| This dissertation makes an effort to understand the influence of residual stresses on the fatigue failure of butt- and socket-welded piping joints. To achieve this objective, a set of experimental fatigue failure data and a numerical scheme are developed. A new observation made from the data is that the recorded strains near the weld toe ratchet continuously, which results in the initiation of fatigue crack(s). A critical analysis of the recorded data indicates that the residual stresses at the welded joints may not relax to zero after a few inelastic cycles, as assumed in the fatigue design methods. The test data also indicate that the fatigue life of materials is reduced by ratcheting.; Thermo-elasto-plastic analyses are performed using ANSYS and ABAQUS. The numerical results are first validated using the temperature and residual stress data in the literature. When the analysis method is applied to the welded piping joints, high tensile residual stresses at or above the yield stress level, are calculated near the weld toe area. Fatigue response analyses demonstrate that the residual stresses do not relax completely after a few inelastic cycles as assumed in the design methods. Because of the deficiency of the constitutive model, it was not possible to simulate the ratcheting in the welded joints. The study concluded that the improvement in fatigue design methods for welded joints requires a constitutive model that can simulate the ratcheting-fatigue failure responses. This, in turn, requires a carefully planned experimental program for developing a systematic set of welded joint fatigue response data and temperature-dependent material response data. |
