Step loading, coaxing and small crack thresholds in Ti-6Al-4V under high cycle fatigue

The conventional method for determining fatigue strength at high cycle counts is to either extrapolate S-N data from lower cycle counts or to generate S-N data in the high cycle count regime using specialized high frequency test machines. As an alternative, a step-loading method may be employed, but the question arises regarding any effect due to coaxing, a phenomenon that causes an elevation of the fatigue stress due to prior cycling below the fatigue limit. Data have been obtained which validate the step-loading methodology in Ti-6AI-4V for determination of both the fatigue limit stress for smooth bars, and crack growth thresholds for cracked bodies. It is shown that fatigue limits for small cracks are not drastically altered from uncracked material when using stress rather than stress intensity as the governing parameter with which to characterize threshold behavior. Some observations on the fatigue process in smooth bars tested at negative stress ratios are presented.