Measurement and modelling of interlaminar shear strength enhancement under moderate through-thickness compression

Interaction between compressive through-thickness stress and out-of-plane shear can enhance the shear stress at which delaminations initiate in composite materials and components. In complex loading situations this may become significant, either resulting in an under prediction of strength if it is not taken into account or even a failure to predict the correct failure mode. This effect is investigated experimentally, analytically and numerically in this work. A symmetric version of the double-notch shear test is proposed for direct measurement of the interlaminar shear strength under moderate through-thickness compression. The test has successfully measured shear strength enhancement under increasing through-thickness compression. A range of failure criteria presented in the literature has been compared to the results obtained. Finally the tests have been modelled using finite element analysis. User defined interface elements which take account of the effect of compression enhancement allow the experimental results to be accurately predicted.