Numerical Prediction of Forming Limit in Hemispherical Punch Bulging by Lemaitre's Ductile Damage Model

Abstract

Prediction of sheet metal forming limits or analysis of forming failures is a very sensitive problem for design engineers of sheet forming industries. In this paper, first, damage behaviour of St14 steel (DIN 1623) is studied in order to be used in complex forming conditions with the goal of reducing the number of costly trials. Mechanical properties and Lemaitre's ductile damage parameters of the material are determined by using standard tensile and Vickers micro-hardness tests. A fully coupled elastic-plastic-damage model is developed and implemented into an explicit code. Using this model, damage propagation and crack initiation, and ductile fracture behaviour of hemispherical punch bulging process are predicted. The model can quickly predict both deformation and damage behaviour of the part because of using plane stress algorithm, which is valid for thin sheet metals. Experiments are also carried out to validate the results. Comparison of the numerical and experimental results shows good adaptation. Hence, it is concluded that finite element analysis in conjunction with continuum damage mechanics can be used as a reliable tool to predict ductile damage and forming limit in sheet metal forming processes.