Predicting the effect of through-thickness compressive stress on delamination using interface elements

The failure of interface elements is typically based on interactive mixed mode criteria for both initiation and propagation of damage. The effect of tensile through-thickness stress is normally taken into account in combination with interlaminar shear stress. When the through-thickness stress is compressive, however, its effect is usually ignored and the failure of interface elements is considered to be pure mode II. Experiments on single-lap, cut-ply and dropped-ply specimens however show that the compressive through-thickness stress can greatly increase the delamination failure stress and cannot be simply neglected.The influence of compressive stress on mode II damage evolution is investigated numerically based on the cut-ply and dropped-ply experiments. A new interfacial failure model with modified failure initiation and propagation criteria is proposed to take the effect of compression on matrix shear strength and mode II critical fracture energy, GIIC, into account. The new model uses one independently determined parameter to relate the compression to the increase in interlaminar shear strength and GIIC. With the new failure criterion applied, two types of cut-ply models and two types of dropped-ply models, using the same input parameter, all produce excellent correlation with experimental delamination stresses. As a validation case the single-lap model was run with the new criterion and the same input parameters and this also achieves very good correlation with the experimental failure stress.