experimental evaluation of inter laminar shear … · as tension, compression and flexural loads...

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Int. J. Mech. Eng. & Rob. Res. 2014 Rathnakar G and H K Shivanand, 2014

ISSN 2278 – 0149 www.ijmerr.com

Vol. 3, No. 4, October, 2014

© 2014 IJMERR. All Rights Reserved

Research Paper

EXPERIMENTAL EVALUATION OF INTER LAMINAR

SHEAR STRENGTH OF FIBRE REINFORCED COMPOSITES

Rathnakar G1* and H K Shivanand2

*Corresponding Author: Rathnakar G � [email protected]

In present day several industries such as aeronautical industries, automobile industries, marineindustries, chemical industries and transportation industries relay on fibre reinforced compositesto manufacture several components such as flaps, doors, gears and other components. Thesefibre reinforced composite laminates are susceptible to de-lamination damage causingcatastrophic failure of the components. The anisotropic nature of laminated composite materialsbecome the source of de-lamination, de-lamination growth is the fundamental issue in theevaluation of laminated composite systems for durability and damage tolerance. Prediction ofthe Inter laminar shear strength of the fibre reinforced composites is important for engineeringapplications yet difficult in nature. The mechanical behaviour of materials and its propertiesbecomes complex with the addition of fibres. These materials subjected to various loads suchas tension, compression and flexural loads are susceptible to mechanical damages which resultsin de lamination in the inter layers of the composites. The effect of de lamination may result inthe catastrophic failure of the component with the increase in the external load. This paper outlines the experimental investigation of inter laminar shear strength as a critical mechanicalproperty of fibre reinforced composite laminate. The main objective of this paper is to find theinter laminar shear strength (trans-plane properties) to study the effect of de lamination in thecomposite laminates and to evaluate inter laminar shear strength of various fibre reinforcedcomposites and to draw appropriate conclusions.

Keywords: Polymer Laminate, Polymer matrix composite, Inter laminar shear test, De-lamination, Shearstress

INTRODUCTION

The fibre reinforced composite materials arepresently used in exhaustive mannerbecause of their enhanced mechanicalproperties. The fibre reinforcing composites

1 Associate Professor, Christ University Faculty of Engineering, Bangalore, India.2 Professor, Department of Mechanival Engineering, UV College of Engineering, Bangalore, India.

with brittle materials, having glass fibres,ceramic, graphite and carbon have potentialto be used in the structural applications(Schwartz MM, 1997). The fibre reinforcedlaminated composites exhibits typical

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mechanical behaviour when loaded, whichis complex in its nature to analyse, theaddition of the reinforcing fibres makes itfurther complex to determine the mechanicalproperties of these materials .The laminatedcomposites are subjected to de laminationat the boundaries making the laminate weakand this will result in the development of acrack and the progression of this crack mayresult in the catastrophic failure of thelaminated structure. The significance of theshear strength lies in the fact that for all typesof composites it is strongly influenced byfactors weakening the interface binds.Boundary interfaces between the fibres andthe matrix, as well as the thin matrix layerbetween the fibres represent the potentialsources of composite weakening, shear forcein these regions can easily cause sheardamages (Kim J et al., 2004). Hence toevaluate the occurrence and progression ofthe de lamination in the laminated compositesInter laminar shear test is an appropriatemethod employed, even though it is criticalto investigate the occurrence and progressionof the crack using this method, does notprovide complete information of the mecha-nical behaviour it provides the initial indi-cations by providing the shear strength of thelaminates. To evaluate the ILSS, three pointbend test ASTM D 2344, also known as shortbeam shear (SBS) is adopted in this experi-mental investigation. Advantage of thismethod is its simplicity and ability of beingperformed rapidly and in much more reliablemanner than other methods. It is easy tomeasure the shear strength of uni-directionallaminates where as the laminatedcomposites with bi woven patterns offerschallenge. From beam theory it is known that

peak inter laminar shear occurs at the midplane of a beam loaded in three point bendingand is inversely related to the specimen widthand thickness. Several methods are deve-loped to evaluate the inter laminar shearstrength (ILSS) including short - beam flexuraltest (K T Kedward, 1972). Ouyang Z et al.,2009) used end notched flexural specimenwith sufficiently long bond length to evaluateinterface shear fracture. (Chen L et al., 2001)analysed the effect of transverse stitching orpinning the fabric across the laminate to holdthe plies together to increase inter laminarshear strength. It is well known that by emplo-ying lamination theory, one can determine thein-plane stress distribution in each layer of alaminate from the knowledge of the appliedforce and moment results. The distributionof the inter laminar shear stresses in eachlayer from the applied shear resultants arenot readily available from the standard lami-nation theory equations. Inter laminar shearproperties are important data required for theanalysis of polymer matrix composite struc-tures, studies are carried out on inter laminarshear behaviour of typical polymer matrixcomposites under high strain rate shearloading. Sierakowski and Chsturvedi [6]adopted the Split Hopkinson pressure bar(SHPB) the most widely used technique todetermine shear properties of compositesunder high strain rate loading. Werner anddharan (Werner S M, Dharan C K H, 1986)carried out inter laminar shear tests on wovengraphite/epoxy using compressive SHPBapparatus. The short beam shear specimenInter laminar shear strength can be calculatedusing the equation (High performance comp-osites source book 2009). Sayers and Harris,1973 used drop weight impact test tech-

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niques for determining the inter laminar shearproperties of carbon/epoxy composites.ASTM 2344 is the recommended standardfor the Apparent inter laminar shear strengthof unidirectional parallel fibre composite byshort beam method. The shear stress iscalculated using the formula.

Where τττττ- is inter laminar shearstress (MPa),

P- (Pmax) -is maximal forceat which speci-men delamination occurs (N),

b- Specimen Width (mm),

d- is specimen thickness

PREPRATION OF SPECIMENS

(MATERIALS)

The composite specimen is fabricated as perstandard procedure and test.

II a. Fabrication of Glass Fiber

Laminates

The fibres chosen were bi-woven glass fibrewith density of 360 gsm. The laminate will becut to the required size and bonded to theglass fibre cloth by using an adhesive madefrom a mixture of LY556 resin & HY 951hardener in proportions of 100:10 by weight.The surfaces will be thoroughly cleaned inorder to ensure that they were free from oil,dirt, etc., before bonding at room temperatureand pressure. The models will be allowed tocure for about 24 hours. Thicknesses of theSpecimens was be maintained at 2 mm & 4mm throughout the experiments for all thespecimens prepared.

II b. Fabrication of Graphite Fiber

Laminates and Carbon fiber Laminates

The material used in this work was plainweave graphite fiber/carbon fiber with epoxyresin as matrix. The table on which laminatewere stacked is cleaned with acetone. Theply stacks were compression molded in a hotplate press under a pressure of 2 bars until atemperature of 120°C reached at a heatingrate of 9°C per minute, after which thepressure was increased to 8 bars and heldat this temperature and pressure for 120minutes. Finally, the laminate was cooleddown to room temperature at a cooling rateof 25°C per minute. The specimens were cutfrom the laminates with a diamond disc cutterand dried in a vacuum oven for at least 24hours. Figure below depicts the vacuumbagging process. Specimens are machinedto minimize the influence of manufacturingrelated defects on test results. The presenceof voids which present may act as the de-lamination points can be reduced by the useof the vacuum bagging.

EXPERIMENTATION

The inter laminar shear strength is the abilityto resist inter laminar deformation under loadfor a material. Three point bend test, alsoknown as short beam shear(SBS) test is oftenused to measure the apparent inter laminarshear strength (ILSS) of compositelaminates. In the short beam shear test, thedetermination of Inter laminar shear stressis based on classical (Bernolli-Euler ) beamtheory. The specimen with the given span issupported between two supports as a simplysupported beam and the load is applied atthe centre by the loading nose producing

τ τ τ τ τ = – ––3 P4 bd

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three point bending at a specified rate. Theparameters for this test are the support span,the speed of the loading, and the maximumdeflection for the test. These parameters arebased on the test specimen thickness andare defined differently by ASTM. Under ASTMD 790 (Kim J et al., 2004), the test is stoppedwhen the specimen breaks. For a beam ofrectangular cross section loaded in threepoint bending, the maximum inter laminarshear stress occurs at the mid thickness ofthe beam between the centre and endsupports.

STANDARD SPECIMEN SIZE

The most commonly used specimen size forASTM is 3.2mm x 12.7mm x 50mm (0.125”x 0.5” x 5.0”). The 3-point Inter Laminar ShearTests were performed in a servo controlledmachine according to the procedure outlinedin ASTM 2334. At least 3 specimens weretested for each thickness of laminate. Thecrosshead speed was maintained at 2mm/min. The tested specimens were examinedusing through visual inspection for failure offibres and matrix.

Figure 1: Closer Look of Specimen Subjectedto Inter laminar Shear Test (ILSS)

Figure 2: [ILSS] Inter LaminarShear Test (Test Rig)

Figure 3: Graphite FibreReinforced Laminate

Figure 4: Carbon FibreReinforced Laminate

RESULTS AND DISCUSSION

Three-point bending test, also known as shortbeam test is used to measure the apparentinter laminar shear strength (ILSS) of fibrereinforced composite laminates. For the sakeof accuracy in determination of the inter

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laminar shear strength, for each combinationof pattern three specimens were testedexperimentally, confirming to the appropriatestandards. For each specimens initial dimen-sions were measured, & then the maximumshear force Pmax (N), that is the force causingthe de lamination of the specimen wasdetermined. The inter laminar shear stressof the composites have been calculated fromthe formulae, the load versus deflectioncurves were plotted using the digital testingmachine. Typical load versus deflectioncurves have been obtained for different typesof fibre reinforced composites. The dataobtained from the independent tests from thethree point shear tests were used in thecalculations. The graph of load versus deflec-tion were linear until the breaking point.During testing, it was observed that observedthat other failure modes including bending.Further it was observed that the damage inthe specimen is characterised by a singlecrack that propagates from a region locatedusually at the axis symmetry. The short beam

test is adopted to conduct inter laminar sheartest because of its simplicity, ease of moun-ting the specimens on the test rig. Duringbending in short beam shear , the loadincreases proportionately with deformation,until a peak load is reached, if the load dropsby 25-30% or more immediately after thepeak load is reached, it is assumed that thespecimen failed in laminar shear and the peakload is then used to determine the apparentILSS.A sharp drop in the load -displacementcurve and an audible cracking soundaccompany catastrophic de lamination, whichis at some times hard to detect with out theaid of a microscope. After the first -ply failure,the load may pick up again and in some casesit may reach the pristine value, refer loadversus displacement curve. The delamination is generally more visible to thenaked eye, and in the large majority of thespecimens it is located near the vertical axisof symmetry, and at the mid plane, actuallyone ply above it.

Table 1: Specimen Details

1.

2.

3.

4.

5.

6.

7.

8.

9.

GL2-01

GL2-02

GL2-03

GR2-01

GR2-02

GR2-03

CA2-01

CA2-02

CA2-03

1.53

1.58

1.51

2.17

2.26

2.24

2.17

2.19

2.33

1.52

1.53

1.51

2.22

2.25

2.19

2.15

2.19

2.28

1.53

1.52

1.53

2.18

2.16

2.36

2.18

2.17

2.19

1.53

1.54

1.51

2.19

2.22

2.26

2.16

2.18

2.26

14.08

14.64

14.37

13.01

13.16

12.99

13.54

12.58

13.42

13.97

14.65

14.40

13.06

13.21

12.94

13.30

12.80

13.59

13.71

14.53

14.31

12.12

13.28

13.01

13.05

12.89

13.63

13.92

14.6

14.36

12.73

13.21

12.98

13.29

12.75

13.54

50

50

50

50

50

50

50

50

50

Sl.No.

SpecimenDetails

Speci-men ID

Thickness(d), mm

1 2 3

AverageThick

ness(d)mm

Width (b), mm

1 2 3

AverageWidth(b)

mm

Lengthmm

Glass FibreReinforcedComposite

GraphiteFibre Rein-forced

Carbon FibreReinforced

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PMax- Maximum load In Newtons ILSS τττττ )- Inter laminar Shear stress MPa.

τ τ τ τ τ = – ––3 P4 bd

Table 2: Load and Shear Strength for 2mm Specimen (Inter Laminar Shear Strength)

1.

2.

3.

4.

5.

6.

7.

8.

9.

GL2-01

GL2-02

GL2-03

GR2-01

GR2-02

GR2-03

CA2-01

CA2-02

CA2-03

216.9

221.3

192.9

296.9

282.2

286.6

308.1

284.9

294.4

210.3

288.5

295.8

7.68

7.38

6.67

7.98

5.68

7.32

8.04

7.68

7.21

7.24

6.99

7.6

4.63

4.56

5.34

3.77

5.55

3.86

4.09

3.93

4.2

4.84

4.39

4.07

Sl.No.

SpecimenDetails

SpecimenID

PMax

N

Averageload,Pavg

N

ILSSτττττ,

MPa

AverageShear

Stress, τττττMPa

Deflectionmm

AverageDeflection

mm

Table 3: Load and Shear Strength for 4mm Specimen (Inter Laminar Shear Strength)

1.

2.

3.

4.

5.

6.

7.

8.

9.

GL4-01

GL4-02

GL4-03

GR4-01

GR4-02

GR4-03

CA4-01

CA4-02

CA4-03

850.7

865.5

810

803.3

922.9

979.1

846.5

934.8

857.4

842.06

901.7

879.5

14.97

16.32

15.14

14.42

15.24

15.88

15.06

15.94

15.43

3.709

3.765

3.435

3.672

2.835

3.337

3.340

2.525

3.003

Sl.No.

SpecimenDetails

SpecimenID

PMax

N

Averageload,Pavg

N

ILSSτττττ,

MPa

AverageShear

Stress, τττττMPa

Deflectionmm

AverageDeflection

mm

Glass Fibre4mm

GarbonFibre4mm

GraphiteFibre4mm

15.47

15.18

15.47

3.63

3.28

2.95

Glass Fibre2mm

GraphiteFibre2mm

GarbonFibre2mm

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Figure 5: Load Vs Displacement (GlassFibre Reinforced Composite 2mm Thick)

Figure 6: Load Vs Displacement (GraphiteFibre Reinforced Composite 2mm Thick)

Figure 7: Load Vs Displacement (CarbonFibre Reinforced Composite 2mm Thick)

CONCLUSION

Inter laminar shear strength of different fiberreinforced polymer composites have beensuccessfully evaluated under short beamshear test. Determination of the flexural shearstrength of unidirectional fiber laminatedcomposites are relatively simple whencompared with the much complicated wovenfiber reinforced composite laminates whichexperiences a variety of different failure anddamage modes before inter laminar shearfailure occurs under short beam shear test.In the load deflection curve, the curve risesgradually (proportionately) and then dropssuddenly showing a distinct failure loaddeveloped in the sample, the curve rises,flattens for some time and then shows a smalldrop. The test helps to understand the delamination principle under static loadingunder short beam test, it is observed that thede lamination occurs in the centre plane atone end of the specimen. The inter laminarshear strength(τ) is calculated using theformula shown. Further it is evident that thecarbon fiber reinforced polymer can withstand maximum load(Inter laminar shearstrength) when compared with the other twotypes of reinforced composites for the samethickness.

ACKNOWLEDGMENT

Authors thankfully acknowledge the Manage-ment, Principal and Head of the Departmentof Mechanical Engineering of their institutionfor their constant encouragement and supportin carrying out this work.

REFERENCES

1. ASTM 2344, ASTM standard D2344/D2344M. Apparent inter laminar shear

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strength of unidirectional parallel fibrecomposite by short beam method; 2000.

2. ASTM D790 M-93, Standard testmethods for flexural properties ofunreinforced and reinforced plastics andelectrical insulating materials, Americansociety for testing and materials, Annualbook of ASTM standards, Vol. 08, No. 1.

3. Chen L (2001), “Anovel Double CantileverBeam Test for Stitched CompositeLaminates” Journal of CompositeMaterial.

4. High performance composites sourcebook (2009), “Gardner Publication Inc.”.

5. K T Kedward (1972), “Fiber ScienceTechnology”, Vol. 5, pp. 85-95.

6. Kim J, Shioya M, Kobayashi H andKaneko J (2004), “Mechanical Propertiesof Woven Laminates and FeltComposites Using Carbon Fibres part1:in-plane Properties” Composite Scienceand Technology, Vol. 64, pp. 2221-2229.

7. Ouyang Z (2009), “Cohesive Zone ModelBased Analytical Solutions for AdhesivelyBonded Pipe Joints Used Under TorsionalLoading”, International Journal of Solidsand Structures, Vol. 46, No. 5, pp. 1205-1217.

8. Sayers & Harris (1973), “Used DropWeight Impact Test Technique forDetermining the Inter Laminar ShearProperties of Carbon/epoxy Composites.

9. Schwartz MM (1997), “CompositeMaterials: Properties, Non DestructiveTesting and Repair”, New Jersey, USA,Prentice Hall Inc., Vol. 1.

10. Sierakowski R L and Chaturvedi S K(1997), “Dynamic Loading andCharacterization of Fiber-reinforcedComposites”, John Wiley & Sons Inc.,New York, pp. 15-99.

11. Werner S M, Dharan C K H (1986), “TheDynamic Response of Graphite Fibre-epoxy Laminates at High Shear StrainRates”, Journal of Composite MaterialsVol. 20, pp. 365-374.

experimental evaluation of inter laminar shear … · as tension, compression and flexural loads...

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