Coupling effects of hardening and damage on necking and bursting conditions in sheet metal forming

New materials are appearing for sheet metal components. Their use in automotive industry require careful analysis of the influence of initial and induced properties on the formability and risks of failure during the process. The proposed approach is based on experimental investigations, mechanical modelling and FE simulations to analyse the effects of hardening and damage on the development of necking, bursting and fracture in sheet metal forming. First, the proposed study focuses on stainless steel sheet metal parts obtained by rolling at different reduction ratios. A new method has been developed to identify the initial elastic modulus from vibration tests on plates and beams. Next conventional tensile tests are used to characterise the initial hardening effect on hardening laws. Then, plastic deformations directly related to stamping conditions are imposed to the specimens. The elastic properties are measured again after unloading from vibrations tests to notice the effect of induced hardening related to processing conditions. From these experimental results, a constitutive model has been developed that takes into consideration both the initial hardening and the current one. It is based on hardening model coupled with a damage one which takes into account both the effects of initial and induced hardening. The damage model affects the elastic properties as well as the plastic ones. After that, the identification of material parameters is realised using the inverse finite elements method based on the procedure developed by the authors. The proposed material models have been implemented into a fully coupled finite element dedicated specifically to the simulation of sheet metal forming process. The limits of formability are estimated through an extension of the linear stability analysis recently developed by the authors. The numerical results have been compared with experiments and the spatial positions of the critical zones at fracture as well as the values of the limiting deformations are well-predicted.