Influence of the Resin on Interlaminar Mixed-Mode Fracture

Both literature review data and new data on toughness behavior of seven matrix and adhesive systems in four types of tests were studied to assess the influence of the resin on interlaminar fracture. Mixed-mode (that is, various combinations of opening Mode I [G_I] and shearing Mode II [G_II]) fracture toughness data showed that the mixed-mode relationship for failure appears to be linear in terms of G_I and G_II. The study further indicates that fracture of brittle resins is controlled by the G_I component, and that fracture of many tough resins is controlled by total strain energy release rate G_T. Regarding the relation of polymer structure and the mixed-mode fracture: high Mode I toughness requires resin dilatation; dilatation is low in unmodified epoxies at room temperature/dry conditions; dilatation is higher in plasticized epoxies, heated epoxies, and in modified epoxies; modification improves Mode II toughness only slightly compared with Mode I improvements. Analytical aspects of the cracked lap shear test specimen were explored. Geometric nonlinearity must be addressed in calculating the G_I/G_II ratio. The ratio varies with matrix modulus, which in turn varies with moisture and temperature.