Tensile flow and instability in galvanized, rolled steel sheet

Abstract

This paper examines flow and instability of a zinc-clad sheet used for the manufacture of car panels. Three forming properties are reported from off-axis tensile testing: (a) anisotropy in the plane of the sheet, (b) tensile instability and (c) homogeneity of deformation. The analysis correlates the directional flow behaviour using the Hill yield function (1)† as a basis for equivalence. This function explains satisfactorily the linear plastic strain paths and the variation in r values observed but does contract flow behaviour to a single curve. A subtangent is derived for the off-axis testpiece in terms of its orientation and two strain ratios referred to its axis. This is used to derive the limiting uniform strain that is found to remain constant in regions exterior to local neck formation. A condition is established for which the principal axes of stress and plastic strain remain coincident. It is shown, from the measured rotations of the principal material axes, that deformations conform to standard strain transformation equations upon removal of rigid-body rotations.