Role of adherend material on the fracture of bi-material composite bonded joints

The aim of this study is to investigate the effect of the adherend material on the mode I fracture behaviour of bimaterial composite bonded joints. Both single-material (steel-steel and composite-composite) and bi-material (steel-composite) joints bonded with a structural epoxy adhesive are studied. Additionally, two adhesive bond-line thicknesses are considered: 0.4 mm (thin bondline) and 10.1 mm (thick bondline). The Penado-Kanninen reduction scheme is applied to evaluate the mode I strain energy release rate. The results show that the mode I fracture energy, G(Ic), is independent of the adherend type and joint configuration (single or bi-material). G(Ic) shows average values between 0.60 and 0.72 N/mm for thin bondlines and 0.90-1.10 N/mm for thick bondlines. For thin bondlines, the failure is cohesive and the similar degree of constraint that is imposed to the adhesive by the high-modulus (i.e., steel) and/or relatively thick (i.e., composite) adherends results in similar values of G(Ic), for both single- and bi-material joint types. For thick bondlines, the crack grows closer to one of the adhesive-adherend interfaces, but still within the adhesive. The results show that the adhesive could deform similarly, although the crack has been constrained on one side by different types of adherends, either a steel or composite.