Upsetting of cylinders: A comparison of two different damage indicators

Two damage indicators for ductile failure, as proposed by Hancok-Mackenzie- Gunawardena and Atkins, are compared with experimental results from literat ure for the upsetting process of cylindrical specimens. It is shown, for th is 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 triaxia lity; the specific mathematical structure of the indicator seems to be of m inor importance. These findings give some guidelines for the practical choi ce of a damage indicator for the stimulation of industrial forming processe s, and suggest the use of the void growth based Hancock-Mackenzie-Gunawarde na indicator even for a certain class of bulk forming processes. In additio n, a slight geometry dependence of the failure lines obtained by the Hancoc k-Mackenzie-Gunawardena indicator is obtained that has so far neither been reported by experimentalists, nor can it be reproduced in a comparable mann er by the Atkins indicator. A surprising correlation is found between these results and those obtained from an entirely different micromechanical anal ysis proposed recently by one of the authors.