A new combined experimental-numerical approach to evaluate formability of rate dependent materials

Regardless of their forms and variations, mechanical stretching tests have been extensively used to generate material forming limit curves that we rely on for designing and executing sheet metal forming operations. Yet the fact that they are typically performed at constant speeds presents a major obstacle in characterising formability limits for rate dependent materials such as lightweight alloys formed at elevated temperatures. In this work, a new hybrid numerical/experimental approach is presented that can be used to construct forming limit diagrams under specified strain rate loading paths in mechanical stretching tests. A new algorithm, coupled with a calibrated constitutive material model, is incorporated into a commercial FE package to control the speed of the deformation according to the desired strain rate loading paths. Simulations are carried out on AZ31 magnesium alloy at selected conditions, while undergoing mechanical stretching according to the Nakazima method. The simulation results are experimentally verified through a set of interrupted tests using several specimen geometries that cover a wide range of biaxial strain ratios. The outcome of this study provides the basis for constructing complete forming limit diagrams that can be accurately assigned to a specific strain rate value.