Observations on fracture in axi-symmetric and three-dimensional cold upsetting of brass

This paper describes the appearance of fractures in cold forming of brass during. axi-symmetric collar upsetting tests and also in the upsetting of hexagonal shaped bars. The latter has been introduced to examine fracture under non-axi-symmetric conditions. The collar tests produced typical ductile fractures but the hexagonal bars displayed fractures similar to brittle fractures even though they were preceded by large plastic strain. Each experimental test was also modelled using the finite-element method to examine local stress/strain conditions at fracture. The simulations of the collar tests showed that fractures appeared in the presence of a circumferential tensile stress together with either a tensile or compressive axial stress. The compressive axial stress was present when fracture occurred at a much lower level of deformation than when the axial stress became tensile. The simulations of the hexagon bar upsetting revealed a maximum plastic strain coincident with the most likely site of fracture initiation. This mixture of differing stress and strain states at fracture leads to the conclusion that a single fracture criterion is unlikely to satisfy all the conditions that lead to fracture. (C) 2002 Published by Elsevier Science B.V.