Mixed mode failure criteria for a glass/epoxy composite and an adhesively bonded composite/composite joint

Mixed mode initiation failure criteria for the delamination of a glass/epoxy composite and its composite/composite bonded joint are compared. Tests were conducted under mode I (DCB), mode II (ENF) and mixed mode I/II (MMB) loading. Mode partitioning was performed using beam theory. The initiation failure point was defined both by the non-linearity point on the load/displacement curve and by the first acoustic signal. The main results concern test stability and fracture energies: crack propagation was unstable in adhesively bonded joint specimens under mode I loading but stable under mode II loads. This is in contrast to the expected behaviour and to that seen in composite interlaminar fracture. Fracture energies are much higher for the joints than for the composite specimens. The fracture envelopes obtained can now be used in the prediction of the failure loads for structural joints.