Influence of Thickness on Shear Properties of Laminated Composites

INTERNATIONAL JOURNAL OF RESEARCH IN AERONAUTICAL AND MECHANICAL ENGINEERING

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January 2014.

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B.V.BabuKiran, Dr. G.Harish 54

ISSN (ONLINE): 2321-3051

INTERNATIONAL JOURNAL OF RESEARCH IN AERONAUTICAL AND MECHANICAL ENGINEERING

Influence of Thickness on Shear Properties of Laminated Composites

1B.V.BabuKiran, 2Dr. G.Harish

1Research Scholar ,2Associate Professor, R & D Center, Department of Mechanical Engineering, University Visvesvaraya College of Engineering, Bangalore.

Abstract

The bending properties of composite materials are often characterized with simply supported beams under concentrated loads. The horizontal shear test with a short-beam specimen in three-point bending appears suitable as a general method of evaluation for the shear properties in fiber-reinforced composites because of its simplicity. In the experimental part of this work, the shear strength of glass fiber-epoxy, Graphite fiber-Epoxy & Carbon fiber –epoxy laminated composite material were investigated with different thickness under three-point-bending test (Short beam Test). In the present study, the composite laminate specimen’s are prepared using the vacuum baggage technique and the specimen are subjected to 3 point bending load on a simply supported pins and the investigation is carried out as per the ASTM standards D2344/D2344M.

This paper examines the influence of thickness on ILSS properties of laminated composites. many structures used in Automobile, Aerospace, Naval and other Transportation vehicle structural parts are subjected to various kinds of loads. These structures are further subjected to bending loads causing Shear stress in the structures. The purpose of this work is to experimentally analyze the progressive failure process of laminated composites subjected to shear loads, Shear loading causes stresses in the composites, which vary through the thickness. Shear properties evaluated are Shear strength and stiffness of the composites system appropriate conclusions was drawn.

Keywords: Laminate; Shear Strength; Short beam test; Three point bending; Stiffness.

1. Introduction The short-beam shear test has become a widely used method for characterizing the interlaminar failure resistance of fiber-reinforced composites. This test method involves loading a beam under three-point bending with certain dimensions so that interlaminar shear failure is induced. The simplicity of the test method makes it a very popular screening tool. The experimental requirements for such a test on a fiber-reinforced composite are simpler than those for a tensile test because the effects of flaws and geometrical stress concentrations are less severe. In addition, a rectangular cross-sectional specimen can be used, and this leads to ease of sample

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preparation. Also, there is no need to provide end tabs on a reduced cross section to ensure failure away from the grips. Harding and Li [1] determined the high strain-rate ILSS of woven carbon/epoxy and glass/epoxy laminates by the tensile split Hopkinson bar (SHB) tests on double-lap shear specimens. Bouetteet al. [2] also attempted to measure the inter laminar shear properties of a unidirectional carbon/epoxy laminate at high strain rates from the tensile SHB tests on single-overlap shear specimens. Mullin and Knoell [3] discussed the theoretical origins of L/t ratios (where L is the span length and t is the nominal thickness) and the effects of material variables and specimen defects such as voids, and Westwater [4] observed that specimens with small L/t ratios often fail by cracks running out to, or past, the support nose and also that the amount of overhang may affect the mode of failure. Daniels et al. [5] demonstrated representative failure modes of carbon-fiber/epoxy-resin composites in short-beam tests but indicated that the same behavior would not necessarily occur for other combinations of materials. Whitney and Browning [6] referred to the limitations of the short-beam shear method.

2. Experimental Investigation

a. Preparation of surface Bi woven glass fiber is used as reinforcement in the form of bi directional fabric and epoxy resin as matrix for the composite material of the laminates specimens. Hand layup process is used to prepare the specimens. Surface of the mould is thoroughly cleaned by removing any dust and dirt from the mould. After the mould surface has been cleaned, the release agent is applied, where the mould surface is coated with silicon wax using a smooth cloth then the thin film of polyvinyl alcohol is applied over the wax surface using sponge. The matrix material used here is epoxy resin which is applies to create bonding between the layers of sheet in the ratio of 100:1.

b. Preparation of the Laminate The first layer of Bi-woven glass fiber cloth (ranging from 0.25 mm to 0.35 mm) is laid and resin is spread uniformly over the cloth by means of brush shown in fig 1.1. The second layer of the cloth is laid and resin is spread uniformly over the cloth by means of brush. After second layer, to enhance wetting and impregnation, a teethed steel roller is used to roll over the fabric before applying resin. Also resin is tapped and dabbed with spatula before spreading resin over fabric layer. This process is repeated till all the 10 layers (2 mm thickness) and 16 layers (4 mm thickness) are placed. No external pressure is applied while casting and curing because uncured matrix material can squeeze out under high pressure. This results in surface waviness (non-uniformed thickness) in the model material. The casting is cured at oven temperature of about 100º C up to 2 hrs & finally removed from the mould to get a fine finished composite plate as shown in the fig 1.2 below.

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c. Preparation of test specimens: After the cure process, the test specimens are cut from the sheet to the following size as per ASTM standards (ILSS Specimen Dimensions as per ASTM D – 2344/2344M) by using diamond impregnated wheel, cooled by running water. All the specimens are finished by abrading the edges on a fine carborundum paper as shown in the Fig 1.3.

Fig1.3 ILSS specimen.

d. Testing Machine: The universal testing machine used in the above test was manufactured by BISS Bangalore. It is a versatile and comprehensive testing machine which can be used as a standalone machine or it can be linked to a remote computer and data analysis software. This machine is designated as PNP-01 and is shown in the below fig.1.4 Flexural and Short beam test can be performed on this machine.

Fig 1.4Universal Testing Machine Fig: 1.5 Specimen Mounted

Fig 1.2 Laminate under curing

Fig 1.1 Bi woven glass cloth

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Machine specifications:

Machine Specifications Fixture Details

Actuator capacity: 10 KN 3-Point Bending Fixture

Stroke:+/-30mm Capacity:15 kN

Vertical daylight:570mm MODEL: BI-10-101

Horizontal day light:40mm

Supply 6 kHz digital servo control

Rate of Loading: 1mm/min

Table 1-Specimen Designation.

Sl. No. Specimen Designation Description

1 CAI/02/01 Carbon Fiber /2 mm thickness/Sample 01

2 CAI/02/02 Carbon Fiber/2 mm thickness/Sample 02

3 CAI/04/01 Carbon Fiber/4 mm thickness/Sample 01

4 CAI/04/02 Carbon Fiber/4 mm thickness/Sample 02

5 GRI/02/01 Graphite Fiber/2 mm thickness/Sample 01

6 GRI/02/02 Graphite fiber/2 mm thickness/Sample 02

7 GRI/04/01 Graphite Fiber/4 mm thickness/Sample 01

8 GRI/04/02 Graphite Fiber/4 mm thickness/Sample 02

9 GLI/02/01 Glass Fiber/2 mm thickness/Sample 01

10 GLI/02/02 Glass Fiber/2 mm thickness/Sample 02

11 GLI/04/01 Glass Fiber/4 mm thickness/Sample 01

12 GLI/04/02 Glass Fiber/4 mm thickness/Sample 02

Table 2-Specimen Designation and Measured Dimensions

SPECIMEN WIDTH1(mm) WIDTH2(mm) WIDTH3(mm) AVG(mm)

CAI

14.35 14.19 14.31 14.28 14.11 14.18 14.22 14.17 13.52 13.76 13.81 13.69 13.38 13.56 13.54 13.49

GRI

14.64 14.9 14.99 14.84 14.69 15.02 15.2 14.97 13.75 13.71 13.7 13.72

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14.34 14.57 14.55 14.48

GLI

14.81 14.49 14.43 14.57 14.32 14.31 14.36 14.33 14.39 14.53 14.6 14.5 14.22 14.12 14.09 14.14

3. Results:

SI SPECIMEN MAX

LOAD(KN) SHEARSTRENGTH

(KN/mm2) SHEAR

MODULUS(MPa)

1 CAI/02/01 2.46 0.064 995.3

2 CAI/02/02 2.4 0.063 987.2

3 CAI/04/01 2.67 0.036 1978.01

4 CAI/04/02 2.66 0.036 1966

5 GRI/02/01 0.62 0.015 388.2

6 GRI/02/02 0.61 0.015 396

7 GRI/04/01 2.13 0.028 781.4

8 GRI/04/02 2.12 0.027 802

9 GLI/02/01 1.45 0.037 292.6

10 GLI/02/02 1.46 0.038 288.4

11 GLI/04/01 1.85 0.023 442

12 GLI/04/02 1.86 0.024 447

Table-1 Results of ILSS test of Glass fiber, Carbon fiber & Graphite with 2mm & 4mm.

Graphs:

Fig 1.6 Carbon fiber 4 mm thick Fig 1.7 Graphite fiber 4 mm thick

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Fig 1.8 Glass fiber 4 mm thick

4. Discussions:

• The Shear properties of the composite specimens have been calculated using the measured data of the Load vs. Displacement Curve.

• Typical load versus deflection curves has been obtained for composite specimens of 2mm and 4mm thick of Glass fiber, carbon fiber and Graphite fiber laminated composites.

• The graphs of load VS Displacement were plotted for the all samples.

• It has been observed that the max value of shear strength for Carbon and the Minimum value of Shear strength for Glass fiber have been observed.

5. Conclusions:

The main conclusions of the experimental investigation of flexural analysis of laminated composite material are as follows:

• Short beam test on Glass fiber, Carbon fiber & Graphite composite specimen was carried out successfully.

• Shear properties are evaluated for Glass fiber, Carbon fiber & Graphite Fiber with two different thicknesses.

• Number of trials was carried out to obtain consistent results and the averages of the results are recorded.

• Load vs. Deformation plot for both types of specimens with varying thickness were recorded.

• Max load, stress, Shear Modulus and Max deformation Values were recorded for both types of specimens to investigate the influence of thickness were analyzed.

Thus, it can be concluded that for same thickness and orientation, carbon fiber provides better Shear properties as compared to glass and graphite under Shear loading conditions.

Acknowledgments

Authors thankfully acknowledge the Principal and the Head of the Department of Mechanical Engineering UVCE Bangalore for their constant encouragement and support in carrying out this work.

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6. References

1. Harding J and Li YL, Determination of inter laminar shear strength for glass/epoxy and carbon/epoxy

laminates at impact rates of strain, Composites Science and Technology 45, 161-171, 1992.

2. Bouette, B., Cazeneuve, C. and Oytana, C.: Effect of strain rate on interlaminar shear properties of carbon/epoxy composites, Composites Science and Technology, Vol. 45, 313-321 (1992).

3. Mullin, J. V.; Knoell, A. C. Mater Res Stand 1970, 10, 16. 4. Westwater, J. V. Am Soc Test Mater Proc 1949, 49, 1092. 5. Daniels, B. K.; Harakas, N. K.; Jackson, R. C. Fibre Sci Technol 1971, 3, 187. 6. Whitney, J. M.; Browning, C. E. Exp Mech 1985, 9, 294. 7. ASTM D 2344/D2344M. Annual Book ASTM Stand 2006.