Effect of Environmental Exposure on the Fracture Properties of an Epoxy Adhesive

An experimental study was carried out about the effect of warm moisture on the fracture energy of an adhesive system formed by steel adherends joined by an epoxy adhesive. The specimens were double cantilever beams, manufactured with the open-face technique. The halves of the specimens (single beams covered by the ‘‘primary’’ adhesive layer) were degraded in a climatic cell at 50 °C and 100% RH, for time periods in the range of 1–5 weeks. At the end of each desired period, the half-specimens were dried, to account for irreversible effects only, and the bonds were completed with the second halves. Static fracture tests were carried out on an Instron 100 kN machine, monitoring crack propagation with a charge-coupled device microscope. The data were processed by means of the Simple Beam Theory to calculate the strain energy release rate and the R-curves were obtained. Similar trends were found for initial and steady critical strain energy release rates: compared to unexposed specimens, great part of the reduction (about -50%, -40%, respectively) occurred in the first week, stabilized values (about -80%, -60%) were observed after 3 weeks. The load-displacement curves decreased likewise. The fracture, mostly interfacial, followed a stepped crack path between the adherends.