A substantial amount of research effort has been applied to the field of environmentally assisted fatigue (EAF) due to the requirement to account for the EAF behaviour of metals for existing and new build nuclear power plants. We present the results of the European project INcreasing Safety in NPPs by Covering Gaps in Environmental Fatigue Assessment (INCEFA-PLUS), during which the sensitivities of strain range, environment, surface roughness, mean strain and hold times, as well as their interactions on the fatigue life of austenitic steels has been characterized. The project included a test campaign, during which more than 250 fatigue tests were performed. The tests did not reveal a significant effect of mean strain or hold time on fatigue life. An empirical model describing the fatigue life as a function of strain rate, environment and surface roughness is developed. There is evidence for statistically significant interaction effects between surface roughness and the environment, as well as between surface roughness and strain range. However, their impact on fatigue life is so small that they are not practically relevant and can in most cases be neglected. Reducing the environmental impact on fatigue life by modifying the temperature or strain rate leads to an increase of the fatigue life in agreement with predictions based on NUREG/CR-6909. A limited sub-programme on the sensitivity of hold times at elevated temperature at zero force conditions and at elevated temperature did not show the beneficial effect on fatigue life found in another study.
A pressurized water reactor primary environment can have a deleterious effect on the fatigue lifetime of austenitic stainless steels. There is a need to develop a greater understanding behind the effect of a pressurized water reactor primary environment on the fatigue behaviour of austenitic stainless steels. One of the ways that we can improve our mechanistic understanding is by carrying out striation spacing analysis. Striation counting is a widely used technique in fatigue failure investigations where it is typically used to infer information on crack progression, including the estimation of propagation rates and number of applied loading cycles. Standardised procedures for performing striation counting are uncommon, especially for environmental fatigue in a high temperature pressurized water reactor primary water environment where differences in fracture surface morphology and oxide coverage can lead to additional complications in performing an analysis. One of the main goals of the EU Horizon 2020 INCEFA-SCALE project is to develop an improved mechanistic understanding of fatigue in these systems through extensive characterisation of laboratory tested specimens. As part of this work, this paper describes the development of a standardised and robust striation counting procedure for the low cycle fatigue of austenitic stainless steels operating in both air and simulated pressurized water reactor environments. Additionally, results are presented from round robin exercises that involved eight partners of the INCEFA-SCALE consortium.
The European project INCEFA-PLUS characterises environmentally assisted fatigue in light water reactor conditions. The project aims at developing a new procedure to assess environmentally assisted fatigue damage susceptibility in nuclear power plant components. The basis for the development of a new fatigue assessment procedure is a major test campaign carried out in eleven different laboratories across Europe which will deliver approximately 200 fatigue tests. The test campaign is based on a common test matrix that was optimized by means of the Design of Experiments method. The initial focus of the project is on the effects and interactions between the factors strain range, environment (air and light water reactor environment), surface finish, hold time, and mean strain. Whereas the bulk of the test program is carried out on a single heat of 304L austentic steel, some tests on different heats of 304L or other austenitic steels allow studying the influence of material variability. To guarantee the quality of the data, the tests are performed according to commonly agreed specifications based on ISO 12106 and each test is validated by a group of experts from within the project. The paper presents the test procedures, provides an overview of the data that has been acquired so far, and gives an outlook on the tests that will be carried out during the final stage of the project.
INCEFA-PLUS (INcreasing Safety in NPPs byCovering gaps in Environmental Fatigue Assessment) project characterized environmentally assisted fatigue of stainless steels in light water reactor environments. During this project more than 200 fatigue tests have been carried out in different laboratories across Europe in air and water environment. Most tests were performed on a single batch of 304L, an austenitic stainless steel alloys employed in NPPs. The tests addressed the effects of strain amplitude, hold time periods, material roughness and mean strain/stress on fatigue endurance. A limited number of tests was carried out on other batches of 304L and on X6 CrNiTi 18 10, a Ti stabilised steel used in VVERs. Additionally, activities on the effects of mean stress under strain control, testing at reduced environmental fatigue correction factor Fen, and different applications of hold time as well as biaxial fatigue tests have been carried out. The data obtained has been collected and standardised in an online environmental fatigue database MatDB.
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