Predicting instability at die radii in advanced high strength steels

Recently, the automotive industry has seen increased use of advanced high strength steels (AHSS) due to superior combinations of strength, ductility, and weldability. However, during stamping of AHSS; fractures are periodically observed along bends of small radii. These fractures have been termed ‘shear fractures’ due to limited localized necking, and fracture on alternating 45° planes, through thickness. Such fractures have proven difficult to predict using traditional measures of formability, such as the forming limit diagram (FLD). The present study outlines an approach to predict shear fractures by instability at die radii, represented by maximum applied tensile force as a function of die radius. Due to a transition from die instability to tensile instability with increasing die radius, material tensile strength is imposed as a limiting condition at large radii. Promising correlations are observed for a wide range of commercially produced AHSS including HSLA450, DP600, TRIP780, DP780, and DP980. Both experimental results and theory suggest a critical radius (normalized by sheet thickness: R/tcrit), above which materials will fail in tension, independent of die radius, and correspondingly localized shear fracture would not occur. The critical R/t value is a measure of formability, since for lower R/tcrit values there is a greater range over which materials exhibit tensile failure, readily predicted by material tensile strength. For the steels analyzed in this study, critical R/t values were found to be dependent primarily on material tensile strength, and to a lesser extent, material yield strength, strain hardening exponent (n value), and instability strain.