1. Introduction

Three dimensional stamping operations are often affected by the insurgence of defects such as shape defects and ductile fractures. So far as the former are regarded, they are usually associated with the insurgence of wrinkles or puckers both on the flange and on the sheet metal between the punch and the die. It should be observed that elastic springback has to be considered as a shape defect occurring during sheet metal stamping processes since it introduces deviations from the desired final shape; consequently, the stamped sheet does not conform to the design specifications and could result in being unsuitable for the application (Makinouchi, 1996).

Springback takes place at the end of the stamping operation after removing the forming tools: actually almost all the sheet forming processes are characterised by a significant amount of deformation introduced by bending mechanics, the distribution of strains along the sheet thickness being strongly inhomogeneous. Such a distribution, together with the elastic-plastic behaviour of the workpiece determines the occurrence of springback after the removal of the forming tools (Mickalich et al., 1988; Lange, 1985).

In modern automotive industries the tendency to reduce the weight of the stamped parts has led to the use of new materials such as high strength steels and aluminum alloys. It is well known from tensile tests that the elastic amount of the total strain, which is recovered if the load is released, is equal to the...