TOUGHENING MECHANISMS IN MIXED-MODE INTERFACIAL FRACTURE

A series of mixed-mode interfacial crack initiation experiments were conducted using thinner glass/epoxy specimens than had been used in the past [Liechti and Chai (1992, J. Appl. Mech. 59, 295-304)]. Three-dimensional effects were eliminated in the sense that the crack front shapes were the same for all mode-mixes. Nonetheless, the thinner specimens were associated with an overall increase in toughness and a noticeable change in the distribution of toughness with mode-mix. Interferometric measurements of normal crack opening displacements (NCOD) revealed values that were lower than predictions based on rate independent and dependent plasticity and even elastic analyses. The lower NCOD were attributed to long range cohesive crack face tractions, possibly due to electrostatic effects. Shielding calculations suggested that the long range tractions were more important than plastic dissipation. Nonetheless, they did not fully explain the shear-induced toughness effect that has been noted for this and other interfaces.