Interfacial mixed-mode fracture of adhesive bonds undergoing large deformation

Interfacial fracture of adhesive bonds undergoing large-scale yielding is studied using a combined experimental/finite-element approach. The full range of in-plane mode mixity is produced over bond thickness ranging from 30 to 500 μm using the scarf and the ENF joint geometries. Novel techniques for introducing pre-cracks and surface decoration, together with in situ observations, facilitate accurate determination of the bond-average and the local shear strains at the crack tip during the onset as well as the rest of the crack propagation event. The crack generally grew along one of the two interfaces of the bond, although the failure was always fully cohesive. The local shear strain at the crack tip is independent of the bond thickness, and, under quasi-static conditions, it remains constant throughout the growth, which make it a viable fracture parameter. This quantity strongly depends on the mode mixity, the sign of the phase angle (i.e., shearing direction) and the crack speed, however.A finite-element analysis is used to obtain the crack tip deformation field for an interface crack in adhesively bonded scarf and ENF joints. Large-strain and quasi-static conditions are assumed. A distinct material model in the fracture process zone that allows for volume change in the post-yield regime is incorporated into the analysis. The local deformation is characterized by a pair of bond-normal and tangential displacements corresponding to the nodal points adjacent to the crack tip. The critical values of these quantities are obtained when the FEM bond-average shear strain at the crack tip becomes equal to its experimental counterpart. The so defined critical local displacements, after an appropriate normalization, seem to conform to a single-valued, linear type interrelationship over the entire range of mode mixity. The fact that this relationship is independent of the bond thickness, and furthermore it encompasses both cases of positive and negative phase angles, makes it a viable candidate for characterizing mixed-mode interfacial fracture under large-deformation conditions.