Experimental studies in plastic wave propagation

Abstract

T he dynamic stress-strain curves of annealed specimens of pure copper, pure aluminium and an aluminium alloy have been measured by experiments in which short bars of these materials were fired from a compressed air gun at a steel pressure bar. It was found that for copper and aluminium the dynamic curves lay appreciably above the static ones, whereas for the aluminium alloy there appeared to be no appreciable rate-of-strain effect. These dynamic results have been used to test the prediction of the strain-rate independent theory of plastic wave propagation and it was found that the theory gave a reliable relation between the permanent strain produced at the end of a bar and its velocity of impact. The theoretical predictions of strain distribution were less satisfactory and there was some indication, in the results for copper at low velocities of impact, that the simple theory needed modification in the manner suggested by Malvern. Some experiments carried out on the effect of repeated impacts showed that the simple hypothesis for strain hardening, namely that a metal behaves elastically up to the highest stress to which it has been previously subjected, led to unrealistic quantitative results when this hypothesis was applied to plastic wave propagation. The maximum stresses that could be propagated elastically along annealed bars of the three metals were measured and found to be in reasonable agreement with the static yield stresses. The anomalous results observed by other workers for copper and aluminium are attributed to the fact that they combined static and dynamic loading. The difference in the stress-strain curves between these two types of loading is then sufficient for the simple theory to become invalid.