Subcritical crack growth at bimaterial interfaces: Part III. shear-enhanced fatigue crack growth resistance at polymer/metal interface

Fatigue crack growth along an Al/epoxy interface was examined under different combinations of mode-I and mode-II loadings using the flexural peel technique. Fatigue crack growth rates were obtained as a function of the total strain energy rate for G_II/G_I ratios of 0.3 to 1.4, achieved by varying the relative thickness of the outerlayers for the flexural peel specimen. Fatigue crack growth resistance of the interface was found to increase with increasing G_II/G_I ratio. Such a shear-enhanced crack growth resistance of the interface resulted in a gradual transition of crack growth mechanism from interfacial at the low G_II/G_I ratio to cohesive at the high G_II/G_I ratio. Under predominantly mode-I loading, the damage in the polymer took the form of crazing and cavitation. In contrast, laminar shear occurred under highly shear loading, resulting in a larger amount of plastic dissipation at the crack tip and improved fatigue crack growth resistance.