PROPOSAL FOR AN IMPROVED ENGINEERING FATIGUE LIFETIME ASSESSMENT CONCEPT

Fatigue life assessment for pressurized components in nuclear power plants (NPPs) is an essential part of the aging management (AM) ensuring safe and long term operation (LTO). For fatigue life assessment different codes and standards provide a variety of methodologies with variable complexity. Major fatigue life influencing factors such as temperature, surface finish, multiaxiality, loading history and others are often considered in a more or less global way by combined overall reduction factors covering multiple mechanisms together. Other effects such as the environment or hold times are often considered not at all or otherwise with high levels of conservatism resulting in large discrepancies between calculated fatigue life and practical experience from power plant operation. In order to reduce this inadequacy a more accurate fatigue lifetime assessment concept including individual fatigue life influencing factors in a mechanistic manner is required. Nevertheless, these amendments are to fit into the existing basic engineering approach of design against fatigue failure as it is implemented in nuclear standards and design codes. In the framework of an ongoing three years German cooperation R&D project with participation of the Materials Testing Institute MPA University of Stuttgart and AREVA GmbH (Erlangen) it is the aim to both improve the state of the art based on an experimental program for some of the main fatigue life influencing factors and on the derivation of a practicable engineering fatigue assessment procedure. Within this fatigue assessment procedure the dominant fatigue life influencing factors are considered individually. The experimental program covers fatigue test results for austenitic and ferritic piping materials including a dissimilar metal weld. Within the testing program strain controlled fatigue tests were performed with and without hold-times in air and high temperature water environments. Smooth and notched specimens provide a database to study the influence of notches and multiaxiality. These results are used to state on the applicability of commonly used failure hypothesis like von Mises and Tresca in comparison to advanced fatigue damage parameters. In addition to constant amplitude strain controlled fatigue testing load spectra were investigated. Thereby fatigue cumulative damage models like Miner's rule can be evaluated. This publication constitutes a follow-up to a previous paper [1] and targets at the presentation of experimental results in conjunction with potentials with an improved fatigue assessment concept. In addition the methodology of the concept is applied to experimental results on fatigue life assessments for piping materials published by other organizations. Requirements for further experimental investigations towards the verification of a closed concept are forinulated.