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

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 where N_i is the number of cycles to crack initiation, Δ_γp ^B is the range of plastic shear strain on Type B planes, σ_n^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.