Fracture R-curve characterization of toughened epoxy adhesives

The R-curve behavior of two different rubber-toughened epoxy adhesives was measured as a function of the mode ratio. A bilinear model was used to characterize the fracture resistance (R-curve) behavior from crack initiation to steady-state crack propagation. Experiments showed that the model parameters depended strongly on the loading mode ratio and the adhesive bondline thickness, but were largely independent of the crack initiation geometry. The results are relevant to the prediction of the crack initiation load and ultimate strength of adhesive joints having relatively short overlap lengths such that a steady-state damage zone cannot develop prior to rupture.