Evaluation of Lower Limit of Fatigue Strength in High Cycle and Very High Cycle Fatigue Regime

In this paper, very high cycle fatigue (VHCF) strengths are first evaluated for the TC17 titanium alloy and the welding joint of the 6005A-T6 aluminum alloy using the up-and-down method (UDM) and continuous testing method (CTM). Then, the influence of the sample size on the lower limit of the high cycle and VHCF strength (10⁷, 10⁸ and 10⁹ cycles) is investigated based on the experimental data and the previous results for aluminum alloys, steels and titanium alloys. It indicates that, compared to the common size of 15 samples used for the UDM, a sample size of 10~14 can generally give an acceptable evaluation of the lower limit of fatigue strength (LLFS) at 90% and 95% survival probabilities (SPs) and 95% confidence for both the UDM and CTM. The absolute value of the relative difference value of the result of 10~14 samples compared to that of 15 samples is generally within 5%. In addition, the UDM could give very dangerous evaluated results, and it fails to evaluate the LLFS in some cases. The CTM deals with all the testing results and gives a safe evaluation of the LLFS. Ten samples and an LLFS at 90% SP and 95% confidence can be preferred for fatigue strength evaluations using the CTM in a high cycle and VHCF regime.