Nonlinear effects in mixed-mode interfacial delaminations

Abstract

The initiation of coplanar, interfacial cracks is considered with a view to examining mixed-mode fracture parameters and criteria. The point-loaded blister configuration provided a simple axisymmetric geometry and table crack growth between glass and an epoxy. Crack opening displacements normal to the interface were measured with submicron resolution using crack opening interferometry. The measurements were made within 2.7 μm from the crack front over a field of view of 500 μm, thus allowing the extent of inelastic behavior to be evaluated. A complementary finite element analysis was used to determine tangential crack opening displacements and other linear elastic mixed-mode fracture parameters, all of which were found to increase with relative increases in mode II component. The results were attributed to related increases in the extent of material nonlinear behavior in the crack front region. The normal crack opening displacement fields in the plastic zone exhibited a power law dependence on radial distance from the crack front. Critical values of normal crack opening displacements evaluated at a distance of 12.7 μm from the crack front were not constant under the range of mixed-mode conditions examined, indicating that any fracture criterion is likely to be a function of the mode-mix for the particular material combination considered here.