Interfacial Crack Propagation

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Abstract
Interfacial crack propagation studies were conducted on specimens of epoxy bonded to aluminum under both tensile and bending loads. The effect of surface roughness of the aluminum on fracture toughness was measured for four different surface finishes: polished, milled, glass-peened and sandblasted. It was found that increased surface roughness resulted in greater fracture toughness and that the interfacial cracks replicated the surface features of the aluminum. Microscopic studies of the failure surfaces indicated that crack propagation occurs in the epoxy near the interface, and that a residue of epxoy remains bonded to the aluminum. The magnitude of the residual stresses due to casting and curing of the epoxy was determined by photoelastic techniques. Methods were developed for analyzing the birefringent pattern in the epoxy to determine the magnitude of the elastic residual stress and the frozen stress. It was found that the residual stresses contributed 15 to 20% of the strain required for crack initiation at the interface.

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