Formability limits in sheet-bulk forming

This paper is focused on the characterization of the fracture limits in sheet-bulk forming. The approach extends to crack opening in mode III (out-of-plane shearing), a digital image correlation-based methodology for determining the fracture forming limits in mode I (tension) and mode II (in-plane shearing). For this purpose, a sheet lengthwise compression test with different end constraints is developed and utilized to obtain the strain loading paths up to fracture in mode III, for the first time directly from sheets. The three fracture forming limits of sheet-bulk forming are first characterized in principal strain space and then transformed into the space of effective strain vs. stress triaxiality by means of an analytical procedure based on anisotropic plastic deformation under proportional loading. A new uncoupled ductile damage criterion is introduced and successfully implemented in an in-house finite element computer program to predict the location where the out-of-plane shearing cracks are triggered. The overall results point out to the difficulty in merging the three different fracture forming limits into a single-branched fracture locus covering the plane stress deformation conditions and the three-dimensional states of stress that are likely to be found in sheet-bulk forming processes.