The stress triaxiality constraint and the Q-value as a ductile fracture parameter

Ductile fracture of metals occurs as a result of nucleation, growth and coalescence of microscopic voids that initiate at inclusions and second phase particles. The main parameters that influence void nucleation and growth, and hence ductile fracture, are the triaxiality factor and the plastic strain. The triaxiality factor is widely used as a constraint parameter. Recent advances highlight the loss of J-dominance in low constraint geometries and the importance of using two-parameter theories, namely J- T and J- Q to characterise near crack front states of yielded crack geometries. In this paper we use three-dimensional finite element models of low constraint geometries to study the variation of the triaxiality factor, plastic strain and Q-value with the deformation level. Comparisons between the triaxiality factor, the plastic strain and the Q-value are made at different distances ahead of the crack front. Our numerical results show that, for a given material, there exists a unique linear relationship between the triaxiality factor and the Q-value that is independent of specimen geometry, dimensions, crack depth and deformation level. This unique relationship shows that the Q-value can be used as a ductile fracture parameter as it parameterises both the stress triaxiality and the plastic strain. It can be concluded that the Q-value and the stress triaxiality factor are equivalent constraint parameters.