On the effect of shear stresses on the fibre failure behaviour in CFRP

Parts made of carbon fibre reinforced plastics (CFRP) are being increasingly used in high performance parts subjected to very high loads. Nevertheless, the failure behaviour of these materials is still not understood completely. One important open question is whether superimposed shear stresses influence the fibre failure due to high fibre parallel tension or compression. Under these loading conditions the predictions of common failure criteria, e.g. Tsai/Wu, Hashin and Puck, are inconsistent with one another. At the Institute of Plastics Processing (IKV) in Aachen, Germany, extensive mechanical tests are conducted to clarify this situation by experimental results. The outcome so far shows that there is no significant effect of a shear stress on the fibre parallel tensile strength. The effect on the fibre parallel compression strength is not clear without ambiguity. A low shear stress does not cause a reduction of the strength, whereas shear of moderate magnitude seems to promote the development of a compression failure of the fibres. Nevertheless, this statement needs to be verified, because the loading case of fibre parallel compression combined with high superimposed shear stress could not be examined so far.