Prediction of Energy Release Rates for Echelon Crack Formation in Mode III Delamination Toughness Tests

The energetics leading to the formation and growth of an array of echelon cracks along a planar delamination front in a split laminate subjected to anti-plane shear loading are studied. The finite element method is used to model both single and multiple echelon cracks that intersect the planar delamination front. Energy release rates are determined along the echelon crack peripheries and along the planar delamination front. Various echelon crack shapes are evaluated in order to represent progressive stages of growth. It is shown that the echelon cracks advance due to primarily mode I conditions, whereas a mode III criterion is appropriate for predicting advance of the planar delamination. It is further shown that mode I advance of the echelon crack and mode III advance of the delamination are competing yet coupled processes, and that the sequence of events predicted by this approach agrees with what has been observed experimentally.