Composite interlaminar shear fracture toughness, GIIc:: Shear measurement or sheer myth?

The concept of G(Ilc) as a measure of the interlaminar shear fracture toughness of a composite material is critically examined. In particular, it is argued that the apparent G(Ilc) as typically measured is inconsistent with the original definition of shear fracture. It is shown that interlaminar shear failure actually consists of tension failures in the resin-rich layers between plies followed by the coalescence of ligaments created by these failures and not the sliding of two planes relative to one another that is assumed in fracture mechanics theory. Several strain energy release rate solutions are reviewed for delamination in composite laminates and structural components where failures have been experimentally documented. Failures typically occur at a location where the Mode I component accounts for at least one half of the total G at failure. Hence, it is the Mode I and mixed-mode interlaminar fracture toughness data that will be most useful in predicting delamination failure in composite components in service. Although apparent G(Ilc) measurements may prove useful for completeness of generating mixed-mode criteria, the accuracy of these measurements may have very little influence on the prediction of mixed-mode failures in most structural components.