Flaw Assessment Procedure for High-Temperature Reactor Components

Abstract

An interim high-temperature flaw assessment procedure is described. This is a result of a collaborative effort between Electric Power Research Institute in the US, Central Research Institute of Electric Power Industry in Japan, and Nuclear Electric plc in the UK. The procedure addresses pre-existing defects subject to creep-fatigue loading conditions. Laws employed to calculate the crack growth per cycle are defined in terms of fracture mechanics parameters and constants related to the component material. The crack growth laws may be integrated to calculate the remaining life of a component or to predict the amount of crack extension in a given period. Fatigue and creep crack growth per cycle are calculated separately, and the total crack extension is taken as the simple sum of the two contributions. An interaction between the two propagation modes is accounted for in the material properties in the separate calculations. In producing the procedure, limitations of the approach have been identified. Some of these limitations are to be addressed in an extension of the current collaborative program.

The Unified Creep-Fatigue Equation for Stainless Steel 316

“…Using the fatigue and creep coefficients found in Section 4.4.2.1, the raw fatigue data (ε p -N) obtained from [25] (T = 873 K and 973 K) are transformed to the reference condition (ε p,ref -N) through Equation (20). The transformed data plotted in Figure 5, which shows that these data can be collapsed into a power-law curve of …”

“…Therefore, the total crack size can be presented as the linear addition of initial crack size and propagative crack size. Normally, the initial crack is identified as the real crack size in the structures before loading, and the propagative crack (Equation (8)) [19,20] can be obtained through Paris's model [21]:…”

“…Then, the plastic strain at reference condition is determined by transforming Equation (17): (20) This transformation would cause all ε p :N data to collapse into one ε p,ref :N curve if the creep function (Equation (18)) and stress function (Equation (19) Next, the relatively simple methods of obtaining the coefficients will be verified on stainless steel 316.…”

A review of a time-dependent fracture life law (or model) based on a proposed multi-scale analysis

“…This classical work has included studies of the effect of the local stress field at the tip of a crack and on specimen geometry as it affects the hydrostatic stress component in the specimen. These results have been used in engineering design codes for structural integrity with methods based on a reference stress σ ref [7][8][9][10][11][12] and other methods including those using a factor η to relate crack tip behaviour to load-displacement response [13,14]. However, the link between physics and engineering is still not secure, because scale effects in engineering structures appears as shown in Fig.…”

“…An algorithm is proposed that links micro-scale damage mechanisms with the prediction of the creep life of large engineering structures. It will be related to engineering codes [7][8][9][10][11][12][13][14] which enables us to expand the applicability of these codes to engineering problems for the prediction of fracture.…”