5. Conclusions
The fatigue initiation properties depend on the orientation of the FS weld. Furthermore, when the FS weld is parallel to the applied load, the FI behaviour is also dependent on the location in the weld. This dependency is caused by the presence of residual stresses, which are negligible perpendicular to the weld (T–L), but significant parallel to the weld (L–T). The microstructure in the FS weld has an influence on the FI behaviour, but this effect is smaller than the influence of residual stresses. A successful detailed FE analysis is performed to understand the influence of the residual stresses in the FS weld on the FI behaviour better. These FE analysis results have been used to predict FI in the weld. It is shown with this prediction that the differences observed between the L–T and T–L FI test results can be attributed to the residual stresses in the weld. Although FE analysis is a powerful tool, it must always be treated with some care. Some assumptions had to be made to model the FS weld by a residual stress field and a yield strength profile. Therefore, the quantitative results will be different in details from the reality. Prediction of the FI behaviour in the FS weld is possible by using the mean stress effect. However, a more realistic representation of the fatigue diagram is required. The modified Goodman relation has been used in this work, but for large changes of mean stress this representation of the fatigue diagram introduces significant errors.
