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

Experiments were conducted to determine the effects of test frequency on th e 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. Abo ve this transition the frequency effect seems to vanish at the lower freque ncies, but it is still apparent at high frequency. In addition, the materia l exhibits ductile behavior above the transition point. The ductile behavio r is characterized by void nucleation and growth and the apparent lack of f atigue crack initiation and growth that is evident at low stress ratios. Sc anning electron microscopy is used to examine the failure mechanisms and tr ansition regions. Several possible explanations are presented to explain th e observed frequency and stress ratio effects. Strain accumulation at high R is observed and is attributed to cyclic strain ratchetting. (C) 1999 Publ ished by Elsevier Science Ltd. All rights reserved.