Influence of small defects on the uniaxial and torsional fatigue strength of 17-4PH stainless steel

The influences of small defects on the fatigue properties of 17-4PH stainless steel under uniaxial and torsional loading are investigated. Experiments were performed with different testing techniques (rotating bending, servohydraulic and ultrasonic fatigue testing) in the high cycle and very high cycle fatigue regime. Fatigue limits under cyclic torsional loading, τa, and cyclic axial loading, σa, were determined. The ratio of τa/σa for smooth specimens is in good accordance with the von Mises criterion. This ratio increases in the presence of defects and becomes close to 1 for defects with area larger than 80 µm. The experimental data are evaluated to determine a predictive approach for the fatigue strength under uniaxial and torsional fatigue loadings.