Influence of stress history on time-dependent spall in metals

S can be defined as a complete or partial separation of a material resulting from the tension induced by the interaction of two rarefaction waves Since the first reported observation of spall by Hopkinson in 1914, the contributions to the field have consisted, for the most part, of the measurement of the stress necessary to produce spall These measurements are generally not in agreement because of appreciable variation in the analyses used to determine the stress as well as in the experimental procedures employed It must be realized that, even though spall occurs in but a few micioseconds, the sequence of events leading to spall is orderly 1 he sequence is initiated by subjecting the material to shock-induced stress, which is usually produced by explosive loading or by impacts The stress waves and their subsequent interactions with material boundaries and with one another ultimately, under proper conditions, pioduce a tension wave The stress waves influence the microstiucture of the material, and thereby condition the mateiial It follows that, if a complete description of spall is to be compiled, each event in the sequence and the response of the material to these events must be considered The description of spall is fai fiom complete However, advances made in the disciplines which parallel the study of spall, such as dislocation theory, ciack propagation, and stress wave analysis, have permitted some insight into the mechanism of spall One of the more significant contributions has been the determination of the dynamic stress-strain relations in one-dimensional strain which describe the stress history and permit, to a limited extent, the reconciliation of the reported data It is with this aspect of the problem that this paper is principally concerned The microscopic mechanisms of defoimation and fracture aie discussed to a lesser extent because of the paucity of infoimation available