The mean path, l, of freshly introduced dislocations in crystals, under the effect of triangular loading pulses τ(t), was shown to depend only upon the pulse amplitude, τm, and to be insensitive to the rate of stress growth. Replacement of triangular pulses, by trapezoidal ones with a constant-load plateau (τ = const) extension of up to 1h, insignificantly affected the l(τm) dependence. The data were explained in terms of the concept of quasi-static relaxation in a non-equilibrium system of dislocations that were subjected to the combined effect of time-dependent applied stresses, τ(t), coordinate-dependent internal stresses, τi(x), and a so-called dry friction, τp, which was due to the pinning of dislocations by point defects. In such a model, the l(τm) dependence was expected to saturate at τm > 2τp. This was observed, in practice, for 0.2τy < τm < 0.3τy  (where τy was the yield stress). This gave an estimate for the pinning stress, τp, which was equal to about 0.1τy. On the basis of the suggested model, a series of experimentally confirmed predictions was obtained. Thus, preliminary treatment of a sample by a series of stress pulses or by holding it in a magnetic field, which transformed the system of fresh dislocations into a more equilibrium state, sharply decreased the density of mobile dislocations which responded quasi-statically to a pulsed load. It was shown that anomalies of dislocation mobility should be observed only in sufficiently pure crystals, where τp was much smaller than τy, and should be absent from contaminated crystals; where τp ~ τy.

Dislocation Dynamics in Pulse-Loaded NaCl Crystals. V.I.Alshits, E.V.Darinskaya, M.V.Koldaeva: Physics of the Solid State, 2001, 43[9], 1703-11