The limiting speeds of dislocations

The purpose of this article is to show that the limiting speeds of moving dislocations are determined by inertial effects, or by viscous drag, at their cores. Simple expressions for the limiting speeds are derived. The standard idea that the speeds are limited by the inertia of the elastic fields, accompanied by Lorentz contractions, is shown to be flawed because it neglects the angular momentum of a moving dislocation; or, equivalently, because it assumes that the motion is steady if the velocity is constant, but this is not possible because the motion creates plastic deformation. Finally, the standard theory considers the dislocation to be moving in an infinite medium but this is not acceptable, because then both the energy and the mass would then be infinite. This article is based on a presentation given in the symposium entitled “Dynamic Behavior of Materials—Part II,” held during the 1998 Fall TMS/ASM Meeting and Materials Week, October 11–15, 1998, in Rosemont, Illinois, under the auspices of the TMS Mechanical Metallurgy and the ASM Flow and Fracture Committees.