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International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 4, July - August (2013) © IAEME
274
A STUDY ON FLEXURAL STRENGTH OF HYBRID POLYMER
COMPOSITE MATERIALS (E GLASS FIBRE-CARBON FIBRE-GRAPHITE)
ON DIFFERENT MATRIX MATERIAL BY VARYING ITS THICKNESS
Mr. M. Nayeem Ahmed1, Dr. P. Vijaya Kumar
2, Dr. H.K. Shivanand
3,
Mr. Syed Basith Muzammil4
1Associate Professor, Dept. of Mechanical Engg., HKBK College of Engineering, Bangalore-
560045, India 2 Professor, Dept. of Mechanical Engg., UVCE, Bangalore- 560001, India
3 Associate Professor, Dept. of Mechanical Engg., UVCE, Bangalore 560001, India
4 Assistant Professor, Dept. of Mechanical Engg., HKBK College of Engineering, Bangalore-560045,
India
ABSTRACT
Composite are occupying the place of conventional materials by meeting the requirements of
industries of not only in aerospace sector but in automotive, mechanical, space, construction
industries and Bio medical applications, but the desire of achieving the higher modulus to density
ratio always remains starved as it requires the maximum output in minimal consumption with better
life expectancy to find the economical means of utilizing the technology for different applications. In
need of which researches have been emerged to obtain the hybrid composites by the combination of
multiple types of materials to obtain the desired strength with less density. For most of the
applications of Automotive, Aerospace and biomedical applications, the flexural strength plays a
crucial role as the structure continuously or repeatedly subjects to point or uniform load, therefore a
study to evaluate the flexural strength by using different types of matrix material and also to optimize
the thickness of lamina is done, and comparisons are made by using different categories of matrix
material on different thickness of laminates.
Keywords: Bending strength of hybrid composites, E- glass-carbon-graphite composite, Graphite-
fibre composites, Hybrid composites, Optimization of thickness of Composite
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ISSN 0976 – 6340 (Print)
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1. INTRODUCTION
Composite materials are new generation materials developed to meet the demands of rapid
growth of technological changes of the industry. Composite materials or composites are engineering
materials made from two or more constituents’ materials that remain separate and distinct on
macroscopic level while forming a single component. It consists of short and soft collagen fibres
embedded in a mineral matrix called apatite. [1]
Composites belong to one of the four categories of structural materials. The other three are
metals and alloys, polymers, and ceramics. In fact composites are not a different category material
but various combinations of 2 or more of the latter 3 categories. This combination is at macroscopic
level such that individual components retain their mechanical properties contributing towards those
of the composite.
On the other hand combination at the microscopic level, such as alloys and solid solutions (at
the atomic level) are not considered as composites. Composites, although more expensive than their
counterpart materials, can demonstrate rather unusual combination of property values which is
difficult to achieve in any one standard material. It is not a random but careful and calculated
combination of materials that leads to a composite that exhibit superior properties than any one
ingredient alone.
While composites can be made out of a number of components, most composites are made of
just two.
One of them is known as matrix phase which is continuous and surrounds the other one
known as dispersed phase. Mechanical properties of composites are a function of those of the
ingredients, as well as their relative fraction amounts, and how the dispersed phase is distributed. The
distribution is characterized by type/shape of the dispersed phase particles, size of the particles, as
well orientation and distribution.
Distribution of fibers in fiber-reinforced composites is varied as per the application or load to
be carried.
These types are
(1) Continuous fiber composite,
(2) Woven composite,
(3) Chopped fiber (whisker) composite and
(4) Hybrid composite.
“Here, an attempt is made to study the bending strength of Hybrid composites, so let us put a light
on hybrid polymer composites”
1.1 HYBRID COMPOSITES Hybrid composites contain more than one type of fiber in a single matrix material. In
principle, several different fiber types may be incorporated into a hybrid, but it is more likely that a
combination of only two types of fibers would be most beneficial [3]. They have been developed as
a logical sequel to conventional composites containing one fiber. Hybrid composites have unique
features that can be used to meet various design requirements in a more economical way than
conventional composites. This is because expensive fibers like graphite and boron can be partially
replaced by less expensive fibers such as glass and Kevlar [4].
Some of the specific advantages of hybrid composites over conventional composites include
balanced strength and stiffness, balanced bending and membrane mechanical properties, balanced
thermal distortion stability, reduced weight and/or cost, improved fatigue resistance, reduced notch
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sensitivity, improved fracture toughness and/or crack arresting properties, and improved impact
resistance [3].
Experimental techniques can be employed to understand the effects of various fibers, their
volume fractions and matrix properties in hybrid composites. These experiments require fabrication
of various composites with the above mentioned parameters, which are time consuming and cost
prohibitive. Therefore, a computational model is created as will be described in detail later, which
might be easily altered to model hybrid composites of different volume fractions of constituents,
hence saving the designer valuable time and resource.
The mechanical properties of hybrid short fiber composites can be evaluated using the rule of
hybrid mixtures (RoHM) equation, which is widely used to predict the strength and modulus of
hybrid composites [5]. It is shown however, that RoHM works best for longitudinal modulus and
longitudinal tensile strength of the hybrid composites. Since, modulus values in a composite are
volume averaged over the constituent micro-stresses, the overall modulus of the composite has little
correlation with the randomness of the fiber location. Strength values on the other hand are not
primarily functions of strength of the constituents; they are however dependent on the fiber/matrix
interaction and interface quality. In tensile test, any minor (microscopic) imperfection on the
specimen may lead to stress build-up and failure could not be predicted directly by RoHM equations
[6].
The computational model presented in this paper takes into account, random fiber location
inside a representative volume element for every volume fraction ratio of fibers, in this case, carbon
and glass. The effect of randomization seems to have considerable effect on the transverse strength
of the hybrid composites. As for the transverse modulus, a semi empirical relation similar to Halpin-
Tsai equations has been derived, with the Halpin-Tsai parameter obtained for hexagonal packing of
circular fibers. Finite element based micromechanics is used to obtain the results, which show a good
match with experimental results for effective modulus for hybrid composites with ternary systems
(two fibers and a matrix) [7]. Direct Micromechanics Method (DMM) is used for predicting strength,
which is based on first element failure method; although conservative, it provides a good estimate for
failure initiation.
1.2 CASE STUDY The purpose of this study was to generate the material database for carbon and glass
composite laminates created by the Hand layup technique and Room Temperature Vacuum bag
molding (RTVBM) process. The material tested was hybrid polymer matrix composite of
composition (E-glass fibres+ carbon fibres + particulate graphite with epoxy resin) composites, two
grades of epoxy resin of different properties and bonding characteristics were used, viz., 5052 and
556 grades. The differences between the two grades of matrix material (Resin+Hardener), are
examined with the variation in ply thickness of (2mm/ply, 3mm/ ply and 4mm /ply). The bending
tests were conducted to study the effect of variation of thickness on strength of composites with
materials of different matrices. The material properties of interest were basic longitudinal and
transverse stiffness and strength, residual stress due to curing, and the effect of bend-twist coupling.
The bend-twist coupling is a feature that can be added to the composite laminate or structure, such
that when it is bent, it will also twist.
2. METHODOLOGY
The basic engineering properties of a composite material can be determined by either
experimental stress analysis (testing) or theoretical mechanics (micromechanics). The
micromechanics approach utilizes knowledge of the individual fibre and resin properties, and the
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proportionality of fibres to the resin in the lamina. A rule of mixtures approach can best be used to
derive the majority of the composite lamina properties. For example the lamina axial modulus is
derived from:
Ex = EfVf + EmVm
Where: Ef is the fibre modulus of elasticity
Em is the matrix (resin) modulus of Elasticity
Vf is the fibre volume ratio
Vm is the matrix volume ratio
Vf + Vm = 1 with zero voids
The fabrication of composite material includes the selection of the required fibre and matrix material,
and collects the appropriate amount of matrix (Resin). (For example, the called-out ratio of say
70:30, requires a ratio of 70% fibre weight to 30% resin weight)
2.1 FIBRE VOLUME AND WEIGHT RATIO RELATIONSHIP While the fibre weight ratio is easily determined by simple weighing, the fibre volume ratio is
quite difficult to determine. Typically, an ASTM test method is employed which requires destruction
of a small sample. However, the determination of fibre volume ratio can be derived from the
fibre/resin weight ratio. The approach is as follows:
Data:
Carbon Fibre: 300 gsm
Glass Fibre: 140 gsm
Carbon Fibre Thickness: 0.17mm
Glass Fibre Thickness: 0.32mm
Specimen calculation for the preparation of Lamina
Required
thickness of the
Lamina
Number of carbon fibre layers
(thickness of fabric: 0.17)
Number of glass fibre layers
(thickness of fabric: 0.32)
Total
Thickness
2mm 4 Layers 0.17*4=0.68mm 4 Layers 0.32*4=1.28
mm
0.68+1.28
=1.96mm
3mm 6 Layers 0.17*6=1.02mm 6 Layers 0.32*6=1.92mm 1.02+1.92
=2.94mm
4mm 8 Layers 0.17*4=1.36mm 8 Layers 0.32*8=2.56mm 1.36+2.56
=3.92mm
Table 2.1: Calculation for specimen preparation
To achieve the appropriate structural performance for a composite material, the fibre volume
ratio plays a crucial role. The engineering designer uses the fibre volume ratio to derive the lamina
properties and thus after lamination, structural properties. But to achieve the required fibre volume
ratio in wet lay-up processes the fabricator requires the fibre (Reinforcement) weight to resin
(Matrix) weight ratio. The expression is dependent on the ratio of the fibre and resin densities. This
relationship clearly identifies the importance of low fibre densities when compared with the resin
density.
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3. EXPERIMENTAL PROCEDURE
3.1 PRE-FABRICATION
Before the fabrication, the fabrics and matrix (appropriate quantity of resin with its hardener
based on calculations done for the required thickness and reinforcement-matrix ratio to be taken) has
to be kept in oven setting the temperature at 600C so that the moisture from resin and fabric (if
present) will be removed, then and the resin and hardener is mixed together and gently stirred, so that
the resin and hardener is properly mixed.
3.2 FABRICATION For the fabrication of polymer matrix composite the required fibres (Reinforcement media)
and Epoxy resin (Matrix material) are to be collected then by applying releasing agent on the work
table mount the releasing layer (Teflon sheet) then again apply the releasing agent and place the first
layer of fabric and wet it then apply the next layer and again wet that follow the same procedure for
all remaining layers, the wetting should be done in such a way that the resin should be distributed
equally on the lamina, care should be taken that there should be no starvation or excess of resin on
the lamina. After the last layer again the resin is applied and covered with Teflon sheet and then the
dead weight is applied over the mold. As the mold is ready it is left to reach the gel time of the resin,
as it reaches the gel time, vacuum is applied by covering the mold by vacuum bag, and is left for
some time to get set so as the resin should be spread equally on mold and excess of resin can be
drawn outside. After the vacuum time it is left as it is at room temperature for 24hrs to cure.
Therefore it is also called as Room Temperature Vacuum Bag Molding (RTVBM).
3.3 POST CURING
As the laminate is ready, it has to be subjected to post curing so the all the layers of the
lamina bond together. This can be achieved by keeping the lamina in oven and set the oven to
increase the temperature gradually to 500C in 15 minutes from room temperature and hold the
temperature for 30 minutes again ramping up to 800C in next 15 minutes and hold the temperature
for 30 minutes again ramp up to 900C in 15 minutes and hold for 30 minutes then ramp up to 120
0C
in 30 minutes and hold for 60 minutes then let the oven cool down slowly to room temperature.
3.4 TESTING
3.4.1 Bending or Flexural test
Bending/ flexural test is one of the fundamental mechanical tests which is required to
evaluate the bending strength of any material which is essential to acknowledged for the design of
any structure, In engineering mechanics, flexure or bending characterizes the behavior of a slender
structural element subjected to an external load applied perpendicularly to a longitudinal axis of the
element. Therefore for evaluation a carefully prepared specimen is subjected to three point load in a
controlled manner. Flexural properties can be measured by the relation of load applied on the
material to deformation (Strain) experienced against the applied load. Figure 3.1 shows the actual 3
point bending of test specimen loaded on testing machine, where as figure 3.2 shows the exactly
represents the terminology of 3 point bending test
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Fig. 3.1: Bending or Flexural test setup Fig. 3.2: 3-Point Bending test
A flexure test produces tensile stress in the convex side (roller support side of the specimen)
and compression stress in the concave side (Loading side of the specimen). This creates an area of
shear stress along the midline. To ensure the primary failure arrival from tensile or compression
stress the shear stress must be minimized. This is done by controlling the span to depth ratio; viz., the
ratio of length of the outer span to height (depth) of the specimen.
3.4.2 Bending test procedure Measure the dimensions of the specimen.
Check the limit of the linear region of the aluminium beam (with no strain gage)
Open the computer and Instron universal test machine and run the associated software.
Prepare the Wheatstone circuit and connect to the cables of strain gages to the defined slot in the
previous experiment “Strain Gage”.
Use the digital micrometre to take sample. It must take 10 samples per a second. Adjust the
associated Instron program with displacement controlled experiment.
Maximum allowed displacement of the specimen is 2mm. After 2 mm it is in plastic region. Also
adjust the software to take 10 Force data per a second.
Run the experiment.
4 TESTING AND EVALUATION:
4.1 BENDING TEST OF SPECIMENS OF THE MATRIX ( EPOXY RESIN OF LY5052): The test was conducted on 2mm, 3mm and 4mm Carbon and glass fiber hybrid laminates.
The data measured from the mechanical testing was used to calculate the maximum load the
specimens can sustain. The table 4.1 shows the values exhibited by the specimens with different
thickness in epoxy resin 5052.
Sl no. Thickness in
mm Ultimate Load in N
Maximum
Displacement in mm
1 2 749.33 14.2
2 3 978.86 11.8
3 4 1869 9
Table 4.1: Bending test results hybrid Composite laminates with resin 5052
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4.1.1 Carbon fibre- Glass fiber- particulate graphite hybrid Composite laminate of epoxy resin
grade 5052 of 2mm thickness
Fig. 4.1: Test specimens of 2mm thick ply before and after bending test
Graph 4.1.1: Load v/s displacement relationship for 2mm thick ply
The above graph represents the load v/s displacement relationship of the bending test of the
2mm thick hybrid laminate. The curve shows a steep linear increase up to a point (650N) and
deforms uniformly, which is the area of yielding, at ultimate point, the material loses its internal
resistance against the acting load, which results in permanent deformation without he excess
application of load on material and hence at some point material gets failed. The maximum
deflection found after failure is 14.2mm.
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0 2 4 6 8 10 12 14 16
Ax
is L
oa
d (
kN
)
Displacement (mm)
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4.1.2 Carbon fibre- Glass fiber- particulate graphite hybrid Composite laminate of epoxy resin
grade 5052 of 3mm thickness
Fig. 4.2: Test specimens of 3mm thick ply before and after bending test
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00
Lo
ad
(k
N)
Displacement (mm)
Graph 4.1.2: Load v/s displacement relationship for 3mm thick ply
The above graph represents the load v/s displacement relationship of the bending test of the
2mm thick hybrid laminate. The curve shows a steep linear increase up to a point (978.6mm) and
deforms uniformly, which is the area of yielding, at ultimate point, the material loses its internal
resistance against the acting load, which results in permanent deformation without he excess
application of load on material and hence at some point material gets failed. The maximum
deflection found after failure is 11.8mm.
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4.1.2 Carbon fibre- Glass fiber- particulate graphite hybrid Composite laminate of epoxy resin
grade 5052 of 4mm thickness
Fig. 4.3: Test specimens of 4mm thick ply before and after bending test
Graph 4.1.3: Load v/s displacement relationship for 4mm thick ply
The above graph represents the load v/s displacement relationship of the bending test of the
2mm thick hybrid laminate. The curve shows a steep linear increase up to a point (1869N) and
deforms uniformly, which is the area of yielding, at ultimate point, the material loses its internal
resistance against the acting load, which results in permanent deformation without he excess
application of load on material and hence at some point material gets failed. The maximum
deflection found after failure is 9mm.
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4.2 BENDING TEST OF SPECIMENS OF THE MATRIX ( EPOXY RESIN OF LY556) The test was conducted on 2mm, 3mm and 4mm Carbon and glass fiber hybrid laminates.
The data measured from the mechanical testing was used to calculate the maximum load the
specimens can sustain. The table 4.2 shows the values exhibited by the specimens with different
thickness in epoxy resin LY 556.
Sl no. Thickness in
mm Ultimate Load in N
Maximum
Displacement in mm
1 2 236.7 13.3
2 3 331.89 10.6
3 4 823.98 8.7
Table 4.2: Bending test results hybrid Composite laminates with resin 556
Graph 4.2.1: Load v/s displacement relationship for 2mm thick ply
Graph 4.2.2: Load v/s displacement relationship for 3mm thick ply
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Graph 4.2.3: Load v/s displacement relationship for 3mm thick ply
The above graphs show the displacement on the application of load. With reference to above
graphs it is seen that the ply of 2mm thickness gets failed on application of bending load with more
displacement and as the thickness of ply increases, the reduction in percentage elongation is found.
5. RESULTS AND DISCUSSIONS
Graph 5.1: Variation of bending load for different thicknesses of Hybrid Composites with different
grades of epoxy resin viz., 5052 and 556 grades
The above graph represents the bending strength of the hybrid composite laminates with
different grades of matrix (epoxy Resin) viz., LY 5052 and LY 556 on different thickness of ply.
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From the above graph it is clearly observed that the resin of grade LY5052 exhibits the
greater strength when compared to the strength of LY 556, which is because of the setting time (gel
time). The gel time of the LY 556 grade resin is very quick (of about 40 minutes) where as for LY
5052 it is late (about 3 hours) which shows that resin 5052 sets steadily and hence the bonding of
which is stronger than the 556 resin which sets quickly.
Also, thickness of the laminate is plays a crucible role in strength of the composites, as it
shown in the above graph that, as the thickness increases, the bending strength increases but this is
not necessary in case of tensile strength, because in bending test the height or thickness of the
specimen experiences the lateral forces i.e, direction of force of application is normal to the thickness
of the specimen, therefore thickness of the specimen determines the bending strength of the ply.
6. CONCLUSION
From the experimentation and results obtained after testing the following conclusion are drawn.
It is found that the Hybrid Composites of LY 5052 as matrix material exhibited more bending
strength when compared to LY 556 as matrix material, irrespective of their thickness.
When the comparison was carried out to study the bending strength between the composite
plies of two grades of matrix materials viz., 5052 & 556 with different thickness, 4mm thick ply has
got the high bending strength when compared to the difference of strengths of 2mm & 3mm thick
plies, which shows the strong bond of 4mm thick hybrid composite lamina.
In this study it is observed that thickness of composites enhances the bending strength,
therefore for the static or dynamic loading applications, section height (thickness) should be more.
By the addition of graphite powder strength is enhanced as it mixes up with the resin and acts
as the reinforcement within the resin.
Addition of graphite in composite enhances the thermal properties of the composite as
graphite is its good conductor.
With this study it is concluded that composition of multiple materials leads to the
improvement in mechanical and thermal properties.
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