Fracture toughness of rubber-modified epoxy adhesives: effect of plastic deformability of the matrix phase

The fracture toughness of adhesively bonded compact tension (CT) and bulk single-edge notched bend (SENB) specimens using rubber-modified epoxy resins was investigated in terms of the relation between the J-integral value and crack extension, whereby the rubber-modified epoxy resins allow to alter the plastic deformability by changing curing temperature. For the SENB specimen, the crack extension is very small and the J-integral value increases with enhanced plastic deformability. On the other hand, for the CT specimen, a considerable crack extension is observed, furthermore, when the crack extension is nearly equal to 0.2 mm, the J-integral value does not vary with curing temperature; however, the slope of the J–(delta)a curve slightly decrease with enhanced plastic defomablity. To elucidate these behaviors, assuming that the resins are governed by the Gursonʹs constitutive equation, the void–volume fractions near the crack tip of both the specimens were calculated by finite element method. It was found that the characteristics of J–R curves can be explained from the results of calculated void– volume fraction.