Influence of load ratio on torsion very high cycle fatigue of high-strength spring steel in the presence of detrimental defects

摘要

The influence of mean shear stress on torsion very high cycle fatigue (VHCF) for internal inclusion-initiated fracture is investigated. Ultrasonic torsion fatigue tests are performed at load ratios R = -1, 0.1, and 0.35 with high-strength spring steel SWOSC-V featuring an increased number of nonmetallic inclusions. Based on Murakami and Endo's (area)~(1/2) parameter model, an equation predicting the torsion fatigue limit in the presence of detrimental defects is rendered. Stress biaxiality is considered using a ratio of torsion to axial fatigue strength of τ_a/σ_a = 0.86. Considering the load ratio sensitivity according to ([1 - R]/2)~α, the value for a is experimentally determined to be 0.550. A fine granular area (FGA) is generated at an artificial surface defect in a fully reversed cyclic torsion test in vacuum. Measured growth rates in the FGA were 10~(-14)-10~(-12) m/cycle, and the formation of the FGA consumed 98% of the VHCF lifetime.