Differential Testing and the Evaluation of Thermal Activation Parameters

Abstract

Thermally activated mechanisms of dislocation motion and interaction account for the dependence of stress on strain, temperature, and time during plastic deformation. Much of the recent activity in identifying particular mechanisms has involved the use of differential type thermomechanical tests. Due to certain basic and recurrent oversights in applying thermal activation theory to such test data, the results are oftentimes misleading. These limitations are identified and explored. Minimum requirements for an accurate characterization of strain history effects are established. A limited amount of self-contained isothermal data, for aluminum and lead, have been obtained in the intermediate strain-rate range which critically test the hypotheses. The present results indicate that the results reported by others may be in substantial error. Much of the discussion presented also applies, in its most basic form, to the evaluation of constitutive models which are not microstructurally based.