In this paper, the ductile fracture mechanism is discussed. The results of the numerical and experimental analyses are used to estimate of the onset of the crack front growth . It is assumed that the ductile fracture in front of the crack starts at the location along the crack front where the accumulated effective plastic strain reaches a critical value. It is also assumed that the critical effective plastic strain depends on the stress triaxiality and the Lode angle. The experimental programme was performed using five different specimen geometries, three different materials and three different temperatures of +20°C, -20°C and -50°C. Using the experimental data and the results of the finite element computations, the critical effective plastic strains are determined for each material and each temperature. However, before the critical effective plastic strain is determined, a careful calibration of the stress–strain curves was performed after modification of the Bai–Wierzbicki procedure. Finally, by analysing the experimental results recorded during the interrupted fracture tests and scanning microscopy observations, the research hypothesis is verified.