Influence of stress triaxiality on the failure behavior of Ti-6Al-4V alloy under a broad range of strain rates

The plastic deformation and failure behavior of metals is significantly affected by loading conditions such as stress state, strain rate, temperature and etc. In this study, the tensile behavior of a dual-phase Ti-6Al-4V alloy was investigated and the influences of stress triaxiality, strain rate and temperature were characterized. Specimens with various initial stress triaxialities (0.33, 0.43, 0.52, 0.62, and 0.74 in this work) were designed and tested under quasi-static and dynamic tension. Digital image correlation (DIC) technique was used to accurately determine the local strain on the surface of the specimen during the process of deformation. Combined experimental-numerical method was adopted to acquire the maximum local strain as well as the local stress triaxiality. Results show that the critical point of the specimen always hides in the center of the cylinder and it is unpractical to measure these values directly, therefore calibration is required for both failure strain and stress triaxiality. Moreover, our results indicate that the flow stress of Ti-6Al-4V is significantly affected by the stress condition. The notched specimens display enhanced engineering flow stress (or load capacity) compared with the smooth cylinder. On the contrary, after calibrated by Bridgman's theory, their true equivalent stresses are actually lower. These findings remind the researchers that special attention should be paid to the effects of stress triaxiality, even the general trend might be wrong.