Extracting post-uniform constitutive behavior from high temperature tensile test data

Constitutive model development for the simulation of hot forming operations is traditionally conducted using hot compression or torsion data, yet neither is possible for thin sheet metal due to the possibility of sample buckling. It is widely recognized to be difficult to extract constitutive behavior from tensile test data at strain levels adequate for simulating metal forming. However, at the temperatures required for warm forming of magnesium alloys, the strain rate sensitivity can be sufficient to delay the onset of catastrophic localized necking up to significant strain levels. This permits the extraction of true stress-true strain data even in cases where modest strain softening, e.g., due to dynamic recrystallization has occurred. A protocol involving on-sample measurements of the strain within a localized region and invoking the Marciniak-Kucinski approach for describing the development of plastic instability is used to constrain the parameters of an internal state variable model due to Bammann, Chelsea, and Johnson.