Аssessment of welded joints impact on structural integrity of a pressure vessel

Previous experience with integrity assessment of welded structures and elements had confirmed the usefulness of numerical simulations, among other things in terms of how rigidity and geometry of such structures affect the stress distribution. This approach worked well for elastic calculations for welded structures, efficiently providing accurate results which were easy to compare with the yield stress levels of selected materials. In the case where stresses are above yield stress levels, the differences in mechanical properties of individual weld regions become a crucial factor in terms of influence on the integrity of the observed structure, unlike the elastic region, where this influence was negligible. In order to obtain proper input data (mechanical tensile properties of all welded joint regions), previouslu developed methodology, that combines experimental and numerical method, is used. Herein the experimental approach provided the strain fields (as obtained by digital image correlation) while specimen was subjected to tensile testing, which were used to improve numerical computational models. In this way, it was possible to accurately determine the mechanical properties of all individual weld regions in an iterative manner. Using the previously described input data, two computational models of a pressure vessel were developed, with and without welded joints. During typical exploitation conditions under service operations, there was no difference between the two models, as the stresses were still in the elastic area. However, once the stresses reached plasticity, considerable differences between these models were observed in the welded joints area. Noticeable stress concentration was present in the zone of welded joints (in a computational model containing welded joints), increasing the stresses by around 50% (stress concentration factor of 1.5) in comparison to computational model without welded joints and without stress concentration. This had shown good agreement with previous research, which involved the application of this methodology to a pipeline, and these two cases were similar, since they involved welded structures made of shell elements loaded with internal pressure, causing membrane stresses.