THE STRESS TRIAXIALITY CONSTRAINT AND THE Q-VALUE AS A DUCTILE FRACTURE PARAMETER

Ductile fracture of metals occurs as a result of nucleation, growth an d coalescence of microscopic voids that initiate at inclusions and sec ond phase particles. The main parameters that influence void nucleatio n and growth, and hence ductile fracture, are the triaxiality factor a nd the plastic strain. The triaxiality factor is widely used as a cons traint parameter. Recent advances highlight the loss of J-dominance in low constraint geometries and the importance of using two-parameter t heories, namely J-T and J-Q to characterise near crack front states of yielded crack geometries. In this paper we use three-dimensional fini te element models of low constraint geometries to study the variation of the triaxiality factor, plastic strain and e-value with the deforma tion level. Comparisons between the triaxiality factor, the plastic st rain and the e-value are made at different distances ahead of the crac k front. Our numerical results show that, for a given material, there exists a unique linear relationship between the triaxiality factor and the e-value that is independent of specimen geometry, dimensions, cra ck depth and deformation level. This unique relationship shows that th e e-value dan be used as a ductile fracture parameter as it parameteri ses both the stress triaxiality and the plastic strain. It can be conc luded that the e-value and the stress triaxiality factor are equivalen t constraint parameters. (C) 1997 Elsevier Science Ltd.