The role of competing timescales of viscoelastic stress redistribution and edge-initiated interfacial cracking in creep failure modes of pressure-sensitive adhesive joints

It is essential to know and understand the interaction between viscoelastic behaviour and interfacial defects in the long-term failure of soft adhesive joints. Investigation of single-lap joints (SLJs) with a pressure-sensitive adhesive (PSA) acrylic revealed mechanical competition between viscoelastic stress redistribution and crack initiation due to interfacial defects. The results of creep tests on PSA joints revealed three failure modes: instantaneous interfacial failure from the edge, progressive damage from the edge, and bulk creep failure, each occurring on very different time scales. Two different time scales were identified as part of the viscoelastic behaviour of the material through stress relaxation analysis, where it was also observed that failures occurring before stress redistribution are similar to joints with poor adhesion; on the other hand, those that proved to be more resistant can withstand up to the longest relaxation time scale. By examining the edge of the SLJs before the test using scanning electron microscopy (SEM), it was possible to observe small imperfections invisible to the naked eye, located at critical points of stress concentration, which were later correlated with how the joint fails.