Example 1: Planar Beam Element with Nonlinear Section Behavior
Remarks
• The coding in this routine is very similar in nature to what would be
coded in the subroutines UMAT and UGENS.
• The routine stores the axial strain, curvature, axial force, and bending
moment at each Gauss point.
– For nonlinear material behavior additional quantities would be
stored.
• The constitutive relation is written in incremental form for easy
generalization to nonlinear section behavior.
• UGENB and UEL must be combined in one external file.
7/01
Writing User Subroutines with ABAQUS
L7.50
ABAQUS
Example 2: Force Control Element
Example 2: Force Control Element
Objective
Implement an active control mechanism in a finite element model.
• The specific objective is to model a deep drawing operation in which
the blank holder force is dynamically adapted during the process.
– The punch force is measured during the deep drawing process.
– If the punch force approaches a value that might cause tearing of
the sheet, the blank holder force is decreased.
– If the punch force decreases significantly below the critical value
for tearing, the blank holder force is increased to minimize the
danger of wrinkling.
7/01
Writing User Subroutines with ABAQUS
L7.51
ABAQUS
Example 2: Force Control Element
The problem is an adaptation of ABAQUS Example Problem 1.3.4, Deep
Drawing of a Cylindrical Cup.
punch
blank holder
Rp = 50 mm
RH = 56.25 mm
R = 13 mm
t = 0.82 mm
r
R = 5 mm
RB = 100 mm
RD = 51.25 mm
die
7/01
Writing User Subroutines with ABAQUS
L7.52
ABAQUS
Example 2: Force Control Element
In this problem a user element is inserted that measures the punch force (by
adding a stiff spring to the rigid body reference node) and, if necessary,
modifies the blank holder force.
u presc
rigid body reference
node for punch
7/01
user element
F presc
rigid body reference node
for blank holder
Writing User Subroutines with ABAQUS
L7.53
ABAQUS
Example 2: Force Control Element
Element Formulation
The element has three nodes, with a single degree of freedom ( u z ) ,
active at each node.
B
C
A
• Nodes A and B are connected by a spring with stiffness K, so the
element internal forces at nodes A and B are
A
B
A
B
F int = – F int = K ( u z – u z ) .
– The stiffness is entered as a user element property.
7/01
Writing User Subroutines with ABAQUS
L7.54
ABAQUS
Example 2: Force Control Element
• The magnitude of the force generated at node C is determined as follows:
– Prescribe the initial value (as an element property).
– Define the target value of the punch force.
– If the spring force has not yet exceeded the target value, keep the
force on node C the same.
– If the spring force exceeds the target value by more than a
user-defined tolerance, decrease the force on node C.
– If the spring force drops below the target value by more than a
user-defined tolerance, increase the force on node C.
• The algorithm uses the value of the spring force at the start of the
increment.
– Consequently, there is no stiffness contribution due to the
application of the force on node C.
7/01
Writing User Subroutines with ABAQUS
L7.55
ABAQUS
Example 2: Force Control Element
Element Definition in the Input File
The following data lines define the user element in the input file:
*USER ELEMENT, TYPE=U1, NODES=3, COORD=2, PROPERTIES=5, VARIABLES=3
2
*UEL PROPERTY, ELSET=FEEDBACK
1.E12, 7.5E4, 1.0E5, 0.04, 0.04
*ELEMENT, TYPE=U1, ELSET=FEEDBACK
1001, 200,500,300
• Five property values are specified: the spring stiffness, the target value
of the punch force, the initial force on the blank holder, the allowable
fractional change in holder force, and the punch force tolerance.
• Three state variables are stored: the punch force, the blank holder force,
and the maximum punch force during the load history.
• The first and the last nodes are the rigid body reference nodes of the
punch and blank holder, respectively.
7/01
Writing User Subroutines with ABAQUS
L7.56
ABAQUS
Example 2: Force Control Element
Coding for Force Control Element Example
c
c Blankholder force control element for deep drawing applications
c
subroutine uel(rhs,amatrx,svars,energy,ndofel,nrhs,nsvars,
1 props,nprops,coords,mcrd,nnode,u,du,v,a,jtype,time,dtime,
2 kstep,kinc,jelem,params,ndload,jdltyp,adlmag,predef,npredf,
3 lflags,mlvarx,ddlmag,mdload,pnewdt,jprops,njprop,period)
c
include ’aba_param.inc’
c
dimension rhs(mlvarx,*),amatrx(ndofel,ndofel),svars(*),props(*),
1 energy(7),coords(mcrd,nnode),u(ndofel),du(mlvarx,*),v(ndofel),
2 a(ndofel),time(2),params(*),jdltyp(mdload,*),adlmag(mdload,*),
4 ddlmag(mdload,*),predef(2,npredf,nnode),lflags(4),jprops(*)
c
c Pick up the input data
c
sPunch
= props(1)
!Spring stiffness
fPunchTarget = props(2)
!Target punch force
fHolderInit = props(3)
!Initial blankholder force
fractHolder = props(4)
!Fractional change allowed
tolPunch
= props(5)
!Tolerance on punch force
7/01
Writing User Subroutines with ABAQUS
L7.57
ABAQUS
c
c
c
Example 2: Force Control Element
Calculate the punch force
fPunchNew = sPunch * (u(1)-u(2))
c
c
c
Generate force vector and
rhs(1,1) = -fPunchNew
rhs(2,1) = +fpunchNew
c
c
c
Generate stiffness matrix
amatrx(1,1)
amatrx(1,2)
amatrx(2,1)
amatrx(2,2)
c
c
c
=
=
=
=
+sPunch
-sPunch
-sPunch
+sPunch
The holder force is only applied during steps 2 and 3
if(kstep.eq.2) then
c
c
7/01
Ramp the punch force to the desired starting value
Writing User Subroutines with ABAQUS
L7.58
ABAQUS
Example 2: Force Control Element
c
fHolder = time(1)*fHolderInit/period
svars(2) = fHolder
rhs(3,1) = -fHolder
else if(kstep.eq.3) then
c
c
c
c
c
Adjust the punch force to control the blankholder force
Values of state variables at start of increment
fPunchOld = svars(1)
fHolderOld = svars(2)
fPunchMax = svars(3)
c
c
c
!Punch force
!Blankholder force
!Maximum blankholder force
Allowed change in blankholder force
dfHolderMax = fractHolder * fHolderOld
c
c
c
Allowed tolerance in the targetforce
dfPunchTol = tolPunch * fPunchTarget
c
7/01
Writing User Subroutines with ABAQUS
L7.59
ABAQUS
c
c
Calculate the holder force
if (fPunchOld.gt.fPunchTarget+dfPunchTol) then
fHolderNew = fHolderOld - dfHolderMax
!Decrease
else if(fPunchMax.lt.fPunchTarget+dfPunchTol .or.
fPunchOld.gt.fPunchTarget-dfPunchTol) then
fHolderNew = fHolderOld
!Keep constant
else
fHolderNew = fHolderOld + dfHolderMax
!Increase
end if
1
c
c
c
Example 2: Force Control Element
Generate holder force vector
rhs(3,1) = -fHolderNew
c
c
c
Update state variables
svars(1) = fPunchNew
svars(2) = fHolderNew
svars(3) = max(fPunchMax,fPunchNew)
end if
c
return
end
7/01
Writing User Subroutines with ABAQUS
L7.60
ABAQUS
Example 2: Force Control Element
Remarks
• No stiffness contribution is associated with the change in blank holder
force because the force does not depend on the solution of the current
increment.
– An additional advantage is that the change in blank holder force
will always be based on a converged solution.
• In principle, this explicit algorithm is only conditionally stable.
– However, since the punch force depends weakly on the blank
holder force and the change in blank holder force is small,
instability is unlikely to occur.
• More sophisticated feedback algorithms are readily implemented.
7/01
Writing User Subroutines with ABAQUS
L7.61
ABAQUS
Example 2: Force Control Element
• The blank holder force can also be made dependent on other solution
variables.
– Subroutine URDFIL can be used to read any solution variable from
the results (.fil) file during analysis.
– The selected variable or variables (for example, the maximum
strain or plastic strain anywhere in the model) can be stored in a
common block, and the variables can be used in the UEL.
• All ABAQUS common block names start with the letter “C,” so name
conflicts can easily be avoided.
7/01
Writing User Subroutines with ABAQUS
L7.62
ABAQUS
Using Nonlinear User Elements in Various Analysis Procedures
Using Nonlinear User Elements in Various
Analysis Procedures
Overview of Procedures
Nonlinear user elements can be utilized in most ABAQUS/Standard
analysis procedures.
• LFLAGS(1) indicates which procedure type is used:
– LFLAGS(1)=11: Dynamic procedure with automatic time
incrementation
– LFLAGS(1)=12: Dynamic procedure with fixed time
incrementation
7/01
Writing User Subroutines with ABAQUS
L7.63
ABAQUS
Using Nonlinear User Elements in Various Analysis Procedures
• To this point in the lecture, the usages described have applied only to
∗STATIC (LFLAGS(1)=1, 2).
• The usage in many procedures is the same or similar to that for
∗STATIC:
∗VISCO
∗HEAT TRANSFER, STEADY STATE
∗COUPLED TEMPERATURE-DISPLACEMENT, STEADY
STATE
∗GEOSTATIC
∗SOILS, STEADY STATE
∗COUPLED THERMAL-ELECTRICAL, STEADY STATE
7/01
Writing User Subroutines with ABAQUS
L7.64
ABAQUS
Using Nonlinear User Elements in Various Analysis Procedures
A special case of static analysis is ∗STATIC, RIKS.
• An additional force vector containing only forces proportional to the
applied loads, as well as the usual force vector and the Jacobian,
must be supplied.
– These additional forces must include thermal expansion effects if
any are present in the element.
• If no forces are applied to the element, the usage is the same as that
for a regular ∗STATIC analysis.
7/01
Writing User Subroutines with ABAQUS
L7.65
ABAQUS
Using Nonlinear User Elements in Various Analysis Procedures
Perturbation Procedures
User elements can also be used in most “linear perturbation” procedures.
• For a static linear perturbation analysis (∗STATIC, PERTURBATION),
a stiffness matrix and two force vectors must be returned by the UEL.
– The value of LFLAGS(3) denotes the matrix to be returned in a call.
– LFLAGS(3)=1: Assembly—return the stiffness matrix for the base
state and the force vector that contains only external perturbation
loads.
– LFLAGS(3)=100: Recovery—return the force vector that contains
the difference between external perturbation loads and internal
perturbation forces:
F
N
N
= ∆P – K
NM
M
∆u .
This force vector is used for the reaction force calculation.
7/01
Writing User Subroutines with ABAQUS
L7.66
ABAQUS
Using Nonlinear User Elements in Various Analysis Procedures
• For a ∗FREQUENCY analysis a stiffness and mass matrix must be
returned by the UEL.
– The value of LFLAGS(3) denotes the matrix to be returned in a call.
LFLAGS(3)=2: Return the stiffness matrix
LFLAGS(3)=4: Return the mass matrix
– No element output is available for user elements utilized with the
∗FREQUENCY option.
7/01
Writing User Subroutines with ABAQUS
L7.67
ABAQUS
Using Nonlinear User Elements in Various Analysis Procedures
– The eigenfrequencies and eigenvectors obtained with the
∗FREQUENCY option can be used in all modal dynamics
procedures:
∗MODAL DYNAMIC
∗STEADY STATE DYNAMICS
∗RESPONSE SPECTRUM
∗RANDOM RESPONSE
• User elements cannot be used in the ∗STEADY STATE DYNAMICS,
DIRECT and ∗BUCKLE procedures.
7/01
Writing User Subroutines with ABAQUS
L7.68
ABAQUS
Using Nonlinear User Elements in Various Analysis Procedures
Transient Analysis
First-order transient effects must be included in UELs that are used with
the following procedures:
∗HEAT TRANSFER (transient)
∗SOILS, CONSOLIDATION
∗COUPLED TEMPERATURE-DISPLACEMENT (transient)
∗COUPLED THERMAL-ELECTRICAL, TRANSIENT
• The heat (pore fluid) capacity term must be included in the flux
vector and the Jacobian.
• If the user element has no heat (pore fluid) capacity, the user element
usage is the same as in the corresponding steady-state analysis.
7/01
Writing User Subroutines with ABAQUS
L7.69
ABAQUS
Using Nonlinear User Elements in Various Analysis Procedures
Transient Heat Transfer Analysis
LFLAGS(1) indicates the transient heat transfer procedure type being
used:
• LFLAGS(1)=32: Transient heat transfer analysis with automatic time
incrementation
• LFLAGS(1)=33: Transient heat transfer analysis with fixed time
incrementation
Additional coding related to the transient terms in the equilibrium
equation is required for the flux vector and Jacobian.
7/01
Writing User Subroutines with ABAQUS
L7.70
ABAQUS
Using Nonlinear User Elements in Various Analysis Procedures
• The flux vector must contain the externally applied fluxes, the fluxes
due to conduction, and the fluxes due to changes in internal energy:
F
N
N
N
N
= F ext + F cond + F cap.
– If the heat capacity matrix C
capacity terms is
N
NM
F cap = – C
is constant, the flux due to the heat
NM
M
∆θ ⁄ ∆t ,
M
where ∆θ is the temperature increment.
– If the heat capacity matrix varies with temperature (such as is the
case during phase transformations), the flux vector must be
calculated from the energy change vector:
N
M
F cap = – ∆U ⁄ ∆t .
7/01
Writing User Subroutines with ABAQUS
L7.71
ABAQUS
Using Nonlinear User Elements in Various Analysis Procedures
• The Jacobian will contain contributions from the conductivity and heat
capacity terms.
– If the heat capacity matrix is constant, the Jacobian has the form
K
where K
NM
NM
+C
NM
⁄ ∆t,
is the conductivity matrix.
– If the heat capacity matrix is a function of temperature, the Newton
algorithm requires the heat capacity at the temperature at the end of
the increment:
C
NM
= C
NM
( θ t + ∆t ) .
For cases in which the heat capacity varies strongly (such as in
case of latent heat), convergence may be difficult.
If the user element contains no heat capacity terms, the formulation for
transient heat transfer is the same as for steady-state heat transfer.
7/01
Writing User Subroutines with ABAQUS
L7.72
ABAQUS
Using Nonlinear User Elements in Various Analysis Procedures
Dynamic Analysis
Second-order transient (inertial) effects must be included in UELs that are
used with direct integration dynamic analysis (∗DYNAMIC).
LFLAGS(1) indicates the dynamics procedure type being used:
• LFLAGS(1)=11: Dynamic procedure with automatic time
incrementation
• LFLAGS(1)=12: Dynamic procedure with fixed time incrementation
Additional coding related to transient terms, sudden changes in velocities
or accelerations, and evaluation of the half-step residual (if automatic
time incrementation is used) is required in the UEL.
7/01
Writing User Subroutines with ABAQUS
L7.73
ABAQUS
Using Nonlinear User Elements in Various Analysis Procedures
The value of LFLAGS(3) indicates the coding being executed and the
matrices to be returned.
LFLAGS(3)=1
Normal time increment. The user must specify the forces and Jacobian
corresponding to the integration procedure used.
• The force vector has the form
F
N
= –M
NM M
u̇˙t + ∆t
NM
N
N
+ ( 1 + α )G t + ∆t – αG t ,
N
where M is the element mass matrix, G is the “static” force
vector, and α is the Hughes-Hilbert-Taylor integration operator.
N
– The static force vector G must also contain the rate-dependent
(damping) terms.
N
– The vector G t must be stored as a set of state variables.
– The parameter α is passed into the subroutine as PARAMS(1).
7/01
Writing User Subroutines with ABAQUS
L7.74
ABAQUS
Using Nonlinear User Elements in Various Analysis Procedures
• The Jacobian has the form
M
NM  du̇˙
------ + ( 1 + α )C
 du
NM  du̇
------ + ( 1 + α )K
 du
NM
where C is the element damping matrix and K
tangent stiffness matrix.
NM
NM
,
is the static
– ( du̇˙ ⁄ du ) and ( du̇ ⁄ du ) follow from the integration operator. For
the HHT operator,
1
γ
( du̇˙ ⁄ du ) = ----------2-, ( du̇ ⁄ du ) = --------- ,
β∆t
β∆t
2
where β = ( 1 ⁄ 4 ) ( 1 – α ) and γ = 1 ⁄ 2 – α are the coefficients
in the Newmark- β operator
– The parameters β and γ are passed into the subroutine as
PARAMS(2) and PARAMS(3).
7/01
Writing User Subroutines with ABAQUS
L7.75
ABAQUS
Using Nonlinear User Elements in Various Analysis Procedures
Remarks
• The coding is simplified considerably if the HHT parameter α =0.
N
– In particular, there is no need to store the static residual vector, G t .
– The variable, α , can be set to zero with the ALPHA parameter on
the ∗DYNAMIC option.
• If the user element has no inertia or damping terms (i.e., if the force
vector does not depend on the velocities and accelerations), the α
parameter can be ignored in the subroutine.
– If the user element includes viscous effects but no inertia terms, the
same approach can be used as for transient heat transfer analysis.
NM
M
The force vector then should contain the term – C ∆u ⁄ ∆t , and
NM
the term C ⁄ ∆t must be added to the stiffness.
– In that case the α parameter can again be ignored.
7/01
Writing User Subroutines with ABAQUS
L7.76
ABAQUS
Using Nonlinear User Elements in Various Analysis Procedures
LFLAGS(3)=5
Half step residual calculation, which is needed only for automatic time
incrementation.
• Only the force vector must be supplied, which has the form
F
N
= M
NM
α N
N
N
u̇˙t + ∆t ⁄ 2 + ( 1 + α )G t + ∆t ⁄ 2 – --- ( G t + G t 0 ) ,
2
N
where G t 0 is the static residual at the beginning of the previous
increment.
N
– G t 0 must be stored as a solution-dependent state vector.
N
N
– The vector u̇˙t + ∆t ⁄ 2 is passed into the subroutine, G t + ∆t ⁄ 2 , and G t
must be calculated.
– It is obvious that this expression simplifies considerably if α =0.
7/01
Writing User Subroutines with ABAQUS
L7.77
ABAQUS
Using Nonlinear User Elements in Various Analysis Procedures
LFLAGS(3)=4
Velocity jump calculation, which will be done at the start of each
dynamic step and after contact changes.
• The purpose of this calculation is to make the velocities conform to
constraints imposed by ∗MPC, ∗EQUATION, or contact conditions
while preserving momentum.
• The Jacobian is equal to the mass matrix, and the force vector should
be set to zero.
7/01
Writing User Subroutines with ABAQUS
L7.78
ABAQUS
Using Nonlinear User Elements in Various Analysis Procedures
LFLAGS(3)=6
Acceleration calculation, which will be done at the start of each
dynamic step (unless INITIAL=NO on the ∗DYNAMIC option) and
after contact changes.
• The purpose of this calculation is to create dynamic equilibrium at
the start of a step or after contact changes.
• The Jacobian is equal to the mass matrix, and the force vector should
contain static and damping contributions only.
7/01
Writing User Subroutines with ABAQUS
L7.79
ABAQUS
Using Nonlinear User Elements in Various Analysis Procedures
Remarks
Implementation of a user element with inertia effects in a dynamic
analysis is fairly complicated. Simplifications to the UEL can be
realized if:
• The ALPHA parameter on the ∗DYNAMIC option is set to zero.
• No inertia effects are included in the user element.
– Inertia effects can be included “indirectly” by overlaying standard
ABAQUS elements on top of user elements.
In this case, the ABAQUS elements should have negligible
stiffness.
For example, it is possible to overlay B23 elements on top of the
beam elements with nonlinear section behavior shown in the first
example.
7/01
Writing User Subroutines with ABAQUS
L7.80
ABAQUS
Workshop Preliminaries
Setting Up the Workshop Directories and Files
If you are taking this seminar in an HKS office, the steps in the following section have already
been done for you: skip to Basic Operating System Commands (p. WP.3). If everyone in your
group is familiar with the operating system, skip directly to the workshops.
The workshop files are included on the ABAQUS release CD. If you have problems finding the
files or setting up the directories, ask your systems manager for help.
Note for systems managers: If you are setting up these directories and files for someone else,
please make sure that there are appropriate privileges on the directories and files so that the user
can write to the files and create new files in the directories.
Workshop File Setup
(Note: UNIX is case-sensitive. Therefore, lowercase and uppercase letters must be typed as they
are shown or listed.)
1. Find out where the ABAQUS release is installed by typing
UNIX and Windows NT: abqxxx whereami
where abqxxx is the name of the ABAQUS execution procedure on your system.
It can be defined to have a different name. For example, the command for the
6.2–1 version might be aliased to abq621.
This command will give the full path to the directory where ABAQUS is installed,
referred to here as abaqus_dir.
2. Extract all the workshop files from the course tar file by typing
UNIX:
abaqus_dir/exec/perl
abaqus_dir/samples/course_setup.pl
Windows NT: abaqus_dir\exec\Perl
abaqus_dir\samples\course_setup.pl
An alternative method is to edit the script course_setup.pl and change the first line
of the script to the local installation of the Perl interpreter or the one available in
abaqus_dir/exec. For example:
#!/usr/bin/perl becomes #!abaqus_dir/exec/perl
7/01
Preliminaries for ABAQUS Workshops
WP.1
ABAQUS
Workshop Preliminaries
3. The script will install the files into the current working directory. You will be asked to
verify this and to choose which files you wish to install. Choose “y” for the appropriate
lecture series when prompted. Once you have selected the lecture series, type “q” to
skip the remaining lectures and to proceed with the installation of the chosen
workshops.
9/01
Preliminaries for ABAQUS Workshops
WP.2
Workshop Preliminaries
ABAQUS
Basic Operating System Commands
(You can skip this section and go directly to the workshops if everyone in your group is familiar
with the operating system.)
Note: The following commands are limited to those necessary for doing the workshop
exercises.
Working with Directories
1. Start in the current working directory. List the directory contents by typing
UNIX:
ls
Windows NT:
dir
Both subdirectories and files will be listed. On some systems the file type (directory,
executable, etc.) will be indicated by a symbol.
2. Change directories to a workshop subdirectory by typing
Both UNIX and Windows NT: cd dir_name
3. To list with a long format showing sizes, dates, and file, type
UNIX:
ls -l
Windows NT:
dir
4. Return to your home directory:
UNIX:
cd
Windows NT:
cd home-dir
List the directory contents to verify that you are back in your home directory.
5. Change to the workshop subdirectory again.
6. The * is a wildcard character and can be used to do a partial listing. For example, list
only ABAQUS input files by typing
UNIX:
ls *.inp
Windows NT:
dir *.inp
Working with Files
Use one of these files, filename.inp, to perform the following tasks:
1. Copy filename.inp to a file with the name newcopy.inp by typing
UNIX:
cp filename.inp newcopy.inp
Windows NT:
copy filename.inp newcopy.inp
7/01
Preliminaries for ABAQUS Workshops
WP.3
Workshop Preliminaries
ABAQUS
2. Rename (or move) this new file to newname.inp by typing
UNIX:
mv newcopy.inp newname.inp
Windows NT:
rename newcopy.inp newname.inp
(Be careful when using cp and mv since UNIX will overwrite existing files without
warning.)
3. Delete this file by typing
UNIX:
rm newname.inp
Windows NT:
erase newname.inp
4. View the contents of the files filename.inp by typing
UNIX:
more filename.inp
Windows NT:
type filename.inp | more
This step will scroll through the file one page at a time.
Now you are ready to start the workshops.
9/01
Preliminaries for ABAQUS Workshops
WP.4
ABAQUS
Workshop 1
User Subroutine FILM
Goals
• To learn how to find the compile and link commands used on your system.
dh
• To see how sensitive the rate of convergence is on the value of ------ .
dθ
dh
• To see if the results are sensitive to the value of ------ .
dθ
Problem Description
User subroutine FILM will be used to define
s 1⁄3
h ( θ ) = 500 θ w – θ i
,
(EQ 1)
s
where θ i = 100° C is the fluid temperature, and θ w is the surface temperature.
The subroutine will be tested on a two-element model, which is shown in Figure W1–1. All
nodes are initially at 77° C.
Convection boundary condition applied with FILM
3
2
θ = 277°
1
Figure W1–1. Two-Element Model
7/01
Writing User Subroutines with ABAQUS
W1.1
Workshop 1: User Subroutine FILM
ABAQUS
Steady-State Solution: Correct Model
The input file film_test-1.inp calculates the steady-state solution for the
problem shown in Figure W1–1.
1. Look at the file film_test-1.f. It is in the directory user_subroutines/
film.
dh
This file contains the correct format for h and ------ .
dθ
dh
Question W1–1: Which variable is assigned the value of ------ ?
dθ
Question W1–2: What is the derivative of the absolute value of a variable; that is,
dy
what is ------ when y = x ?
dx
2. Submit the input file film_test-1.inp to ABAQUS. The command to submit the
file is
abaqus j=film_test-1 user=film_test-1.f
By default on our system, the analysis will run as a background process. Look at the
contents of the film_test-1.log file to see how the analysis is progressing.
3. Look at the number of iterations ABAQUS needed for each increment of the analysis;
this information can be found in either the status (.sta) or the message (.msg) files.
Enter the total number of iterations that ABAQUS needed during the analysis to obtain
the solution in Table W1–1.
Question W1–3:
Where is the easiest place to find the total number of iterations used
during an analysis?
4. If you want to provide the functionality of a user subroutine to another user, but you do
not want to provide them with the source code, you will have to compile the user
subroutine prior to running ABAQUS. However, to do this, you need to know what the
proper compile command is for your system.
Question W1–4:
How can you find out the proper compile command for your
system?
Optional:
1. Once you find the proper compile commands for your system, try precompiling the user
subroutine.
2. Create symbolic links to the ABA_PARAM.INC file. The syntax to create this link is
found in the communications (.com) file; it will vary from system to system.
7/01
Writing User Subroutines with ABAQUS
W1.2
Workshop 1: User Subroutine FILM
ABAQUS
3. Submit the input file and the precompiled user subroutine to ABAQUS:
abaqus j=film_test-2 input=film_test-1 user=film_test-1.o
4. Compare the wall clock times for the two simulations. How much time was saved?
Steady-State Solution: Model with Error 1
1. Submit the input file film_test-e1.inp to ABAQUS using the subroutine in
film_test-e1.f.
dh
This model contains an error in the definition of ------ :
dθ
h(2) = third*500.0d0*(abs(temp-sink))**(-two*third)
The correct definition of h(2) should contain a term, a1, which is defined by the sign
of temp-sink:
a1 = sign(1.0, temp-sink)
h(2) = a1*third*500.0d0*(abs(temp-sink))**(-two*third)
Question W1–5:
Do you think this is a significant error?
2. Look at the number of iterations ABAQUS needed for each increment of the analysis.
Enter the total number of iterations that ABAQUS needed during the analysis to obtain
a solution in Table W1–1.
3. Compare the final temperature values from this analysis with those from the
film_test-1 analysis.
Steady-State Solution: Model with Error 2
1. Submit the input file film_test-e2.inp to ABAQUS using the subroutine in
film_test-e2.f
dh
This model contains an error in the definition of ------ :
dθ
h(2) = third*500.0d0*(abs(temp-sink))**(-third)
The correct definition of h(2) should have the term (abs(temp-sink)) raised to
the – 2 ⁄ 3 power:
h(2) = a1*third*500.0d0*(abs(temp-sink))**(-two*third)
2. Look at the number of iterations ABAQUS needed for each increment of the analysis.
Enter the total number of iterations that ABAQUS needed during the analysis to obtain
a solution in Table W1–1.
7/01
Writing User Subroutines with ABAQUS
W1.3
Workshop 1: User Subroutine FILM
ABAQUS
3. Compare the final temperature values from this analysis with those from the
film_test-1 analysis.
Question W1–6:
Were there any differences in the results? Were they significant?
Steady-State Solution: Model with Error 3
1. Submit the input file film_test-e3.inp to ABAQUS using the subroutine in
film_test-e3.f.
dh
This model contains an error in the definition of ------ :
dθ
h(2) = third*500.0d0*(abs(temp-sink))**(two*third)
The correct definition of h(2) should have the term (abs(temp-sink)) raised to
the – 2 ⁄ 3 :
h(2) = a1*third*500.0d0*(abs(temp-sink))**(-two*third)
2. Look at the number of iterations ABAQUS needed for each increment of the analysis.
Enter the total number of iterations that ABAQUS needed during the analysis to obtain
a solution in Table W1–1.
3. The analysis has failed to converge.
Question W1–7:
Why does the analysis fail to converge?
Table W1–1
Model
Number of Iterations
film_test-1.inp
film_test-e1.inp
film_test-e2.inp
film_test-e3.inp
7/01
Writing User Subroutines with ABAQUS
W1.4
Answers to Workshop 1
ABAQUS
Answers 1
User Subroutine FILM
Question W1–1:
Answer:
Question W1–2:
Answer:
Question W1–3:
Answer:
dh
Which variable is assigned the value of ------ ?
dθ
The second position in the array H, H(2), is given the value.
dy
What is the derivative of the absolute value of a variable; that is, what is -----dx
when y = x ?
The derivative of the absolute value of a variable is either – 1 or +1
depending on the sign of the variable.
Where is the easiest place to find the total number of iterations used during
an analysis?
The easiest place to find the total number of iterations is in the summary at
the end of the message file.
ANALYSIS SUMMARY:
TOTAL OF
1
5
45
45
45
0
0
0
0
0
0
0
1
INCREMENTS
CUTBACKS IN AUTOMATIC INCREMENTATION
ITERATIONS
PASSES THROUGH THE EQUATION SOLVER OF WHICH
INVOLVE MATRIX DECOMPOSITION, INCLUDING
DECOMPOSITION(S) OF THE MASS MATRIX
ADDITIONAL RESIDUAL EVALUATIONS FOR LINE SEARCHES
ADDITIONAL OPERATOR EVALUATIONS FOR LINE SEARCHES
WARNING MESSAGES DURING USER INPUT PROCESSING
WARNING MESSAGES DURING ANALYSIS
ANALYSIS WARNINGS ARE NUMERICAL PROBLEM MESSAGES
ANALYSIS WARNINGS ARE NEGATIVE EIGENVALUE MESSAGES
ERROR MESSAGES
THE SPARSE SOLVER HAS BEEN USED FOR THIS ANALYSIS.
7/01
Writing User Subroutines with ABAQUS
WA1.1
ABAQUS
Answers to Workshop 1
The status file does not provide a total number of iterations used in an
analysis if there are any cutbacks in the increment size and may not provide
the correct number if there are any severe discontinuity iterations.
Question W1–4:
Answer:
How can you find out the proper compile command for your system?
You could look in the communications (.com) file, or you could use the
abaqus help=environment command. If you look in the
communications file, you will find the following lines:
ln -s /usr/abaqus60/abqver/sgi4000/../../release/sgi4000/
r5-07-001/site/aba_param_dp.inc aba_param.inc
ln -s /usr/abaqus60/abqver/sgi4000/../../release/sgi4000/
r5-07-001/site/aba_param_dp.inc ABA_PARAM.INC
.
.
.
echo Compiling... film_test-e3.f
f77 -G 0 -mp -c -mips2 -32 -O2 film_test-e3.f
The symbolic links (ln -s commands) are needed to include the
ABA_PARAM.INC file during the compile command. It is important to use
all the flags and settings on the compile command that are found by either
method.
7/01
Question W1–5:
Answer:
Do you think this is a significant error?
It is not likely to be a significant error because the sign of the derivative will
s
always be positive in this example ( θ w – θ i > 0 ).
Question W1–6:
Answer:
Were there any differences in the results? Were they significant?
There were differences between the two analyses; however, the differences
were minor errors in the fourth significant figure of the numbers.
Writing User Subroutines with ABAQUS
WA1.2
ABAQUS
Question W1–7:
Answer:
7/01
Answers to Workshop 1
Why does the analysis fail to converge?
The analysis fails to converge because the rate of convergence is very slow
and ABAQUS continues to cut back the increment size. The rate of
dh
convergence is slow because the value calculated for ------ contains a large
dθ
error.
Writing User Subroutines with ABAQUS
WA1.3
ABAQUS
Workshop 2
User Subroutine UMAT:
Tangent Stiffness
Goals
• To investigate the effect of the tangent stiffness matrix on convergence.
• To see how a poorly developed material model can cause problems in simulations that
are under load control.
Problem Description
The tangent stiffness matrix calculated in user subroutine UMAT plays a critical role in the
Newton-Raphson algorithm in ABAQUS/Standard. Errors in the formulation of the tangent
stiffness matrix will result in analyses that require more iterations and, in some cases, diverge.
In this workshop the importance of calculating tangent stiffness correctly is explored by
analyzing a small problem under uniaxial tension.
Both displacement and load control simulations are considered. The material response being
modeled is elastic-plastic; Mises plasticity is assumed.
The analysis is carried out by modifying the material stiffness calculated with user subroutine
UMAT, which effectively introduces an error into the tangent stiffness matrix. To build
confidence in user subroutine UMAT, the problem is first solved using the built-in Mises
plasticity algorithm in ABAQUS/Standard.
Analyses with the ∗PLASTIC Material Model
1. Submit the file umat/axi1.inp to ABAQUS. This input file is set up for a
displacement-control analysis and uses the Mises plasticity routine that is built into
ABAQUS.
7/01
Writing User Subroutines with ABAQUS
W2.1
ABAQUS
Workshop 2: User Subroutine UMAT: Tangent Stiffness
2. Enter the number of increments and the total number of iterations required to complete
this displacement control analysis in Table W2–1. Under the heading “Flag Setting”
enter “built in.”
Table W2–1
Displacement Control
Flag Setting
Number of Increments
Number of Iterations
Load Control
Flag Setting
Number of Increments
Number of Iterations
3. Copy axi1.inp into another file name, and modify it as follows: remove the nonzero
displacement boundary conditions, and add a negative pressure loading of 40000 on the
element in the axial direction.
4. Submit the new file to ABAQUS.
5. Repeat Step 2, except put the results in the load control portion of the table.
Displacement-Control Analyses with the UMAT Material Model
Now solve the same problem using user subroutine UMAT.
• The material constants used in the user subroutine are specified with the ∗USER
MATERIAL option.
• Seven material constants are specified in this problem.
• The seventh material constant is used as a flag in user subroutine UMAT: when it is set
to 1, UMAT returns the actual (i.e., correct) material stiffness; when it is set to 0, UMAT
returns only the elastic stiffness.
7/01
Writing User Subroutines with ABAQUS
W2.2
Workshop 2: User Subroutine UMAT: Tangent Stiffness
ABAQUS
1. The file umat/axi2.inp contains the input data for the model, and the file
umat/iso_mises_umat.f contains the source code for the UMAT. The file
axi2.inp is set up for a displacement-control analysis; the seventh material constant
is set to 1, which provides ABAQUS/Standard with the correct tangent stiffness.
2. Submit the input file.
3. Compare the results of this analysis with those obtained for the displacement-control
analysis using the built-in Mises plasticity routine.
4. Enter the flag setting, number of increments, and the total number of iterations required
to complete the analysis in the displacement-control section of Table W2–1.
5. Modify the material constants so that the seventh constant under the ∗USER
MATERIAL option is set to 0, which provides ABAQUS/Standard with the incorrect
tangent stiffness.
6. Repeat Step 2–Step 4.
Question W2–1:
Are the results obtained with the modified stiffness matrix correct?
Load-Control Analyses with the UMAT Material Model
Now repeat the analysis under a state of load control.
1. Copy axi2.inp into another file name, and modify it as follows: remove the nonzero
displacement boundary conditions, and add a negative pressure loading of 40000 on the
element in the axial direction.
2. Set the seventh material constant to 1.
3. Submit the job. Enter the flag setting, number of increments, and the total number of
iterations required to complete the analysis in Table W2–1.
4. Repeat Step 3 when the seventh material constant is set to 0.
Question W2–2:
7/01
What can you say about the difference in the convergence behavior
of this problem when a pressure loading is applied instead of a
boundary condition?
Writing User Subroutines with ABAQUS
W2.3
Answers to Workshop 2
ABAQUS
Answers 2
User Subroutine UMAT: Tangent
Stiffness
Question W2–1:
Answer:
Are the results obtained with the modified stiffness matrix correct?
Yes. Errors in the tangent stiffness will only affect the convergence
behavior, not the final result. When the elastic stiffness is used instead of
the true stiffness, the number of iterations is increased. The results (when
obtained) are still correct.
Question W2–2:
What can you say about the difference in the convergence behavior of this
problem when a pressure loading is applied instead of a boundary
condition?
Displacement-control problems are more stable than load-control
problems. When the elastic stiffness is used instead of the true stiffness, the
displacement-control analysis ran successfully but the load-control analysis
failed.
Answer:
7/01
Writing User Subroutines with ABAQUS
WA2.1
Answers to Workshop 2
ABAQUS
Table W2–1
Displacement Control
Flag Setting
Number of Increments
Number of Iterations
built in
6
8
1
6
8
0
6
21
Load Control
Flag Setting
Number of Increments
Number of Iterations
built in
9
28
1
9
28
0
6*
59*
*Did not run to completion.
7/01
Writing User Subroutines with ABAQUS
WA2.2
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