Various kinds of thermal fatigue failure modes exist in nuclear power plant components. Main causes of thermal loads are structural responses to fluid temperature changes. These phenomena have complex mechanisms and so many patterns, that their problems still occur even though well-known issues. Among them, this paper treats high cycle thermal fatigue of branch pipes as the typical mode. Firstly, experimental and analytical researches are explained for thermal load evaluation. Through them, both numerical and kinematic methods were developed. Next chapter describes thermal fatigue strength studies on both crack initiations and propagations. They revealed the similarities of thermal crack initiations with mechanical ones and frequency characteristics of crack propagation. Finally, current status and future challenges are discussed for evaluation of actual plants.

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To investigate effects of cyclic frequency on high cycle fatigue properties of SFVQ1A, uni-axial fatigue tests were conducted under several stress ratios and cyclic frequencies. In the case of stress ratio of R = -1, longer fatigue lives and higher fatigue strength were observed under higher frequency condition in comparison to those under lower frequency condition. Strain ranges during the fatigue tests were measured, and the difference of range was not observed at the same stress amplitude in both cyclic frequency conditions. The result indicated that the strain rate can affect fatigue properties. However, the difference of fatigue lives and strength did not appear under R = 0.1. The strain ranges were relatively small, and the effects of strain rate probably became smaller than those under R = -1. Effects of cyclic loading were observed under R = 0.5, and strength in the cyclic loading condition became higher than that in the static loading condition. Strain rate should play an important role in the fatigue properties of the material.

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Ultrasonic fatigue tests were carried out for a low alloy steel, SFVQ1A, under stress ratios R = -1, -0.5 and 0 at a cyclic frequency of 20 kHz in order to investigate the giga-cycle fatigue strength properties. Fatigue limits obtained for the R values well corresponded to Gerber diagram. Comparing with the data in literatures, the ultrasonic fatigue strength depended on the cyclic frequencies. Peened specimens showed interior fracture, however, no reduction in the fatigue strength was found due to the change in the fracture mechanism. The fracture origins were complex type inclusions, which consists of Al₂O₃, MgO, MnS and CaS with the sizes ranging from 10.7 to 70.3 μm. The effects of inclusions on the S-N curves were discussed in conjunction with the influence of cyclic frequency.

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The study considered the relation between fatigue strength property and damage mechanism in GFPC having modified resin/fiber interface by observing the fracture, and the fatigue testing. 1) Damage was initiated at the fiber end in the early stages of the fatigue process, and debonding propagated along the fiber interface. 2) In fatigue testing, the number of cycle difference in a high stress is based on the difference in a damage mechanism.

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In recent years, Japanese nuclear power plants experienced multiple large earthquakes, such as Niigata-ken Chuetsu-Oki Earthquake in 2007 and the Tohoku District - off the Pacific Ocean Earthquake in 2011. Therefore, it is very important to assess the structural integrity of reactor piping under such a large earthquake when a crack exists in the piping. In this study, an effect of excessive compressive loading on the crack growth behavior of piping materials has been evaluated through cracked plate testing. It was observed that excessive compressive loading had an acceleration effect on crack growth rate. The evaluation method using J-integral to evaluate the crack growth behavior under the condition beyond small-scale yielding has been proposed for the acceleration effect of excessive compressive loading on crack growth rate. It was indicated that the acceleration effect by excessive compressive loading simulating a large earthquake could be evaluated using the proposed method.

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In order to examine the fine granular area (FGA) formed timing in under rotating bending fatigue for high strength steel, the rotating bending tests were carried out under decremental step loads for high carbon chromium bearing steel. The decremental step loads were three conditions, the ratio of cumulative number of stress cycles at the high stress level were 2, 4, 7% against the average of the fatigue life in the interior inclusion induced fracture, and were 80% against the average of the fatigue life in the surface induced fracture. From the experimental results, FGA was observed around the inclusion at the fracture surface of the interior induced fracture specimens. The size of FGA and the stress intensity factor range of FGA were almost similar to each other for every condition.

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Micro-macro combined creep damage simulation on the basis of the grain-boundary-fracture-resistance model was applied to the creep damage simulation of Mod.9Cr-1Mo steel welds. Firstly material parameters for simulation were identified and distribution of number density of small defects through wall thickness was simulated for large-size tensile creep specimen at 923K. Distribution with high peal at subsurface coincided with observed results, and the applicability of the simulation to the micro damage in the welded joint was demonstrated.

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Rupture speeds (Mach numbers) obtained by ordinary laboratory fracture experiments using monolithic brittle linear elastic materials are by far lower than those predicted by classical theories of mechanics and inferred from seismic inversions. Some seismological recordings even imply the existence of supersonic (supershear) rupture speeds. Here, in order to possibly explain these discrepancies, dynamic mode-I fracture in hyperelastically behaving rubber materials is experimentally investigated. Utilizing a high-speed digital video camera system, rupture (crack) initiation and propagation process is recorded. The preliminary results clearly show that if the magnitude of static crack-parallel stress is relatively large and comparable to that of remote mode-I loading stress, the crack propagates surprisingly straight and, even without the existence of material heterogeneities, the crack front accelerates from subsonic to a constant supersonic speed to capture the wave front generated upon rupture initiation.

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In this study, we focused our attention on microstructure control and improvement of mechanical properties of induction tempering S45C steel using super rapid induction heating system. We examined the effect of tempering time on tensile properties. The tempering temperature was changed in 3 steps, and tempering time was changed in 2 steps. As a result, it was found that the QT steel obtained from super rapid induction tempering had both high strength and high ductility. The super rapid induction tempering is effective to increase the fracture strain and reduction of area. The reason for this phenomenon was explained in that cementite particle of super rapid induction tempering steel became small and dispersed finely in comparison with usual tempering steel. Finally, this paper proposed effective technique to improve mechanical properties of S45C steels.

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Fatigue tests of bolted joint were carried out under constant amplitude loading. In all tests, bolts were broken at the root of the first thread where the nut and bolt were engaged. The relationship between the stress amplitude, which was defined as the load divided by the effective cross-sectional area of the bolt, and the number of cycles to failure showed a curve similar to a typical S-N curve. To estimate the axial force of bolt in bolted joint, the compressive force of bolted joint was detected by using the self-made load cell during the fatigue test. The maximum compressive force decreased as the cycle increased, while the minimum compressive force was equal to zero independent upon the load cycle. The axial loading pattern applied to the bolt could be estimated based on the change of maximum compressive force and the test load amplitude. The fatigue tests of bolt were carried out under the estimated loading pattern. The fatigue life of bolt was in good agreement with the life of bolted joint.

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This paper deals with the analysis of the stress intensity factor for interfacial crack in dissimilar materials by using the finite element method. In the present method, the stress values at the crack tip calculated by FEM are used under the same mesh pattern and the stress intensity factors are evaluated from the ratio of stress values at the crack-tip-node between a given and a reference problems. This method is based on the fact that the singular stress field near the interface crack tip is controlled by the stress values at the crack tip calculated by FEM. As the reference problems, a single interface crack in an infinite bi-material plate subjected to tension and shear is selected in this study. The usefulness of the present analysis is verified by the comparing the singular stress distribution of the given problem with that of the reference problem. The calculation shows that the present method has the sufficient accuracy in the interface crack problems under various boundary conditions.

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High strength steels are used for various important components such as rolling bearings to ensure safety. Strength designs for these components require material properties at the local areas. The objective of the present study is to evaluate the local mechanical properties of high strength steels by the dual-indenter method. Non-dimensional Π function is developed for 118 degree trigonal pyramid indenter using finite-element method. Dual-indenter method is conducted by indenters with the apex angles of 115 and 118 degrees for SUJ2 and SUJ3. The results reveal that good agreements are achieved between stress-strain curves of tensile testing and those of the dual-indenter method. The local mechanical properties are evaluated by the dual-indenter method for induction-hardened and carburized components. There are some differences in stress-strain curves at the locations of the components by the influence of the heat treatments.

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In recent years, single lap joints have been widely used to bond dissimilar material members particularly in aircraft and automobile structures. However, the debonding easily happens at the corner of the single lap joint because of the singular stress, and it is necessary to evaluate the interface strength for the single lap joint. This paper discusses the effects of adhesive thickness and overlap length on the intensity of singular stress fields under arbitrary material combinations. A useful method is proposed for calculating the normalized intensity of singular stress by using FEM with varying the adhesive thickness and overlap length. Here, two problems are considered together by applying the same mesh at the corner for different adhesive thickness or overlap length. Then it is found that the normalized intensity of singular stress is almost independent of mesh size. The normalized intensity of singular stresses are indicated under arbitrary material combinations. The results show that the intensity of singular stress decreases with decreasing adhesive thickness and increasing overlap length. The increment and decrement are different depending on material combinations between adhesive and adherent.

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To improve the fatigue strength and resistance to stress corrosion cracking, peening is the effective technique because of the introduction of compressive residual stress. As penetration depth can varies with frequency and electromagnetic properties are varied with the induced stress, an electromagnetic method is proper to evaluate the peening efficiency nondestructively. The piezoresistive effect and Villari effect cause the variation of electromagnetic properties like as the electrical resistivity with the introduction of stress. It was reported that the electromagnetic properties were varied with processing time of peening and depth where the compressive residual stress was introduced. In this paper, to establish the evaluating method of peening intensity and homogeneity after peening, austenitic stainless steel, Japanese Industrial Standards (JIS) SUS316L treated by cavitation peening which causes little variation of surface roughness and microstructure were evaluated by means of an eddy current method with coil scanning. In results, the peening intensity can be evaluated by the eddy current method.

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This paper describes novel integrated molding for fabrication of functionally-graded (FG) syntactic epoxy foams to control distribution of the mechanical properties for highly impact energy absorption. In order to control mechanical properties, the density distributions in the FG epoxy foams were graded by floating phenomenon of the light-weight micro-balloons in the matrix epoxy resin during the curing process. By the turning procedure of the mold before grading the micro-balloons, the FG epoxy foams having the gradual and continuous distributions of density in wide range could be produced. The static compression tests of the fabricated FG epoxy foams suggested that the foams had high absorption of mechanical energy since the foams collapsed progressively due to the grading of the density distribution.

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Mg alloys can be used for reducing the weight of various structural products, because of their high specific strength. In order to use Mg alloys for manufacturing vehicles, it is important to investigate the deformation mechanism and transition point for optimizing the materials and vehicle design. In this present study, we investigated the transition of deformation mechanism during high temperature uniaxial tensile deformation and forming limit by conical cup test. As a result, formation of fine grain and texture controlled by warm rolling process of AZ31 Mg alloys, we have excellent formability comparison with A5083 alloy at warm to high temperature deformation. Deformation mechanism changes beyond the temperature at 523 K because of a change in the stable behavior and transition in the deformation mechanism that is mainly domained by grain boundary sliding.

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Using a conductive soft Si probe for atomic force microscopy as an electrode of nanomanipulater installed in a transmission electron microscope, we successfully measured forces generated by one-way shape memory effect of a carbon nanotube (CNT). We revealed from the measured forces that the recovery moment is about several tens aNm for a 3.5 nm-diameter CNT consisting of four walls. The measured recovery forces are around thousand times stronger than the output of nano-sized bio-molecular actuators. This finding indicates that a CNT has an advantage as high-power actuators.

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A basic study to upgrade long-term reliability and durability of a high-pressure hydrogen gas valve was carried out in anticipation of increase of the use of hydrogen utilization machines and infrastructures. The morphology of the surface damage in the sealing components, the damaging mechanism and the relation between development of the damage and leakage were investigated. The surface damage in hydrogen gas was caused by surface plastic flow and transfer of material. In addition, fatigue crack and following delamination were caused by repetitive contact. Small amount of the surface damage didn't cause leakage. When the surface damage reached a certain amount of depth or width, steep increase of the leakage was caused. Although the meaning of the limitation of the depth or width of the surface damage to cause the rapid increase of leakage is not fully understood, it was recognized that holistic studies to minimalize surface damage including contact conditions, shape of the sealing components and material are required in terms of wear but also in terms of fatigue.

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The production of new materials by mixing together two or more raw materials is a useful method of reusing industrial waste. The purpose of this study is to produce glass-ceramics, with wollastonite with superior mechanical and chemical properties, from the industrial wastes of fly ash, concrete sludge, and glass cullet. First, two targets with different chemical compositions were selected from the wollastonite crystallization area in the phase equilibrium diagram of CaO-Al₂O₃-SiO₂, and the mixing ratios of industrial wastes were determined. Some batches were prepared by mixing the wastes at the respective weight ratio. Glass samples were produced by melting the batches at high temperatures and then quenching the melt. The amorphous glasses produced were reheated in a range of crystallization temperatures to convert them to glass-ceramics. The results of SEM observation and X-ray diffraction (XRD) analysis of the produced materials showed wollastonite to be the main crystal, with other crystals of anorthite or calcium silicate. The mechanical and chemical properties of the produced glass-ceramics, i.e., hardness, fracture toughness, and acid resistance, were investigated. It was clarified that the properties were improved after the conversion of the amorphous glass to the glass-ceramic for both types of glass-ceramic, and the wollastonite precipitated glass-ceramic containing a small amount of anorthite crystal exhibited better mechanical and chemical properties than that containing calcium silicate.

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It will be easily possible to evaluate the magnitude of strain by simply observing the color of a film, if there is a film whose color is changed by the dynamic strain. In this study we attempted to visualize strain by developing a film whose color changes with the magnitude of its strain. Strain visualization can be realized by converting the voltage produced by a piezoelectric polymer film to heat and converting this heat to color change. We experimentally developed three types of strain visualization film combining piezoelectric polymer, polyimide and thermosensitive films. The polyimide film converted voltage to heat and the thermosensitive film converted heat to color change. The piezoelectric polymer film generated voltage, when cyclic strain was applied. We attached one of the films to a test piece and carried out cyclic applied load experiments. The piezoelectric film generated small electric current. Therefore we installed a piezo amplifier into the system. We found that it was possible to visualize strain using a piezo amplifier, and piezoelectric polymer, polyimide and thermosensitive films.

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A new highly sensitive strain measurement method has been developed by applying the change of the electronic conductivity of CNTs, which is caused by the distortion of the electronic band structure. A two-dimensional strain distribution sensor was developed by using thin-film processing. Finely area-arrayed multi-walled carbon nanotubes (MWCNTs) were grown on a silicon wafer by applying a chemical vapor deposition method, and the area-arrayed MWCNTs were interconnected by the lift-off process of titanium thin film. The height of the grown MWCNT was about 250 μm, and size of the bundle of the grown CNT was 40 μm square, and the pitch of the bundle was 1000 μm. Uni-axial pressure was applied to the sensor, and it was found that the gauge factor of the developed sensor was about 100. Thus, this sensor is usefull for detecting the two-dimensional distribution of compressive strain in micro-scale.,

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In this study, an efficient technique for satisfying multi-point constraints in the parallel conjugate gradient method is proposed. Our strategy is to eliminate the dependent degrees of freedom from the system matrix equations (Ku = f ) by using the constraint conditions (Bu = 0 ). The elimination process is implemented in the conjugate gradient solver in such a way that the “MPC preconditioning matrix” is multiplied at each iteration step. In addition, a preconditioning based on the system matrix without multi-point constraints is introduced. The proposed method was implemented in a parallel finite element structural analysis software and its performance was measured. In a practical example problem with over one million degrees of freedom, both the number of iterations and computational time for convergence were reduced by about 94% compared to the penalty method. Parallel performance was also measured and 7.7 times speed-up was achieved with eight MPI processes. The result shows the effectiveness of the proposed method against large-scale assembled structural analyses.

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A high-temperature ultrasonic fatigue testing system was developed to evaluate the gigacycle fatigue properties of single-crystal superalloys used in aircraft engine turbine blades. In this development, a commercial ultrasonic fatigue testing machine was considerably modified to achieve high-temperature fatigue testing. The developed system took account of temperature dependency of Young's modulus, and also had a function to evaluate the Young's modulus. In order to protect the testing system from the heat of a specimen, straight and round rods were interpolated between the testing system and the specimen. Other modifications achieved accurate control of temperature, edge displacement and resonance frequency, which were necessary for accurate control of stress amplitude. The testing system was first applied to a heat-resistant steel at 650 °C to check its accuracy, and next to single-crystal superalloy samples at 1000 °C. As a result, the ultrasonic fatigue testing showed good agreement with the conventional fatigue testing, demonstrating the high accuracy of the developed system. In these results, the single-crystal superalloys showed no fatigue limit, indicating gigacycle fatigue tests to be necessary.

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A structural health monitoring system using multiple fiber Bragg grating sensors (FBG sensors) was developed. The system was designed to measure a large and a fast strain change and also to measure acoustic emissions ( AE ) simultaneously. The strain up to 1% and up to 100 kHz was considered. A multiple fiber ring laser was adopted as a light source, which consists of a multiple erbium-doped fiber amplifier (EDFA), an optical circulator, optical couplers and FBG sensors. Multiple fiber ring lasing wavelengths depended on strains loaded to FBG sensors. A CFRP beam bending test was carried out to confirm the possibility of simultaneous measurement of both strain and AE signals from a single FBG sensor. In the test, signals from a conventional electric resistive strain gage and a piezo-electric AE sensor were revealed equivalent to those from the FBG sensor. The system will be applied to development of composite structures in aerospace field such as Epsilon Launch Vehicle.

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In the previous work, the authors formulated the balance laws of mass, momentum, angular momentum and energy of the lattice element used for recrystallization. These laws were summed up over a phase in a representative volume element (RVE) and averaged in the RVE so as to develop the discrete balance laws for single phase. Furthermore, the balance law of angular momentum was separated into a bulk and a lattice parts through the orderestimation with the representative lengths both in macroscopic and microscopic scales. In this paper, the RVE converges on a material point so that the laws are rewritten in the integration form. When the laws are summed up all over the phases and averaged in them, the balance laws of mass, momentum, angular momentum and energy for nuclei and matrix as mixture are formulated, using an useful theorem proposed for the mixing summation of unsteady terms. At this time, the macroscopic part of the balance law for angular momentum results in the usual equation of angular momentum, so that the stress tensor keeps symmetry even if the lattice rotation is considered. While, the microscopic one is localized as an equation of spin angular momentum for lattice, which is suggested to be equivalent to the evolution equation of crystal orientation in KWC type phase-field model. Moreover, the increase law of entropy for mixture is also formulated. During this process, the entropy flux is defined by use of relative mass flux and chemical potential of phase transformation.

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The purpose of this study is to clarify the strain distribution within deep brain and influence of brain shape factors on the strain distribution by using three dimensional physical head models during rotational impact. Three different shaped brain models were constructed; actual human brain shape model, no-ventricle model and simplified shape model. Angular acceleration pulse, whose peak of 4500 rad/s² with 8 ms pulse duration, was induced to the models. After rotating 60 degree, all models were decelerated with 1500 rad/s² peak with 30 ms duration. As for the strain distribution within deep brain inducing actual human brain, strain concentrations were measured at corpus callosum and brain stem. This was due to constraint of cerebrum brain rotational motion by falx and tentorium and hollow shape of ventricle. The maximum principal strain at brainstem in no-ventricle brain model was larger than the actual brain shape model. However the strain distributed near the corpus callosum in no-ventricle brain model was smaller than the actual brain shape model. The strain in the simplified brain shape model was smaller compared with other brain models. Therefore the cerebral ventricle relieved strain at brainstem and increased the strain near the corpus callosum. On the other hands, the sulci has a influences of increasing strain within deep brain.

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