Step loading for very high cycle fatigue

TLDR: In this paper, some of the unique test equipment being applied to the study of high cycle fatigue in our laboratory are reviewed. Data are presented on the long-life fatigue behaviour of Ti-6Al-4V which validate both the step-loading methodology and indicate the trend of fatigue strength as a function of cycle number in the very high-cycle fatigue regime.

Abstract: 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 special high frequency test machines. In the process of generating S-N data, runout tests often occur where no failure is obtained within the allotted time for a test. Such data points are normally indicated on an S-N plot with an arrow, but no information is available as to whether failure would ever occur. As an alternative, and as a method for getting a data point for every test, a step-loading method may be employed. In this paper, some of the unique test equipment being applied to the study of high cycle fatigue in our laboratory are reviewed. Data are presented on the long-life fatigue behaviour of Ti-6Al-4V which validate both the step-loading methodology and indicate the trend of fatigue strength as a function of cycle number in the very high cycle fatigue regime. The absence of coaxing is demonstrated by comparing S-N data with step-loading data. Applicability of high frequency testing to the determination of fatigue crack growth thresholds is also demonstrated.