Transient analysis of dislocation emission from a growing crack

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A n exact transient solution for screw dislocation emission in arbitrary directions from a Mode III crack growing due to SH-wave diffraction is performed. An emission criterion based on the dislocation force is then imposed to obtain the instant of emission, the distance traveled by the emitted dislocation prior to its arrest, and the instant of arrest. The formulae for these quantities are examined for dependence on parameters such as emission angle, material yield stress and crack and dislocation speeds. Dynamic effects are clear, and order-of-magnitude evaluations suggest that the quantities are on a micromechanical scale, but not insignificant. In particular, the emission instant behavior indicates that purely brittle fracture is difficult and, moreover, that the instant itself might be incorporated in the emission criterion. Indeed, general suggestions for rendering the criterion more precise are noted. The solution and formulae are then modified to study multiple emission, post-arrest behavior, emission in the crack plane and the response of the dynamic stress intensity factor. These studies indicate that dislocation shielding is enhanced in a transient analysis but decreases after dislocation arrest occurs, and that dislocation arrest can be momentary.