EFFECT OF FIBER LOADING ON MECHANICAL BEHAVIOR OF

EFFECT OF FIBER LOADING ON MECHANICAL BEHAVIOR OF
EFFECT OF FIBER LOADING ON MECHANICAL BEHAVIOR OF
CHOPPED GLASS FIBER REINFORCED POLYMER COMPOSITES
A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF
Bachelor of Technology in Mechanical Engineering
BY
BIJESH K
(Roll Number: 10603052)
DEPARTMENT OF MECHANICAL ENGINEERING
NATIONAL INSTITUTE OF TECHNOLOGY
ROURKELA 769008
May 2010
EFFECT OF FIBER LOADING ON MECHANICAL BEHAVIOR OF
CHOPPED GLASS FIBER REINFORCED POLYMER COMPOSITES
A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF
Bachelor of Technology in Mechanical Engineering
BY
BIJESH K
(Roll Number: 10603052)
Under the guidance of
Prof. Sandhyarani Biswas
DEPARTMENT OF MECHANICAL ENGINEERING
NATIONAL INSTITUTE OF TECHNOLOGY
ROURKELA 769008
May 2010
DEPARTMENT OF MECHANICAL ENGINEERING
NATIONAL INSTITUTE OF TECHNOLOGY
ROURKELA 769008
CERTIFICATE
This is to certify that the thesis entitled “Effect of Fiber Loading on
Mechanical Behavior of Chopped Glass Fiber Reinforced Polymer
Composites” submitted by Bijesh K (Roll Number: 10603052) in partial
fulfillment of the requirements for the award of Bachelor of Technology in the
department of Mechanical Engineering, National Institute of Technology,
Rourkela is an authentic work carried out under my supervision and guidance.
To the best of my knowledge, the matter embodied in the thesis has not
been submitted to elsewhere for the award of any degree.
Place: Rourkela
Date:
Prof. Sandhyarani Biswas
Mechanical Engineering Department
National Institute of Technology
Rourkela-769008
i
DEPARTMENT OF MECHANICAL ENGINEERING
NATIONAL INSTITUTE OF TECHNOLOGY
ROURKELA 769008
ACKNOWLEDGEMENT
It gives me immense pleasure to express my deep sense of gratitude to my
supervisor Prof. Sandhyarani Biswas for her invaluable guidance, motivation,
constant inspiration and above all for her ever co-operating attitude that
enabled me in bringing up this thesis in the present form.
I am extremely thankful to Prof. R. K. Sahoo, Head, Department of
Mechanical Engineering and Prof. K. P. Maity, Course Coordinator for their
help and advice during the course of this work.
I express my sincere gratitude to Prof. B. B Verma, Head, Metallurgical and
Materials Engineering Department for providing the necessary facilities in the
department.
I thankful to Sri Rajesh Pattnayak and Sri Hembram of Metallurgical and
Materials Engineering Department for their support & help during my
experimental work.
I am greatly thankful to all the staff members of the department and all my well
wishers, class mates and friends for their inspiration and help.
Date:
Bijesh K
Roll No: 10603052
ii
ABSTRACT
Chopped strand mat glass fibre reinforced polymer composites is widely used
in many industrial applications particularly in the automotive industry due to
advantages such as low weight, ease of processing, price and noise
suppression. Although a great deal of work has been reported in the literature
which discuss the mechanical behavior of fiber reinforced polymer composites,
however, very limited work has been done on effect of fiber loading on
mechanical behavior of chopped glass fiber reinforced epoxy composites.
Against this background, the present research work has been undertaken, with
an objective to explore the potential of chopped glass fiber as a reinforcing
material in polymer composites and to investigate its effect on the mechanical
behavior of the resulting composites. The present research work thus aims to
develop chopped glass fiber based polymer composites and study the influence
of fiber loading on their mechanical behavior. Finally the morphology of
fractured surfaces is examined by using scanning electron microscopy (SEM)
after tensile, impact and flexural tests.
iii
CONTENTS
Chapter No.
Chapter 1
Description
1. INTRODUCTION
Page No.
1-10
1.1. Overview of Composite Materials
1.2. Classification of composites
1.3. Advantages of Composites
1.4 Applications of composites
1.5 Fiber Reinforced Polymer Composites
Chapter 2
2. LITERATURE SURVEY
11-14
2.1 Objectives of the Research Work
Chapter 3
3. MATERIALS AND METHODS
15-20
3.1. Specimen Preparation
3.2. Mechanical Testing
3.3. Scanning Electron Microscopy
Chapter 4
4. MECHANICAL CHARACTERISTICS OF
21-28
COMPOSITES: RESULTS & DISCUSSIONS
4.1. Mechanical Characteristics of Composites
4.2. Effect of Fiber Loading on Micro-hardness
4.3. Effect of Fiber Loading on Tensile Properties
4.4. Effect of Fiber Loading on Flexural Strength
4.5. Effect of Fiber Loading on Impact Strength
4.6. Surface Morphology of the Composites
Chapter 5
5. CONCLUSIONS
29-30
5.1. Scope for Future Work
REFERENCES
31-36
iv
LIST OF TABLES
Table 3.1. Designation of Composites
Table 4.1. Mechanical properties of the composites
v
LIST OF FIGURES
Figure 3.1 Chopped glass fiber reinforced epoxy composite
Figure 3.2 Tested specimens
Figure 3.3 Experimental set up and loading arrangement for the specimens for
tensile test and three points bend test.
Figure 3.4 SEM Set up
Figure 4.1 Effect of fiber loading on micro-hardness of the composites
Figure 4.2 Effect of fiber loading on tensile strength of composites
Figure 4.3 Effect of fiber loading on tensile modulus of composites
Figure 4.4 Effect of fiber loading on flexural strength of composites
Figure 4.5 Effect of fiber loading on impact strength of composites
Figure 4.6 SEM of fracture surfaces of all the chopped glass fiber reinforced
epoxy composites
vi
Chapter 1
INTRODUCTION
1
CHAPTER 1
INTRODUCTION
1.1.
Overview of composite materials
The concept of composite materials is ancient: to combine different materials
to produce a new material with performance unattainable by the individual
constituents. An example is adding straw to mud for building stronger mud
walls. Some more recent examples, but before engineered materials became
prominent, are carbon black in rubber, steel rods in concrete, cement/asphalt
mixed with sand, fiberglass in resin etc. In nature, examples abound: a coconut
palm leaf, cellulose fibers in a lignin matrix (wood), collagen fibers in an
apatite matrix (bone) etc.
A composite material consists of two or more physically and/or chemically
distinct, suitably arranged or distributed phases, with an interface separating
them. It has characteristics that are not depicted by any of the components in
isolation. Most commonly, composite materials have a bulk phase, which is
continuous, called the matrix, and one dispersed, non-continuous, phase called
the reinforcement, which is usually harder and stronger. The function of
individual components has been described as:
• Matrix phase
The primary phase, having a continuous character, is called matrix. Matrix is
usually more ductile and less hard phase. It holds the dispersed phase and
shares a load with it.
• Dispersed (reinforcing) phase
The second phase (or phases) is embedded in the matrix in a discontinuous
form. This secondary phase is called dispersed phase. Dispersed phase is
usually stronger than the matrix, therefore it is sometimes called reinforcing
phase.
2
Many of common materials(metal alloys, doped Ceramics and Polymers mixed
with additives) also have a small amount of dispersed phases in their structures,
however they are not considered as composite materials since their properties
are similar to those of their base constituents (physical properties of steel are
similar to those of pure iron).
There are two classification systems of composite materials. One of them is
based on the matrix material (metal, ceramic, polymer) and the second is based
on the material structure:
1.2. Classification of composites
 Based on matrix material
Metal
Matrix Composites (MMC):
Metal
Matrix Composites are
composed of a metallic matrix (aluminum, magnesium, iron, cobalt, copper)
and a dispersed ceramic (oxides, carbides) or metallic (lead, tungsten,
molybdenum) phase. Metal Matrix Composites have many advantages over
monolithic metals like higher specific modulus, higher specific strength, better
properties at elevated temperatures, and lower coefficient of thermal expansion.
Because of these attributes metal matrix composites are under consideration for
wide range of applications viz. combustion chamber nozzle (in rocket, space
shuttle), housings, tubing, cables, heat exchangers, structural members etc.
Ceramic
Matrix Composites (CMC):
Ceramic
Matrix Composites are
composed of a ceramic matrix and embedded fibers of other ceramic material
(dispersed phase). One of the main objectives in producing ceramic matrix
composites is to increase the toughness. Ceramic fibers, such as Alumina and
SiC (silicon carbide) are advantageous in very high temperature applications,
and also where environmental attack is an issue. Since ceramics have poor
properties in tension and shear, most applications as reinforcement are in the
particulate form (e.g. zinc and calcium phosphates) Ceramic Matrix
Composites (CMC’s)
used in very high temperature environments, these
3
materials use a ceramic as the matrix and reinforce it with short fibres, or
whiskers such as those made from silicon carbide and boron nitride.
Polymer Matrix Composites (PMC):
Most commonly used matrix
materials are polymeric. The reasons for this are two-fold. In general the
mechanical properties of polymers are inadequate for many structural purposes.
In particular their strength and stiffness are low compared to metals and
ceramics. These difficulties are overcome by reinforcing other materials with
polymers. Secondly the processing of polymer matrix composites need not
involve high pressure and does not require high temperature. Also equipments
required for manufacturing polymer matrix composites are simpler. For this
reason polymer composites developed rapidly and soon became popular for
structural applications. Polymer composites are used because overall properties
of the composites are superior to those of the individual polymers. They have a
greater elastic modulus than the neat polymer but are not as brittle as ceramics.
Polymeric matrix composites are composed of a matrix from thermoset
(unsaturated
polyester,
epoxy
or
thermoplastic
polycarbonate,
polyvinylchloride, nylon, polystyrene and embedded glass, carbon, steel or
Kevlar fibers (dispersed phase).
The potential applications of polymer composites include consumer goods
(sewing machines, doors, bathtubs, tables, chairs, computers, printers, etc),
sporting goods industry (golf shafts, tennis rackets, snow skis, fishing rods,
etc.), aerospace industry (doors, horizontal and vertical stabilizers, wing skins,
fin boxes, flaps, and various other structural components), marine applications
(passenger ferries, power boats, buoys, etc.), automotive industry (bumper
beam, seat/load floor, hood radiator support, roof panel and land transport
systems like cars, trucks and bus bodies, railway coach components, containers
and two and three wheelers ), construction and civil structures (bridges,
columns doors, windows and partitions and for translucent roofing sheets,
prefabricated modular
houses and buildings etc.), industrial applications
4
(industrial rollers and shafts, bushings, pump and roller bearings, pistons, robot
arms and others).
 Based on reinforcing material structure
Classification of composites: three main categories:
•
particle-reinforced (large-particle and dispersion-strengthened)
•
fiber-reinforced (continuous (aligned) and short fibers (aligned or random)
•
structural (laminates and sandwich panels)
 Particulate Composites:
Particulate Composites consist of a
matrix
reinforced by a dispersed phase in form of particles. These are the cheapest
and most widely used. They fall in two categories depending on the size of
the particles:
 Composites with random orientation of particles.
 Composites with preferred orientation of particles.
Dispersed phase of these materials consists of two-dimensional flat platelets
(flakes), laid parallel to each other.
 Fibrous Composites:
Short fiber reinforced composites:
Short-fiber reinforced composites consist of a matrix reinforced by a dispersed
phase in form of discontinuous fibers (length < 100*diameter). They are
classified as
 Composites with random orientation of fibers.
 Composites with preferred orientation of fibers.
Long-fiber reinforced composites:
Long-fiber reinforced composites consist of a matrix reinforced by a dispersed
phase in form of continuous fibers.
 Unidirectional orientation of fibers.
 Bidirectional orientation of fibers (woven).
5
 Laminate Composites
When a fiber reinforced composite consists of several layers with different
fiber orientations,it is called multilayer composite.
1.3. Advantages of Composites
Advantages of composites over their conventional counterparts are the ability
to meet diverse design requirements with significant weight savings as well as
strength-to-weight ratio. Some advantages of composite materials over
conventional ones are as follows:
• Tensile strength of composites is four to six times greater than that of
steel or aluminium (depending on the reinforcements).
• Improved torsional stiffness and impact properties.
• Higher fatigue endurance limit (up to 60% of ultimate tensile strength).
• 30% - 40% lighter for example any particular aluminium structures
designed to the same functional requirements.
• Lower embedded energy compared to other structural metallic materials
like steel, aluminium etc.
• Composites are less noisy while in operation and provide lower
vibration transmission than metals.
• Composites are more versatile than metals and can be tailored to meet
performance needs and complex design requirements.
• Long life offer excellent fatigue, impact, environmental resistance and
reduce maintenance.
• Composites enjoy reduced life cycle cost compared to metals.
• Composites exhibit excellent corrosion resistance and fire retardancy.
• Improved appearance with smooth surfaces and readily incorporable
integral decorative melamine are other characteristics of composites.
• Composite parts can eliminate joints / fasteners, providing part
simplification and integrated design compared to conventional metallic
parts.
6
1.4. Applications of composites
Composites, it is the fastest growing "materials" market segment. Sporting
goods, Aircraft, automobile, shipbuilding, are just a few examples.
Tennis rackets, golf clubs, bumpers, door panels, dashboard, even engine
components of modern automobiles; look closely at a Boeing 777etc. Some
applications are given below,
o Paints and coatings
o Electrical systems and electronics
o Aircraft industry, Ex: Doors and elevators
o Consumer and marine applications
o Aerospace applications
o Chemical industry, Ex: Tanks, Pipes, Pressure vessels
o Automotive body frames, engine components.
1.5. Fiber Reinforced Polymer Composites
Because of their favourable properties (e.g. high specific tensile and
compressive strength, controllable electrical conductivity, low coefficient of
thermal expansion, good fatigue resistance and suitability for the production of
complex shape materials), fibre-reinforced composites are very widely used.
According to papers, they have become the alternatives of conventional
structural materials such as, steel, wood or metals in many applications.
Typical areas of composite applications are car industry, aircraft fabrication,
wind power plant, boats, ships, etc. During the human history, composites
made occasionally large breakthroughs in construction and other materials.
Nowadays, the situation has been the same with modern fibre-reinforced
composites for which mass production of polymers provided stable background
[1–8].
Generally, reinforced plastic composites consist of different reinforcement
materials in a polymer matrix that are classified as thermoplastic and
thermoset. Thermosets are polymers which undergo a curing reaction or
7
chemical cross-linking where a resin with a relatively low molecular weight is
converted into another with a high molecular weight. It is important to remark
that thermosets are much more rigid than the commonly used bulk
thermoplastics (e.g. HDPE, LDPE, PP, PS, etc.). The choice of thermoset
matrices is considerable and the commonest groups of them are polyesters,
epoxies, phenolics and polyimides [9]. Epoxys are the most widely used
thermosetting resins because of its easy processing. Polyesters could not be
applied for technological purposes without reinforcing because of low strength
and brittleness, but they are intensively used for composite matrices [10,11].
The glass-fibre (GF) composites are the most widespread among fibrereinforced materials due to their favourable mechanical and economical
characteristics. For industrial application, the E- and S-type glass fibres are the
most commonly used because they have the most favourable cost-mechanical
properties relationships. Thermoset composites have been applied in the last
1940s in aircraft industry for the first time. Those materials were laminated
polyester composites, and the first application was the cover of radar antennas
because there was a need for such non-metallic materials that allowed radio
waves through free from distortions. The manufactured parts were found to
have better weight/volume ratio than the ones made from metallic materials.
Since then thermoset composites have been applied as construction materials.
Current civil aircraft applications have concentrated on replacing the secondary
structure with fibrous composites, where the reinforcement material has either
been carbon, glass, Kevlar, or hybrids of those [12].A great deal of work has
been published on glass fiber reinforced polymer composites. However, very
limited work has been done on effect of fiber loading on mechanical behaviour
of chopped glass fiber reinforced epoxy composites. Against this background,
the present research work has been undertaken, with an objective to explore the
potential of chopped glass fiber as a reinforcing material in polymer
composites and to investigate its effect on the mechanical behaviour of the
resulting composites. The present work thus aims to develop this new class of
8
fibre based polymer composites with different fiber loading and to analyse their
mechanical behaviour by experimentation.
******
9
Chapter 2
LITERATURE SURVEY
10
CHAPTER 2
LITERATURE SURVEY
This chapter outlines some of the recent reports published in literature on
mechanical behaviour of glass fiber reinforced polymer composites with
special emphasis on chopped glass fiber reinforced polymer composites.
Composite materials are nowadays employed in many engineering structures,
such as helicopter and wind turbine rotor blades, boat hulls, and buildings,
implying the application of variable loadings for long time spans. This raises
the question of their fatigue behaviour, whose importance is increasingly
appreciated also in the fixed-wing aircraft industry, where fatigue life has not
been a major issue in the past, due to the low working strains used in practical
components. Significant efforts have been devoted toward the use of
lightweight structures to increase energy efficiencies in various industrial and
commercial sectors [13-16].
Fiber-reinforced composites have found numerous applications in aerospace
industry for their high specific strength and specific stiffness [17]. However,
the cost of traditional composite materials is also considerable. Random
chopped fiber-reinforced composites (RFCs) have emerged as promising
alternative materials for lightweight structures due to their low cost and mass
production capabilities. Their potential application in, for example, automotive
industry has been documented [13, 14, 16]. In order to expand their use,
accurate material characterization is required. The main difficulty in fully
exploring the capabilities of the RFCs lies in the apparent impediment to
effectively model their geometry at the micro-level for high fiber volume ratios
(35-40%). This difficulty becomes even more obvious at high aspect ratio
fibers.
11
Glass-fiber-reinforced composites (or glass-fiber reinforced plastics, GFRP)
have seen limited use in the building and construction industry for decades [1820]. Because of the need to repair and retrofit rapidly deteriorating
infrastructure in recent years, the potential for using fiber-reinforced
composites for a wider range of applications is now being realized [21-26].
These materials offer excellent resistance to environmental agents and fatigue
as well as the advantages of high stiffness-to weight and strength-to-weight
ratios when compared to conventional construction materials. However, one of
the obstacles preventing the extensive use of composites has been a lack of
long-term durability and performance data. Although there have been
numerous studies of fatigue and environmental fatigue with composite
materials in the past three or four decades, most of those devoted to structural
composites have focused on aerospace applications. Reviews on the fatigue
behavior for composite materials can be found in literature [27-30].
Mechanical properties of fibre-reinforced composites are depending on the
properties of the constituent materials (type, quantity, fibre distribution and
orientation, void content). Beside those properties, the nature of the interfacial
bonds and the mechanisms of load transfer at the interphase also play an
important role. If the building parts of composites differ in physical form and in
chemical composition either, only a weak interaction can be developed at the
interface. For improving the adhesion between the matrix and the fibres, there
are varieties of modification technique depending on the fibre and matrices
type.
The reported studies on short fiber reinforced composites by different
investigators are found to have focused mostly on the strength properties of the
composites. Beyerlein et al. [31] have described the influence of fiber shape in
short fiber composites. Kari et al. [32] have evaluated numerically the effective
material properties of composites with randomly distributed short fibers. Hine
et al. [33] have presented a numerical simulation of the effects of fiber length
distribution on the elastic and thermoelastic properties of short fiber
12
composites. Fu et al. [34] have studied the flexural properties of misaligned
short fiber reinforced polymers by taking into account the effects of fiber
length and fiber orientation. Recently, efforts to reduce the weight of
automobiles by the increased use of plastics and their composites, have led to a
growing penetration of short-fibre-reinforced injection-moulded thermoplastics
into fatigue-sensitive applications [35,36]. In general, short-fibre/polymermatrix composites are much less resistant to fatigue damage than the
corresponding continuous-fibre-rein- forced materials, mainly because the
weak matrix has to sustain a greater proportion of the cyclic load [37].
Chopped strand mat (CSM) glass fibre-reinforced polyester (GRP) is widely
used in pressure vessel and pipe line systems for the chemical industry. Glass
mat thermoplastics (GMTs) are being increasingly used in the automotive
industry due to advantages such as low weight, ease of processing, price and
noise
suppression
[38].
The
hot
stamping
of
glass-mat-reinforced
thermoplastics, GMT, is of great interest to the automotive industry [39-44].
Few reaserch has been done on chopped glass fiber reinforced polymer
composites. Durability based design criteria for a chopped glass fiber
automotive structural composite has been studied by Corum et al. [45].
Interlaminar shear fracture of chopped strand mat glass fibre reinforced
polyester laminates has been studied by Zhang et al. [46]. Monotonic and
tension–tension fatigue tests were carried out on E-glass chopped-strandmat/polyester composites, varying the flexibiliser content by weight in the
matrix in the range 0-30% [47]. In a previous paper [48], the static and fatigue
behavior of a polyester resin with different proportions of flexibiliser was
analysed. In this work, the same resin system considered was used to fabricate
four chopped-strandmat/polyester (CSM) composites, which were subjected to
monotonic and repeated-tension fatigue tests. The fibre volume fraction was
kept low, to highlight the role played by the matrix in the mechanical response
of the composite.
13
A study on numerical generation of a random chopped fiber composite RVE
and its elastic properties has been done by Pan et. al. [49]. A study on theory of
fabrication-induced anisotropy of chopped-fibre/resin panels martin has been
done by Martin [50]. A study on chopped glass and recycled newspaper as
reinforcement fibers in injection molded poly (lactic acid) (PLA) composites
has been done [51].
Although a great deal of work has been reported in the literature which discuss
the mechanical behavior of fiber reinforced polymer composites, however, very
limited work has been done on effect of fiber loading on mechanical behaviour
of chopped glass fiber reinforced epoxy composites. Against this background,
the present research work has been undertaken, with an objective to explore the
potential of chopped glass fiber as a reinforcing material in polymer
composites and to investigate its effect on the mechanical behaviour of the
resulting composites. The present work thus aims to develop this new class of
polymer composites with different fiber loading and to analyse their
mechanical behaviour by experimentation.
2.1. Objectives of the Research Work
The objectives of the project are outlined below.
• To develop a new class of chopped glass fiber based polymer
composites.
• To study the effect of fiber loading on mechanical behaviour of chopped
glass fiber reinforced epoxy based composites.
• Evaluation of mechanical properties such as: tensile strength, flexural
strength, tensile modulus, micro-hardness, impact strength etc.
******
14
Chapter 3
MATERIALS AND METHODS
15
CHAPTER 3
MATERIALS AND METHODS
This chapter describes the details of processing of the composites and the
experimental procedures followed for their mechanical characterization. The
raw materials used in this work are
1. E glass fiber (chopped strand mat)
2. Epoxy resin
3. Hardener
3.1. Specimen preparation
Chopped strand mat glass fiber (Figure 3.1) is reinforced with Epoxy LY 556
resin, chemically belonging to the ‘epoxide’ family is used as the matrix
material. The glass fiber, epoxy resin and the hardener are supplied by Ciba
Geigy India Ltd. The fabrication of the composites is carried out through the
hand lay-up technique. The low temperature curing epoxy resin (Araldite LY
556) and corresponding hardener (HY951) are mixed in a ratio of 10:1 by
weight as recommended. Composites of three different compositions such as
30wt%, 40wt% and 50wt% glass fiber are made and the designations of these
composites are given in Table 3.1. The cast of each composite is cured under a
load of about 50 kg for 24 hours before it removed from the mould. Then this
cast is post cured in the air for another 24 hours after removing out of the
mould. Specimens of suitable dimension are cut using a diamond cutter for
mechanical testing. Utmost care has been taken to maintain uniformity and
homogeneity of the composite.
16
Figure 3.1 Chopped glass fiber reinforced epoxy composite
Table 3.1 Designation of Composites
Composites
Compositions
C1
Epoxy (70wt%)+ glass fiber (30wt%)
C2
Epoxy (60wt%)+ glass fiber (40wt%)
C3
Epoxy (50wt%)+ glass fiber (50wt%)
3.2. Mechanical Testing
After fabrication the test specimens were subjected to various mechanical tests
as per ASTM standards. The tensile test and three-point flexural tests of
composites were carried out using Instron 1195. The tensile test is generally
performed on flat specimens. A uniaxial load is applied through both the ends.
The ASTM standard test method for tensile properties of fiber resin composites
has the designation D 3039-76. Micro-hardness measurement is done using a
Leitz micro-hardness tester. A diamond indenter, in the form of a right pyramid
with a square base and an angle 1360 between opposite faces, is forced into the
material under a load F. The two diagonals X and Y of the indentation left on
the surface of the material after removal of the load are measured and their
arithmetic mean L is calculated. In the present study, the load considered F =
24.54N. Low velocity instrumented impact tests are carried out on composite
specimens. The tests are done as per ASTM D 256 using an impact tester. The
charpy impact testing machine has been used for measuring impact strength.
17
Figure 3. 2 shows the tested specimens for flexural, tensile, impact and
hardness test respectively. Figure 3 .3 shows the experimental set up and
loading arrangement for the specimens for three point bend test.
(a)
(b)
(c)
(d)
Figure 3.2 Tested specimens
18
Figure 3.3 Experimental set up and loading arrangement for the specimens for
tensile test and three points bend test.
3.3. Scanning electron microscopy (SEM)
The scanning electron microscope (SEM) JEOL JSM-6480LV (Figure 3. 4)
was used to identify the tensile fracture morphology of the composite samples.
The surfaces of the composite specimens are examined directly by scanning
electron microscope JEOL JSM-6480LV. The samples are washed, cleaned
thoroughly, air-dried and are coated with 100 Å thick platinum in JEOL sputter
ion coater and observed SEM at 20 kV. Similarly the composite samples are
mounted on stubs with silver paste. To enhance the conductivity of the
samples, a thin film of platinum is vacuum-evaporated onto them before the
photomicrographs are taken.
19
Figure 3.4 SEM Set up
******
20
Chapter 4
MECHANICAL CHARACTERISTICS OF
COMPOSITES: RESULTS & DISCUSSIONS
21
CHAPTER 4
MECHANICAL CHARACTERISTICS OF COMPOSITES: RESULTS &
DISCUSSIONS
This chapter presents the mechanical properties of the chopped glass fiber
reinforced epoxy composites prepared for this present investigation. Details of
processing of these composites and the tests conducted on them have been
described in the previous chapter. The results of various characterization tests
are reported here. This includes evaluation of tensile strength, flexural strength,
impact strength and micro-hardness has been studied and discussed. The
interpretation of the results and the comparison among various composite
samples are also presented.
4.1. Mechanical Characteristics of Composites
The characterization of the composites reveals that the fiber loading is having
significant effect on the mechanical properties of composites. The properties of
the composites with different fiber loading under this investigation are
presented in Table 4.1.
Table 4.1 Mechanical properties of the composites
Composites Hardness
(Hv)
Tensile
Tensile
Flexural
Impact
strength
modulus
strength
energy
(MPa)
(GPa)
(MPa)
(KJ/m2)
C1
18.4
83.20
5.81
56.12
12
C2
19.1
137.70
5.95
107.20
14.5
C3
24.2
122.40
6.23
160.30
15.5
22
4.2. Effect of fiber loading on Micro-hardness
The measured hardness values of all the three composites are presented in
Figure 4.1. It can be seen that the hardness value of chopped glass fiber
reinforced epoxy composites is increasing gradually with the fiber content.
With the increase in fiber loading from 30wt% to 50wt% the hardness is found
to have increased from about 18.4 Hv to 24.2Hv.
30
25
20
Micro 15
Hardness
10
(Hv)
5
0
30%
40%
50%
Fiber Loading(wt.%)
Figure 4.1 Effect of fiber loading on micro-hardness of the composites
4.3. Effect of fiber loading on Tensile Properties
The test results for tensile strengths and moduli are shown in Figures 4.2 and
4.3, respectively. It can be seen that the tensile strength of chopped glass fiber
reinforced epoxy composite is more in case of composite with fiber content up
to 40%. However further increase in fiber content the tensile strength value is
decreasing. The increase of tensile strength may be due to the good
compatibility of fiber and epoxy resin. But further increase in fiber content the
strength is decreasing due to epoxy resin is not sufficient to wet the fiber. From
Figure 4.3 it is clear that the fiber content has significant effect on tensile
modulus of composites. Previous reports [18, 19] demonstrated that normally
the fibers in the composite restrain the deformation of the matrix polymer,
reducing the tensile strain. So even if the strength decreases with fiber addition,
23
the tensile modulus of the composite is expected to increase. The same result
has been observed for the chopped glass fiber reinforced epoxy composites.
160
140
120
100
Tensile 80
Strength
60
(MPa)
40
20
0
30%
40%
50%
Fiber Loading(wt.%)
Figure 4.2 Effect of fiber loading on tensile strength of composites
8000
7000
6000
5000
Tensile 4000
Modulus 3000
(MPa)
2000
1000
0
30%
40%
50%
Fiber Loading(Wt.%)
Figure 4.3 Effect of fiber loading on tensile modulus of composites
4.4. Effect of Fiber loading on Flexural Strength
Figure 4.4 shows the comparison of flexural strengths of the composites
obtained experimentally from the bend tests. Composite materials used in
structures are prone to fail in bending and therefore the development of new
composites with improved flexural characteristics is essential. It is interesting
to note that the fiber content has significant effect on tensile modulus of
24
composites. From the figure it is clear that the flexural strength value of
chopped glass fiber reinforced epoxy composites is increasing significantly
with the fiber loading.
180
160
140
120
100
Flexural 80
Strength
60
(MPa)
40
20
0
30%
40%
50%
Fiber Loading(Wt.%)
Figure 4.4 Effect of fiber loading on flexural strength of composites
4.5. Effect of fiber loading on Impact Strength
Effect of fiber loading on impact energy values of different composites is
shown in Figure 4.5. High strain rates or impact loads may be expected in
many engineering applications of composite materials. The suitability of a
composite for such applications should therefore be determined not only by
usual design parameters, but by its impact or energy absorbing properties.
From the figure it is observed that resistance to impact loading of chopped
glass fiber reinforced epoxy composites is increasing gradually with the
increase of fiber loading.
25
Impact
Energy
(KJ/m2)
18
16
14
12
10
8
6
4
2
0
30%
40%
50%
Fiber Loading(wt.%)
Figure 4.5 Effect of fiber loading on impact strength of composites
4.5. Surface morphology of the composites
The fracture surfaces study of chopped glass fiber reinforced epoxy composite
after the tensile test, flexural test and impact test has been shown in Figures.
From the above analysis Figure 4.2, we can conclude that the Tensile strength
chopped glass fiber reinforced composites depends on the interfacial properties
between fiber and matrix. During the Failure process of chopped glass fiber /
matrix interface, adhesion bonding prevails prior to debonding whereas
frictional stress provides resistance to slip and pullout after debonding.
Therefore, both the bond stress and frictional stress betweenfibers and matrix
have effects on the chopped glass
fiber pull
-out force and pull-out energy
(Figure 4.6a). However, the interfacial adhesion stress can be improved by the
increase of fiber loading from 30wt% to 40wt% as shown in Figure 4.6b. But
on further increase of fiber loading from 40wt% to 50wt% the tensile strength
decrease drastically as shown in Figure 4.2, which will be decreased because of
the decreasing contact surface between fiber and matrix. Thus, it appears that
the decrease of the tensile stress for some composites can be explained by the
decrease of the interfacial bond stress.
Figures. 4.6c shows the fracture surface SEM micrographs of flexural strength
test specimens with 30wt.% of the chopped glass fiber loading. As seen in the
26
Figure 4.6c, fibres normal to the loading direction were observed to fail due to
expansions of other fiber as in chopped glass fibers are arranged randomly.
Also, matrix cracking and fiber-fiber interface cracks are visible for the
specimens. Figure 4.6d shows the fracture surface of the composite specimens
with 50wt.% fiber loading. As seen in the Figure 4.6d,fibres along the loading
directions were observed to buckle and fractured fibres formed fiber kinks
locally. Also, longitudinal splitting, along the interlaminar region is visible. It
was also observed between tensile and flexural test, the strength property of the
chopped glass fiber composites may not shows promising results whereas,
flexural strength results increases with increase in fiber loading. This may be
due to the lower inter laminar strength of the composites made with chopped
glass fiber reinforced composites.
Examination of impacted specimens (Figure 4.6e) reveals a very similar
damage pattern to that seen in tensile samples (Figures 4.6a,b), in particular,
mainly matrix damage at energies of up to 15.5kJ/m2 (for 50wt% fiber
loading), accompanied by fiber breakage at 12kJ/m 2 (or more) for 30wt% fiber
loading (Figure 4.6e). Ignoring boundary conditions and misalignment of
fibers, the fiber structure of can be considered similar when impacted in the
out-of-plane direction. Thus the damage caused by low velocity impact in these
three composites would be expected to be very similar for the same impact
energies. Although the impact damage is very similar for the two different fiber
geometries, the effect of this damage on subsequent tensile tests is significantly
different, due to the highly anisotropic nature of thesefiber reinforced This is
consistent with the observation of matrix damage (cracking, debonding and
delamination) at low energies (up to about 12kJ/m2), accompanied byfiber
breakage at impact energies of 15.5kJ/m2 or greater as shown in Fig. 4.6f. The
matrix damage caused by lower energy impact would be expected in case of
30wt% fiber loading to affect the matrix dominated material, since the matrix
also plays major load bearing supports to the composites. However, such
matrix damage would not be expected to have a signi
ficant e ffect on the fiber
27
dominated carrying most of the load remain undamaged. This would explain
ficant
the observed critical impact energy of 12kJ/m2, below which no signi
reduction in residual properties is evident. The higher the impact energy, the
more fiber breakage occurred, and lower the residual tensile strength of the
composites.
(a)
(b)
(c)
(d)
(e)
(f)
Figure 4.6 Scanning electron micrographs of chopped glass fiber reinforced
epoxy specimens after tensile, flexural and impact testing.
******
28
Chapter 5
CONCLUSIONS
29
CHAPTER 5
CONCLUSIONS
This experimental investigation of mechanical behaviour of chopped glass fiber
reinforced epoxy composites leads to the following conclusions:
 The successful fabrication of a chopped glass fiber reinforced epoxy
composites with different fiber loading has been done by simple hand
lay-up technique.
 It has been noticed that the mechanical properties of the composites
such as micro-hardness, tensile strength, flexural strength, impact
strength etc. of the composites are also greatly influenced by the fiber
loading.
 The fracture surfaces study of chopped glass fiber reinforced epoxy
composites after the tensile test, flexural test and impact test has been
done. From this study it has been concluded that the poor interfacial
bonding is responsible for low mechanical properties.
5.1. Scope for Future Work
There is a very wide scope for future scholars to explore this area of research.
This work can be further extended to study other aspects of such composites
like effect of fiber type, fiber orientation, loading pattern, fiber treatment on
mechanical behavior of chopped glass fiber reinforced polymer composites and
the resulting experimental findings can be similarly analyzed.
******
30
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36
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