Investigation on stress indices C3, K3, C'3 for gross structural discontinuities of piping systems

The paper presents the first results of a research activity promoted in Italy by ENEA and carried out in the framework of the ENEA-ANSALDO consortium GENESI aimed at assessing the aging of gross discontinuities such as reducers, transitions and penetrations subjected to severe thermal transient as those produced by sudden admittance of steam (or hot water) into piping at ambient temperature, which can result in low cycle fatigue failure. In particular, the paper addresses the problem of design by rule, as per ASME NB 3600, where stress indices, or intensification factors, (SIF) are needed and a full-scale testing is mandatory to validate them. Here SIFs are derived through a refined 2D-3D finite elements analysis for 20 different geometries only 9 of which were covered by the ASME Code, the others being without any available SIF. Results indicate that not always the ASME factors are conservative and that an integrated approach to the problem is required were SIFs evaluated by calculations must be checked on full scale structures and non-destructive examination procedures applied to demonstrate the capability to detect the early appearance of a tiny crack produced by thermal stresses in complex geometries. (author)