NUMERICAL AND EXPERIMENTAL INVESTIGATION OF INTERLAMINAR SHEAR FAILURE IN 3D WOVEN COMPOSITES USING SHORT BEAM SHEAR TEST METHOD

Abstract

Inter-laminar shear strength (ILSS) is an important mechanical property for structural composites loaded in shear. The higher ILSS translates into higher delamination damage resistance. This property is measured using short beam shear testing standards such as ASTM D2344. This standard is only approved for use with 2D woven composites, however in practice it is also being used for 3D woven composites. While the use of ASTM D2344 is well established for 2D woven composites, its applicability for 3D woven composites is arguable. Thus in the present work, the stress and damage states, resulting from short beam shear loading of 3D woven angle interlocked GFRP composites, have been investigated both numerically and experimentally. For the numerical investigation, FE analysis was carried out to simulate testing conditions of ASTM D2344. Equivalent orthotropic material definition was used and the analysis was carried out as a single step implicit analysis. The FE analysis shows that unlike the 2D woven composite specimens the 3D woven composite specimen under short beam shear test loading experience a highly non-uniform interlaminar shear stresses in the gauge section. It also shows that for 3D composites the use of classical beam theory to calculate ILSS from failure load (as suggested in ASTM D2344) is also not valid because the uniform through thickness parabolic stress distribution predicted by the theory is not achieved for 3D woven composites. From the FE analysis it is clear that for 3D woven composites a mixed state of stress rather than a pure shear state cause failure if specimen dimensions are kept same as those suggested for 2D woven composites specimen in ASTM D2344. It is also shown that the variation of interlaminar shear stress across specimen length and width is minimized if the specimen thickness is increased i.e. length to thickness ratio and width to thickness ratio is reduced. The experimental observations also show that the failure mechanisms are different from the one observed in case of 2D woven composites. The failure propagates along the through thickness warp weaver yarns instead across specimen. By increasing the thickness of the specimen, the ILSS value in the warp direction is increased however the failure mode is still different as instead of having one major delamination along the mid-plane one can detect delamination at all interfaces which may be joined by a crack travelling along the through thickness yarn. ix Thus this study has established that if ASTM D2344 is used for determination of ILSS for 3D woven composites, then appropriate changes in specimen dimensions must be made, acceptable failure mode needs to be redefined and the ILSS values for warp and weft direction must be specified separately as these values are markedly different from each other.