An Empirical Stretch Bending Failure Criterion for AHSS

The failure mechanism in stretch bending over a small die radius for Advanced High Strength Steels (AHSS), commonly referred as “shear fracture”, has rendered the Forming Limit Diagrams (FLD) fail to predict it based on the initiation of a localized neck. As shown in previous studies using a Stretch-Forming Simulator (SFS) and Bending Under Tension (BUT) test, shear fracture depends not only on the radius-to-thickness (R/T) ratio but also on the tension/stretch level applied to the sheet during bending. Although the stress-base empirical fracture limit criterion was developed for various AHSS grades, the fracture limit was not well implemented in the computer simulations to predict stretch bending fracture. In this paper, the new developed experimental analysis is conducted on the modified bending under tension test to further investigate the stretch bending fracture mechanism under the production die condition. Various AHSS grades including DP590, DP780, DP980 and DP1180 are included in the study. Based on numerous experimental results, the maximum shear stress at failure, the thinning strain and strain gradient across the die radius are obtained for all test materials. Results demonstrate that the presence of the large strain gradient is the cause for fracture in stretch bending AHSS over a small die radius. The maximum shear stress at failure and the limit thinning strain on the die radius in the stretch bending condition are determined and used as the new fracture criteria, which can be easily implemented in the computer simulations.