Generalized procedures for the use of plane strain and elastic-plastic fracture mechanics options in the modernization of nuclear standards

Abstract

In the design of nuclear power plants and the selection of required structural materials, the assurance of reliability in operation is an essential consideration. The need for analytical criteria for defining the adequacy of fracture toughness is particularly acute for pressure vessel materials. The 1972 revision to the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (Section III) has adopted linear elastic fracture mechanics (LEFM) methods as a means of assuring fracture-safe operation for nuclear vessels and components. It is noted that LEFM can be used to analyze only the behavior of metals subject to plane strain constraint (i.e. brittle behavior), while many steels when used in structural applications will behave in a ductile fashion. Thus, in the modernization of nuclear codes, there is the additional need to include the full range of fracture mechanics options for system design, that is, elastic-plastic and fully plastic fracture mechanics as well as the linear elastic procedures. The choice of a particular toughness regime for application of the metal (e.g., plane strain or elastic-plastic) can then be made by the designer or regulatory body. It follows that this decision will have a major implication on the selection of nuclear structural materials. This paper describes recent developments in the means for defining the full range of plane strain, elastic-plastic, and plastic fracture mechanics options available to the designer. Comparisons are made between these options and the fracture toughness requirements of the ASME Nuclear Code (Section III). Exisiting dynamic plane strain, K Id , data for structural metals are analyzed in concert with dynamic tear (DT) test trends. The limited temperature region of K Id applicability for these materials is shown to presage the elastic-plastic regime through which sharply increasing stress is required for fracture propagation whereby a leak-before-fail condition is ultimately attained. This phenomenon highlights the need to extend the analytical capabilities for fracture assurance into the non-brittle regime. The DT test is an effective engineering tool which, like the crack opening displacement (COD) concept, can be used to define the elastic-plastic and plastic-constraint transitions. The DT test procedure is fully rationalizable in terms of section size parameters and can be used independently or together with the K Id -temperature trend to predict the onset of the elastic-plastic and plastic regimes as a function of temperature and section thickness.