Study of the transition from strain localization to fracture of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloy by experiments and phase-field modeling

Many metal materials possess the transition from strain localization to fracture, the so-called post-necking behavior, during large deformation under tensile load. It is crucial to understand and characterize the transition for the application of these materials. In this paper, the stress state dependence of the transition from strain localization to fracture of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloy is characterized and analyzed by carrying out experiments and phase-field modeling. Firstly, tensile tests for smooth and notched round bars were conducted, in which the notched specimens have different notch radius. It was observed in the tests that the sharper the notch, the shorter the transition process, which can be explained as that high stress triaxiality would shorten the transition process. To further quantify the effects of stress triaxiality in the transition process, the phase-field fracture model involving the nucleation, growth and coalescence of micro-voids is developed to simulate and characterize the damage evolution observed in experiments. The simulations show that as the stress triaxiality increases, the material undergoes less plastic deformation, whether in the initiation of strain localization or the transition process. More interestingly, it is found that there exists a critical value of stress triaxiality, which can be considered the characteristic point of brittle-ductile transition. Above this point, the material has almost no plastic deformation before localization, and brittle fracture occurs directly. This work is meaningful for evaluating the safety of metal structures under complex loads.