On the prediction of the effect of process parameters upon forming limit strains in sheet metals

Plasticity analysis of sheet metal forming requires a detailed knowledge of the influence of process parameters on the stress-strain relationships from yielding up to localized necking, for accurate prediction of forming limits. Achievable strain and stress-strain relationships are sensitive to modulations in process parameters, chiefly temperature and strain rate. However, the effects of changes in strain rate and temperature are often complex as they also depend on the levels of strain, strain rate and the temperature employed. Such variations could be either triggered by the process dynamics of the forming operation or imposed for optimal exploitation of the material ductility. In this study, the influence of such process parameter modulations upon formability has been theoretically modelled, following the Sing-Rao prediction approach. The limit strains thus predicted compare favourably with experimental results for a drawing steel, thus validating the present formalism. This approach can also be adopted to accommodate non-linear straining conditions. Thus, theoretical modelling of strain-path-dependent forming limits, which has not been explored adequately so far, now becomes feasible. (C) 1999 Elsevier Science Ltd. All rights reserved.