Abstract Previous transient studies of dislocation emission from cracks have treated constant-speed motions which satisfy a dislocation force-based criterion that is, in effect, associated with micromechanical strain-hardening. This analysis requires, instead, that the force necessary to produce glide away from the crack be equal to, but not exceed, the yield stress level. As an illustration, an exact transient solution for the emission of screw dislocation from a mode III crack subjected to SH-wave diffraction is generated. Invoking the new criterion leads to an equation of motion for the dislocation. The equation shows that the dislocation initiates at an essentially zero speed, accelerates to a near-critical speed, and then decelerates to arrest at a finite distance from the crack edge. Expressions for parameters such as the instant of initiation and the arrest distance are also obtained, and studied as functions of material properties such as yield stress and Burgers vector magnitude.