Nondestructive evaluation of a surface slit and a fatigue crack by surface SH waves has been studied. By adjusting incident angle of SH waves probe, surface SH waves passing through the surface region of materials are received. In this study, diffraction of surface SH waves received by double-probe technique was used. Firstly, the specimens introduced surface slit were inspected by surface SH waves. With the results that the amplitude of surface SH waves decreased with the increment of slit depth in the depth over half wavelength of SH waves, but the amplitude is constant in the depth under half wavelength of SH waves. On the other hand, the arrival time of surface SH waves increased with the increment of slit depth in every depth. Furthermore, the changes of the amplitude and the arrival time to the increment of slit depth were calculated based on Fresnel-Kirchhoff diffraction integral. A comparison between the measured values and the calculated ones shows that the change of the arrival time is useful for evaluating surface defect. Secondly, the evaluation method of a fatigue crack in the round bar using the artificial circle osculating fatigue crack was proposed, and the growth behavior of fatigue crack, which initiated at an artificial small hole by rotating-bending fatigue, was evaluated with the change of the arrival time. By applying the evaluation results to Paris' law, the validity of our method was confirmed.

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We tried to evaluate the plastic zone size and fracture behavior by analyzing the surface's thermal image of heat generation which was exhausted by plastic deformation. By continuous comparing the change of surface's temperature distributions between thermography method and thermocouples method, we showed the validity of the thermal image of heat generation under plastic deformation by using thermograophy method and the effective thermal image processing method. Furthermore CCD camera was used to observe the process of plastic deformation on the surface, the relations between CCD image and the thermal image was discussed.

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On fatigue crack propagation characteristics of recycling FRTP, the recycling evaluation using infrared thermometer and thermal analysis equipment is carried out. In this paper an explanation is given why the crack tip temperature of recycled fiber reinforced thermo-plastics(FRTP) is higher than that of its virgin material,and the deteriorative mechanism fatigue crack propagation property of recycled materials is investigated by measuring their crystallinity. The main results are as follows ; (1) Recycling process causes the decrease in crystallinity of fiber-reinforced PA46 because of its matrix resin. (2) It is caused by the decrease in crystallinity in recycling process that the mechanical properties(yield strength and elastic modulus) of recycled materials are inferior to chose of virgin material. The increase in hysterics heating for recycled materials is caused by the decrease in mechanical properties. The thermal conductivity of its matrix resin is reduced by the decrease in crystallinity. Therefore the crack tip temperature of recycled materials is higher than that of virgin material. (3) Comparing with virgin material, the recycled materials have higher crack tip temperature and lower mechanical properties so that their fatigue crack propagation property is deteriorated.

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It is important to estimate the distribution of intensity of a magnetic field for application of magnetic method to industrial nondestructive evaluation(NDE). This paper introduces the development of a system(Magnetic camera) that can measure the distribution of intensity of a magnetic field quantitatively in a short time without spatially restricted device like an automation movement device. Magnetic camera consists of magnetic source, sensor head, magnetic lens, A/D converter, data processor, monitor, and each part corresponds to light source, CCD sensor, optical lens, processor, video recorder and monitor in digital video camera relatively. Leakage magnetic flux near the crack on the magnetized specimen could be amplified by 3-dimensional magnetic fluid and zoom in and out of measurement area. The distribution of a magnetic field could be measured quantitatively at same interval with constant lift-off by the 2-dimensional arrayed magnetic sensors. In addition, multiple A/D converters were introduced in order to input the intensity of a magnetic field from sensor head to computer at high speed. Advantages of magnetic camera are demonstrated using a slit on plate specimen and modified steel reinforcement in concrete.

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Although the resonant ultrasound spectroscopy (RUS) is useful for efficient testing of surface and inside of objects, the resonance property (resonance frequency and mode amplitudes) is influenced by the boundary conditions of supports. To eliminate this influence, we propose a floating resonance method where the resonance property is evaluated after floating the objects, using the non-contact measurement by laser ultrasonics. In a preliminary application to bearing balls, we succeeded to float a 8 mm diameter steel bearing ball by controlled air flow and to improve the reproducibility of resonance frequency to better than 0.005 %.

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We found a new type of dislocation motion in highly oriented pyrolytic graphite during subsurface observation by ultrasonic atomic force microscopy. As a load was applied to the tip, dislocations of apparently edge type (Frank partial dislocations) moved to the direction of climb over distances of tens of nanometers, and returned to the original position as the load was removed. To explain this motion, we propose a possible model where the extra half-plane of the dislocation is elastically compressed to shorten its length due to the applied load by the tip.

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In dynamic atomic force microscopy (AFM) for detecting local elastic properties of samples, it is desirable that the contact resonance of an AFM cantilever is sensitive to tip-sample contact stiffiiess. This paper presents a unique cantilever with its mass concentrated as a way of enhancing the sensitivity. The cantilever is made up of a commercially available normal cantilever and a tungsten particle adhesively attached to the free end. Spectra of the contact vibrations are measured for three sample materials. The mass-concentrating cantilever whose tip is in contact with a sample exhibits a distinguishing spectrum containing several resonant peaks. A meaningful resonant peak affected significantly by contact stiffness can be found out among them because the other trivial peaks coincide with those for the cantilever free from a sample. The experiments and the elemental beam theory prove that the attachment of a concentrated mass provides an enhanced sensitivity, even for stiff materials, when such a meaningful peak focused on. If the attached mass is about four times larger than the distributed (cantilever) mass, the resonant peak obeys a spring-mass model. This means that the cantilever with its mass concentrated can provide the maximum sensitivity.

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A classification method of the flaw type is discussed by utilizing the ultrasonic scattering amplitudes from flaws in an elastic body. The method is based on the combination of two linearized inverse scattering methods of Born and Kirchhoff inversions. The Born inversion is sensitive to the volumetric flaw but not to a crack-like flaw. The Kirchhoff inversion reacts to both surfaces of volumetric and crack-like flaws. The combined use of the Born and Kirchhoff inversions leads to the classification of flaw type. To show the versatility of the method, aluminum specimens that contain a circular cavity and artificial cracks are prepared, and the backscattered waveforms are acquired from the experimental measurement. The measured waveforms are processed and the waveforms are fed into the Born and Kirchhoff inversions. The results show that the crack-like flaw can be discriminated from the volumetric flaw. It is also shown that the scattered wave component in the low frequency range is important for the present classification method.

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It is required to evaluate nondestructively depth of surface-breaking cracks in structures. In this paper, the self-calibrating technique by laser-based ultrasound is used to measure the depth of surface-breaking crack. Optical generation of ultrasound produce well defined pulse with a reliable frequency content. They are broad banded and are suitable for the measuring of attenuation and scattering over a wide frequency range. The self-calibrated surface signal transmission data shows a good sensitivity as practical tool for assessment of surface-breaking crack depth. It is suggested that the relationship between the signal transmission and crack depth can be used to predict the surface-breaking crack depths in structures.

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After a crack on a metal surface has been detected, determining its depth becomes an important practical issue for the fracture analysis and repair considerations. Reliable sizing of tightly closed small cracks was beyond the ability of standard NDE techniques. This limitation has recently been removed by the development of a new ultrasonic evaluation method, where the ultrasonic testing is considered as an inverse problem. An important issue associated with the present development is to analyze the quantitative characterization of the cracks from the perspective of crack length. The present paper is on the investigation of the influence of crack length on the ultrasonic determination of depths of small surface cracks, together with their extent of crack closure. Evaluation results of open as well as tightly closed small surface cracks with different crack lengths verify the method to be a powerful tool for evaluating the depths without involving any information regarding the crack length.

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Sensitive evaluation of the crack, which might be originally present or developed during operation is imperative for assessing the integrity of structural components. In an attempt to overcome the limitations of usual techniques for enhancing the sensitivity of ultrasonic nondestructive evaluation of tightly closed small cracks, a new angle beam approach of shear wave with 50^0 incidence in the solid is proposed. Fatigue cracks vertically situated on the back wall of stainless steel specimens are considered. As a generator and receiver of ultrasound, a single flat pulse-echo transducer in water immersion process is used. The highly sensitive features of the proposed fully automated and computer controlled technique are compared with those of recently developed ones. Based on crack corner echo, cracks ranging 0.1-4.66 mm are tested, and echo amplitudes are analyzed considering the simultaneous effects of crack size and crack closing stress. The proposed technique offers a good possibility of quantitative evaluation of crack size and closing stress by using inverse analysis of crack corner echo response.

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Eddy current testing (ECT) is used for the in-service inspection of tubes in steam generators of pressurized water reactor type nuclear plants. In order to increase the testing speed, arrayed multi-coil probes are proposed. Newly developed arrayed multi-coil probes are described in this paper. Because of the multi-coil arrangement, it is possible to detect the whole round of the tube without rotation. High detectability is achieved. The signals of this probe are predicted by the edge based finite element method, and comparing between experimental and numerical results show good agreement.

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Nondestructive methods for measurement and evaluation of fatigue damage in austenitic stainless steel 316NG, and ferritic steel SCM420 have been investigated. The ICFPD (Induced Current Focusing Potential Drop) method is useful to quantitatively estimate the micro crack initiation-growth and fatigue damage. In order to apply the ICFPD method to in-plant conditions, it is not sufficient to study about sensors or the analysis of properties for fatigue-damaged materials. In this paper the usefulness of ICFPD is examined for complicated structure and in-plant component.

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During operation, different defects initiate and grow in machine components due to fatigue, SCC, etc., and the reliability of components decreases with operating time. The performance and safety have to be maintained by regular and adequate inspection techniques and appropriate treatment. Furthermore these concepts have to be extended to evaluate residual life of components. The potential drop (PD) technique utilizing electromagnetic induction is a nondestructive inspection technique based on electromagnetic technique. This technique has been applied to the detection and sizing of both front and back surface defects in a laboratory. Unfortunately, most nondestructive inspection (NDI) and evaluation (NDE) techniques are strongly affected by many actual field parameters such as the skill of operator, the measurement environment, the shape of the measured object, etc. Especially, the technical skill of the operator is the most important parameter, and the way to investigate the effect of this parameter on the detectability of a NDI technique is by utilizing a probability of detection (POD) curve. In this study, by analyzing the POD curve obtained from operators trained for four hours to examine a welded specimen, the effect of the technical skill of the operator on the detectability of the induced current PD technique is investigated.

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In order to realise an advanced nondestructive inspection technique characterised by high sensitivity and high accuracy, a new method has been developed for the detection and characterisation of surface cracks in metals by means of micro antennas. The principle of this new technique is described here. A new probe combined with a conductive wire is developed with the aim of easier and quicker measurement. Experiments were performed for the detection and characterisation of artificial surface defects in austenitic stainless steel specimens. Also, fatigue cracks in paramagnetic and ferromagnetic materials were detected by use of this new probe.

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To evaluate the depth of small fatigue cracks under a condition of no contact and without any coupling medium, a novel microwave technique was demonstrated. An open-ended coaxial line sensor was used to increase the spatial resolution and the ratio of signal to noise. Closed fatigue cracks were detected successfully and a W-shaped characteristic signal was obtained. A dual frequency technique was proposed to evaluate the depth of small fatigue cracks of which situation regarding crack closure is unknown. The evaluated results are shown to agree well with the actual values.

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The paper presents the experimental investigation on the fracture of thermal barrier ceramic coating (TBC) at high temperature. The fracture was induced by temperature gradient along TBC system thickness direction and oxidation between TBC and bond coat. Laser heating method was used to simulate the operating state of TBC system. Both micro-observation and acoustic emission (AE) detection revealed that fatigue crack was in two forms: surface crack and interface delamination. One can understand the failure mechanism from the plane of TBC surface temperature and substrate surface temperature. It was found that the life of thermal fatigue was reduced by the formation of alumina at interface. On the other hand, it was found that the temperature gradient between inner and outer surface of specimen accelerated the growth of alumina layer.

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Recently, thermal barrier coatings (TBC) have been used in advanced gas turbine plants to improve the durability and performance of turbine blades. Usually, TBCs consist of an inner layer of metallic bond coating (MCrAlY) and an outer layer of ceramic top coating. According to several studies, the failure of such coatings is induced by thermal stress due to the formation of thermally grown oxide (TGO) at the interface between the TBC and MCrAlY. Therefore, it is important to investigate the oxidation behavior of the interface at high temperatures. In this work, the TGO, which forms at the interface, is characterized in detail. Moreover, for the purpose of understanding oxide formation at the interface, the oxidation behavior of specimens with and without TBC are compared. The TBC specimens are thermally aged at high temperature to simulate the surface temperature of the first rotating blades of 1500deg.C class gas turbine plants. After aging, TGO formed at the interface. The TGO formed is observed to consist of two different oxide layers. The thickness of the two oxide layers increases with the aging time. As a result of a comparison between the specimens with and without TBC, the TGO thickness of the specimen with TBC is observed to be thicker than for the specimen without TBC. These specimens exhibited different oxidation behaviors, which are caused by specimen dependent preferential growth of one of the oxide layers for the specimen with TBC.

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Two-layer plasma sprayed coatings on a Ni base superalloy have been used in advanced gas turbine plants to improve their durability and efficiency. Generally, the coatings consist of an inner layer of a metallic bond coating (MCrAlY) and an outer layer of 8wt% yttria-stabilised zirconia (YSZ) as a thermal barrier coating (TBC). After aging, a thermally grown oxide (TGO) was formed at the interface between the YSZ and the MCrAlY. The thickness of the TGO increased with aging time. Due to the difference between the thermal expansion coefficients of the Ni base superalloy, the YSZ top coating and the TGO, the presence of the TGO layer leads to degradation of the TBC by mechanisms such as cracking, delamination or spalling. Therefore, the capability to carry out a nondestructive evaluation (NDE) of the formation and the growth of the TGO is important. An in-situ NDE was implemented by means of an impedance spectroscopy (IS) technique at 1000□. Meanwhile, a great deal of attention has been paid to the oxidation behaviour of the TBC/MCrAlY interface. However, the role of oxide formation in terms of the TBC top layer is still not clearly understood. In our previous work, we revealed that the oxidation behaviour of specimens with and without TBC is quite different, in spite of their being subjected to the same aging conditions. The oxidation behaviour of the TGO for both specimens was investigated using an IS technique. The results showed that the impedance of the specimen with TBC was smaller than that of the specimen without TBC, although the specimen with TBC had a thicker oxide layer than the one without TBC.

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A small punch creep(SP-Creep) test using miniaturized specimen(10×10×0.5mm) has been described for the development of the new creep test method for structural components operated at high-temperature and pressure. The SP-Creep test technique has been applied to 9CrlMoVNb steel used as valve material in boiler components. The creep test temperatures were varied at 575℃〜625℃, and the applied loads at 40kg〜100kg. The overall deformations of SP-Creep curve were definitely depended on creep test conditions and showed the creep behaviors of three regions like those of conventional uniaxial tensile creep curve. The steady state creep rate (ε^・_<qf-ss>) in SP-Creep test increased with increase of applied load and creep temperature. Furthermore, the power law's exponent decreased with increase of creep temperature, and the activation energy(Q_<SPC>) obtained from SP-Creep test decreased as applied load increases. These behaviors were similar to those of conventional uniaxial creep test. Also, a predicting equation of SP-Creep rate was suggested and a good agreement between experimental and calculated values was shown. And several creep results of 9CrlMoVNb steel were compared with those of 2.25Cr-1Mo and 1Cr-0.5Mo steels that has been widely used as boiler tube and header materials, respectively.

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A small punch creep testing technique using miniaturized specimen with a dimension of 10 × 10 × 0.3^t mm has been demonstrated for evaluating change in creep property of high temperature components due to long-term service operation. This technique was applied to SUS 316 HTB secondary superheater boiler tube actually used for 100,600 h in a fossil power plant. The overall shapes of the creep curves determined by the small punch creep test were similar to those obtained from a conventional uniaxial creep test. That is, they exhibited clearly three creep stages. The extent of secondary creep stage and rupture time t_r decreased with a increase of testing load level. The creep rupture life of the service-exposed tube was shorter than that of the unexposed tube at a temperature of 650℃ and at high load levels, i.e. 338 N and 408 N. However, to the contrary, the former became longer than the latter at relatively low load levels, i.e. 234 N and 286 N. The difference in rupture life between them had a tendency to increase with a decrease of testing load level. These small punch creep behaviors of SUS 316 HTB superheater boiler tubes have been examined from a metallographic point of view.

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TiAl inter-metallic compound is considered to be an effective material for high temperature structural materials. Since, this material is considered to be a creep brittle like material, it is necessary to construct the law of prediction of the life of creep crack growth. Furthermore, there are several types of TiAl with various microstructure such as full lamellar, duplex and γ-base TiAl. Therefore, it is necessary what type of TiAl is more suitable for practical application. In this paper, the characteristics of creep crack growth rate and its life for Near Gamma Cast TiAl-Fe-V-B alloys with lamellar structure were investigated and they were compared with those for full lamellar TiAl with large grain size.

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Low cycle fatigue crack initiation test of notched 0.4 compact tension specimen(notch tip radius of 1mm) was carried out at 800℃ in air and in vacuum under fully reversed tension-compression loading. Microcracks at the notch root surface were observed using a scanning electron microscope by interrupting the fatigue test. The relation between number of cycles to crack initiation N_i (defined as the number of cycles to 0.5mm long crack on the notch root surface) and apparent stress intensity factor K_a is linear against log-log plot. N_i in air is about ten times smaller than in vacuum. It was found that the crack in air initiates at the oxidized lamellar, while the crack in vacuum initiates independent upon the microstructure. The very short fatigue crack initiation life in air is due to the fast oxidation and microcrack initiation at the lamellar(Ti_3Al).

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For gas turbine engines, safe life methodology has historically been used for fatigue life management of failure critical engine components. The safe retirement limit is necessarily determined by a conservative life procedure, and many components which are discarded have a long residual life which is thereby forgone. The objective of this study is to introduce the damage tolerant design concept into the life management for aircraft engine component instead of conservative fatigue life methodology which has been used for both design and maintenance. Crack growth data were collected on a nickel base superalloy which have been subjected to combined static and cyclic loading at elevated temperatures. Stress analysis for turbine disk was carried out. It was estimated that remaining life of turbine disk component under various temperature and conditions using creep-fatigue crack growth data. As the result of life assessment, it was confirmed that retirement for cause concept was applicable for the evaluation of useful remaining life of retired turbine disk which had been designed based on conventional fatigue life methodology.

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Platinum and its alloys are widely used in the glass-melting industry because of their high melting point, high resistance to oxidation and inertness with molten glass. Since they are usually used above 1273K, it is important to improve their mechanical properties at high temperatures above 1273K. For the further improvement of high temperature properties, it is essentially important to clarify the basic mechanical properties of platinum and its alloys at elevated temperatures. In this study, fatigue reliability at elevated temperature of platinum rhodium alloys were examined by the elucidation of fracture mechanism. Main results were follows ; (1) S-N characteristics of PtRh alloys were almost arranged in the straight line. (2) From the fractography using SEM, it was indicated that fatigue fracture mechanism was similar to the static destruction in the high stress side, on the other hand, that was effected by creep in the low stress side.

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Tensile test was carried out in air at RT and 900℃ of as-received and air-exposed tensile specimen of CMSX-10 ([001] tensile loading direction). Cross section and gauge length of the specimen is 2mm wide × 2mm thick and 10mm long, respectively. Air-exposure was done at 1100℃ for 24 h, 240 h and 1000 h. At RT, yield strength decreased with increasing exposure time, while ultimate tensile strength was almost same. At 900℃, with exposure time, both yield strength and ultimate tensile strength decreased. Work hardening was recorded in the as-received, while no work hardening in the 240 h and 1000 h exposed specimens. Fracture surface was observed by a scanning electron microscope. Shear fracture takes place at RT, while flat fracture at 900℃. The difference of the fracture surface between RT and 900℃ is discussed.

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The fatigue strength has been investigated about the plastic deformed specimens by roller-working in Ti-6Al-4V alloy with a notch by 4 kinds of plastic deformation ratio. The results in this test are summarized as follow: (1) Though the hardness number of roller-worked materials are higher than those of non-deformed Ti-6Al-4V alloy, the fatigue limit of roller-worked materials becomes deteriorated in spite of roller-working. Low work-hardening ratio of Ti-6A1-4V alloy may not contribute to the improved hardness and residual stress. (2) Fatigue fracture had occurred in the boundary (weak part) between plastic deformed area and non-deformed area where tensile residual stress would exist. (3) The results of the SEM observation, the roller-worked specimen showed crack initiating from the one point on the surface. It shows two-step fracture modes, which are shear fracture mode and normal one. (4) The fatigue limit of at the plastic deformed area seemed to be improved. It is necessary to further study the qualitative analysis of residual stress at the plastic deformed area.

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According to the statistical analysis by one of the authors, more than 90% of failure is caused by fatigue, and more than 90% of failure causes are initiated from the notched portion. It is very important to improve the fatigue strength of the notched portion in the practical components. Roller working is one of the effective plastic working method to improve the fatigue strength of materials such as carbon steels. In this study, roller work process was used to notched pure aluminum specimens for estimating its effect on fatigue properties. The result of fatigue test indicates that the fatigue strength by 10^7 cycles of specimens after roller working increases by 30% compared to non-roller worked specimens, but with the increase of roller working value, the fatigue strength by 10^7 cycles decreases gradually. The crack propagation behavior was also investigated by taking replicas and observations under an optical microscope. The change in fatigue strength was discussed according to the concept of compressive residual stress at the notch root and the surface condition after roller working.

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This paper investigates striation configuration(H/s) under 2-step loading condition. Here, H is striation height and s is striation width. Under condition that vary from low maximum load to high maximum load, no acceleration of crack growth was visible, further H/s remained similar to values found under constant max. load. When load varied from high to low, retardation appeared immediately after varying the load. Striation configuration immediately after varying the load either flat or the H/s value dropped remained to a level much smaller than that under constant max. load. Once crack growth reached about 500μm, striation configuration recovered from the previous flat condition and the H/s value returned to the value found under constant max. load.

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Effects of small defects on the fatigue strength of Ti-6Al-4V alloy were investigated by tension-compression fatigue tests. The artificial holes having 50, 200, 400 and 1000μm diameters were introduced onto the surface of a series of specimens (Series A). Another series of specimens (Series B) were prepared to investigate the effect of a burr or pre-crack, which was introduced at the edge of the artificial hole. The fatigue limit was defined by two ways depending on the threshold conditions for Series A and B: (1) the threshold for crack initiation from the edge of the hole for series A, and (2) the threshold for crack propagation from the burr of the hole or the pre-crack tip for Series B. As a result of the fatigue tests, the fatigue limits of Series A specimens were 20〜60MPa higher than that of Series B. This is because the crack initiation stress of titanium alloys is very high but once a crack initiates at the edge of a hole, the nonpropagating crack is hard to exist in the microstructure of Ti-6Al-4V, Therefore, the √<area> parameter model underestimated the fatigue limit of Series A but it predicted accurately the fatigue limit defined by the crack-growth threshold for Series B. Besides, the fatigue life was also decreased by the burr. This is because the burr easily became the crack initiation site and reduced the number of cycles to crack initiation.

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The fatigue strength of steel is generally proportional to the hardness up to a certain point. However, the relationship between hardness and fatigue strength is more complicated for the case of high strength material, mainly because the defects in the material sensitively reduce the strength. The threshold notch depth a_<n th>, which is the limit not to reduce the fatigue strength, is obtained by the fatigue test. The surface layer is observed by using scanning electron microscope and electron probe micro analyzer to clarify the characteristics of latent defects. A strength evaluation method for the carburized gear is proposed using those results.

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Low cycle fatigue tests are performed on high strength structural steels that be developed for submarine material. The relation between absorbed plastic strain energy and numbers of cycle to failure is examined in order to predict the low cycle fatigue life of structural steels by using plastic strain energy method. The cyclic properties are determined by a least square fit techniques. The life predicted by the plastic strain energy method is found to coincide with experiment data and results obtained from the Coffin-Manson method. Also the cyclic behavior of structural steels is characterized by cyclic softening with increasing number of cycle at room temperature. Especially, low cycle fatigue characteristics and microstructural changes of structural steels are investigated according to changing tempering temperatures. In the case of PFS steels, the ε-Cu is formed in 550℃ of tempering temperature and enhances the low cycle fatigue properties.

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Cantilever type rotational bending fatigue tests were carried out in super-long fatigue life range for high carbon chromium bearing steel. The fatigue properties and the fatigue crack initiation were investigated. The stress intensity factor calculated from the area of rough surface region of "Facet" in the fish-eye was independent on the stress amplitude and was almost a constant. This fact suggests that when the stress intensity factor reached to a constant of 5.4 MPam^<1/2>, the fatigue crack propagate depend on the stress amplitude. On the other hand, it is found by SEM observation, that the fracture in the mode of "Inclusion on Subsurface", is caused by a dark region near the specimen surface. There is a small inclusions caused by segregation on subsurface in this dark region. The crack initiates outside of the dark region, same as the mode of the mode of " Fish-eye" that the crack initiates in the region of "Flat".

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In this study, the fatigue growth behavior of the transverse crack, which is the most dangerous damage among the various types of rail defects, was investigated using the notched keyhole specimen under constant amplitude and variable amplitude loads. Fatigue limit of the smooth specimen in rail steel at R=0 was 110MPa. And, in the region of the low stress amplitude (i.e. long life), most portions of the total fatigue life were occupied by the fatigue crack initiation life. The fatigue strength under variable amplitude load was converted to the equivalent fatigue strength based on the Miner's rule, which was estimated about 9% lower than that under constant amplitude load.

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A hypothesis of fatigue plastic adaptation is proposed as a basic concept on strength of fatigue macrocrack initiation. The hypothesis represents that plastic deformation, which is caused in a surface layer by slip bands inherent in fatigue, is equal to maximum elastic deformation in a pertinent location and the deformation is generated in a surface layer of an unnotched specimen and at a notch root of a notched one. Parallel multi-bar models are devised based on the hypothesis in order to simulate cyclic stress behaviors in a surface layer and at a notch root, respectively. Consequently, an equivalent stress ratio R_<EQ> is derived from consideration of mechanical behaviors in the models as a stress parameter for correspondence between fatigue macrocrack initiation strength in unnotched and notched specimens. The parameter R_<EQ> is applied to published experimental data on notched plates and notched round bars as well as authors' fatigue test data on round bars, and the correspondence between notched and un-notched specimen fatigue data is investigated. As a result, availability of the parameter of the equivalent stress ratio R_<EQ> is proved.

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Multiaxial fatigue life prediction under irregular loading is studied on two materials exhibiting shear fracture(SNM440, SNCM630). Fatigue tests are conducted at room temperature under tension-torsion, proportional and nonproportional loading with constant and variable amplitudes. The Kandil-Brown-Miller (KBM) parameter, the Smith-Watson-Topper (SWT) parameter and an energy-based parameter are used with the rainflow cycle counting method and Miner's rule of damage accumulation. The fatigue life is calculated based on the strain profiles on the critical plane for the KBM and SWT parameters and based on the maximum damage for the energy parameter. The energy-based parameter is found to provide best correlation with experimental lives. It was also found that the normal strain on the maximum shear strain plane does not affect fatigue life of SCM440 significantly.

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Eutectoid steels are used for variable kinds of industrial components. In addition, these components are usually pre-strained before use during production process or machining one. The object of this study was to investigate the microscopic behavior of eutectoid steel during static tension, pre-strain process and to evaluate the effect of pre-strain on the fatigue strength of the steel. The main results obtained in this test are as follows: (1) There is no obvious yield point of eutectoid steel. (2) Under the pre-strain process (ε_p=7%), micro-cracks initiate in the surface of specimen. (3) All fatigue limits of different pre-strain ratio are lower than the original one. (4) There are no obvious fatigue limits variations for different small pre-strain ratio. (5) Fatigue cracks initiate from slip line, or micro-cracks that are generated in the process of plastic strain.

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We demonstrate the validity of J_e, which comprehensively describes the effects of specimen geometry and loading type, in predicting the fatigue life of auto seat belt anchor panel. We first simplify the heat affected zone model to reduce the number of finite elements. We then establish finite element models reflecting the actual overload behavior of 3 types of seat belt anchor specimens. Using elaborate finite element models, we obtain the effective crack driving parameter J_e composed of its ductility-dependent modal components. It is confirmed that the J_e concept successfully predicts the fatigue life of multi-spot welded panel structures represented by auto seat belt anchors here.

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In order to investigate the slip behavior near an interface in high cycle fatigue, a tension-compression fatigue test is carried out using a two-phase (α/γ) stainless steel bicrystal, which consists of ferritic (Fe30Cr) and austenitic (Fe11Cr19Ni) single crystals. Since the yield stress of α-phase is much higher than that of γ-phase, the crystallographic slip takes place only in the γ-phase. Characteristic slip, which is not expected from the Schmid factors, appears in the γ-phase near the interface. The stress analysis by a finite element method(FEM) reveals that the resolved shear stress, τ_<rss>, of the specific slip systems near the interface increases due to the constraint of deformation at the boundary. The characteristic slip takes place where the magnitude of Trss exceeds the critical stress of 31 MPa. This agrees well with the critical resolved shear stress, τ_<crss>, in the γ-single crystal. In short, the slip near the interface is governed by the local shear stress in the high cycle fatigue. The analytical result and the experimental observation point out that the increase of τ_<rss> on the specific slip system is confined in a region near the interface on the surface.

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In order to investigate fatigue crack behavior of the hard-shot peening (HSP) treated Type 316L austenitic stainless steel, the rotating bending fatigue test were carried out. As for a series of studies for rotating bending fatigue properties in the HSP-treated type 316L, the mechanism of fatigue strength improvement was investigated and estimation method of HSP-treated austenitic stainless steel was suggested. In this study, observation of a longitudinal section with the optical microscope, a fractographical observation with the SEM, surface crack observation with the replication technique and non-destructive inspection with the SH-ultrasonic wave were performed for discussing the crack behaviors in detail. As the results of these investigations, the fatigue crack behavior at the early fatigue propagation stage was clarified. (1) Although the fatigue crack initiated at the surface in early stage of fatigue, the crack does not propagate during the fatigue process until just before the final fracture. (2) The fatigue crack propagates toward inside of the radial direction at first, and followed gradually changing to axial direction at a depth of about lOOμm from surface. (Generation of axial-direction crack) (3) Secondary fatigue crack initiates from the axial-direction crack and propagates toward inside of the radial direction.(4) After the fatigue crack propagated radically for a certain depth, the surface fatigue crack propagated quickly at the final fracture.

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In this paper, tensile behavior and low cycle fatigue behavior of 316L stainless steel were investigated. This material is currently favored structural material for several high temperature components such as the liquid metal cooled fast breeder reactor (LMFBR). Research was performed at 550℃, 600℃ and 650℃ since working temperature of 316L stainless steel in a real field is from 400℃ to 650℃. From tensile tests performed by strain control with 1×10^<-3> /s, 1×10^<-4> /s and 1×10<-5> /s strain rates at each temperature, negative strain rate response (that is, strain hardening decreases as strain rate increases) and negative temperature response were observed. Strain rate effect was relatively small compared with temperature effect. LCF tests with constant total strain amplitude were performed by strain control with a high temperature extensometer at RT, 550℃, 600℃, 650℃ and total strain amplitudes of 0.3%〜0.8% were used and test strain rates were 1×10<-2> /s, 1×10<-3> /s and 1×10<-4> /s. A new energy based LCF life prediction model which can explain the effects of temperature, strain amplitude and strain rate on fatigue life was proposed and its validity was verified by analyzing the test results and comparing with currently used models.

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In order to investigate corrosion fatigue characteristics in weldment of multi-pass welded A106 Gr B steel pipe, corrosion fatigue tests were performed in the various stress frequencies and 3.5mass% NaCl solution at room temperature. And corrosion fatigue characteristic curves were represented by using crack closure concept. Obtained results are as follows; When load frequency is 1Hz, the crack opening point is transited in the region of K_<max> = 20 〜 32MPa√<m> . In the lower stress intensity factor range, the crack opening point is higher than that of air. But, in the higher stress intensity factor range, it was lower than that of air. And in the case of 0.1 Hz and 0.01 Hz, the crack opening point gradually decreased to K_<min> with K_<max> increase

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In order to clarify the effect of thermal sprayed (WC-Cr-Ni) cermet coating on the corrosion fatigue strength of high strength SKD11 steel, rotating bending corrosion fatigue tests were conducted in a 2 mass% ZnSO_4 aqueous solution. It was found that the corrosion fatigue strength of the thermal sprayed (WC-Cr-Ni) cermet coated specimen with 100μm thickness was 318.6MPa at 2×10^7 cycles and was about three times larger than that of the SKD11 base metal at the same number of cycles. Corrosion fatigue life tests were also conducted at rotating bending stress of 342.5 MPa on (WC-Cr-Ni) cermet coated specimen with coating thickness of 50 to 150μm to investigate the effect of coating thickness. It was found that the effective thickness of this coating was about 100 μm. The major cause of the improvement of corrosion fatigue strength by thermal sprayed (WC-Cr-Ni) cermet coating is the insulation of SKD11 steel from the permeation of ZnSO_4 aqueous solution. Impregnation of fluorine contained resin into the thermal sprayed (WC-Cr-Ni) cermet coating improved again corrosion fatigue strength at 2×10^7 cycles. However impregnation of fluorine-contained resin was ineffective at higher stress region due to the exfoliation from cermet coating.

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Surface modified Al-Si sintered alloy by diamond like carbon (DLC) coating surface modification was investigated into the property of the fatigue brittle fracture. By DLC coating, fatigue strength of Al-Si sintered alloy was improved. The strength in the interface between the Al matrix and Si-particle, resistance to crack initiation, was improved. The main factors were as follows. (1) The defects in the surface were healed because the surface was covered with DLC film. (2) In DLC coating material, the debonding on Si-particle was inside crack. The stress intensity factor was lower than that of virgin material. (3) The debonding on Si-particle was controlled by the compressive residual stress on DLC film. This compressive stress gradually hardened the surface of material, and controlled the crack growth.

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Three point bending fatigue tests were carried out to clarify the effect of Arc Ion Plating (AIP) coating on fatigue strength. The Arc Ion Plating was performed using Cr cathode and N_2 gas. The SUS304 substrate was employed in this study. Coating was carried out under the following conditions; the arc current was changed from 50 to 200 A. The bias voltage was changed from -50 to -500 V. The gas pressure was changed from 1.3 to 5.3 Pa. The results obtained were as follows: (1) The diameter and number of macro particle decreased with increasing the arc current and the bias voltage, also increasing the gas pressure. (2) The hardness of CrN thin film decreased with increasing the bias voltage. (3) When the arc current, the bias voltage and the gas pressure were constant, the fatigue strength of coated specimens for thin film was greater than for thick film.

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To improve the fatigue reliability for aluminum alloy (A6061) by the Fine Particle Bomberding (FPB) surface modification, the influence of the modification on fracture mechanism was studied experimentally. By FPB surface modification, the fatigue strength of A6061 was improved, and the fatigue limit of the surface modified material could be increased by 20%. The detailed observations have revealed that fatigue crack growth in a surface modified specimen showed a tensile mode not a shear mode with virgin material. The fatigue crack initiation and propagation of the surface modified specimen was restricted by the hardening and compressive residual stress.

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The combined impact experiment at high strain rates is necessary to establish the dynamic constitutive equation. It seems that the experimental data for the purpose are not enough due to the difficulty of the experiment. In this research, a combined impact testing machine was proposed, which was possible for various combined impact test of tension and torsion. Especially, the testing machine can realize the impact loading and inverse loading in order to examine the Bauschinnger effect at high strain rates. This machine is based on SHPB method, and has two clamp devices that are released in the time lag of 100 μsec by connecting the two clamps to a rod. Using the testing machine, some efficiency tests were done. The results showed that the rise time of the impact load is about 50 μsec and the present machine is valid as a combined impact testing machine. Moreover, some impact experiments were conducted with short aluminum specimens. It was confirmed that the Bauschinnger effect appeared under the impact loading as same as the static loading.

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Since laminate films used for liquid packaging bags are produced by laminating very thin single films, they are easily influenced by ambient temperature; bags often break due to impact load during loading and unloading of packaged products. In this study, impact tensile tests were carried out at relatively low impact tensile speeds between v=0.20 and 1.40 m/s using a rotary impact tensile apparatus developed by us, with ambient temperature varied to six temperatures, i.e., t=0, 10, 20, 30, 40 and 50℃. The results indicate that the impact tensile strength of NY/EVA is negligibly influenced by the impact tensile speed at an ambient temperature of t=50℃, while those of NY/XA and PET/XA are influenced by the impact tensile speed.

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The Present paper proposed a new constitutive equation that is applicable to proportional and non-proportional cyclic combined loading at the wide range of strain-rate from static to impact by introducing the inner-state variables. Using the variables, we can describe the behavior of materials without distinction between loading and unloading. Namely in this equation the loading state is described by the angle between the inner-state variable increment tensor and the stress tensor. Using this constitutive equation, some dynamic and static stress-strain relations for cyclic combined loadings of tension and torsion were calculated. As a result, Bauschinger effect can be expressed naturally for impact tensile load and unload as well as static one.

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