Yuichiro Nomura

Final Proposal of Environmental Fatigue Life Correction Factor (Fen) for Structural Materials in LWR Water Environment

Low cycle fatigue life of structural materials diminishes remarkably as functions of various parameters in high temperature water simulating LWR coolant. Such reduction was estimated by the fatigue life reduction factor (Fen ) and the equations to calculate Fen were developed and have undergone revision over the past ten years. The authors have endeavored to establish the method assessing fatigue damage at LWR power plants for the past 13 years in the Japanese EFT (Environmental Fatigue Tests) project under the financial support from the JNES (Japan Nuclear Safety Organization). The project terminated at the end of March in 2007. Final proposals of Fen equations were established for carbon, low-alloy, and austenitic stainless steels and nickel base alloys based on all the data obtained in the project. As the results, a small change in saturated strain rate for carbon and low-alloy steels in highly dissolved oxygen water and newly revised equations including slight change in saturated strain rate for stainless steels in BWR water as well as those for nickel base alloys were proposed. The difference between revised and previous model is essentially not large.Copyright

Fatigue Crack Growth Curve for Austenitic Stainless Steels in PWR Environment

Although reference fatigue crack growth curves for austenitic stainless steels in air environments and boiling water reactor (BWR) environments were prescribed in JSME S NA1-2002, similar curves for pressurized water reactors (PWR) were not prescribed. In order to propose the reference curve in PWR environment, fatigue tests of austenitic stainless steels in simulated PWR primary water environment were carried out. According to the procedure to determine the reference fatigue crack growth curve of BWR, which of PWR is proposed. The reference fatigue crack growth curve in PWR environment have been determines as a function of stress intensity factor range, Temperature, load rising time and stress ratio.Copyright

Applicability of the Modified Rate Approach Method Under Various Conditions Simulating Actual Plant Conditions

The fatigue life in elevated temperature water is strongly affected by water chemistry, temperature and strain rate. The effects of these parameters on fatigue life reduction have been investigated experimentally. In transient condition in an actual plant, however, such parameters as temperature and strain rate are not constant. In order to evaluate fatigue damage in actual plant on the basis of experimental results under constant temperature and strain rate condition, the modified rate approach method was developed. As a part of the EFT (Environmental Fatigue Tests) project, the study was conducted in order to evaluate the applicability of the modified rate approach to the case where temperature and strain rate varied simultaneously. It was reported in the previous papers (1,2) that the accuracy of modified rate approach is about factor of 2. Various kinds of transient have to be taken into account of in actual plant fatigue evaluation, and stress cycle of several ranges of amplitude has to be considered in assessing damage from fatigue. Generally, cumulative usage factor is applied in this type of evaluation. In this study, in order to confirm applicability of modified rate approach method together with cumulative usage factor, tests were carried out by combining stress cycle blocks of different strain amplitude levels, in which temperature changes in response to strain change in a simulated PWR environment.Copyright

Fatigue Crack Growth Rates for Nickel Base Alloys in Air

When defects are detected by in-service inspections of the nuclear power plants in Japan, allowable flaws are evaluated according to the Rules on Fitness-for-Service for Nuclear Power Plants of Japan Society of Mechanical Engineers (JSME maintenance rules). The fatigue crack growth analysis is an important part of the flaw acceptance evaluation. However FCGR curves for nickel base alloys in air are not provided in the current JSME maintenance rules.This paper describes the evaluation of the fatigue tests in air of nickel base alloys and formulation of the FCGR curves for the JSME rules on Fitness-for-Service. From the test results, the temperature and stress ratio are not dominant factors. However, as based on the form of the FCGR curve of NUREG/CR-6721, the proposed curves are shown to be functions of these parameters. Threshold of stress intensity factor is introduced in the curves by taking into account crack closure.Copyright

Effect of Surface Finish and Loading Conditions on the Low Cycle Fatigue Life of Stainless Steel Welded Piping in PWR Environment

NUREG/CR-6909 of USA and JSME of Japan proposed new rules for evaluating environmental effects in fatigue analyses of reactors components. These rules were established from a lot of fatigue data with polished specimens under simple loading condition. The effects of surface finish or complex loading condition were reported in some papers, but these data were obtained with the simple shaped specimens.In order to evaluate the effects of surface finish and loading condition and to confirm the applicability of the proposed rules to actual components, Low Cycle Fatigue tests are performed in PWR environment with the specimens cut from 316 austenitic stainless steel welded piping. The pipes are machined to have three levels of surface finish condition and the load pattern simulating the thermal stress is applied to specimens.In this study, the effect of surface finish on fatigue life is included to be small for 316 austenitic stainless steel welded piping. Considering the insensitive region in the current evaluation rule, predicted accuracy is increased and possibility of improving the current rule is indicated.Copyright

High Strain Rate Effects on Environment Assisted Fatigue for Austenitic Stainless Steels in PWR Environment

Current Japanese fatigue evaluations in the PWR environment are conducted using JSME S NF1 in which some parameters (temperature, strain rate, dissolved oxygen, etc.) are evaluated as influencing factors. However, it is assumed that the JSME Code would be conservative in high-strain-rate regions because the environmental factor (Fen) is evaluated by extrapolating experimental test results from a low strain rate (up to 0.4%/sec). In this study, experimental data are obtained additionally and Fen for high-strain-rate regions are reevaluated. As a result, it is confirmed that Fen at a strain rate of 1.0%/sec is about one half of that in the JSME Code. Further, marshaled experimental data obtained from the EFT project, which are classified according to stainless grade and material charge, indicated that the strain rate at Fen = 1 can be lower than 3%/sec for all austenitic stainless steels and similar to the current test result.Copyright

Examination of Factors in the Modified Rate Approach Method Under Various Conditions

Fatigue life in elevated-temperature water is affected by water chemistry, temperature, and strain rate. To evaluate these effects, the environmental fatigue life correction factor was established. And to evaluate fatigue damage in actual plants where factors such as temperature and strain rate are not constant, the modified rate approach method was developed. In order to confirm the applicability of these methods, several tests were carried out under a condition in which strain rate changes in response to temperature and fatigue life could be evaluated with an accuracy of a factor of 3, but conservatism was observed. In this evaluation, conservatism of environmental fatigue prediction is studied. To minimize conservatism in environmental fatigue evaluation, four factors are examined. As a result of examination, we conclude that an improvement the environmental fatigue life correction factor and application of a strain range insensitive to the environment may reduce conservatism, and that investigation into the mechanism of reduction in fatigue life is necessary for further improvement.© 2011 ASME

Effects of Continuous Strain Rate Changing on Environmental Fatigue for Stainless Steels in PWR Environment

The strain rates in actual transients of operating plants are not constant and changing momentarily. A large number of fatigue tests under stepwise strain rate changing conditions were performed to develop a method for evaluating fatigue life under varying strain rate conditions. Based on these test results, a strain base integral model known as the named the modified rate approach method was developed and verified. However it was reported recently that in the case of sine wave fatigue tests in BWR environment, the fatigue life was two to six times longer than the fatigue life predicted by the modified rate approach method. For this paper, in order to confirm whether the same tendency is observed in PWR environment, fatigue tests of sine wave were performed of SS316 in simulated PWR water environment. As the result, the difference of fatigue life by sine wave test reported in BWR environment was not observed between experimental and predicted life in PWR environment and the modified rate approach method was applicable under continuous strain rate changing condition.Copyright

Effects of Strain Holding and Continuously Changing Strain Rate on Fatigue Life Reduction of Structural Materials in Simulated LWR Water

The fatigue life reduces remarkably with reduction in strain rate in simulated light water reactor (LWR) water but the effects of strain wave form on this reduction are still not clear. This paper provides fatigue life data obtained from stepwise strain rate change tests, sine wave tests and strain holding tests. The effects of varying strain rate on fatigue life reduction can be estimated very well by the modified rate approach (MRA) method in the case of the step wise strain rate changing as shown in authors’ previous papers [1, 2, 3, 4, 5]. In the case of sine wave, however, the fatigue life reduction is much less compared to that predicted by the MRA method. The mechanism of such difference is not clear and the quantitative assessment of the fatigue life reduction caused by irregular strain wave form in actual transient seems impossible. The current MRA method gives always conservative assessment for sine wave straining and thus it is judged that this method need not be revised any more. The fatigue life reduction caused by strain holding at the peak of straining cycle in simulated BWR water had been reported in the previous paper [6]. In actual thermal transients, however, strain is not usually held at the peak of straining cycle but at the point somewhat reduced from the peak after the stabilization of temperature. In considering this phenomenon, additional fatigue tests in which the strain was held at the point somewhat reduced from the peak were carried out. In such conditions, the fatigue life reduction caused by strain holding disappeared. The similar fatigue tests with peak strain holding were also carried out in simulated PWR water and no fatigue life reduction can be observed. Considering the effects of strain holding on fatigue, the model for evaluating fatigue life reduction in LWR water was revised.Copyright

Effect of Factors on Fatigue Life in PWR Water Environment

It is known that the fatigue life in elevated temperature water is substantially reduced compared with that in the air (1–4) . Although the key parameters that have an effect on fatigue lives are strain rate and temperature in PWR water environment, it is necessary to consider the other factors on fatigue life for accurate evaluation. The effects of many factors on fatigue life have been investigated experimentally in the EFT project of Japan Nuclear Energy Safety Organization (JNES). Many tests have been done for carbon, low alloy, stainless steel and nickel-based alloy, and the environmental fatigue life equation that evaluates quantitative factor influencing the fatigue life has been proposed. In this paper, in order to evaluate effects of material structure difference between base metal and weld metal, strain amplitude, strain rate, strain ratio, temperature, sulfur content in steel, aging, water flow rate and strain holding, fatigue tests were performed in simulated PWR water environment.Copyright