321 stainless steel (SS) is a stabilized grade of austenitic stainless steel and is widely used in Light Water Reactors (LWRs). 321 SS constitutes vital structural material for systems and components such as piping, heat exchangers and shielding material.
In the current study, cyclic deformation, fracture and the resultant stress-life fatigue curves of 321 SS under engineering-stress and strain controlled cycling are compared. This study is summarized as follows:
(1) In stress-controlled LCF tests at 633 K, 321 SS shows significant ratcheting in spite of the fully reversed loading (i.e. zero mean-stress) for stress amplitudes above 195 MPa. The direction of ratcheting strongly depends on the initial ramp-direction of first cycle.
(2) Stress-controlled LCF deformation induced high accumulated plastic strains and large striation widths on fractographs in comparison to strain-controlled LCF deformation.
(3) Fatigue lives under stress-controlled cycling are lower than those of strain-controlled LCF tests, for stress amplitudes above 195 MPa.
(4) The stress-life curve obtained from stress-controlled tests is 2–3 orders of magnitude lower in comparison to the S-N curve derived from strain-controlled data as per the ASME procedure. The discrepancy is attributed to the strain-life data translation (using predefined Young’s Modulus), and differences in deformation and damage development in stress and strain-controlled LCF tests.