Fracture R-curve of a toughened epoxy adhesive as a function of irreversible degradation

Open-faced double cantilever beam (DCB) specimens of a toughened epoxy-aluminum adhesive system were degraded over a relatively wide range of temperature, relative humidity (RH) and exposure time, dried and tested to characterize the irreversible evolution of the mixed-mode fracture resistance curves (R-curves). The water diffusion properties of the bulk adhesive were modeled using an earlier sequential dual Fickian (SDF) model for the same adhesive in order to predict the adhesive water content. Three temporal stages of degradation possessing different R-curve and fracture surface characteristics were observed. In general, the steady-state critical strain energy release rate (Gcs), the rate of toughening (dGcr/da) and the length of the rising part of the R-curve decreased with increasing exposure temperature, RH and water concentration, while the initiation Gc (Gci) remained unchanged. It is hypothesized that crack initiation is governed by the properties of the epoxy matrix and that the toughening action of rubber particles does not become appreciable until after a certain amount of crack extension (more than about 50 μm in the present case). The irreversible degradation of fracture toughness was found to be insensitive to the phase angle, which simplifies the construction of the fracture toughness envelope for a given level of degradation. These effects were incorporated into a new R-curve degradation model which has an application in the R-curve prediction for closed joints having nonuniform degradation.