Prediction of Fatigue life on High-Strength Steels in Very High Cycle Regime

Abstract

Three kinds of fatigue failure mode are appeared in a S-N curve of high strength steel depending on applied stress amplitude level, such as surface inclusion induced failure mode in high stress amplitude level and short fatigue lifetime, subsurface inclusion induced failure mode without GBF area and that with GBF area in low stress amplitude level and high and/or very high cycle fatigue (VHCF) regime. Fatigue life in the VHCF regime is controlled by formation of the GBF area. Method to predict the fatigue lifetime and/or S-N curve of high-strength steels in the VHCF regime was discussed. It was assumed in this study that small fatigue crack growth in the GBF area was followed by Paris power law. It was shown that there was a linear relationship in the double logarithmic diagram between value of exponent, m, and constant, C, in the power law obtained from the S-N data of some high-strength steels tested under rotating bending and axial loading condition. Also, the exponent, m, showed the linear relationship with the Vickers hardness of the materials in double logarithmic diagram. Based on these experimental facts, new prediction of fatigue lifetime in the VHCF regime was proposed in form of Basquin's equation. Predicted fatigue lifetime and experimental result accorded well together, as compared to the prior proposed predictions.