The conventional forming limit curve (FLC) has been widely and successfully used as a failure criterion to detect localized necking in stamping. However, in stamping advanced high strength steels (AHSS), under certain circumstances such as stretching-bending over a small die radius, the sheet metal fails much earlier than predicted by the FLC. This type of failure on the die radius is commonly called “shear fracture.”
In this paper, the laboratory Stretch-Forming Simulator (SFS) and the Bending under Tension (BUT) tester are used to study shear fracture occurring during both early and later stages of stamping. Results demonstrate that the occurrence of shear fracture depends on the combination of the radius-to-thickness (R/T) ratio and the tension/stretch level applied to the sheet during stretching or drawing. Based on numerous experimental results, an empirical shear fracture limit curve or criterion is obtained. It is observed that all data points above this shear fracture limit curve show shear fracture while data points below this line show tensile failure (localized necking) regardless of the test methods used. The empirical shear fracture limit curve for several AHSS is obtained. This failure criterion can be used in computer simulations to predict the shear fracture phenomenon in the entire deformation process involved in a sheet metal stretching or drawing over a die radius.
