An investigation of the effects of stress ratio on the micromechanisms of and short fatigue crack growth in Ti-6Al-4V

This paper presents the results of a recent study of the effects of positive stress ratios on the propagation of long and short fatigue cracks in mill annealed Ti-6Al-4V. Differences between the long fatigue crack growth rates at positive stress ratios (R = K-min /K-max = 0.02-0.8) are attributed largely to the effects of crack closure. Microstructurally short fatigue cracks are shown to grow at stress intensity factor ranges below the long crack fatigue threshold. Anomalously high fatigue crack growth rates and crack retardation are also shown to occur in the short crack regime until the short crack data merge with the long crack growth rate data in the Paris regime. Differences between the long and the short crack behavior are associated with differences in fatigue crack growth mechanisms.