Ratcheting-fatigue behavior and life prediction of Z2CN18.10 austenitic stainless steel elbow

Abstract

The ratcheting fatigue behavior of Z2CN18.10 austenitic stainless steel elbow under internal pressure and cyclic loading was studied. The evolution of axial and hoop ratcheting strains at four different positions, respectively at intrados (0°), 45°, crown (90°) and extrados (180°) on the outer surface of the elbow was discussed. It is shown that the hoop ratcheting strain of the elbow is always greater than the axial ratcheting strain. Under cyclic test with force control, the order of ultimate hoop ratcheting strain is intrados (0°)>crown (90°)> 45°> extrados (180°). Under cyclic test with displacement control, the order of ultimate hoop ratcheting strain is 45°> intrados (0°)> crown (90°)> extrados (180°). Under cyclic tests with both force and displacement control, the crown (90°) of the elbow leaked first, which is the critical point to evaluate ratcheting fatigue life. Afterward, the ratcheting fatigue life is predicted based on ASME code, and it is found that the prediction results tend to be non-conservative. Then, a modified Manson-Coffin equation considering ratcheting strain is proposed to predict fatigue life of elbow well.