NET SECTION PLASTIC COLLAPSE ANALYSIS OF TWO-LAYERED MATERIALS AND APPLICATION TO WELD OVERLAY DESIGN

摘要

The net section plastic collapse equations currently used for piping flaw evaluation in ASME Section Ⅺ, IWB-3600 are derived based on thin shell theory assuming properties for a single material. Because of the wide use of weld overlays to repair piping flaws in both BWRs and PWRs, there is a need to investigate the effect of multi-layered materials on the net section plastic collapse equations since weld overlays are typically applied with the weld material being different than the underlying base material. In addition, the larger section of the weld overlay provides additional load-carrying capability. A question also arises as to the application of appropriate loads and Z-factors if the underlying material is low-toughness material and the weld overlay has high toughness, not requiring consideration of thermal expansion stresses and Z-factors. The inherent assumptions that are made may lead to over-sizing of the overlay which can increase the welding time during the overlay implementation and subsequently the outage time.In this paper, an approach is developed for inclusion of thermal expansion loads and Z-factors for a weld overlaid circumferentially cracked section. Limit load equations are derived assuming a two layered material, both with and without taking advantage of the greater area and section modulus associated with the weld overlay. The equations for the allowable applied loads include consideration of both limited and complete circumferential flaw extent into the compressive zone of the section. The formulations are then applied to the design of weld overlays which are used to repair flawed piping. The evaluation shows that there can be significant difference in the allowable piping stresses as compared to those based on use of simplified uniform material assumptions.