Frequency and stress ratio effects in high cycle fatigue of Ti-6Al-4V

Experiments were conducted to determine the effects of test frequency on the high cycle fatigue (HCF) behavior of Ti-6Al-4V. Specimens were tested at a range of frequencies for three different stress ratios (0.1, 0.5, and 0.8). Results indicate a frequency dependence that varies with stress ratio (R). At low R, the fatigue strength increases with increasing frequency. As R is increased, this trend continues until a transition zone is reached. Above this transition the frequency effect seems to vanish at the lower frequencies, but it is still apparent at high frequency. In addition, the material exhibits ductile behavior above the transition point. The ductile behavior is characterized by void nucleation and growth and the apparent lack of fatigue crack initiation and growth that is evident at low stress ratios. Scanning electron microscopy is used to examine the failure mechanisms and transition regions. Several possible explanations are presented to explain the observed frequency and stress ratio effects. Strain accumulation at high R is observed and is attributed to cyclic strain ratchetting.