Fatigue Strength and Internal Crack Growth Behavior of High Strength Steel under Variable Amplitude Stressing in Very High Cycle Regime

Abstract

Ultrasonic fatigue tests were carried out for high carbon chromium bearing steel, SUJ2, under two-step variable amplitude stressing at a frequency of 20 kHz in order to investigate the growth of an interior crack. Since it is important to clarify the damage mechanisms under variable amplitude stressing for practical applications, fatigue tests were conducted under repeated two-step variable amplitude stressing. An inclusion was observed at the initiation site of fracture. Optically dark area (ODA) and fish-eye were observed around the inclusion. Sizes of ODA and fish-eye depended on the stress intensity factor range. Interior crack growth rate was slower than the surface one due to vacuum environment. Cumulative damage values calculated by Miner's rule were relevant to the size of inclusions under repeated two-step variable amplitude stressing. Characteristics of interior fatigue crack growth were determined by the fracture surface observations for beach marks and the ODA sizes. Based on the results, the formation mechanism of ODA was discussed in conjunction with the reports by the other researchers.