Study of composite joint strength with carbon nanotube reinforcement Faulkner, Susan D.

Study of composite joint strength with carbon nanotube reinforcement Faulkner, Susan D.
Calhoun: The NPS Institutional Archive
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Thesis Collection
2008-09
Study of composite joint strength with
carbon nanotube reinforcement
Faulkner, Susan D.
Monterey, California. Naval Postgraduate School
http://hdl.handle.net/10945/3995
NAVAL
POSTGRADUATE
SCHOOL
MONTEREY, CALIFORNIA
THESIS
STUDY OF COMPOSITE JOINT STRENGTH WITH
CARBON NANOTUBE REINFORCEMENT
by
Susan D. Faulkner
September 2008
Thesis Advisor:
Second Reader:
Young W. Kwon
Scott W. Bartlett
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4. TITLE AND SUBTITLE Study of Composite Joint Strength with Carbon
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Nanotube Reinforcement
6. AUTHOR(S) Susan D. Faulkner
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13. ABSTRACT (maximum 200 words)
Strengthening of composite joints is a topic of recent research. The benefits of using locally applied carbon
nanotubes to reinforce a carbon fiber composite joint were studied. The effect of carbon nanotubes on enhancing the
fracture toughness and joint interface strength was investigated by performing Mode I, Mode II, and Mixed Mode
I/Mode II fracture with and without carbon nanotubes applied locally at the joint interface. Furthermore, the effects
of seawater absorption on Mode II fracture were investigated. Finally, an optimization of carbon nanotube
concentration was performed. During the study, the image correlation technique was used to examine the fracture
mechanisms altered by the introduction of carbon nanotubes. The experimental study showed that carbon nanotubes
can increase the fracture toughness of the composite interface significantly, especially for Mode II, including a
physical change in the fracture mechanism.
14. SUBJECT TERMS Carbon Nanotubes, CNT, Carbon Fiber Composite, Fracture Mechanics, Joint
Strength Enhancement, Reinforcement, Mode I, Mode II, Mixed Mode I/Mode II
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Approved for public release; distribution is unlimited
STUDY OF COMPOSITE JOINT STRENGTH WITH CARBON NANOTUBE
REINFORCEMENT
Susan D. Faulkner
Lieutenant, United States Navy
B.S., United States Naval Academy, 2000
Submitted in partial fulfillment of the
requirements for the degree of
MASTER OF SCIENCE IN MECHANICAL ENGINEERING
from the
NAVAL POSTGRADUATE SCHOOL
September 2008
Author:
Susan D. Faulkner
Approved by:
Prof. Young W. Kwon
Thesis Advisor
Scott W. Bartlett
Second Reader
Knox T. Millsaps
Chairman, Department
Engineering
iii
of
Mechanical
and
Astronautical
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iv
ABSTRACT
Strengthening of composite joints is a topic of recent research. The benefits of
using locally applied carbon nanotubes to reinforce a carbon fiber composite joint were
studied. The effect of carbon nanotubes on enhancing the fracture toughness and joint
interface strength was investigated by performing Mode I, Mode II, and Mixed Mode
I/Mode II fracture with and without carbon nanotubes applied locally at the joint
interface. Furthermore, the effects of seawater absorption on Mode II fracture were
investigated. Finally, an optimization of carbon nanotube concentration was performed.
During the study, the image correlation technique was used to examine the fracture
mechanisms altered by the introduction of carbon nanotubes. The experimental study
showed that carbon nanotubes can increase the fracture toughness of the composite
interface significantly, especially for Mode II, including a physical change in the fracture
mechanism.
v
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vi
TABLE OF CONTENTS
I.
INTRODUCTION........................................................................................................1
A.
BACKGROUND ..............................................................................................1
B.
LITERATURE SURVEY................................................................................1
C.
OBJECTIVES ..................................................................................................3
II.
COMPOSITE SAMPLE CONSTRUCTION............................................................5
A.
SAMPLE SPECIFICATION ..........................................................................5
1.
Materials ...............................................................................................5
2.
Construction Techniques ....................................................................6
B.
HAND LAY-UP TECHNIQUE ......................................................................6
C.
VACUUM
ASSISTED
RESIN
TRANSFER
MOLDING
TECHNIQUE .................................................................................................10
III.
PHASES OF RESEARCH ........................................................................................17
A.
PHASE I..........................................................................................................17
B.
PHASE II ........................................................................................................17
C.
PHASE III.......................................................................................................17
D.
PHASE IV.......................................................................................................17
E.
PHASE V ........................................................................................................18
IV.
TESTING....................................................................................................................19
A.
OVERVIEW...................................................................................................19
B.
MODE I...........................................................................................................19
C.
MODE II .........................................................................................................20
D.
MIXED MODE I/MODE II ..........................................................................21
E.
SEAWATER ABSORPTION EFFECTS ....................................................22
V.
RESULTS AND DISCUSSION ................................................................................23
A.
MODE I...........................................................................................................23
B.
MODE II .........................................................................................................24
C.
MIXED MODE I/MODE II ..........................................................................33
D.
SEAWATER ABSORPTION EFFECTS ....................................................34
E.
CNT OPTIMIZATION .................................................................................38
VI.
CONCLUSIONS AND RECOMMENDATIONS...................................................41
APPENDIX A: MODE I DATA ...........................................................................................43
PHASE III...................................................................................................................43
APPENDIX B: MODE II DATA ..........................................................................................45
PHASE III...................................................................................................................45
PHASE IV...................................................................................................................45
APPENDIX C: SEAWATER ABSORPTION EFFECTS DATA .....................................47
PHASE III...................................................................................................................47
APPENDIX D: PHASE V DATA .........................................................................................49
vii
PHASE V ....................................................................................................................49
LIST OF REFERENCES ......................................................................................................51
INITIAL DISTRIBUTION LIST .........................................................................................53
viii
LIST OF FIGURES
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
Figure 14.
Figure 15.
Figure 16.
Figure 17.
Figure 18.
Figure 19.
Figure 20.
Figure 21.
Figure 22.
Figure 23.
Figure 24.
Figure 25.
Figure 26.
Figure 27.
Figure 28.
Figure 29.
Sample geometry ...............................................................................................5
Side view of bottom plate ..................................................................................8
Image of composite sample prior to cure...........................................................8
Image of composite sample curing under vacuum ............................................9
Image of cured bottom layer after surface preparation with delamination
insert attached ....................................................................................................9
Side view of bottom plate with CNT ...............................................................10
Side view of constructed sample......................................................................10
Layers of carbon fiber fabric stacked on peel ply............................................13
Inlet tubing set-up ............................................................................................13
Outlet tubing set-up with resin trap .................................................................14
Vacuum applied during approximately 10 minute wait...................................14
Resin flow through carbon fiber layers, showing inlet and outlet tubing........15
Resin flow through carbon fiber layers............................................................15
Double cantilever beam test for Mode I (i.e., crack opening) fracture............20
Three point bending test for Mode II (i.e., shearing mode).............................21
Mixed Mode I/Mode II test apparatus [From Ref. 13] ....................................22
Mode I Normalized GI Values .........................................................................24
Image of transverse normal strain just prior to Mode I (opening mode)
crack propagation.............................................................................................24
Mode II Normalized GII Values.......................................................................26
Representative load versus extension plot for Mode II (shear mode) testing
of non-reinforced sample (The point of crack propagation is marked with
an X.) ................................................................................................................27
Representative load versus extension plot for Mode II testing of CNT
reinforced sample.............................................................................................27
Mode II Normalized GII Values for Phase IV samples....................................28
Initial crack propagation of resin only sample (Crack propagated from the
initial crack tip.) ...............................................................................................29
Initial crack propagation of CNT reinforced sample (The internal crack
was nucleated away from the initial crack tip. Then the internal crack
grew to meet the initial crack tip as the load increased.) .................................30
Plot of shear strain from Digital Image Correlation System for Mode II
(i.e., shearing mode).........................................................................................30
Mode II crack surface of non-reinforced sample (Note the crack
propagated through resin. In some areas the resin failed and in others the
resin pulled away from the fibers.) ..................................................................31
Mode II crack surface of CNT reinforced sample (Note the crack
propagated through the fibers and through a neighboring fiber layer in one
region.) .............................................................................................................32
Schematic of secondary bond with CNT .........................................................33
Plot of Mixed Mode I/Mode II data.................................................................34
ix
Figure 30.
Figure 31.
Figure 32.
Figure 33.
Figure 34.
Seawater absorption weight tracker for Phase III samples ..............................35
Mode II Normalized GII Values for Phase III seawater soaked samples.........36
Bending failure of Phase IV seawater soaked sample (side view) ..................37
Bending failure of Phase IV seawater soaked sample (top view)....................38
Mode II Normalized GII Values for CNT Optimization samples ....................39
x
LIST OF TABLES
Table 1.
Table 2.
Detailed hand lay-up sample construction procedure........................................7
Detailed VARTM sample construction procedure ..........................................12
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xii
ACKNOWLEDGMENTS
First and foremost, I would like to thank Dr. Young Kwon for his mentorship
during the course of this research and throughout my graduate studies.
Thank you to Erik Rasmussen, Scott Bartlett, Doug Loup, and Tim Dapp from the
Naval Surface Warfare Center Carderock Division (NSWCCD) team for “Advanced Hull
Materials & Structures Technology (AHM&ST)” who provided crucial funding,
materials, and technical guidance.
Chris Hicks from the Northrop Grumman Ship Systems Advanced Capabilities
Group, Science and Technologies – Composites is also appreciated for providing
technical guidance, specifically in the area of moisture effects testing.
xiii
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xiv
I.
A.
INTRODUCTION
BACKGROUND
In recent years, large composite structures have been incorporated into naval
vessels to increase operational performance while lowering ownership costs [1]. The
trend continues with new projects, such as the superstructure for DDG 1000.
In
particular, carbon fiber composite material provides high strength and stiffness while
maintaining low weight. The joints of these large composite structures are the weakest
point due to discontinuity of fiber reinforcement. The joints therefore have the largest
failure rate [2]. Strengthening the composite joint will increase the strength of the entire
composite structure. Research has shown that varying joint geometry can increase joint
strength [3]. However, changing the joint geometry can depend on the loading condition.
Ship structures undergo a variety of loading conditions, so varying the geometry is not
always the ideal method of strengthening the joint. Another type of reinforcement is
therefore required. Carbon nanotubes, with high strength and stiffness, provide a means
to locally reinforce the joint while not sacrificing the integrity of the composite material.
Carbon nanotubes (CNT) are allotropes of carbon with a hexagonal lattice
structure like graphite. The lattice structure forms a tube with nano-sized diameter. CNT
can be several millimeters in length. They can be either single-walled or multi-walled,
meaning an inner cylinder lies within the outer cylinder [4]. Although many strides have
been made in the manufacture of CNT, they are still quite expensive. CNT have an
extremely high elastic modulus (greater than 1 TPa) yet are lightweight [5]. Therefore,
they are ideal for strengthening composite materials.
B.
LITERATURE SURVEY
The elastic modulus of carbon nanotubes (CNT) is greater than one TPa, and CNT
are 10 to 100 times stronger than the strongest steels [5]. The high strength and relatively
low weight of CNT make them a prime candidate for composite material reinforcement.
Much research has been performed documenting the ability of CNT to reinforce a variety
of matrix materials such as various polymers and ceramics. One such study found high
1
interfacial shear stress and stronger interfacial adhesion between multi-walled CNT
(MWNT) to epoxy than epoxy to epoxy. The same study found no increase in tensile
strength due to MWNT reinforcement [6]. Another study explored the use of several
different types of carbon nanotubes in a polymer composite. Young’s modulus was
doubled as a result of the reinforcement. The same study indicated that low diameter
multi-walled carbon nanotubes were the ideal CNT for reinforcement due to their surface
area characteristics [7].
Many studies have been conducted to determine the type of bonds formed
between CNT and epoxy. The general conclusion is that CNT bond in three main ways:
micromechanical interlocking, chemical bonding, and van der Waals bonding. While the
CNT surface is quite smooth, it has been proposed that there are local non-uniformities in
the CNT such as kinks, bends, and changes in diameter.
It is at these local non-
uniformities where micromechanical interlocking occurs [6].
Chemical bonding is
possible, but it is not guaranteed [8]. Finally, van der Waals bonding certainly occurs,
but a relatively weak bond forms.
One study also proposes the effects of thermal
properties. The coefficient of thermal expansion of CNT is much higher than that of the
polymer matrix. As a result, residual compressive thermal stress is present after the
polymer matrix hardens. This thermal stress results in closer contact between the CNT
and polymer, which in turn increases micromechanical interlocking and non-bond
interactions [6].
While the effects of uniform incorporation of carbon nanotubes within a polymer
structure have been studied, only one study has documented the results of local
reinforcement of a carbon fiber composite with CNT.
The research focused on a
composite scarf joint, which is applicable to the U.S. Navy. Several types of CNT were
tried, including various multi-walled CNT as well as bamboo structured CNT.
Additionally, two different CNT concentrations were used. The study found that under
compression testing, the carbon fiber composite scarf joint was stronger when reinforced
with CNT [9].
2
C.
OBJECTIVES
The research presented in this paper builds on the aforementioned study.
Widespread use of carbon nanotubes throughout a ship superstructure is too costly for the
United States Navy. However, local reinforcement of the structure at its weakest points
is possible. The fracture toughness of the locally reinforced joint must be studied to
determine the impact of reinforcement. The purpose of this research is to determine the
critical energy release rate, G, and crack propagation characteristics of CNT reinforced
and non-reinforced carbon fiber/vinyl ester resin composite samples during Mode I,
Mode II, and Mixed Mode I/Mode II fractures. Additionally, the effects of seawater
absorption on Mode II critical energy release rate were studied. Finally, an optimization
of CNT concentration was performed. A wide variety of samples were tested to show
conclusively the impact of CNT reinforcement on fracture toughness. Sample sets varied
in geometry and construction technique. The two construction techniques employed were
hand lay-up and Vacuum Assisted Resin Transfer Molding (VARTM).
This research is in support of the Naval Surface Warfare Center Carderock
Division (NSWCCD) team for “Advanced Hull Materials & Structures Technology
(AHM&ST).” The seawater absorption testing was completed in support of Northrop
Grumman Ship Systems Advanced Capabilities Group, Science and Technologies –
Composites.
3
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4
II.
A.
COMPOSITE SAMPLE CONSTRUCTION
SAMPLE SPECIFICATION
Five sets of carbon fiber samples were constructed during the course of this
research. Each set of samples consisted of resin only samples and CNT reinforced
samples so results could be compared. Size and construction technique of the samples
varied, which will be discussed later. However, the basic sample construction remained
the same throughout the research.
Samples consisted of carbon fiber composite
specimens with a secondary bond at the interface layer and a pre-existing edge crack, as
shown in Figure 1. The presence of the secondary bond is required to mimic joint
construction. When constructing the scarf joint, one side is constructed and cured. The
other side is then constructed directly on top of the existing side.
a 2h
L
Figure 1.
Sample geometry
where:
L = length
2h = thickness
a = initial crack length
1.
Materials
A vinyl-ester matrix base, DERAKANE 510-A was used with TORAY T700CF
carbon fiber weave. These materials were selected by the Naval Surface Warfare Center
Carderock Division (NSWCCD) team for “Advanced Hull Materials & Structures
Technology (AHM&ST)” since they are used for naval vessels. Hardening chemicals are
5
required to cure the resin. The hardening chemicals are Methyl Ethyl Ketone Peroxide
(MEKP) and Cobalt Naphthenate (CoNap). These chemicals were used in concentrations
recommended by the manufacturer of DERAKANE 510-A. A hardening time of 60
minutes was selected to allow ample time for sample construction.
With ambient
temperature between 70˚F and 80˚F, the combination of hardeners consisted of 1.25 %wt
MEKP and 0.20 %wt CoNap to achieve the desired hardening time.
2.
Construction Techniques
Two construction techniques were used during the research. First, a hand lay-up
technique was employed. This is a relatively simplistic method of constructing carbon
fiber composite specimens which involves minimal laboratory equipment. After proving
the theory that fracture toughness is affected by CNT reinforcement, a more complex
technique was employed. Vacuum Assisted Resin Transfer Molding (VARTM) is one of
several construction techniques used in industry, thereby making it a logical choice of
construction technique. While it involved more laboratory equipment and extensive trial
and error to create suitable samples, it was imperative to prove local CNT reinforcement
would be both useful and feasible by industry. Both the hand lay-up and VARTM
techniques will be discussed in detail.
B.
HAND LAY-UP TECHNIQUE
A detailed description of the hand layup procedure is provided in Table 1. In
summary, a bottom carbon fiber plate was constructed first and cured. The bottom plate
was then sanded and cleaned with acetone. Next, a wax paper insert of thickness 0.0038
cm (0.0015 in) was placed across the bottom plate for the initial crack. Next, acetone
was used as a dispersing agent for CNT. This study used conventional multi-walled CNT
with diameter 30nm+/-15nm and length 5-20μm. CNT surface concentration was 7.5
g/m2. The selection of CNT as well as the selection of acetone as the dispersing agent
was based on results from compression testing of CNT reinforced scarf joints [9]. After
the acetone dried, the top plate was constructed on top of the bottom plate, forming a
secondary bond between plates. After curing, samples were cut using the Jet Edge
waterjet cutter.
6
Samples then underwent a post-cure treatment. Although the resin is mostly
cured after 12 hours, it continues to cure over long periods of time. It is possible that
material properties may change over time. Therefore, samples underwent a six-hour
post-cure at 140°F to mimic long-term curing.
Table 1.
Detailed hand lay-up sample construction procedure
Step 1 Attach a layer of porous non-permeable ply and peel ply to aluminum
plate to serve as base for composite layup.
Step 2 Cut carbon fiber fabric to desired size. Four layers of carbon fiber
fabric were used to achieve desired thickness.
Step 3 Manually apply resin compound to each sheet of carbon fiber fabric * See
using a foam brush.
Figs.
2 and 3
Step 4 Immediately following completion of layup, wrap the composite in one
layer of peel ply, one layer of porous non-permeable ply, and one layer
of buffer ply.
Step 5 Place composite plate in airtight vacuum bag. Apply vacuum. * See
Vacuum removes trapped air in the composite structure and promotes Fig. 4
absorption of excess resin by the buffer ply.
Step 6 After 12-hour cure, remove the vacuum and composite plate.
One-half of the sample has been constructed.
Step 7 Sand the top of the composite plate with 100 grit sand paper to roughen
the surface.
Step 8 Clean with acetone and allow acetone to dry fully.
Step 9 Attach delamination insert to desired area of composite plate.
* See
Fig. 5
Step 10 Disperse CNT on top of composite plate and allow dispersing agent * See
(acetone) to dry.
Fig. 6
Step 11 Construct top layer of sample by repeating steps 2-6.
* See
Fig. 7
Step 12 Cut samples using Jet Edge waterjet cutter.
Step 13 Post-cure samples at 140°F for six hours.
* Phases
III-V
only
7
Multiple
carbon fiber
and resin
layers
Figure 2.
Figure 3.
Side view of bottom plate
Image of composite sample prior to cure
8
Figure 4.
Figure 5.
Image of composite sample curing under vacuum
Image of cured bottom layer after surface preparation with
delamination insert attached
9
Dispersed CNT
Initial Crack Insert
Bottom plate
Figure 6.
Side view of bottom plate with CNT
Top plate
Figure 7.
C.
Side view of constructed sample
VACUUM ASSISTED RESIN TRANSFER MOLDING TECHNIQUE
A detailed description of the Vacuum Assisted Resin Transfer Molding
(VARTM) procedure is provided in Table 2.
In summary, the VARTM technique
involves pulling resin through the layers of carbon fiber with a vacuum. Samples were
constructed in the same manner as when using the hand lay-up technique, meaning a
bottom carbon fiber plate was constructed first and cured. The bottom plate was then
sanded and cleaned with acetone. When using the VARTM technique, Teflon film of
thickness 0.0051 cm (0.002 in) was used as the delamination insert. Acetone was again
used as the dispersing agent for applying CNT. CNT surface concentration was 7.5 g/m2.
After the acetone dried, layers of carbon fiber were stacked on the bottom plate and
infused with resin. After curing, samples were cut using the Jet Edge waterjet cutter.
Samples then underwent a post-cure treatment.
10
There was some concern that the CNT would be displaced when pulling the resin
through the layers of carbon fiber. However, the CNT remained in place. This was a
significant finding, since VARTM is a popular method for constructing carbon fiber
composites in industry. No special technique will be needed when applying CNT locally.
The CNT can simply be dispersed on the desired area and VARTM can be conducted.
11
Table 2.
Detailed VARTM sample construction procedure
Step 1 Place a layer of peel ply on glass to serve as base for composite
construction. Glass must be at least 1.27 cm (0.5 in) thick.
Step 2 Cut carbon fiber fabric to desired size. Five layers of carbon
fiber fabric were used to achieve desired thickness.
Step 3 Stack carbon fiber fabric on top of peel ply.
Step 4 Place a second layer of peel ply on top of carbon fiber fabric.
Place a sheet of distribution media on top of peel ply.
Step 5 Set up resin inlet and outlet tubing. Adequate tubing is
required to ensure resin is not pulled into the vacuum source. A
resin trap on the outlet side is recommended.
Step 6 Attach plastic sheet using putty/tape. Plastic sheet will act as a
vacuum bag.
Step 7 Perform vacuum check and fix vacuum leaks. Vacuum of 26
inches Hg should be obtained. Continue applying vacuum.
Step 8 Mix resin and hardeners. A cure time of 60 minutes was used
for this research.
Step 9 Wait approximately 10 minutes. Immediately after being
mixed with hardeners, the resin produces air bubbles. Wait
until air bubbles are no longer being produced.
Step 10 Allow resin to flow into carbon fiber layers. Flow speed may
be adjusted by adjusting vacuum. However, vacuum of 10
inches Hg should be maintained.
Step 11 When carbon fiber layers are infused with resin and resin
accumulates in the outlet tubing, clamp resin inlet to ensure air
is not pulled into the sample. Infusion time depends on sample
size and thickness. During this research, infusion time was
roughly 5-10 minutes.
Step 12 Maintain vacuum until resin hardens.
Step 13 Allow sample to cure at least 12 hours before removing sample
from VARTM set up. Construction of bottom plate is
complete.
Step 14 Sand the top of the composite plate with 100 grit sand paper to
roughen the surface.
Step 15 Clean with acetone and allow acetone to dry fully.
Step 16 Attach delamination insert to desired area of composite plate.
Step 17 Disperse CNT on top of composite plate and allow dispersing
agent (acetone) to dry.
Step 18 Construct top layer of sample by repeating steps 1-13.
Step 19 Cut samples using Jet Edge waterjet cutter.
Step 20 Post-cure samples at 140°F for six hours.
12
* See Fig. 8
* See Figs. 9
and 10
* See Fig. 11
* See Figs. 12
and 13
* See Fig. 2
* See Fig. 5
* See Fig. 6
* See Fig. 7
* Phases III-V
only
Figure 8.
Layers of carbon fiber fabric stacked on peel ply
Figure 9.
Inlet tubing set-up
13
Figure 10.
Figure 11.
Outlet tubing set-up with resin trap
Vacuum applied during approximately 10- minute wait
14
Figure 12.
Resin flow through carbon fiber layers, showing inlet and outlet tubing
Figure 13.
Resin flow through carbon fiber layers
15
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16
III.
A.
PHASES OF RESEARCH
PHASE I
Phase I was completed as a learning experience. Ten samples were constructed to
practice the hand lay-up technique and dispersion of CNT. The samples were then tested
to learn how to use the test equipment.
B.
PHASE II
Phase II was completed to test the theory that fracture toughness is affected by
CNT reinforcement. This phase consisted of large samples constructed via the hand layup technique. Samples were nominally 2.5 cm wide, 0.75 cm thick, and 40.5 cm in
length. The large sample size was chosen so readily available laboratory equipment
could be used during testing. The samples were tested in both Mode I and Mode II and
critical energy release rate, G, was calculated.
C.
PHASE III
After proving the theory, samples were constructed via the hand lay-up method
and tested according to applicable ASTM standards.
Sample size was reduced to a
nominal 2.5 cm wide, 0.5 cm thick, and 14.0 cm in length. The purpose of this phase of
research was to ensure sample size did not affect the impact of CNT reinforcement on
fracture toughness. Additionally, a six-hour postcure at 140°F was conducted to mimic
long term curing of the sample. The postcure was conducted on all subsequent phases.
Testing was also expanded to include Mode I, Mode II, and Mixed Mode I/Mode II
testing. Additionally, the effects of seawater absorption were studied during this phase of
research.
D.
PHASE IV
Once the theory was proven using the hand lay-up technique, samples were
produced using the VARTM technique.
The VARTM technique requires extensive
laboratory supplies, and is one of the common techniques used in industry. The purpose
of Phase IV was two-fold. First, a method of locally dispersing the CNT in the carbon
17
fiber composite was devised. Secondly, Mode II testing was repeated to ensure the
effects of CNT reinforcement were not affected by the VARTM procedure. The effects
of seawater absorption were also studied during this phase of research.
E.
PHASE V
The final phase of research determined an optimum concentration of CNT. The
effect of “banded CNT” was also studied. Previously, all CNT reinforced samples were
constructed with CNT dispersed on the entire fracture surface.
However, CNT
reinforcement during this phase only extended 6 cm from the crack tip. “Banding” the
CNT was done to determine the effect of localized reinforcement. Additionally, three
concentrations of CNT were used: 5 g/m2, 7.5 g/m2, and 10 g/m2.
constructed via the VARTM technique. Mode II testing was completed.
18
Samples were
IV.
A.
TESTING
OVERVIEW
Samples were tested using an Instron Tension/Compression Machine (Model
Number: 4507/4500) with 10 kN load cell. Series IX computer software was used to
control displacement and record displacement and load values. All tests were performed
at the rate of 2.54 mm displacement per minute (0.1 in/min). Additionally, a Digital
Image Correlation System was employed to record images during testing at the rate of 1
image per second. The Digital Image Correlation System was also used to measure strain
fields around the crack during the crack initiation and growth.
B.
MODE I
The applicable ASTM Standard was followed for Mode I testing. Mode I testing
consisted of a double cantilever beam (DCB) test as shown in Figure 14 [10]. Piano
hinges, used to apply the load, were attached to each sample using a commercially
available 2-part epoxy. The following equation was used to determine critical energy
release rate, GI, through the Modified Beam Theory method [10]:
GI =
3Pδ
2ba
where:
P=load when crack propagates
δ =load point displacement
b=sample width
a=initial delamination length
19
P δ
a
P Figure 14.
C.
Double cantilever beam test for Mode I (i.e., crack opening) fracture
MODE II
No applicable ASTM Standard exists for pure Mode II fracture toughness testing.
Mode II testing consisted of a three point bending test as shown in Figure 15. Because
the crack lies in the midplane of the beam, only shear stress is applied to the crack. The
following equation was used to determine Mode II critical energy release rate, GII [11]:
3Pc 2 ⎡ 2 0.2h 2 E11 ⎤
GII =
⎢a +
⎥
64bE11 I ⎣
G13 ⎦
where:
Pc=critical load when crack propagates
h=1/2 total thickness
b=sample width
a=initial crack length
I=
bh3
12
The selection of the critical load was based on both observation of crack
propagation and a local maximum or slope change in the load versus displacement curve.
20
a
P
L
Figure 15.
L
Three point bending test for Mode II (i.e., shearing mode)
Calculation of the Mode II critical energy release rate, GII, was repeated using a
compliance approach with the following equation [12]:
9a 2 Pc 2C
GII =
2b(2 L3 + 3a 3 )
where:
Pc=critical load when crack propagates
C=compliance
a=initial crack length
b=sample width
L=1/2 span length
It can be shown that the two methods are equivalent. The first method clearly
delineates the contribution from transverse shear deformation. However, the first method
requires material properties to be known as well as precise measurement of height and
thickness of the sample. The second method, the compliance approach, does not require
material properties to be known. Instead, the material properties are indirectly measured
via the experimentally determined compliance. The contribution from transverse shear
stress is also imbedded in the compliance measurement. Both equations were used to
compute GII for the present study.
D.
MIXED MODE I/MODE II
The applicable ASTM Standard was used to guide Mixed Mode I/Mode II testing.
Mixed Mode I/Mode II testing requires a special test rig as shown in Figure 16 [13].
21
Piano hinges, used to apply the load and secure the sample in the test rig, were attached
to each sample using a commercially available 2-part epoxy. Multiple equations are
necessary to calculate the Mixed Mode I/Mode II critical energy release rate. These
equations can be found in the applicable ASTM Standard [13].
Figure 16.
E.
Mixed Mode I/Mode II test apparatus [From [13]]
SEAWATER ABSORPTION EFFECTS
To test the effects of seawater absorption on local CNT reinforcement, samples
were soaked in seawater until saturation and then tested in Mode II. Seawater was mixed
using substrate conforming to ASTM Standard D1141-98 and samples were soaked at
room temperature, nominally 70-80 degrees Fahrenheit [14]. Dimensions and weight of
each sample were recorded prior to soak.
Seawater absorption was tracked by
periodically weighing each sample during soaking. When weight no longer changed
significantly, the samples were determined to be saturated and Mode II testing was
conducted as described previously.
22
V.
A.
RESULTS AND DISCUSSION
MODE I
Mode I testing showed a small improvement in GI when the interface joint was
reinforced with CNT. Figure 17 displays the average values of normalized GI for resin
only samples and CNT samples from Phase III. Included in Appendix A are values of GI
for each sample. Standard deviation is also shown in the figure. Similar results were
obtained for Phase II. Mode I crack propagation characteristics were also observed with
no discernable difference between the CNT reinforced and non-reinforced samples.
Since CNT reinforcement does not lead to a significant improvement of GI, no further
Mode I testing was completed.
The Digital Image Correlation System was used to plot normal strain
perpendicular to crack orientation because the normal stress is the cause of crack
opening. A representative image just prior to crack propagation is shown in Figure 18.
CNT reinforced and non-reinforced images were very similar.
After testing, the samples were fully broken to inspect the cracked surface. Mode
I samples revealed little difference between CNT reinforced and non-reinforced samples.
Both CNT reinforced and non-reinforced samples had crack growth through the resin
layers where the initial cracks were located.
23
Figure 17.
Mode I Normalized GI Values
0.00069
-0.00081
Figure 18.
B.
Image of transverse normal strain just prior to Mode I
(opening mode) crack propagation
MODE II
Mode II testing resulted in a significant increase in GII for the samples reinforced
with CNT.
Figure 19 displays the normalized average values of GII for Phase III
specimens. Again, standard deviation is also shown in the figure. As displayed by the
standard deviation, the lowest CNT reinforced value is higher than the highest non24
reinforced value. Additionally, the average CNT reinforced GII value was 27.6% higher
than the average resin only GII value. Appendix B includes GII values for each sample.
The average value of GII varied between the Phase II and Phase III. The average
value of GII for Phase II was 83% higher than that of Phase III. There are three potential
causes for the discrepancy. First, Phase III samples underwent a post-cure treatment
while Phase II samples did not. Over time, the material properties of carbon fiber
composite may change due to continued curing of the resin. The post-cure treatment
accelerates the long term curing.
The second factor may be degradation of the
uncatalyzed, uncured resin as a function of time.
While CNT reinforced and non-
reinforced samples in each phase were constructed at the same time, the two sample sets
were fabricated several months apart. Finally, the specimen dimensions were different.
Phase III sample size conformed to the ASTM Standard, while Phase II samples were
larger. The ASTM standard is probably designed for aerospace laminates with thin layers
and unidirectional fibers or tight fabric. The 9oz woven fabric from 12K rovings may be
‘too coarse’ for a smaller specimen size, resulting in a different value for Mode II critical
energy release rate. Subsequent study should investigate the respective impact of postcure treatment, resin degradation, and sample size.
Although the quantitative values of GII were different from sample set to sample
set, the effect of CNT reinforcement remained the same. The average CNT reinforced GII
value was 27.3% higher for Phase II and 27.6% higher for Phase III.
25
Figure 19.
Mode II Normalized GII Values
The Mode II critical energy release rate calculation was then repeated using a
compliance approach. Compliance was determined from the linear region of the load
versus displacement plot. Representative plots are shown in Figures 20 and 21. A linear
regression was used to obtain the slope of the linear region.
propagation is marked with an X.
26
The point of crack
Figure 20. Representative load versus extension plot for Mode II (shear mode) testing
of non-reinforced sample (The point of crack propagation is marked with an X.)
Figure 21.
Representative load versus extension plot for Mode II testing of CNT
reinforced sample
27
Repeating the calculation using a compliance approach showed a similar
improvement in GII. The average GII value of CNT reinforced samples was 30.5% higher
than that of the non-reinforced samples. Additionally, the lowest CNT reinforced value
was higher than the highest non-reinforced value.
Since the method of locally reinforcing the samples with CNT significantly
increased the value of Mode II critical energy release rate, testing was repeated using
samples constructed via the VARTM technique since the VARTM technique is
commonly employed by industry. Similar results were obtained when testing VARTM
samples produced in Phase IV. Figure 22 displays the normalized average values of GII
for Phase IV specimens. Calculated via the compliance approach, the average GII value
of CNT reinforced samples was 31.6% higher than that of the non-reinforced samples.
The implementation of local CNT reinforcement is promising due to these consistent
results.
Figure 22.
Mode II Normalized GII Values for Phase IV samples
28
Qualitatively, the observed crack propagation was significantly different between
the CNT reinforced and non-reinforced samples. In the non-reinforced samples, crack
propagation began at the tip of the initial crack. However, this did not occur in the CNT
reinforced samples. As the load increased, a crack began to occur away from the initial
crack tip, perhaps in an area of lower CNT concentration, i.e., a weaker strength zone.
Eventually, this new crack grew to meet the initial crack.
This result was widely
observed in the CNT reinforced samples. Figures 23 and 24 display images of the
observed crack propagation. This phenomenon was observed in all phases of research. A
representative image from the Digital Image Correlation System is shown in Figure 25.
Shear strain is plotted at the onset of crack initiation since the shear stress is the cause of
crack growth in Mode II.
Initial crack tip
Initial crack propagation
Figure 23. Initial crack propagation of resin only sample
(Crack propagated from the initial crack tip.)
29
Note: No connection yet
Initial crack tip
Internal crack propagation
Figure 24. Initial crack propagation of CNT reinforced sample (The internal crack
was nucleated away from the initial crack tip. Then the internal crack grew to
meet the initial crack tip as the load increased.)
-0.03
0
Figure 25.
Plot of shear strain from Digital Image Correlation System for Mode II
(i.e., shearing mode)
After testing, the samples were fully broken to inspect the cracked surface. Mode
II crack propagation of the non-reinforced samples occurred at the interface of the initial
crack site. In some areas, the joint interface bond was broken through the resin while in
others the resin was pulled away from the fibers, as shown in Figure 26.
30
The CNT reinforced samples failed much differently. The CNT reinforced the
resin, making it stronger. It is important to note that the CNT themselves did not
fracture. The CNT bonded to the resin, blocking crack propagation. As a result, the
crack propagated through the fibers, at times through a different layer than the initial
crack layer.
The critical energy release rate for CNT reinforced samples is higher
because the crack propagated through the carbon fibers vice resin. Figure 27 shows an
image of the cracked surface.
Resin
Failure
Figure 26. Mode II crack surface of non-reinforced sample (Note the crack
propagated through resin. In some areas the resin failed and in others the resin
pulled away from the fibers.)
31
Fracture
through
neighboring
fiber layer
Broken fibers
Figure 27. Mode II crack surface of CNT reinforced sample (Note the crack
propagated through the fibers and through a neighboring fiber layer in one
region.)
CNT reinforcement was significant during Mode II failure while not significant
for Mode I. A possible explanation is given below for the application of CNT as an
interface bond. When the CNT are applied, there are two surfaces to which they bond:
cured resin and wet resin. The long polymer chains of the wet resin entangle the CNT.
While the cured resin is not necessarily a smooth surface, the CNT do not have the
opportunity to become entangled in the polymer chains because the resin is already cured.
When the sample is cured, the CNT are entrapped in the polymer chains that were wet
when the CNT were applied, as shown in Figure 28.
When a force is then applied normal to the bottom layer, the CNT have little
effect, as in Mode I testing. However, when a force is applied along the surface, such as
during Mode II testing, there is a mechanical interlocking between CNT and polymers,
which makes the bond not easily broken. Then, the crack propagates through the fibers
vice through the resin. As a result, the critical energy release rate is higher due to CNT
reinforcement.
32
Long polymer
chains – wet
resin
Bottom layer –
cured resin
CNT
Figure 28.
C.
Schematic of secondary bond with CNT
MIXED MODE I/MODE II
Mixed Mode I/Mode II testing was conducted with the intention of determining a
“best fit” formula for Mixed Mode I/Mode II calculations. However, when conducting
the testing, technical problems arose. To conduct the test, piano hinges must be affixed
to the samples to both apply load and secure the samples in the test apparatus. During the
course of the test, the epoxy used to affix the piano hinges failed. Therefore, testing
could not be completed. Crack propagation did occur in two non-reinforced samples, but
results were inconclusive. Figure 29 displays Mode I, Mode II, and Mixed Mode I/Mode
II data for non-reinforced samples.
No CNT reinforced sample data was achieved for
Mixed Mode I/Mode II testing.
33
Figure 29.
D.
Plot of Mixed Mode I/Mode II data
SEAWATER ABSORPTION EFFECTS
Phase III samples were used to determine the effects of seawater absorption on
Mode II critical energy release rate. Weight of each sample was tracked periodically
during soaking, as shown in Figure 30. The samples were deemed saturated when no
significant weight change occurred. In this case, the samples were tested after 91 days of
soak. Samples were removed from the seawater, patted dry, and tested. It should be
noted that the CNT reinforced samples absorbed slightly less seawater. During the hand
lay-up process, a small amount of CNT migrates from the interface layer to neighboring
layers. The resin near the CNT does not absorb as much seawater, resulting in a smaller
percentage weight change. However, results of the testing were not significantly affected
by the small difference in percentage weight change.
34
- Resin Only
- CNT
Figure 30.
Seawater absorption weight tracker for Phase III samples
The results of moisture effects testing were similar to Mode II testing with no
seawater absorption. CNT reinforcement resulted in a 35.6% increase in GII as shown in
Figure 31. Again, Figure 31 displays average, normalized values. Standard deviation is
also shown. Furthermore, the non-normalized values were similar to values for dry
samples. It can therefore be concluded that soaking the carbon fiber composite samples
in seawater did not affect Mode II fracture toughness. GII values for each sample are
included in Appendix C. Soaking the samples in seawater also did not affect the impact
of localized CNT reinforcement.
35
Figure 31.
Mode II Normalized GII Values for Phase III seawater soaked samples
To further study the effects of seawater absorption, samples constructed via the
VARTM technique in Phase IV were soaked and tested. Again, the samples were
deemed saturated when no significant weight change occurred. In this case, the samples
were tested after 64 days of soak. Samples were removed from the seawater, patted dry,
and Mode II testing was completed.
The percentage weight change for Phase IV samples was slightly lower than that
of Phase III samples. The difference is due to the relative concentrations of resin and
carbon fiber fabric. Phase III samples were produced via the hand lay-up technique, and
therefore contain relatively more resin than Phase IV samples, which were produced via
the VARTM method. Since seawater is absorbed by the resin, the Phase III samples had
a higher percentage weight change. Since Phase IV samples were constructed via the
VARTM technique, the non-reinforced and CNT reinforced samples absorbed nearly the
same amount of seawater. There was no difference in percentage weight change between
resin only and CNT samples, as in the Phase III samples.
36
Phase IV seawater soaked samples yielded very different results from Phase III
seawater soaked samples. During Mode II testing, the majority of samples failed in
bending, as shown in Figures 32 and 33. The seawater absorption resulted in an overall
stiffness reduction of the composite material, causing the samples to bend. The crack did
propagate after bending failure initiated. However, the bending failure resulted in a shift
of the neutral axis, meaning the initial crack was no longer on the neutral axis. When the
three point bending test is conducted, the initial crack must lie on the neutral axis to
determine pure Mode II critical energy release rate. As a result, Mixed Mode I/Mode II
crack propagation occurred, and the calculation of Mode II critical energy release rate is
no longer valid [15]. It is necessary to extract Mode I and Mode II energy release rates
from the test results. However, the calculation requires the correct data of bending failure
just before the interface crack propagation. In order to avoid the bending failure, it is
recommended to have thicker specimens for future testing. Thick specimens will allow
the interface crack to propagate before failure caused by bending stress.
Bending
Failure
Figure 32.
Bending failure of Phase IV seawater soaked sample (side view)
37
Bending
Failure
Figure 33.
E.
Bending failure of Phase IV seawater soaked sample (top view)
CNT OPTIMIZATION
The main purpose of Phase V samples was to optimize the concentration of CNT.
To achieve this goal, three concentrations of CNT were used: 5 g/m2, 7.5 g/m2, and 10
g/m2. As with all sample sets, non-reinforced samples were constructed and tested as a
reference point. Mode II testing was completed since prior phases determined CNT
reinforcement significantly affects Mode II fracture toughness. The results of Mode II
testing are shown in Figure 34 along with standard deviation. As shown, 7.5 g/m2 of
CNT is the optimal concentration, which is consistent with the previous study on CNT
compression strength improvements [9]. Again, the lowest value of GII for samples
reinforced with 7.5 g/m2 CNT is higher than the highest value of non-reinforced samples.
The GII value for each sample is included in Appendix D.
38
Figure 34.
Mode II Normalized GII Values for CNT Optimization samples
The secondary purpose of Phase V was to determine the effect of “banding” CNT.
“Banding” refers to only reinforcing a part of the interface area on the sample. All other
sample sets involved using CNT to reinforce the entire secondary bond between the top
and bottom plates. Phase V samples were only reinforced in the area extending 6 cm
from the initial crack tip. “Banding” CNT may be applicable to repair of carbon fiber
composite components when only a localized area requires reinforcement. The Mode II
critical energy release rate, as calculated via the compliance method, resulted in 18.8%
increase due to CNT reinforcement with 7.5 g/m2 CNT concentration. The drop from
roughly 30% found in previous sample sets is due to “banding” the CNT vice reinforcing
the entire secondary bond.
39
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40
VI.
CONCLUSIONS AND RECOMMENDATIONS
In conclusion, critical energy release rate, G, and crack propagation
characteristics of a pre-existing crack were studied in carbon fiber composite samples.
Five phases of research were completed, each consisting of non-reinforced samples and
samples reinforced with carbon nanotubes. Mode I (i.e., opening mode), Mode II (i.e.,
shearing mode) and Mixed Mode I/Mode II crack propagation were studied. Mode I
testing determined no significant increase in GI due to CNT reinforcement. Also, no
differences in crack propagation were observed. However, Mode II testing determined a
significant increase in GII due to CNT reinforcement. Additionally, two qualitative
differences were noted during Mode II testing as stated below:
1. CNT reinforced samples displayed crack nucleation and growth away from the
initially existing crack tip. As load increased, these cracks propagated to meet the
existing initial crack. For non-reinforced samples, crack propagation occurred from the
existing initial crack tip.
2. Crack propagation occurred across the fibers in CNT reinforced samples.
Conversely, crack propagation in non-reinforced samples occurred due to resin failure.
Additional research was conducted to determine the effects of seawater absorption
and optimize the concentration of CNT. Seawater absorption was found to have no effect
on Mode II fracture toughness. The optimal concentration of CNT was found to be 7.5
g/m2.
Finally, the VARTM technique was implemented to ensure local CNT
reinforcement is feasible using current manufacturing practices. It was determined that
the dispersed CNT remain in place while the carbon fiber layers are infused with resin.
Further research is necessary to determine the impact of CNT reinforcement in
Mixed Mode I-Mode II failure. In actual structures, the stress will rarely be purely Mode
I or Mode II. Further research is also needed to determine feasible manufacturing
practices for local CNT dispersion.
41
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42
APPENDIX A: MODE I DATA
PHASE III
Resin Only
Sample
1
2
3
4
5
GIC (N/m)
2.84E+02
2.61E+02
2.56E+02
2.29E+02
2.90E+02
CNT Reinforced
Sample
1
2
3
GIC (N/m)
2.76E+02
2.83E+02
3.17E+02
43
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44
APPENDIX B: MODE II DATA
PHASE III
Resin Only
Sample
1
2
3
4
5
6
GIIc (N/m)
1.33E+03
1.61E+03
1.52E+03
1.38E+03
1.66E+03
1.62E+03
CNT Reinforced
Sample
1
2
3
4
5
6
GIIc (N/m)
1.91E+03
2.02E+03
1.94E+03
1.80E+03
2.07E+03
2.17E+03
PHASE IV
Resin Only
Sample
1
2
3
4
5
GIIc (N/m)
1.50E+03
1.38E+03
1.48E+03
1.51E+03
1.49E+03
CNT Reinforced
Sample
1
2
3
4
GIIc (N/m)
2.00E+03
1.78E+03
1.90E+03
2.08E+03
45
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46
APPENDIX C: SEAWATER ABSORPTION EFFECTS DATA
PHASE III
Resin Only
Sample
1
2
3
4
GIIc (N/m)
1.30E+03
1.90E+03
1.28E+03
1.37E+03
CNT Reinforced
Sample
1
2
3
4
GIIc (N/m)
1.88E+03
2.06E+03
1.87E+03
2.12E+03
47
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48
APPENDIX D: PHASE V DATA
PHASE V
Resin Only
Sample
1
2
3
4
5
GIIc (N/m)
1.55E+03
1.54E+03
1.57E+03
1.66E+03
1.57E+03
5 g/m2CNT Reinforced
Sample
1
2
3
4
5
GIIc (N/m)
1.64E+03
1.47E+03
1.57E+03
1.33E+03
1.38E+03
7.5 g/m2CNT Reinforced
Sample
1
2
3
4
5
GIIc (N/m)
1.89E+03
1.85E+03
2.12E+03
1.65E+03
1.87E+03
10 g/m2CNT Reinforced
Sample
1
2
3
4
GIIc (N/m)
2.04E+03
1.94E+03
1.76E+03
1.66E+03
49
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50
LIST OF REFERENCES
[1]
A. P. Mouritz, E. Gellert, P. Burchill, K. Challis, “Review of Advanced
Composite Structures for Naval Ships and Submarines,” Composite Structures, v.
53, pp. 21-41, July 2001.
[2]
B. Jones, “VT Durability and U of Maine Fracture Program Seminar: Introduction
to Navy Joints Design,” NSWC Carderock Division, February-March 2006.
[3]
V. K. Ganesh, T. S. Choo, “Modulus Graded Composite Adherends for SingleLap Bonded Joints,” Journal of Composite Materials, v 36, pp. 1757-1767, 2002.
[4]
William D. Callister, Jr, Materials Science and Engineering: An Introduction, ed.
7, John Wiley and Sons, Inc, New York, 2007, pp. 433.
[5]
E.T. Thostenson, Z. Ren, and T. Chou, “Advances in the Science and Technology
of Carbon Nanotubes and Their Composites: A Review,” Composites Science and
Technology, v. 61, pp. 1899-1912, June 2001.
[6]
M. Wong, M. Paramsothy, X.J. Xu, Y. Ren, S. Li, and K. Liao, “Physical
Interactions at Carbon Nanotube-Polymer Interface,” Polymer, v. 44, issue 25, pp.
7757-7764, December 2003.
[7]
M. Cadek, J.N. Coleman, K.P. Ran, V. Nicolose, G. Bister, A. Fonseca, J.B.
Nagy, K. Szostzk, F. Beguin, and W.J. Blau, “Reinforcement of Polymers with
Carbon Nanotubes: The role of nanotube surface area,” Nano Letters, v. 4, no. 2,
pp. 353-356, February 2004.
[8]
L. S. Schadler, S.C. Giannaris, P.M. Ajayan, “Load Transfer in Carbon Nanotube
Epoxy Composites,” Applied Physics Letters, v. 73, no. 26, pp. 3842-3844,
December 1998.
[9]
Y. W. Kwon, R. Slaff, S. Bartlett, and T. Greene, “Enhancement of Composite
Scarf Joint Interface Strength through Carbon Nanotube Reinforcement,” Journal
of Materials Science, doi: 10.1007/s10853-008-2689-8, 2008
[10]
ASTM Standard D 5528-01, “Mode I Interlaminar Fracture Toughness of
Unidirectional Fiber-Reinforced Polymer Matrix Composites,” March 2002.
[11]
J. R. Reeder, “An Evaluation of Mixed-Mode Delamination Failure Criteria,”
NASA Technical Memorandum 104210, February 1992.
51
[12]
M. Todo, T. Nakamura, K. Takahashi, “Effects of Moisture Absorption on the
Dynamic Interlaminar Fracture Toughness of Carbon/Epoxy Composites,”
Journal of Composite Materials, v 34, pp. 630-648, 2000.
[13]
ASTM Standard D 6671/D 6671M-06, “Standard Test Method for Mixed Mode IMode II Interlaminar Fracture Toughness of Unidirectional Fiber Reinforced
Polymer Matrix Composites,” March 2006.
[14]
ASTM Standard D 1141-98, “Standard Practice for the Preparation of Substitute
Ocean Water,” August 2008.
[15]
J. G. Williams, “On the Calculation of Energy Release Rates for Cracked
Laminates,” International Journal of Fracture, v 36, pp. 101-119, 1988.
52
INITIAL DISTRIBUTION LIST
1.
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2.
Dudley Knox Library
Naval Postgraduate School
Monterey, California
3.
Graduate School of Engineering and Applied Sciences
Naval Postgraduate School
Monterey, California
4.
Erik A. Rasmussen
Naval Surface Warfare Center Carderock Division
West Bethesda, Maryland
5.
Scott W. Bartlett
Naval Surface Warfare Center Carderock Division
West Bethesda, Maryland
6.
Douglas C. Loup
Naval Surface Warfare Center Carderock Division
West Bethesda, Maryland
7.
Chris A. Hicks
Northrop Grumman Ship Systems
Gulfport, Mississippi
8.
Joe Johnson
Integrated Composites Inc.
Marina, California
9.
John Dickie
Integrated Composites Inc.
Marina, California
10.
John McWaid
Integrated Composites Inc.
Marina, California
11.
Professor Young W. Kwon
Naval Postgraduate School
Monterey, California
53
12.
Professor and Chairman Knox T. Millsaps
Naval Postgraduate School
Monterey, California
13.
Susan D. Faulkner
Naval Postgraduate School
Monterey, California
54
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