Crack Initiation Under Low-Cycle Multiaxial Fatigue in Type 316L Stainless Steel

Abstract

Low-cycle fatigue tests corresponding to fatigue life range between 103 and 105 cycles were carried out at room temperature on one heat of 316 L austenitic stainless steel. These tests included: (i) reversed tension-compression, (ii) reversed tension-compression with a superimposed steady torque, (iii) pulsated tension-compression with a stress ratio (R σ ) such that −0.5<R σ <0, (iv) reversed and pulsated tension-compression with a superimposed steady internal pressure. In tests (ii), the torsional ratcheting effect was measured. SEM observations were used to determine the number of cycles corresponding to Stage I crack initiation and the orientation of Stage I microcracks. It was observed that the in-depth growing Type B shear microcracks were most damaging. A simple criterion is proposed Ni=No(Δγp B)α•(σnB)β where N i is the number of cycles to crack initiation, Δγ p B is the range of plastic shear strain on Type B planes, σn B is the maximum normal stress acting on these planes, N o ,α and β are parameters adjusted from the Manson-Coffin law and reversed cyclic stress-strain behavior.