Measurement of adhesive joint fracture properties as a function of environmental degradation

The degradation of the fracture strength of two epoxy adhesives was measured using a new approach to accelerated aging. 'Open-faced' specimens were prepared by applying adhesive to aluminum plates and, after curing, exposing them to a range of temperatures and humidities. At various times, the adhesive layer (in either the 'wet' or 'dry' states) was bonded to a second aluminum adherend to form a double-cantilever-beam fracture specimen. The critical strain energy release rate was then measured at several mode ratios by ensuring that the crack followed a path in the 'primary' adhesive. This approach yielded fracture data which unambiguously corresponded to a particular, uniform state of degradation. Moreover, the rate of water absorption was greatly accelerated. The 'wet' state fracture data illustrated the combined effects of water plasticization (reversible upon drying) and degradation, while the 'dry' state data showed only the irreversible effects of degradation. (C) 1998 Elsevier Science Ltd. All rights reserved.