Mean Stress Effect on Fatigue Behavior of Austenitic Stainless Steel in Air and LWR Conditions

In this paper, mean stress influence and cooperative effects associated with light water reactor (LWR) environmental factors were studied through load-controlled fatigue tests with cylindrical hollow specimens. Positive (+10, +50 MPa) and negative (-10, -20 MPa) mean stresses were applied and both showed beneficial influence on fatigue life due to cyclic hardening, which results in smaller strain amplitude under the same stress amplitude. The increase in fatigue life was found to depend on mean stress and testing environments. The increase (1.8-2.2x) in fatigue life with +50 MPa mean stress in boiling water reactor/hydrogen water chemistry (BWR/HWC) is smaller than that in air (3.0-3.4x), while the increase with-20 MPa mean stress in both environments is approximately similar. -20 MPa mean stress enhances the resistance to crack initiation as observed with optical microscopy (OM) observations of fracture surfaces and wall cross-sections of tested specimens.