The equivalent strain in high pressure torsion
Abstract
The results of experiments carried out by means of high pressure torsion (HPT) are frequently described in terms of the Eichinger ``equivalent'' strain. When employed in conjunction with equivalent stresses derived using von Mises in conjunction with the Fields and Backofen analysis, the values of the work done/unit volume associated with the Eichinger-based flow curves are shown to be only a small fraction of the actual work done. By contrast, the values of the work done/unit volume that correspond to the shear stress/shear strain as well as the von Mises flow curves are in full agreement with the torque/twist work done/unit volume. When the principle of work conjugacy is employed, the Eichinger strain-based work done values are necessarily in agreement with the shear stress/shear strain and torque/twist results, but then the ``equivalent'' stresses are unrealistically high. It is concluded that only the von Mises equivalent stresses and strains should be employed in the description of HPT results and that the Eichinger ``equivalent'' strain should not be used for this purpose.