INCEFA-PLUS Findings on Environmental Fatigue

INCEFA-PLUS is a five year project supported by the European Commission HORIZON2020 programme. The project concluded in October 2020. 16 organisations from across Europe have combined forces to deliver new experimental data which is being used to develop improved guidelines for assessment of environmental fatigue damage to ensure safe operation of nuclear power plants. Within INCEFA-PLUS, the effects of mean strain and stress, hold time, strain amplitude and surface finish on fatigue endurance of austenitic stainless steels in light water reactor environments have been studied experimentally. This document constitutes a Reference Book compiling the research developed within the INCEFA-PLUS Project. It provides a comprehensive overview of the tasks performed, and it also presents the background and the assumptions taken to develop the INCEFA-PLUS experimental and analytical works. It compiles and orders documents and contributions from INCEFA-PLUS partners.This project has received funding from the Euratom research and training program 2014-2018 under grant agreement No 662320

Cyclic behavior and fatigue of stainless surge line material

Fatigue of Niobium stabilized austenitic stainless steel (X6CrNiNb1810mod) was studied using specimens extracted from a solution annealed and quenched φ360×32 mm pipe, which fulfils all the German KTA material requirements for primary components in BWR and PWR. An experimental strain life fatigue curve was determined as a base line for component specific evaluations and for comparison with the Langer and Chopra curves, which are the basis of the ASME III and NRC RG 1.207 design criteria. Stress strain responses in strain controlled tests were carefully measured to clarify the fatigue mechanisms. In the LCF regime our data lie within a common scatter band between the Langer and Chopra curves. When approaching HCF regime (&amp;gt; 100 000 cycles), the curves deviate and Chopra curve becomes highly conservative for this material. Strain controlled fatigue tests ranging to 10 million cycles in RT air indicated an endurance limit behavior for this alloy.</jats:p

Research on Hold Time Effects in Fatigue of Stainless Steel:Simulation of Normal Operation Between Fatigue Transients

In PVP2011-57942 we reported improved endurance in fatigue tests with intermediate annealing to roughly simulate steady state operation between fatigue transients in NPP components. Quantification of this effect is in focus of our continued research on fatigue performance of niobium stabilized stainless steel (1.4550, X6CrNiNb1810mod). Similar effect is expected in nuclear power plants during normal operation — e.g. in a PWR surge line or in pressurizer spray lines. Holds affect cyclic stress strain response. Stress amplitude, tensile mean stress and apparent elastic modulus are increased immediately after a hold, while decreased by cycles in between. Axial shortening is measured during hot holds at zero stress. This all suggest cyclic accumulation of lattice defects and recovery during holds. Recovery may occur through thermally activated dislocation migration together with diffusion, grouping and annihilation of lattice defects. More than one thermally activated processes control the rates of contraction during hold periods at elevated temperatures. Hold hardening delays crack formation by preventing plastic strain localization, in components also on macroscopic level. A mechanism informed model is sought for transferring laboratory data to real plant components in terms of improving accuracy of numerical fatigue usage assessment. Anticipated mechanisms behind gradual changes in material responses are discussed in relation to quantitative effects of holds.</jats:p