Recent Developments of Advanced Calculation Concepts for the Fatigue Assessment of Power Plant Components

The Fatigue lifetime assessment of pressurized components e.g. in nuclear power plants (NPPs) is an essential part of the aging management (AM) ensuring safe and reliable long term operation (LTO). The practically applied calculation procedures of different international codes and standards are widely affected by the basic principles formulated in the criteria document [1]. Those criteria are still fundamental for the fatigue assessment of pressure vessel and power plant components and there is good reason to target at improvements in the consideration of the major factors of influence on the fatigue lifetime while keeping the well-proven methodological framework. This paves the way to related future design code amendment proposals. In practice, there are often large discrepancies between calculated fatigue life and practical experience from power plant operation, where the operating experience reveals much higher fatigue lifetimes as their predictions based on laboratory tests and conservative consideration of major influencing factors (plastification by Ke-factors, Environmentally Assisted Fatigue (EAF) by FEN-factors) in the calculation approach. In the framework of a cooperative research program (see e.g. [2], [3] and [4]) including various subprojects Framatome GmbH, Erlangen, and the Materials Testing Institute MPA Stuttgart have been developing improvements to the fatigue lifetime assessment methods in the framework of the well established engineering approach [1]. These improvements are consolidated by the results of experiments on specimens of ferritic and austenitic stainless steels and austenitic stainless steel welds as well as component tests are performed under laboratory and operating conditions to improve fatigue assessment. The following aspects are going to be pointed out - Consideration of hold time and environmental (EAF) effects including the threshold and fatigue limit behavior - Relevance of multiaxial non-proportionality effects for typical power plant operational loading histories - Consideration of the cyclic elastic-plastic deformation behavior - Integration of the fatigue damage parameter approach into the calculation concept (particularly modified fatigue damage parameter of Haibach and Lehrke PHL,mod and fatigue damage parameter PJ) - Low-Cycle (LCF), High-Cycle (HCF) and Very High-Cycle (VHCF) fatigue behavior, interaction and related damage accumulation effects including the transient fatigue limit. The paper gives an overview of the state of the calculation concept.