Experimental evaluation of mixed-mode fracture in adhesive bonds

Previous tests have shown that the mode II and mode III fracture energies of adhesive bonds coincide, with the nominal value dictated by post-yield shear deformation in the interlayer. This suggests that the complete mixed-mode fracture behavior may be dellineated by determining the interaction curve in either the G_I-G_II or G_I-G_III plane. A DCB-type specimen capable of delivering the entire opening versus shearing toughness spectrum in a single test was used. A brittle and a ductile epoxy resin were evaluated, with the adhesive thickness varying from a few micrometers up to 0.6 mm. Excluding very thin bonds, the mixed-mode fracture curve was approximately bilinear; when the applied energy release rate in shear, G_s, was relatively small, the total fracture energy equaled G_IC but otherwise, the fracture curve decreased essentially linearly with increasing G_S. In the case of the ductile adhesive, the transition in trends occurred when G_S was approximately 55 percent of the shearing fracture energy. When the bond thickness was decreased to a few micrometers, the mixed-mode curve displayed a concave shape, with mode interaction occurring promptly. SEM analysis and analytical considerations suggest that this change in mixed-mode behavior was due to the development of a triaxial state of stress in the interlayer. Based on previous fracture studies of the individual fracture modes in adhesive bonds and laminated composites, the present results should be also applicable to mixed-mode interlaminar fracture of laminated composites.