Ductile fracture criteria and its prediction in axisymmetric drawing

Ductile fracture occurs due to micro-void nucleation, growth and finally co alescence into micro-crack. The ductile fracture criteria (P.F. Thomason, D uctile Fracture of Metals, Pergamon, 1990; S. Dhar et al., A continuum dama ge mechanics model for void growth and micro-crack initiation, Engineering Fracture Mechanics 53 (1996) 917) developed based on the microscopic phenom ena of void nucleation, growth and coalescence along with a simple criterio n (N.V. Reddy et al., Central bursting and optimal die profile for axisymme tric extrusion, ASME Journal of Manufacturing Science and Engineering 118 ( 1996) 579) based on the concept of the hydrostatic stress component at a po int in the deformation zone falling to zero and compressive elsewhere are u sed to predict the fracture initiation in drawing (i.e. central bursting). Even though the first two criteria are based on microscopic description, th e material parameters required are available fora few steels only and their determination involves difficult metallurgical experimentation. The above criteria used along with the results of Eulerian Rigid-Plastic and Elasto-P lastic; formulations are presented in this paper. Finite element formulatio ns for obtaining the generalized strain distribution and for obtaining the damage distribution by using the critical damage criteria are also presente d. The present study shows that predictions based on the simple criterion a re in good agreement with the experimental as well as numerical results pub lished earlier and are, in general, conservative. Further, comparison of th e predictions of the three criteria shows that the hydrostatic stress crite rion is highly conservative and hence safe for die design. (C) 1999 Elsevie r Science Ltd. All rights reserved.