Microstructural parameters influencing shear band propagation and, ultimately, fracture in steels are reviewed for two types of test. These are the plane strain tension characteristics of spheroidized steels and the mixed mode I/III J-resistance behavior of a rotor steel. In the former case, shear band propagation is associated with voids forming at carbide particles because of incompatibility effects. In the latter case, the mixed mode I/III toughness is less than that in either pure mode I or pure mode III because local plastic flow in the path of the crack produces damage in the form of voids by a process analogous to the first case. A model for the interaction of a shear band and particles, suggested by these results, is proposed in the form of a discontinuous tilt wall of dislocations that breaks away and propagates past the particles. The model is shown to be consistent with several experimental observations, both of shear band characteristics and of the stress required to propagate the band.

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