Shock Plasticity Reconciled with Uniform High-Rate Plasticity

Recent papers describe plastic flow in metals as departing from a relatively rate-independent regime to a highly rate sensitive regime at rates above about 10⁴ s⁻¹. Analysis of recently published data in copper suggests that the departure is due to the breakdown of collective dislocation motion which prevents the formation of the larger slip bands, forcing the stress to rise. As the rate increases still further, the stress required to generate dislocation loops in the perfect lattice is reached. Then a new regime is established, in which a Lϋders band propagates through the metal. This band might well be related to what is often called ‘adiabatic shear’, characteristic of the plastic response at very high rates. The characteristic power laws corresponding to the different regimes can be rationalised.