Strain ageing effects and stress relaxation in iron during shock loading

Abstract

The influence of prestraining and ageing upon the stress relaxation, which follows the elastic precursor wave during shock loading of iron, has been determined experimentally. Prestraining through the Lüders strain is observed to eliminate this stress relaxation under shock loading. Suitable time-temperature ageing treatments after prestraining cause a reappearance of the shock stress relaxation. Thus the influence of prestraining and ageing upon the stress relaxation in iron is qualitatively the same as the influence of these treatments upon the upper to lower yield point drop in conventional low strain rate tensile tests. Quantitative differences between the influence of strain ageing upon the shock stress relaxation and the conventional yield drop are observed. The return of the shock stress relaxation by ageing of prestrained iron (containing both nitrogen and carbon) appears to be governed by the diffusion of carbon while the return of the conventional yield drop appears to be governed by the diffusion of nitrogen. Further, for a given ageing temperature, the critical ageing time for the return of shock stress relaxation is from one to two orders of magnitude longer than the critical ageing time for return of the conventional yield drop. The relationship of these results to the current theories of dislocation behavior in iron containing small amounts of carbon and nitrogen is discussed.