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/epox y composite and its composite/composite bonded joint are compared. Tests we re conducted under mode I (DCB), mode II (ENF) and mixed mode I/II (MMB) lo ading. Mode partitioning was performed using beam theory. The initiation fa ilure point was defined both by the non-linearity point on the load/displac ement curve and by the first acoustic signal. The main results concern test stability and fracture energies: crack propagation was unstable in adhesiv ely bonded joint specimens under mode I loading but stable under mode II lo ads. This is in contrast to the expected behaviour and to that seen in comp osite interlaminar fracture. Fracture energies are much higher for the join ts than for the composite specimens. The fracture envelopes obtained can no w be used in the prediction of the failure loads for structural joints. (C) 2000 Elsevier Science Ltd. All rights reserved.