The effect of forming history and strain rate on forming limit diagrams

Abstract

Metal sheets produced from higher strength steels are now extensively used in the automotive industry. Products formed from these materials are used mainly for parts expected to resist high impact loading. This report presents FLD’s determined at punch velocities from 2.08*10^-4 m/s up to 22.5 m/s. A special experimental stand, able to produce striking velocities up to 50 m/s, had to be built for these measurements. Limiting strains for selected materials are evaluated at different stress states. The effect of the forming history was evaluated on specimens already deep drawn and then loaded by high velocity impact till fracture. Different punch velocities, magnitude and direction of the first draw have influenced the limiting strains. Higher strength steel TRIP steel RAK 40/70 was used for the experiments. The research results should be used in the motorcar industry for design, manufacturing and crash test simulation.