Investigation of a stress concentration factor for analytical stress based failure criterion

Localized necking concerns have traditionally been predicted in sheet metal forming by comparing the strain state within a material to the associated strain based forming limit curve. However, the strain path dependence of this failure criterion is a major concern. Alternatively, an analytical model was developed by Derov et al. to predict a stress based forming limit curve and shown to exhibit less strain path dependence. A critical stress concentration factor, defined as the ratio of the effective stress in the base material to the effective stress in the necking region, characterizes the failure condition for this model. An investigation of the model with past experimental data suggests this critical stress concentration factor is a material parameter. An experiment aimed at validating this observation was performed using a Marciniak test on a steel and aluminum alloy. Preliminary results for steel are discussed. A study was also conducted investigating the use of a stress based failure criterion in numerical simulations to predict failure for a Nakajima test. Finally, the design of a biaxial loading stage which will be used to measure in-plane stresses in-situ is presented