A study on flexural strength of hybrid polymer composite materials e glass fib

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

INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING

AND TECHNOLOGY (IJMET)

ISSN 0976 – 6340 (Print)

ISSN 0976 – 6359 (Online)

Volume 4, Issue 4, July - August (2013), pp. 274-286 © IAEME: www.iaeme.com/ijmet.asp

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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



276

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



277

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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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)



2mm thick hybrid laminate. The curve shows a steep linear increase up to a point (978.6mm) and




deflection found after failure is 11.8mm.



282






2mm thick hybrid laminate. The curve shows a steep linear increase up to a point (1869N) and




deflection found after failure is 9mm.



283

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





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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.



285

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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A study on flexural strength of hybrid polymer composite materials e glass fib

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