Upsetting of Cylinders: A Comparison of Two Different Damage Indicators

Two damage indicators for ductile failure, as proposed by Hancock–Mackenzie–Gunawardena and Atkins, are compared with experimental results from literature for the upsetting process of cylindrical specimens. It is shown, for this specific example, that quite similar results can be obtained from highly different damage indicators as long as they allow for the accumulation of damage proportional both to the equivalent strain and to the stress triaxiality; the specific mathematical structure of the indicator seems to be of minor importance. These findings give some guidelines for the practical choice of a damage indicator for the simulation of industrial forming processes, and suggest the use of the void growth based Hancock–Mackenzie–Gunawardena indicator even for a certain class of bulk forming processes. In addition, a slight geometry dependence of the failure lines obtained by the Hancock–Mackenzie–Gunawardena indicator is obtained that has so far neither been reported by experimentalists, nor can it be reproduced in a comparable manner by the Atkins indicator. A surprising correlation is found between these results and those obtained from an entirely different micromechanical analysis proposed recently by one of the authors.