Thermal fatigue crack may initiate at mixing tee where high and low temperature fluids flow in and mix. This paper shows some results of numerical simulations that were conducted to evaluate temperature fluctuation in mixing tee using some turbulence models. Accuracy of these simulations was examined for a velocity condition of WATLON experiments performed at Japan Atomic Energy Agency: wall jet condition that the jet from the branch pipe flowed along the main pipe wall. Time-averaged temperature distribution with LES and DES agreed well with the experimental results, but the intensity of temperature fluctuation in the wake ofjet had some differences. RANS model, such as SST model, can't simulate the fluctuation around the jet although the same mesh, boundary conditions and time step were used in the calculation.

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Fatigue cracks can be initiated at tee junction where fluids of different temperature flow in. In order to prevent the crack initiation, JSME issued the guideline for evaluating pipe system. In the guideline, the fatigue damage is estimated according to linear damage accumulation law, which is usually applied for low-cycle fatigue. In this study, problems to be solved were discussed to validate thennal stress and the fatigue damage assessment procedure of the JSIVIE guideline. Based on the analytical results for the thermal striping at a tee junction, the cyclic load was characterized. Then, loading patterns for simulating the fatigue damage were proposed. Possibility of introducing the crack growth analysis in the fatigue damage assessment was also discussed.

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High cycle thermal fatigue is one of postulated failure modes at the pipes of the cooling systems of the nuclear power plants. We examined this phenomenon and identified heat transfer coefficient, wall thickness and frequency of temperature fluctuation of fluid as dominant parameters. Thorough the fmite element analysis, we could obtain the curves to evaluate the effects of dominant parameters. And using these curves and theoretical equations, we proposed the frame work of evaluation method of high cycle thermal fatigue. As a result, the evaluation has found to be more accurate than current guideline of JSME and have proper safety margin against the actual phenomenon.

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This paper describes the effects of stress ratio on the ΔK_<th> improved by overload. High-tension steel (HT540) and aluminum alloy (A7075) were prepared as test materials. Tensile overload was applied to compact tension specimen, subsequently fatigue tests were carried out at stress ratio S=0.1 or 0.5. The main conclusions are as follows; (1) The value of ΔK_<th> increased with increasing of the tensile overload. (2) The effect of overload on ΔK_<th> did not differ between HT540 and A7075. (3) The improvement ratio of ΔK_<th> S=0.1 was larger than that at S=0.5.

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This study proposes an innovative method for aesting fatigue crack, and making harmless the cracks by overload. By overloading, compressive residual stress is introduced ahead of the crack The residual stress reduce K values and arrest the crack. By theory, ΔK_<th> increase with increasing Kov (K value by overload) and the nominal threshold values were given by the equation when stress ration was negative: Δ^NK_<th>^s=C+DKov. Where C is ΔK_<th>^o and D is a proportional constant and 0.24〜0.27. Experimental results showed that D showed good agreement with theory. Thus, if Δ^NK_<th>^s is enough larger than applied K and ΔK values, crack is not able to grow and can be made harmless.

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Recently, incidents of SCC in Ni-base alloy weld metal of BWR components have been reported. Structural integrity assessment should be. performed based on the Rules on Fitness-for-Service for Nuclear Power Plants of the Japan Society of Mechanical Engineers Code (JSME FFS Code). However, fatigue crack growth analysis for Ni-base alloy is impossible since the fatigue crack growth rate curves in air and BWR environment are not prescribed yet in JSME FFS Code. In this study, fatigue crack growth tests in air were performed for Ni-base alloy weld metal. Based on the test data, fatigue crack growth rate curves with ΔK_<th> were investigated. The curve in Paris' region was regarded as the same curve despite the difference of the test temperature and the stress ratio. On the other hand, ΔK_<th> was regarded to depend on the test temperature and the stress ratio.

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There are several code and standards, handbooks and guidelines for the nuclear power plant maintenance in Japan, the US and EU. They include SCC and fatigue crack growth curves for crack growth calculation. In this paper the authors selected five kinds of codes, standards and guidelines and compared their fatigue crack growth curves for choice of the suitable curves. The feature of each curve was summarized and quantitatively evaluated. JSME maintenance rule and ASME code provide the fatigue crack growth formulae for both ferritic and austenitic steels and consider the environmental effects in some cases. The FITNET curves are categorized in many kinds of metal whereas the FKM guideline and WES procedure provide the common properties applicable to steels generally.

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Effect of wall-thinning dimensions on the strength of piping elbows against internal pressure and in-plane bending was examined through the finite element analysis considering material nonlinearity and geometrical nonlinearity. The pipe used for simulation was MS SMTP370 80A Sch160. Assuming the wall-thinning area was fixed to relatively narrow area, the effect of depth and position of wall-thinning was investigated. The capped elbow specimen was first subjected to a constant internal pressure, and second subjected to the close-mode in-plane bending until the prescribed displacement The maximum load was defmed as the collapse load. From the simulation results, we found that the effect of relatively narrow wall-thinning on the collapse load was small under the internal pressure and close-mode in-plane bending.

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The beam theory has been used for the limit load evaluation for straight pipes with surface flaws. In case of elbow pipe, however, the beam theory gives just an approximated solution because of the three-dimensional defomiation combining the beam bending and plate bending. In this study, we searched for the boundary, within which the approximation based on the beam theory was applicable. To determine the boundary, the bend radius and wall thickness was systematically changed, and the reasonable solutions were obtained by the fmite element analysis using the solid element with the reduced integration to avoid the numerical locking. The variation of error in the extrados and intrados surface stress with respect to the pipe dimensions was numerically clarified and 10%-error bound was fixed as the applicability boundary. The explicit formulation of the boundary was found to judge the applicability of beam theory to the elbow pipes.

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Effect of wall-thinning area dimensions on the flexural buckling load was addressed through the finite element linear buckling analysis. The width and length of rectangular wall-thinning were systematically changed, and the variation of elastic flexural buckling load was calculated. From the simulation result, the flexural buckling load considerably decreased when the wall-thinning area exceeded the value of mean radius in length and 180 deg in width. Further investigation using the nonlinear finite element analysis, which considered the internal pressure with the material nonlinearity and geometrical nonlinearity, showed that the internal pressure made the plastic collapse load decrease. Moreover, we found that the limit load was valid even in case that the plastic flexural buckling, that is, the local buckling occurred for wider wall-thinning.

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This paper describes comparison of observed SCC crack depth in PLR piping system and crack growth estimation. Crack depth estimated using crack growth rate curves, based on the recent R&D results and the prescribed in the Rules on Fitness-For-Service in JSME, envelopes observed data.

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High chromium steel is expected as a material of the primary coolant pipes for the next generation of sodium cooled fast breeder reactor in Japan, whereas austenitic stainless steel is used for that of Monju. A lot of elbows in the piping system should be necessary for Monju because of relaxation of thermal stress. Thermal expansion of high chromium steel is lower than that of austenitic stainless steel, thus numbers of elbows shall be reduced, by using this material. Piping system is fabricated to connect by welding process. Mechanical properties, for example, hardness and stress-strain characteristic curve, of the weld metal and heat-affected zone (HAZ) differ from base metal in the welded joint. The welded joint of high chromium steel has naturally softened HAZ due to multiple welding heat cycles. Tensile strength limit in a welded joint is affected by HAZ softening, and it is desirable to consider the softening effect and to increase tensile strength not in HAZ but in a whole welded joint. The effect of HAZ softening effect on tensile strength limit of the welded pipe joint is investigated in this paper.

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For the development of Advanced Ultra Super Critical power plant, it is important to improve creep damage and life assessment methods for welded part, especially welded part consisted of Ni-based superalloy/Heat resisting steel. In this study, for the welded joint ofAlloy617/CrMoV and 12%Cr/CrMoV, creep crack growth rate test were conducted to investigate the creep crack growth behavior. As a result, the different behavior which was thought to be a cause of the different multi-axial stress field, was confirmed.

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Low cycle fatigue tests were conducted using 100A elbow pipe specimens with or without local wall thinning. Local wall thinning was machined on the inside of the exirados test elbows to simulate metal loss due to flow-accelerated corrosion or liquid droplet impingement erosion. Low cycle fatigue test were carried out under displacement control with an inner pressure of 9 MPa. To simulate seismic events, low cycle fatigue tests were carried out on elbow pipe subjected to cyclic overloads. Regardless of local wall thinning, fatigue life of overload pipe was not so different from that of the non-overload pipe in appearance. Miner's rule can be applied to evaluate fatigue life of the elbow pipes with and without wall thinning, even if overload is applied.

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We developed a numerical method for predicting a distribution of compressive residual stress on a material surface after water jet peeing (WJP). WJP applies the bubble collapse energy of cavitating jet to the change of residual stress from tensile to compressive for preventing the occurrence of stress corrosion cracking. We estimated the impulsive bubble pressure that varied in microseconds by using a numerical simulation with 'bubble flow model'. The bubble collapse energy was evaluated by the cavitation intensity calculated from the bubble pressure. The cavitation intensity was predicted when the cavitating jet was impinged slantingly on cylinders, and was compared with the measured compressive residual stress after WJP. The distribution of the cavitation intensity corresponded qualitatively to that of the compressive residual stress on the cylinder surface.

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In order to prevent stress corrosion cracking (SCC) initiation at inner surface of butt-welding region of small diameter piping, the residual stress improvement method was developed. In this method, butt-welding region is quenched by cooling water after heated in air. During quenching, a large thermal stress is generated by the temperature difference between inner and outer surfaces. When the thermal stress exceeds the yield stress of piping material, compressive stress can be introduced at the inner surface due to tensile yielding. In this study, we confirmed the effectiveness of this method by measuring residual stress at the inner surface of butt-welding region of small diameter piping after applying this method.

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Reducing the residual tensile stress near the area of a pipe that is affected by weld heat inhibits both of occurence and growth of SCC in stainless steel. We propose a way of reducing tensile residual stresses on the inner surface of small-bore stainless steel pipe welds used in nuclear plants, the delta-T stress improvement method, in which the wall of a pipe is heated by an external heating device and the inner surface of the pipe is rapidly cooled with water. We analytically and experimentally surveyed the mechanisms of using this method to reduce residual stress. In both analytical and experimental results, tensile residual stresses on the inner surface of the small-bore stainless steel pipe with a nominal outside diameter of 60.5 mm were reduced. Furthermore, the analytical and experimental results correspond well, and thus provide good confirmation that this method can be used to reduce tensile residual stress.

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This study describes residual stress distribution of large-bore and heavy-walled stainless steel pipe with butt-welded joint as measured by inherent strain method in the form of function, in which the inherent strain distribution is represented in the form of function. The deviation of the most probable residual stress value was small indicating accurate measurement. The detail feature and 3D-distribution of the residual stress generated within the heavy-walled pipe with welded joint can be evaluated in this method. It was found that the residual stress distribution had more complicated shape as the aperture and thickness of the pipe was larger.

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Laser peening is one of effective technique to prevent SCC initiation. However, it has possibilities that compressive residual stress appended by laser peening is affected by applied stresses during plant operation. In this study, in order to evaluate the effect of applied stress on the compressive residual stress of weldment after laser peening, laser peening were conducted for weldment test specimen of SUS316L and NCF600, and their surface residual stress were measured by X-ray diffraction method before and after loading. In case of applying stress larger than 0.2 %proof stress, it was confirmed that compressive stress remained and the behavior of relaxation in heat affected zone is same as that of base metal.

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Fatigue crack propagation (FCP) tests were performed using CT specimens of extruded magnesium alloy, AZ61, under controlled humidity. Humidity accelerated FCP rates in low AK region. After allowing for crack closure, the environmental effect still existed, where FCP rates were the fastest in 30%R.H. condition, then in the higher humid conditions and in dry air, in decreasing order. The acceleration could be attributed to both anodic dissolution and hydrogen embrifflement at the fatigue crack tip.

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Fatigue tests were carried out using extruded Mg alloy AZ31 to study effect of negative load ratio on crack propagation behavior. In the tests, R ratio was changed into the negative value. When the minimum stress exceeds over the value of the compressive yield strength of the material, the rate of crack propagation accelerated suddenly due to occurrence of the twinning.

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Loading frequency dependence of near-threshold fatigue crack growth behavior at elevated temperature, was examined in the air and in a vacuum in Ni-based Superalloy. It was found that crack growth behavior at 800℃, 850℃, and 900℃ in the air and 900°C in a vacuum was dependent on loading frequency, while crack growth behavior at 800°C and 850℃ in a vacuum was independent of loading frequency.

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Experiments has been perfonried for the growth of fatigue cracks in mode III (anti-plane shear) in circumferentially notched specimens subjected to cyclic torsion with dynamic mode I loading. The influence of material and stress ratio on the fatigue crack growth was investigated. Almost fracture surfaces of the specimens were showed factory roof fracture, which was typical for Mode III failure. However, when the degree of overlap was 90°, the surfaces were flat on the whole whatever the material was.

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The general method is not always effective for the measurement of fatigue small crack growth properties by relation of accuracy and a device characteristic. Non-contact and the highly precise measurement are expected by the use of a magnetostriclion effect to apply in this study The small crack growth properties measurement by a magnetic field change with the magnetostiiction effect that was one of a characteristic of magnetism of feomagnetic substance was studied. Using a coil, a magnetic field change with magnetosthction effect was detected, and the waveform change of the output was inspected The experiment result was revealed that the small crack affects a magnetic field change by amagnetostriction effect.

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In order to clarify the formation factors of fine concavo-convexo region on the fracture surface of sub-surface fractures in very high cycle fatigue, crack growth tests of Ti-6Al-4V were conducted in ultra high vacuum and the effects of vacuum environment and repeating number of cycles on fracture surfaces were investigated. As a result, the fine concavo-convexo regions were formed in vacuum and the area ratio of them increased with increasing number of cycles. The fine concavo-convexo regions; however, were not formed in crack growth tests in air. These results coincided with the formation factors of ODA in high strength steel, therefore, it was concluded that long term cyclic loadings in vacuum-like environment around sub-surface crack is the main reason for the formation of fine concavo-convexo fracture surfaces regardless of materials.

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Loading frequency effects on FCGR and fracture surface morphology were investigated on a hydrogen-charged specimen of a pre-strained low carbon steel. Test results showed difference tendency of loading frequency effects between charged-hydrogen and hydrogen environment. Tests in a low pressure nitrogen showed that decreasing loading frequency revealed increasing FCGR. However, in a hydrogen gas FCGR revealed decreasing tendency as well as an uncharged specimen. Fracture surface morphology showed that the change of FCGR in this region related to the transition of ductile to brittle-like fracture surface or the reverse.

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Turbine runner is one of important components of hydraulic power generation. In this study, stainless cast steel (SCS6) which used in hydraulic power generation for several decades was used for the material. And, in order to clarify the fatigue crack propagation behavior of stainless cast steel under the constant loading, fatigue crack propagation test was carried out by using CT specimen. The results are summarized as follows: it was observed that liner relationship between da/dN and ΔK, da/dN and ΔK_<eff>. It was clear that there was two kinds of fracture surface for crack growth rate.

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In order to claiify the fundamental fatigue strength characteristic of precipitation strengthening type stainless steel: A286, fatigue tests were performed. In the fatigue test, the authors paid attention to the behavior of small fatigue cracks in order to make clear fatigue strength properties more than usual fatigue test. As a result of the fatigue test, the fatigue strength characteristic of A286 became clear as shown below. The crack of a size according to a ciystal grain diameter initiates. The cracks are easy to initiate and propagate by Mode II. The fatigue crack does not become a non-propagating crack. Many cracks initiate and propagate even if the stress is around the fatigue limit. The cracks join together and it becomes a long crack, and fmally results in a failure. This is the reason why the fatigue failures occur even after 10^7 cycles.

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It has been recently revealed by the authors that the fatigue damage accumulation of low alloy steel can be detected by the change in hydrogen desorption characteristic, witich is measured by the thennal desorption spectrometry after charging the steel with hydrogen. In this study, the optimization of hydrogen charging condition was examined to improve the sensitivity for detecting the damage. The cathodic electrolysis for hydrogen charging was performed in 1 mol/L NaOH aqueous solutions with a wide variety of concentrations of NH_4SCN for various periods. Experimental results revealed that the amount of desorbed hydrogen, C_H, increased significantly with increasing NH_4SCN concentration before 0.1 mass % and it had a maximum value wiien the charging time was 12 h. The increase in C_H obtained under the modified charging condition with fatigue life was more pronounced compared with that under the conventional condition.

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Effects of irradiation damage on the micro-crack initiation following the slip band formation and growth under low cycle fatigue in the proton-irradiated SUS316L austenitic stainless steel was investigated for improving the prediction method of the micro-crack initiation life under irradiation. The slip band spacing and the micro-crack initiation life of the specimen irradiated up to 0.25 dpa at 230℃ were about twice larger and one-fifth shorter than those of the unirradiated specimen, respectively. The micro-crack initiation in the unirradiated specimen occurred at the grain boundary, trans-crystalline, twin boundary and slip band. While, that occurred only at the twin boundary and slip band in the irradiated specimen. The number density of the slip band increased rapidly at the micro-crack initiation life ratio of 0.2〜0.4, and then it tended to saturate. These trends were independent of the irradiation.

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An ultrasonic fatigue test is an effective measure to obtain very high cycle fatigue lives, but it contains some problems due to its high test frequency. One of problems is self-heating of test pieces which hinders accurate fatigue data collection. In this study, an attempt was made to measure the amount of self-heating in the ultrasonic fatigue test specimen and to estimate the temperature distributions by FE analysis. Though the proposed method required coefficient of heat transfer which can be obtained by the combination between FE analysis and simple experiments, it showed good estimation of the specimen surface temperature in steady state. The method also made it possible to estimate the upper limit condition to perform the ultrasonic fatigue test without self-heating.

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Stress intensity factor of fatigue crack tip after two fatigue cracks had connected it was estimated by conventional method using replication technique under rotating bending load. The stress intensity factor was calculated with the surface crack length which was obtained by replica film observation and aspect ratio which was obtained by some way. In the other hand, stress intensity factor was obtained by converting from crack propagation rate of crack tip using the Paris low by micro computed tomography (μCT) with synchrotron radiation of SPring-8. As a result of the comparing both value, the value by conventional method was larger than the value by μCT method.

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In order to investigate the high cycle fatigue properties of extruded ZK60 and AZ80 magnesium alloys, axial tension-compression (R=1) fatigue tests were carried out at room temperature. The duplex S-N characteristics containing of different S-N curves were obtained for both alloys. The fatigue strengths at 10^9 cycles of ZK60A and AZ80A magnesium alloys were 120MPa and 110MPa, respectively. The fatigue strengths were lower than that of 0.2% proof stresses for both alloys. The fatigue crack initiated at the specimen surface in low cycle regime. On the other hand, the fatigue crack initiated at the interior Al-Mn system inclusion in very high cycle regime.

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Giga-cycle fatigue properties were investigated for an ultrafine-grained steel processed by warm tempforming. The fatigue properties were compared with conventional tempered martensite steel. Fatigue tests were carried out at frequencies of 20Hz and 20kHz, using servo-hydraulic and ultrasonic fatigue testing machines, respectively. Both tempformed and conventional steels showed surface fracture in short life regions and fish-eye fracture in long life regions. The fish-eye facture origins were oxide inclusions comprised of Si, Mg, Ca other than Al. Difference of fatigue strength was not notable between the tempformed and conventional steels, i.e. the fatigue strength of the tempformed steel was as high as that of conventional steel.

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Giga-cycle fatigue properties were investigated for high-strength spheroidal graphite cast iron. Fatigue tests were carried out using ultrasonic fatigue testing at 20 kHz and conventional fatigue testing at 30 Hz used specimens with 3 and 6 mm diameters under stress ratio of R=-1. As the results, frequency effects were very small in the both of the 3 and 6mm specimens. Accordingly, ultrasonic fatigue testing could be applicable to spheroidal graphite cast iron. Decrease in fatigue strength in the giga-cycle region were a little independent of specimen size. Moreover, effect of specimen size on fatigue strength was not remarkable. However, the furacture sites were graphite and microshrinkage in 3mm specinens, while all of them were rnicroshrinkage in 6mm specimens.

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Fish eye failure that is unique fatigue fracture occurs in very long life region of high strength steel. The fatigue life of Fish eye failure dominated ODA (Optically Dark Area) around the fracture origin. A granular looking area of ODA is visible close to the crack initiation, Fatigue life of Fish eye failure is almost exclusively spent propagating the fatigue clack in ODA. Therefore, to predict fatigue life of Fish-eye failure is important to know ODA of formative mechanism. In this study, fatigue and tensile test was performed in vacuum that simulated environment of subsurface failure and compared the result by the test condition. The observation of Process when ODA is formed on the precrack tip was examined in order to make clear a mechanism of ODA formation. When the tensile stress of 800MPa or less was given, the remarkable growth of the ODA crack was not seen. Crack growth rate has depended on maximum stress by range of ΔK≦4MPam. Under the fixed maximum stress of value, initiated fatigue cracks became non-propagating cracks.

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Corrosion fatigue tests and corrosion weight-loss tests in 3% NaCl solution were conducted using the anodized Mg alloy to study a correlation between corrosion fatigue strength and corrosion resistance of the material.

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Ultrasonic corrosion fatigue tests were conducted for 2.5%Mo containing high strength austenitic stainless steels in 3% NaCl aqueous solution. The reduction of corrosion fatigue strength at 10^9 cycles for heat treated steel and TMCP made steel was 12.1 and 19.5%, respectively. Corrosion pit was observed on corrosion fatigue crack initiation area. Striation was predominantly observed on crack propagation area both in air and in 3%NaCl aqueous solution. It can be concluded that the reduction of corrosion fatigue strength of 2.5%Mo containing high strength austenitic stainless steels is due to the corrosion pit formation at corrosion fatigue crack initiation area.

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The crack propagation under variable stress differs from that under constant amplitude stress. In order to clarify the effects of hydrogen gas environment on the fatigue crack propagation behavior, bending fatigue tests were carried out on a SUS304 in H2_ gas of 0.1 MPa and in air. Hydrogen gas accelerated the fatigue crack propagation rate for σ_L=50 MPa, but lowered it for 60, 80, 100MPa. The cause of the reduction was the development of the crack closure in H_2 gas.

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In the present paper, in order to investigate the effects of hydrogen on the strain aging of low carbon steel, Vickers haidness tests were carried out on 0.13 % carbon steel. A couple of specimens with a laige scale plastic zone were aging heat-treated; one was a specimen after hydrogen chaiging, the other was a specimen without hydrogen charging. Viekers hardness tests showed that the Vickers hardness of a hydiogen-chaiged specimen was lower than that of an uncharged specimen. This implies that hydiogenacts as an inhibitorof strain-aging hardening oflowcarbon steel. It also implies that hydrogencould affect the non-propagating behaviorthrough inhibiting strain aging.

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High cycle fatigue strength of spheroidal graphite cast iron was remarkably increased by radical nitriding. By nitriding, thick and hard diffusion layer was formed and the compressive residual stress was induced. So, initiation of a fatigue crack was suppressed. In order to clarify the state of defects in fracture origin, fracture section was observed by using SEM. As a result, fracture starts at microshrinkages that exist on the neighboring surface at high stress rebels. On the other hand, fracture originated from inner microshrinkages and abnormal carbons at low stress rebels. And distance from the surface that the defects exist was deep as the stress decreased.

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Rotating bending fatigue tests were conducted for a series of plasma-nitrided Fe-C-Mn steels in order to investigate the effect of manganese contents. 5 types of steels were prepared, i.e., 0.0Mn, 0.5Mn, 1.0Mn, 2.0Mn and 4.0Mn. Microstractures of the steels were fenite-pearlite except 4.0Mn whose microstructure was martensite. 2.0Mn and 4.0Mn of the nittided specimens had a hardened layer of 0.5 to 1 mm in depth, while the no hardening was observed in 0.0Mn, 0.5Mn and 1.0Mn of the nitrided specimens. In case of 0.0Mn, 0.5Mn and 1.0Mn, the nitrided specimen showed surface fracture and increase of fatigue strength was veiy small. The 2.0Mn nitrided specimen showed surface fracture and fatigue strength were largely improved by plasma-nilriding. On the other hand, in case of 4.0Mn, the nittided specimen showed internal fracture beneath the hardened layer and the fatigue strength were lower than that of the 2.0Mn nitrided specimen.

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The objective of the present paper is to propose a test method that can evaluate fretting fatigue properties of aluminum wheels for heavy vehicles. Four-point bending fatigue test conducted on bolted joint specimens with wheel and hub parts fixed by bolt and nuts. As a result, specimens fractured at the contact edge between nut and wheel part due to fretting fatigue as well as actual wheels. Fatigue limit of specimens is slightly higher than that of actual wheels. The small difference of fatigue limit is discussed from the view on probability of fracture.

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To predict fatigue life under variable loading accurately, it is important to understand the damage factors determining fatigue life. In this study, the damage factors of high-cycle fatigue were investigated for Type 316 stainless steel. Fully-reversed axial fatigue tests were performed in ambient air at room temperature. In order to evaluate the applicability of linear damage accumulation rule, 2 step stress tests were performed. It was shown that the linear damage accumulation rule did not always evaluate the fatigue life conservatively. Furthermore, the fatigue limit was decreased in the 2 step stress test In order to evaluate the role of cracking on the fatigue life, the surface layer of specimens was removed after the first step of the 2 step stress tests. The fatigue life was extended by the surface removal, although it was less than the life of the undamaged specimen. It was revealed that the fatigue limit of surface removed specimen decreased due to cyclic softening caused by the cyclic loading in the first step, and that the fatigue life of surface removed specimen was equivalent to that obtained by strain controlled tests. It was concluded that the fatigue life of Type 316 stainless steel should be predicted based on the strain amplitude rather than the stress amplitude even for the high-cycle regime.

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For application to the commercialized fast reactor, JSFR, creep fatigue evaluation method was improved to consider holding position during creep in the previous study. In the previous study, the validity of the improved method was confirmed by using the uniaxial tests only. In this study, for the validation of the improved method, we conducted the thermal transient tests of reactor vessel model. As the result, based on the elastic analysis, the method predicts the crack initiation life with enough margins. In addition, based on the inelastic analysis, the method predicts the crack initiation life with high accuracy.

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Fatigue tests were conducted for 1800 MPa-class spring steels at various stress ratios. For comparison, similar fatigue tests were conducted for conventional steels whose tensile strength was lower than 1200 MPa. The spring steels exhibited fish-eye fractures, and the origins of these fractures were oxide, TiN and the matrix itself. In contrast, the conventional steels never exhibited fish-eye fractures. The fatigue strength of these steels decreased monotonously as the stress ratio increased, when the fatigue strength was evaluated in tenns of stress amplitude. With these results, application of conventional methods to evaluate mean stress effects, such as modified-Goodman diagram, a power law of a stress ratio and so on, were discussed in case of fish-eye fracture of high-strength steel. As the result, the difference between the spring and conventional steels was negligible. This means that the conventional methods were applicable to the case of fish-eye fracture of high-strength steel within the scope of this research.

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