Effect of loading frequency on very high cycle fatigue behavior of 40CrNiMoA

Purpose Given the high effectiveness of the ultrasonic fatigue testing method for very-high-cycle fatigue (VHCF) studies, this work aims to investigate the effect of test frequency and, more importantly, to identify corresponding solutions where necessary. Design/methodology/approach A comparative study of two fatigue testing methods was conducted to investigate the effect of frequency in the VHCF regime. A complementary theoretical analysis was performed to elucidate the influence of strain rate on fatigue properties. Findings This steel exhibits a frequency effect under the two fatigue testing methods. At identical stress levels, ultrasonic loading (UL) causes reduced damage accumulation per cycle. This reduction influences both fatigue crack initiation and propagation, resulting in extended fatigue life. A fatigue life conversion model, accounting for differences in cyclic strain rates between the frequencies, effectively aligned the ultrasonic fatigue data with low-frequency experimental data. Originality/value The frequency effect in ultrasonic fatigue testing represents a fundamental research area where conflicting results exist in the literature regarding VHCF. This work directly addresses these discrepancies in fatigue data. By proposing a novel model to convert fatigue results between testing frequency and potential service frequency, it offers significant implications for both the broader application of UL methods and VHCF research.