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The question of whether a dislocation can accelerate through the shear-wave speed "barrier" is addressed by analyzing the transient motion at the instant when the velocity equals the shear-wave speed in the presence of acceleration. The stresses carried by the forming Mach wave fronts depend on the acceleration at this instant, and the energy required to push the dislocation through the shear-wave speed barrier is determined by means of the "contour-independent" dynamic J integral, which defines the self-force on a moving defect, and is obtained as a function of the acceleration as it crosses the barrier. For decelerating motion through the shear-wave speed barrier this energy is released as dissipation.

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