History Effects in Polycrystalline FCC Metals Subjected to Rapid Changes in Strain Rate and Temperature
A review is presented of available experimental data on strain rate and temperature history effects for FCC polycrystalline metals together with new experimental results on copper and lead. The latter were obtained in experiments using a torsional split Hopkinson bar. The procedure involved incremental loading of the specimen, i.e. loading first at a low constant strain rate up to a predetermined initial shear strain at which a higher strain rate is superimposed without unloading the specimen. In all cases very prononunced strain rate history effects are observed; these are stronger in lead at room temperature than in aluminum or copper. It is evident from all the data presented that history effects play an important role which cannot be neglected in deriving constitutive relations to describe the plastic behavior of metals. It is shown in this paper that the influence of strain rate or temperature on the flow stress can be divided into two parts. The initial part is due to the existing work-hardened structure at that strain level, while the second is associated with the formation history of that structure. A possible explanation for these effects lies in dynamic recovery processes which take place during the slower deformation before the imposition of the rapid change in strain rate or temperature.
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