Strain-rate sensitivity and effects of strain rate in sheet forming

Abstract

The strain-rate dependencies of flow strength and strain-hardening behaviour are particularly important in sheet-forming operations because they influence strain distribution and necking resistance. Measurements made in uniaxial and biaxial tensile tests on low-carbon steels have shown that strain-rate sensitivity is a function of both strain and strain rate, and that it varies with dissolved interstitial solute content. It is concluded that some simple constitutive equations which are commonly used to describe flow-strength–strain-rate relationships do not provide generally reliable bases for predictions of limit strains in sheet-forming operations. Variations in the plastic behaviour of the workpiece material which resultfrom changes in the speed of pressing can have important influences on performance in practical forming operations, but pressing speed–formability relationships are usually complicated by simultaneous changes in process variables such as lubricant behaviour and the extent of deformation heating. Evidence provided by small-scale stretch-forming and deep-drawing tests show that, while some of the more important effects of interaction between speed-dependent material and process variables can be rationalized in qualitative terms, there is a needfor more searching analysis of the interacting system as a whole under conditions which are representative of industrial sheet-forming processes.