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A fractographic study of Mode II fracture surfaces has been conducted for the purpose of identifying the microstructural mechanisms responsible for fracture in the edge-sliding mode. A compact shear (CS) specimen developed by the authors was employed to generate the fracture surfaces and also to establish Mode II fracture toughness values (KIIc) for Ti-6Al-4V, A533-B steel, and several aluminum alloys. In all tests, one of the two edge cracks sustained complete Mode II fracture while the other exhibited only a limited amount of subcritical crack growth. Mode II fracture surfaces, which were unique in appearance, have been examined by optical and scanning electron microscopy.
It was determined that shear (or parabolic) microvoid coalescence was the dominant mechanism for Mode II fracture. The smallest dimples were about ½ to 1 µm in diameter and were about the same size for all alloys studied. However, a significant portion of the fracture surfaces were abraded against their mating crack surfaces so that no initiation mechanisms could be observed in these regions. It was established that most of the surface abrasions were created by the relative sliding of the fracture surfaces during unstable fracture rather than the crack initiation process. Some tendency for brittle fracture on crystallographic planes of maximum shear was also observed.
fractography, microstructure, fracture tests, crack propagation, edge-sliding mode, shear microvoid coalescence, Mode II fracture
Associate research professor, School of Engineering and Applied Science, The George Washington University, Washington, D.C.,
Research mechanical engineer, Federal Highway Administration, Washington, D. C.,