Based on the precedent design fatigue curves and recent fatigue data obtained from materials with different mechanical properties, new design fatigue curves with high general versatility in air have been developed by The Japan Welding Engineering Society (JWES). Structural materials with different tensile strength are utilized in fatigue tests to verify the validity of these design fatigue curves and discuss the mean stress effect. The materials employed in this study are austenitic stainless steel (SS) SUS316LTP, carbon steel (CS) STPT370, low-alloy steels (LASs) SQV2A and SCM435H, all of which are used in the structural components of nuclear power plants of Japan. The best-fit curves (BFCs) are formulated by using the parameter of tensile strength to describe the relationship between strain (stress) amplitude and fatigue life [1]. The results of fully reversed axial fatigue tests conducted with small-scale test specimens of those materials in air at ambient temperature show good agreement with the developed BFCs. The results of fatigue tests also indicate that the mean stress effect is remarkable in materials with higher tensile strength. The applicability of Modified Goodman and Smith-Watson-Topper (SWT) approaches to the design fatigue curves is compared and discussed when considering mean stress effect. The correction of mean stress effect with SWT approach shows a good agreement with the developed BFCs.

In order to develop new design fatigue curves for carbon steels & low-alloy steels and austenitic stainless steels and a new design fatigue evaluation method that are rational and have clear design basis, Design Fatigue Curve (DFC) Phase 1 subcommittee and Phase 2 subcommittee were established in the Atomic Energy Research Committee in the Japan Welding Engineering Society (JWES). The study on design fatigue curves was actively performed in the subcommittees. In the subcommittees, domestic and foreign fatigue data of small test specimens in air were collected and a comprehensive fatigue database (≈6000 data) was constructed and the accurate best-fit curves of carbon steels & low-alloy steels and austenitic stainless steels were developed. Design factors were investigated. Also, a Japanese utility collaborative project performed large scale fatigue tests using austenitic stainless steel piping and low-alloy steel flat plates as well as fatigue tests using small specimens to obtain not only basic data but also fatigue data of mean stress effect, surface finish effect and size effect. Those test results were provided to the subcommittee and utilized the above studies. Based on the above studies, a new fatigue evaluation method has been developed.

UK very high integrity (VHI) component classification includes design, manufacturing, and inspection requirements that go beyond those established in ASME BPVC Sec. III Subsection NB [1]. One of these requirements is to ensure the component is tolerant of manufacturing defects. This can be demonstrated using a Defect Tolerance Assessment (DTA) based on two parameters fracture mechanics method. The brittle fracture parameter of this assessment requires the analysis of stress occurring in the component against the plane strain fracture toughness, K IC of the material. This work focuses on the practical determination of K IC for materials chosen for a Boiling Water Reactor (BWR) Main Steam Piping (MSP) and Main Steam Isolation Valve (MSIV), which carbon steel seamless pipe SA-106 Grade C and carbon steel casting SA-216 Grade WCB, are respectively. These materials are usually tested by Charpy impact testing specified in [1], but there are not many studies reporting their K IC , and there is not enough information concerning actual piping and valve materials. Thus the authors implemented fracture toughness testing using J-resistance curve according to ASTM E 1820 [2] for test pipe and test casting block simulating actual MS Piping and MSIV, and evaluated K IC (J) to be used in DTA. K IC (J) is evaluated from elastic-plastic fracture toughness, J IC , gained from the J-resistance curve, and equivalent to K IC [3]. K IC (J) corresponds to K JIc in ASTM E 1820. There were some cases, however, in which valid J IC values could not obtained, because of the materials high toughness, test specimen size limitations, and uneven final crack sizes. When valid J IC can’t be obtained, retesting or remanufacturing would significantly affect plant construction schedule. Hence, alternative evaluation methods by which J IC can certainly be obtained are desired. In this study, the authors focused on two types of alternative J IC evaluation methods. The first one is the Stretch Zone Width (SZW) method, in which J IC is calculated from SZW measurements of crack tip plastic blunting on fracture toughness test specimens. The SZW method was well studied in the 1970s, and experimental data showed a clear correlation between J IC values obtained from J-resistance curves and J IC values obtained from SZW measurements [4]. The second method is by correlation of J IC with the energy absorbed during Charpy testing. As represented by Rolf’s study [5], it has been reported that there are correlations between Charpy absorbed energy and K IC for high tensile strength steels. In this study, the validity of the SZW method was first evaluated by comparing its results with J IC obtained from J-resistance curves. Then, the applicability of the J IC values to DTA of actual products was discussed. Finally, by comparing Charpy absorbed energy and K IC (J) , the validity and applicability of K IC determination method with Charpy absorbed energy was discussed.

Published works on studying effects of specimen size on fatigue endurance limit and fatigue life were reviewed. Specimen size effect is apparent in many bending fatigue tests. However, axial tension and compression loading fatigue tests showed that the fatigue limit decreased slightly with increasing specimen size. Design margin for the size effect on fatigue limit under axial tension and compression loading condition that is the condition of the data used to construct Design Fatigue Curve (DFC) is not considered to be necessary. If it is considered conservatively, the value less than 1.1 is enough. Axial tension and compression loading fatigue tests using cylindrical specimens having different diameters showed that the fatigue life increased with increasing specimen diameter. This phenomenon is considered to occur due to the crack growth life which is not negligible in relatively large diameter specimens. Based on this, design margin factor on number of cycles is not necessary to be considered in DFC. These design margins based on the axial tension and compression loading fatigue test results are less than the values in current design codes. This indicates that the current design fatigue curves in the codes have large margins. These considerations will be verified after the large scale fatigue tests that are planned in Design Fatigue Curve (DFC) subcommittee in the Atomic Energy Research Committee in the Japan Welding Engineering Society.

Fatigue evaluation methods have been proposed based on environmental fatigue test results regarding parameters selected for simulating Boiling Water Reactor (BWR) and Pressurized Water Reactor (PWR) conditions. The effects of strain wave form have been discussed by comparing experimental fatigue life with predicted fatigue life evaluated by modified rate approach (MRA) method. The applicability of the MRA method has been verified extensively by the environmental fatigue tests with strain rate changing conditions consisting of combined constant strain rates. However, different results have been obtained for a sine strain wave in simulated BWR and PWR conditions. More study for evaluating the applicability of MRA method was required by evaluating with continuous strain rate conditions such as a sine wave. For the purpose of verification, two approaches were applied. One is performing the environmental fatigue tests with the sine strain wave in simulated BWR condition. The other is to evaluate the low cycle thermal fatigue test performed in simulated BWR condition because the wave form of this test contains continuous strain rate changing condition. MRA method was indicated to be applicable to predict fatigue lives under these kinds of continuous strain rate changing conditions. All of the studies including this study verifying the applicability of the MRA method were performed with small specimens having the well polished surfaces in the gage length. These results indicate that the evaluation by the MRA method includes the synergistic effect between the water environment and the transient. However, the synergistic effects with the surface roughness and the component size are not known. Design margin derived by the multiplication of the sub-factors of environment, surface roughness and component size may be conservative. The evaluation of the conservatism is considered to be beneficial.

In order to develop and propose new design fatigue curves for austenitic stainless steels, carbon steels and low alloy steels that are rational and have clear design basis, Design Fatigue Curve (DFC) subcommittee has been established in the Atomic Energy Research Committee in the Japan Welding Engineering Society and the study on design fatigue curves are going on. This paper introduces the plan and status of the activities of the DFC subcommittee.

Recently, incidents of SCC in Ni-base alloy weld metal of BWR components have been reported. When the defects are detected by inspection, structural integrity assessment should be performed for the technical judgment on continuous service based on the Rules on Fitness-for-Service for Nuclear Power Plants of the Japan Society of Mechanical Engineers Code (JSME FFS Code). The structural integrity assessment includes fatigue crack growth analysis. However, fatigue crack growth analysis for Ni-base alloy is impossible since the fatigue crack growth rate curves in air and the BWR environment are not prescribed yet in the JSME FFS Code. The curve in air environment is needed for the structural integrity assessment of the flaw when the embedded flaw repair which is one of the repair techniques to isolate the defect from water environment by seal welding is applied. In this study, fatigue crack growth tests in air environment were performed for Ni-base alloy weld metal. Based on the test data, fatigue crack growth rate curves with Δ K th of Ni-base alloy weld metal were investigated. It is found that fatigue crack growth data in the Paris region hardly depend on the test temperature and the stress ratio, whereas data around Δ K th are dependent on them. Hence, the curve in the Paris region was regarded as the same curve despite the difference of the test temperature and the stress ratio. The minimum Δ K of final crack growth data in the Δ K decreasing test was adopted as Δ K th of the curve.

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 ( F en ) and the equations to calculate F en 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 F en 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.

Fatigue tests in simulated LWR environment of carbon and stainless steels were performed under high water flow rates between 7 to 10 m/s. For carbon steel, high flow rate of water clearly mitigated the environmental effect on the fatigue life at the high sulfur concentration of 0.016% which caused high environmental effect on a fatigue life. On the contrary, high flow rate of water slightly enhanced the environmental effect at the low sulfur concentration at or less than 0.008% which caused very low environmental effect. These results suggested that the environmental fatigue life under various flow rate conditions should be determined by the combination between the mitigating effect caused by flushing of the severe local environment and the enhancing effect caused by increase in corrosion potential. Low alloy steel showed the similar behavior as carbon steel. For stainless steel, flow rate had little effect on the fatigue life of type 316NG stainless steel. It suggested that there was no role of water flushing. For type 304 and 304L stainless steel, fatigue life has a tendency to decrease with increase in water flow rate. Fatigue lives of type 304 stainless steel under high flow rate of 7 to 10 m/s were shorter than those predicted by proposed fatigue life prediction equation by the Japanese EFT committee. This effect should be considered in an evaluation of environmental fatigue. No water flow effect was found in cast stainless steel.

The environmental fatigue life of carbon steel is influenced by BWR conditions such as water temperature, dissolved oxygen concentration, water flow rate and so on. These parameters change inconstantly during operation of BWR plants and strain rate changes in the structural components due to the temperature change. In general, fatigue life evaluation equations have been formulated based on the fatigue data obtained under constant conditions. To apply these equations to evaluate the fatigue life of actual components, a study is necessary to confirm the applicability of the proposed equations to the changing conditions. In this study, fatigue tests were performed under changing conditions of strain amplitude, strain rate, temperature, dissolved oxygen concentration and water flow rate. It was confirmed that the proposed fatigue life equation could predict the fatigue life under changing conditions.

The fatigue life of austenitic stainless steel has recently been shown to undergo remarkable reduction with decrease in strain rate and increase in temperature in water. Either of these parameters as a factor of this reduction has been examined quantitatively and methods for predicting the fatigue life reduction factor F en in any given set of conditions have been proposed. All these methods are based primarily on fatigue data in simulated PWR water owing to the few data available in simulated BWR water. Recent Japanese fatigue data in simulated BWR water clearly indicated the effects of the environment on fatigue degradation to be milder than under actual PWR conditions. A new method for determining F en in BWR water was developed in the present study and a revised F en in PWR water is also proposed based on new data. These new models differ from those previously used primarily with regard to the manner in which strain amplitude is considered to affect F en in the environment.

The flow rate of water flowing over a steel surface is considered to be one of the most important factors influencing the fatigue life of the steel, because the water flow produces differences in the local environment. The effect of the water flow rate on the fatigue life of carbon, low alloy, and austenitic stainless steels was therefore investigated experimentally. Fatigue testing of low (S = 0.008 wt%) and high (S = 0.016 wt%) sulfur content carbon steels and a low alloy steel was performed at 289°C for various dissolved oxygen concentrations (DO) of less than 0.01 and 0.05, 0.2, and 1 ppm, and at various water flow rates. Three different strain rates of 0.4, 0.01, and 0.001%/s were used in the fatigue tests. For high sulfur carbon steel (S = 0.016 wt%), the effect of a high water flow rate on mitigating fatigue life reduction was more clearly observed at a lower strain rate, irrespective of the DO. This effect of high water flow rate was most notable at a DO of 0.2 ppm, which was the DO level that produced a significant sulfur effect. This indicates that the mechanism responsible for the mitigation of fatigue life reduction is the flushing effect of the water, which eliminates the locally corrosive environment. For high sulfur carbon steel (S = 0.016 wt%), no benefit of a high water flow rate was found at a DO of 0.01 ppm. This was because the environmental effect is insignificant at this low DO level. For low sulfur carbon steel (S = 0.008 wt%) and low alloy steel (S = 0.008 wt%), a high water flow rate had little effect on mitigating fatigue life reduction even at a DO of 0.2 ppm. This indicates that the sulfur is much less influential in low sulfur steel than in high sulfur steel. Fatigue testing of Type 316 nuclear grade stainless steel (316NG) and Type 316 stainless steel (SUS316) was performed at 289°C and 320°C for DO levels of less than 0.01 and 0.05, and 0.2. For austenitic stainless steel, no mitigating effect at a high water flow rate was found. It should be noted rather that there is a possibility that a high water flow rate decreases the fatigue life because a tendency to a slight decrease in fatigue life with an increasing flow rate was observed.

A database of fatigue strength in air and high temperature water environments for the materials used in LWR structural components has been developed. It includes the fatigue data acquired in the EFT project entrusted by METI, Japan, as well as public data obtained from domestic and overseas literature. One of the features of the database is that it includes about 2,500 images such as scanning electron micrographs of the fractured surface of the specimen after fatigue test. Another feature is that the program can search data, draw graphs such as S-N curves and hysteresis loops automatically, and make calculations of the hysteresis energy equivalent to the area of hysteresis loop, for example. The structure of the database, the contents of the data compiled, and the functions of the program are presented with actual examples, and the characteristics of the database are discussed.

Fatigue tests in simulated LWR environment of carbon and stainless steels were performed under high water flow rates between 7 to 10 m/s. For carbon steel, high flow rate of water clearly mitigated the environmental effect on a fatigue life at the high sulfur concentration of 0.016 wt% which caused high environmental effect on a fatigue life. On the contrary, high flow rate of water slightly enhanced the environmental effect at the low sulfur concentration at or less than 0.008 wt% which caused very low environmental effect. These results suggested that the environmental fatigue life under various flow rate conditions should be determined by the combination between the mitigating effect caused by flushing of locally severe environment and the enhancing effect caused by increase in corrosion potential. To understand those effects, effects of sulfur concentration on fatigue life for various DO condition were formulated. And corrosion potential under low and high flow rate condition was measured during the fatigue test. Environmental correction factor, F en , which is the ratio of fatigue lives derived from the fatigue life at room temperature in air divided by that in water to be used for the fatigue life prediction at high flow rate condition was assumed based on the MITI guideline equation and considering the hypothetical fatigue life under sulfur free condition and high corrosion potential condition. This assumption was agreed very well with the test data. For stainless steel, flow rate had little effect on a fatigue life of type 316 stainless steel. It suggested that there was no role of water flushing. For type 304 stainless steel, fatigue life has a tendency to decrease with increase in water flow rate. Fatigue lives of type 304 stainless steel under high flow rate of 7 to 10 m/s were shorter than those predicted by MITI guideline equation. This effect should be considered in an evaluation of environmental fatigue.

Low cycle fatigue life of carbon and low alloy steels reduces remarkably as functions of strain rate, temperature, dissolved oxygen and sulfur in steel in high temperature water simulating LWR coolant. A model for predicting such fatigue life reduction was first proposed in the early 1980s and since then has been revised several times. The existing model established in 2000 is used for the MITI Guideline [6] and the TENPES Guideline [7] which stipulate procedures for evaluating environmental fatigue damage at LWR plants in Japan. This paper presents the most recent environmental fatigue evaluation model derived based on additional fatigue data provided by the EFT Project over the past five years. This model differs not significantly with previous version but does provide more accurate equations for the susceptibility of fatigue life to sulfur in steel, strain rate, temperature and dissolved oxygen. Test data on environmental fatigue of nickel base alloys are available only to a limited extent and there is yet no model for predicting fatigue life reduction in such an environment. The EFT Project has made available considerable environmental fatigue test data and developed a new model for calculating F en of nickel base alloys. The contribution of environment to fatigue of nickel base alloy is much less compared to that in austenitic stainless steel.

The fatigue life of carbon steel is influenced by Boiling Water Reactor (BWR) conditions such as water temperature, dissolved oxygen concentration, water flow rate and so on. These parameters changes constantly during the operation of BWR plants and strain rate changes in the structural components occur because of temperature changes. In general, fatigue life evaluation equations have been formulated based on fatigue data obtained under constant conditions. Before using these equations to evaluate the fatigue life of actual components, a study is necessary to make sure they are applicable to changing conditions. In this study, fatigue tests were performed under changing strain amplitude, strain rate, temperature, dissolved oxygen content, and water flow rate conditions. It was confirmed that the proposed fatigue life equation could predict the fatigue life under practical changing conditions.