2.5.1 Mechanical Fatigue Parameters
In general, mechanical fatigue behaviour is influenced by different parameters, such as material, temperature, strain range, load ratio, stress or strain rate. Other parameters (including fluence, fluence rate, hold-time and cold-work) have been investigated to identify synergisms with other ageing mechanisms or to evaluate models, non-destructive examination techniques or fabrication/improvement techniques (in the scope of projects such as 7210-MA/131, 7210-MA/823, 7210-MA/951, GRETE, NET and MATTER).
The fatigue design curves derived from experimental data include adjusting factors for the stress and the number of cycles. These adjusting factors take into account variations, such as material composition, microstructural differences (including grain size, inclusions, orientation within a forging or plate, cold-work and heat treatment), temperature and flow rate.
An overview of fatigue S-N data for carbon steels, low alloy steels and austenitic stainless steels is given in different reports which discuss the environmental influence on fatigue. Nevertheless, data for mechanical fatigue in air also are included and dependencies are discussed [NUREG-CR-6583] [NUREG-CR-6787] [NUREG-CR-6909]. The temperature range of the performed experiments does not cover operating conditions for all components of advanced gas-cooled reactors (AGRs).
These experimental data indicate that fatigue life of austenitic stainless steels is independent of temperature up to 400 °C. Also variation in strain rate in the range of 0.4 – 0.008 %/s has no effect on fatigue lives up to 400 °C which covers typical temperatures for components in light-water reactors (LWRs). The fatigue life of cast austenitic stainless steels is similar to wrought ones [NUREG-CR-6787] [NUREG-CR-6909].
The fatigue lives of carbon steel and low alloy steels depend on steel type, temperature, orientation in case of rolled specimens and strain rate. In general, the fatigue life of carbon steels and low allow steels is comparable. For both types, life is decreased by a factor of ~1.5 when the temperature is increased from room temperature to 300 °C. In experiments, the influence of the strain rate on mechanical fatigue depended on the material composition and showed no unique effect for the material class [NUREG-CR-6583] [NUREG-CR-6909].
The effect of a roughened surface is comparable for carbon, low alloy and austenitic stainless steel. The fatigue life of roughened samples is decreased showing the largest effect for austenitic stainless steels with a factor of ~3 [NUREG-CR-6815].
Concerning EC research in the timeframe FP4 to FP7, the effects and parameters, sometimes in combination, that have been investigated for some austenitic stainless steels include temperature, hold-time, fluence, fluence rate, microstructure and cold-work.
In [JRC8609], mechanical fatigue experiments with the stainless steel AISI 316 have been performed at different temperatures (373, 473 and 573 K) under simultaneous proton irradiation (6 x 10-7 dpa s-1 and fluences < 1 dpa) as well as without in the framework of the NET project. With increasing test temperature, the number of cycles necessary for crack initiation, Ni, decreases for the irradiated and unirradiated samples, but Ni is always lower for the latter. The difference of Ni between irradiated and unirradiated specimens decreases slightly with increasing test temperature, whereas the crack growth rate increases for both irradiated and unirradiated samples. Normally most components of LWRs or AGRs experience only moderate or no fluence, so that the experimental data under irradiation are not relevant for operating European nuclear power plants (NPPs) and components not close to the core (core internals) but the temperature range of experimental data from unirradiated tests covers both LWR as well as AGR operating conditions.
Experimental creep-fatigue data on grades of AISI 316 for two different temperatures (550 °C and 600 °C) at strain ranges from 0.35 % to 1.0 % and with different hold times have been assessed within the MATTER project to validate a creep-fatigue model [Utili2012]. These parameters are only relevant for gas cooled reactors (GCRs) like the AGRs in the UK.
2.5.3 Load Profile
Different types of load profile can be applied during fatigue testing (such as sinusoidal, triangular and rectangular) with different frequencies. Also, different load ratios can be used or hold-times can be applied to adapt the experiments to be more representative of operating conditions. Depending on other conditions (such as temperature and irradiation) synergisms of fatigue with other ageing mechanisms like creep can occur.
18.104.22.168 Effect of Hold-Times
The effect of various hold-times (20, 50 and 1000 s) has been investigated in [JRC12412] [JRC16627] in the framework of the NET project for the austenitic stainless steel AISI 316L in combination with deuteron irradiation (fluence rate x10-6 dpa s-1 and 6 x 10-6 dpa s-1). The hold-time was applied at the minimum strain value during irradiation to promote irradiation creep. The values of the hold-times were chosen to simulate the operating conditions of fusion experiments. The simulation of common operating conditions in NPPs (such as start-up and shut-down processes) requires longer hold-times.
Much longer hold-times have been used in experiments from the MATTER project. To validate a creep-fatigue model which has been developed in this project, experimental creep-fatigue data were generated for grades of AISI 316 at two different temperatures (550 °C and 600 °C); at strain ranges from 0.35 % to 1.0 %; and hold-times from 6 to 600 minutes (360 to 36,000 s). The range of cycles to failure was from 450 to 200,000 [Utili2012]. The experimental conditions are comparable to the operating conditions of AGRs.
2.5.4 Fluence Rate
In [JRC12412] [JRC16627], mechanical fatigue experiments with 20 % cold-worked and annealed AISI 316L material at 400 °C and under simultaneous deuteron irradiation conditions (fluence rate 1 x 10-6 dpa s-1 and 6 x 10-6 dpa s-1) have been performed. Different hold times have been applied at the maximum stress value to investigate synergisms of fatigue with irradiation creep. The fatigue lifetime of the cold-worked material was significantly reduced by the presence of irradiation creep whereas the effect was negligible for the annealed material.
Differences in the microstructure of materials can occur for different reasons, including alloying elements, welding, annealing, and cold-work. Some projects investigated the influence of cold-work by comparing cold-worked samples with as-received or annealed samples.
Differences in microstructure and the effect on fatigue life have been investigated within the European Coal and Steel Community (now RFCS) funded projects 7210-MA/131,7210-MA/823 and 7210-MA/951, where crack growth rates have been investigated for the base metal, the heat-affected zone and the weld metal for an austenitic stainless steel AISI 304L (AISI 308 weld material) and a duplex stainless steel [EUR19972].