Influence of the mode mix of precracking on the delamination toughness of laminated composites

A study is described that evaluated the effect of the mode mix used to create a precrack on the perceived delamination toughness of laminated composites. Mode I double cantilever beam, mode II end-notched flexure, and mixed-mode symmetrically delaminated single leg bending tests were first performed on unidirectional graphite/epoxy specimens with mode I precracks. The tests were then repeated on new unidirectional specimens with mode II precracks. Through an examination of the mechanisms involved, it is shown that toughness values for materials that do not exhibit fiber bridging will be most accurate, at all mode mixities, if test specimens are precracked at the same mode mix as the test itself.