Crack initiation mechanisms of Ti6Al4V in the very high cycle fatigue regime

High frequency fatigue tests were carried out with a 20 kHz ultrasonic testing facility to investigate the cyclic deformation behavior of Ti6Al4V in the Very High Cycle Fatigue (VHCF) regime in detail. The S,Nf-curve at the stress ratio R = −1 shows a significant decrease of the tolerable stress amplitude and a change from surface to subsurface failures in the VHCF regime for more than 107 cycles. Quantitative microscopic investigations of the distribution of the α- and β-phase of Ti6Al4V show that inhomogeneities in the phase distribution exist at subsurface crack initiation sites. This could be proven in scanning electron microscopic as well as light microscopic investigations. Beside the primary fatigue crack additional defects like micro cracks and crack clusters were observed in fatigued specimens. SEM-investigations of specimens loaded up to 1010 cycles without macroscopic failure show irreversible microstructural changes and defects in the subsurface volume. Two step tests were performed to evaluate the influence of fatigue induced defects observed in specimens which did not fail within 1010 cycles.