Gurson–Tvergaard–Needleman model is widely used to describe the three stages of ductile tearing: nucleation, growth and the coalescence of micro-voids. The aim of this article is to study the relationship between volume fraction of voids and the fracture strain _f. The effects of the volume fraction of nucleation, f_N, and the critical volume fraction, f_c, were analysed. These parameters play crucial roles in the process of ductile damage. A phenomenological analysis is carried out to study the relationship between the different void volume parameters and the fracture strain _f. A method is proposed for the determination of f_N and f_c, knowing the experimental fracture strain _f. The experimental parameters are extracted from the load–diametric contraction curve of an axisymmetric notched tensile bar test AN2.