Fracture predicting in bulk metal forming

An important concern in forming is whether the desired deformation can be accomplished without failure of the work material. This paper describes the utilization of ductile fracture criteria in conjunction with the finite element method for predicting failures in cold bulk metal forming. Four previously published ductile fracture criteria are selected, and their relative accuracy for predicting and quantifying fracture initiation sites is investigated. Experiments with ring, cylindrical, tapered and Banged upset samples are performed to investigate the validity of the workability criteria under conditions of stress and strain similar to those usually found in bulk metal forming processes. The implementation of ductile fracture criteria into a rigid-plastic finite element computer program is presented. Local stress and strain distributions throughout the deformation are computed and compared with experimental measurements. A general good agreement is found. However, only two of these workability criteria have successfully predicted the location at which fracture initiates for all the upset tests performed in this work. The paper concludes with a discussion of the importance of the critical damage at fracture to remain independent from the technological processes.