Fatigue properties of titanium alloys disclosed in NIMS fatigue data sheets

ABSTRACTNational Institute for Materials Science fatigue data sheets disclose fatigue data on Ti-6Al-4 V alloys, pure titanium, and Ti-6Al-4 V ELI alloys. The fatigue tests applied were low-cycle, high-cycle and gigacycle types. Low-cycle tests revealed that fatigue lives evaluated by strain amplitudes were roughly the same between the alloys despite differences in tensile strength. Ti-6Al-4 V alloys revealed cyclic softening, while pure titanium revealed cyclic hardening. The cyclic yield stress of the titanium alloys was higher than that of conventional steels. In contrast to the results for pure titanium, the high-cycle fatigue tests on Ti-6Al-4 V alloys revealed internal fractures without conventional fatigue limits. Internal fractures were more frequent at high-stress ratios than at R = –1. Internal fractures from interiors created fish-eye patterns on the fracture surfaces, whereas no fish-eye patterns were observed in internal fractures from sub-surfaces. The internal crack initiation sites revealed clear facets at high-stress ratios without inclusions. The gigacycle fatigue tests on Ti-6Al-4 V alloys revealed the results of ultrasonic fatigue testing at 20 kHz to be equivalent to those for conventional fatigue testing at 120 Hz in cases where internal fractures occurred. Fatigue failures at over 109 cycles were very rare, suggesting the presence of new fatigue limits in the gigacycle region. Degradation of gigacycle fatigue strength was very minor at R = –1, while at high-stress ratios degradation was marked, with the modified Goodman line entering the danger zone, revealing this behaviour to be the main shortcoming of Ti-6Al-4 V alloys.