Mode II interlaminar fracture of carbon/epoxy multidirectional laminates

This paper reports an experimental study on the mode II interlaminar fracture of carbon/epoxy multidirectional laminates. A 3D finite element analysis was first performed to define appropriate stacking sequences for end-notched flexure (ENF) specimens with starter delaminations on θ/−θ and 0°/θ interfaces. The analysis concerned the shape of widthwise distributions of the mode II strain energy release rate, GII, the level of mode-mixity, the effect of residual stresses on GII and the applicability of data reduction schemes. The experimental GIIc values increased with the ply angle θ for both θ/−θ and 0°/θ specimens. These trends were found to be in agreement with an average interlaminar stress fracture criterion. However, this criterion proved to be of limited practical usefulness, as its predictions were dependent on a characteristic length. Therefore, GIIc measurements on multidirectional specimens remain essential for the application of accurate fracture mechanics based design criteria.