Microcracks, and the static and dynamic elastic constants of annealed and heavily cold-worked metals

Abstract

The existence of sub-microscopic cracks in heavily cold-worked metals has been suggested in explanation of the very large discrepancies in dislocation densities estimated from changes in density and electrical resistivity, and release of stored energy on annealing such materials. It is shown that microcracks will produce changes in the elastic constants of a material comparable with - but somewhat larger than - the change in electrical resistivity, rather than with change in density. Measurements have been made, both statically and dynamically, of Young's modulus, the shear modulus and Poisson's ratio for annealed and heavily cold-worked aluminium, copper and nickel. No changes in moduli were found that could be ascribed to the presence of microcracks. An unexpected result is a very large and consistent difference between the bulk moduli, and Poisson's ratio, determined statically and dynamically - a difference very much greater than can be accounted for by the usual thermodynamic relationship between isothermal and adiabatic moduli.