Study on the relationship between the root welding residual stress and the root-failure fatigue strength of Plug welded specimens

It is difficult to examine the relation between the root notch welding residual stress (WRS) and the root-failure fatigue strength because of the difficulty in the direct root WRS measurement. The root WRS can be calculated by thermal elastic plastic finite element analysis (TEPFEA), but its accuracy has not been fully verified yet. In this study, root-failure fatigue tests, in which direct root WRS measurement can be performed, are carried out. Plug welded (PW) specimens with backing plates are used in those tests, and the root WRS in as welded (AW) and stress relieved (SR=PWHT) specimens are measured using X-ray diffraction (XRD) method by cutting off the backing plate. The measured WRSs are compared with those calculated by TEPFEA. It is found that the root WRSs in the PW specimens estimated by TEPFEA become much larger than those measured when creep strain is neglected. The SR specimen’s fatigue strength improvement ratio is estimated by using the WRS effect formulas developed for toe-failure cases (IIW Fatigue Recommendations and MIL-HDBK-5D). The estimated improvement ratio shows fair agreement with that measured.