Mixed mode I + II interlaminar fracture of carbon/epoxy laminates

Abstract

This paper describes a study of the mixed mode I + II fracture of carbon/epoxy unidirectional and multidirectional specimens with symmetric, quasi-symmetric and markedly asymmetric legs. A beam model was developed for predicting the compliance and total strain energy release rate, G. Model predictions agreed well with three-dimensional finite element results for double cantilever beam, end-notched flexure and mixed mode bending tests. The mode mix was evaluated from a virtual crack closure technique limit analysis, which overcomes ambiguity in delaminations between differently oriented plies. Experimental specimen compliances were in good agreement with beam model predictions. Initiation critical strain energy release rates Gc were plotted against the mode II ratio GII/G. Fracture loci of specimens with symmetric and quasi-symmetric legs had practically identical shapes, showing a linear Gc increase with GII/G > 25 %. Moreover, Gc values of multidirectional specimens with quasi-symmetric and markedly asymmetric legs were very similar.