Comparison of Fatigue Behavior in Rotating Bending and Torsional Fatigue of Cast Stainless Steel

Rotating bending and torsional fatigue tests were carried out on specimens of cast stainless steel. The initiation and propagation processes of microcracks were investigated using the plastic replica method. The main results are summarized as follows. (1) The critical cracks appeared at a microshrinkage cavity in rotating bending fatigue, but not in torsional fatigue. Therefore, the ratio of fatigue limits in cast stainless steel (τw/σw≒0.77) is larger than that of defect-free isotropic carbon steel (τw/σw≒0.65). (2) The crack growth laws in rotating bending and torsional fatigue are expressed by dl/dN∝σ7.8l and dl/dN∝τ9.0l, respectively. (3) The difference between the dependences of Nf on the stress amplitude in rotating bending fatigue and in torsional fatigue can be explained by the above crack growth laws.