Structural ceramics are being considered for use in a variety of fields due to their excellent properties. However, ceramics are very sensitive to defects and minute surface defects can affect fracture strength. In addition, the relationship between surface defects and fracture strength exhibits non-linearity, and thus, the conventional linear elastic fracture mechanics may not predict fracture strength. Therefore, there is a need for an accurate method to determine the effect of surface defects on the fracture strength of ceramics. In this study, we developed a method that extends the model proposed by the authors to incorporate fracture strength reduction due to surface defects. This method can reproduce the non-linearity of the relationship between surface defects and fracture strength of ceramics. Then, we discuss about the competing relationship between internal and surface defects. It can also estimate the allowable size of surface defects by combing with the Weibull distribution of fracture strength.

The application of concurrent cathodic hydrogen charging SSRT is being investigated as an alternative to SSRT in high-pressure hydrogen gas, which is used to evaluate the hydrogen compatibility of metallic materials. There are some differences in the stress-displacement behavior and the fracture surface between the two tests. These differences can be attributed to the difference in the timing of the onset of hydrogen entry in the two environments. In SSRT of low-alloy steel in high-pressure hydrogen gas, the amount of absorbed hydrogen up to the yield stress is small and increases rapidly after plastic deformation, Whereas, since in concurrent cathodic hydrogen charging, hydrogen entry is initiated as soon as begins, it is necessary to determine an apporopriate start time. The surface changes of SCM435 specimens during SSRT in NaOH solution were analyzed by electrochemical measurements, and the breakdown of the oxide film that inhibits hydrogen entry during SSRT in high-pressure hydrogen gas was discussed. As a result, the impedance at low frequencies decreased immediately after the yield point, which was attributed to the decrease in the reaction resistance of the oxide film. The rest potential decreased immediately after the yield point and immediately after the fracture, which were attributed to the exposure of the fresh surface due to the breakdown of the oxide film. From these results, it was clear that hydrogen charging in the concurrent cathodic hydrogen charging SSRT should be started just after the yield point.

In a high pressure hydrogen gas environment, there is a risk of "hydrogen embrittlement (HE)" by hydrogen entered to the steels. Therefore, many experiments of steels in high pressure hydrogen gas have been conducted such as tensile, fatigue and so on. However, there are little knowledge of influence of the material microstructure on HE. There is a possibility of HE susceptibility of steels depending on the material microstructure. Therefore, objective of this development is clarification of the microstructure to excellent HE resistance. As a result of Slow Strain Rate Tensile (SSRT) test under hydrogen environment, tempered martensite structure with finely cementite dispersed was most excellent in HE resistance. The reason for this is considered that the generation of micro voids, which is the initiation of surface cracks affecting ductility of SSRT, was suppressed.

In order to utilize underground space as a site for energy extraction and material circulation, it is a prerequisite to ensure the mechanical reliability of subsurface systems as well as aboveground systems. For this purpose, it is essential to address the issues related to strength of materials in rocks under the unique underground environment. In this presentation, research activities concerning the underground development, such as design and formation of engineered geothermal reservoirs by using hydraulic stimulation technology, are reviewed and future issues are discussed. The topics to be covered include modeling of the crack propagation behavior in underground systems under mechanical loads and characterization of subsurface complex crack systems using the fluid flow behavior in rocks.

A cylindrical rock-core obtained by borehole drilling expands radially into a slightly elliptical shape along its transverse cross-section when the rock sample is relieved of its in-situ anisotropic stress by drilling. The magnitudes and orientation of in-situ stresses can, in principle, be estimated from the circumferential variation of core expansion. However, it is not possible to evaluate the expansion from a core collected in a conventional way, with a single drilling bit, since it will be in its expanded state when extracted and lacks information about its required pre-expansion dimensions. To overcome this problem, we propose an innovative way of core drilling referred to as Dual-bit Coring. This approach results in a single core that has two signature transverse cross-sections: the first has an elliptical shape resulting from core expansion and the second records the initial circular shape prior expansion. We simulated numerically the process of cutting a rock column out as a core by the Dual-bit Coring method and examined how the column deforms during core drilling.

The paper discuss the way of dicision on stress intensity factor in the vicinity of crack on the center of plate which is a part of orthotropic cantilever on tension load and bending load by means of finite element analysis and virtual crack closure integral method.

Development of geological resources requires understanding fluid flow in the underground. It is necessary to characterize fracture networks in the underground that works as flow paths. However, the fracture network structures are complex and difficult to analyze. Therefore, we quantified the fracture structures using persistent homology, which is one of the topological data analysis methods. In this study, we verified whether it is possible to estimate the number of flow paths from the tracer response. The results showed that it is possible to estimate the number of flow paths using tracer responses, but the method still needs to be improved to apply to complex fracture networks.

Reinjection of wastewater from geothermal power plants has been standard in geothermal development. Since reinjected water, however, may cool production areas, assessment of injected-water flow within subsurface structures is required. Suzuki et al. (2022) successfully validated an estimation of flow-path surface areas with long-term temperature data by thermal breakthrough experiments using a fracture structure created by a 3D printer. In this study, we improved their experiment to fit with the assumption of the mathematical model in the estimation method. As a result, we could improve the experimental setup and verified the estimation and prediction of future temperature change based on short-term temperature data.

A closed-form solution of complex potentials representing the elastic fields in an orthotropic circular disk of uniform thickness subjected to a self-equilibrium concentrated-counter-forces acting in the diameter direction is derived. Then the stress intensity factors at a tip of a straight crack located at the center of the orthotropic disk subjected to a counter-force compression was calculated using a derived solution and the body force method. The results of a present study are useful for evaluating a tensile strength of brittle materials such as anisotropic rocks.

In geothermal development, fluid is injected into subsurface fractures in reservoirs to improve productivity. At that time, understanding tectonic stress is important to estimate the portion where the permeability is improved. Tectonic stress can be measured in a borehole, but it requires drilling of the borehole, and the measurement in the deep borehole is costly and time-consuming. In this presentation, the tectonic stress measurement method using induced AE (Acoustic Emission) is introduced, where fracture planes are estimated from accurately estimated AE source distribution and the estimated possible slip directions of the fracture planes.

When investigating the long-term performance of various rock engineering projects, such as the geological disposal of high-level radioactive waste (HLW), enhanced geothermal system (EGS), it is essential to accurately predict the permeability evolution within fractured rocks due to coupled thermal-hydraulic-mechanical-mechanical (THMC) phenomena including fracture generation and subsequent geochemical creep. In this paper, we introduced our coupled THMC simulator that can explicitly introduce the fracture surfaces to describe the permeability change within fractured rocks as realistically as possible. The numerical results of long-term change in rock permeability during a disposal period of HLW computed by our simulator, showed that the permeability reduction with time was observed only within specific fractures where the geochemical creep (i.e., pressure dissolution) had been activated depending on the heterogeneous properties of each fracture. It may suggest that our simulator has a possibility to reveal the detailed spatial distribution of permeability within rock masses containing the newly generated fractures.

It is important to evaluate materials' strength and damage state in material mechanics. A series of inspection techniques called non-destructive testing is used to examine the state of such materials non-destructively. Non-destructive testing is an inspection technique that examines harmful damage to the interior or surface of an object without destroying the object. It can be broadly classified into methods to detect internal flaws by injecting radiation or ultrasonic waves into the object and methods to detect surface flaws by passing an electric current or magnetic flux near the surface. Non-invasive inspection is a method similar to non-destructive testing—however, non-invasive inspection targets humans, not objects. Non-destructive testing is used for our safety and security, and non-invasive testing is used for our health. Although non-destructive testing and non-invasive inspection are not directly related to geophysical exploration, they share many of the same underlying principles. Geophysical exploration targets subsurface structures, including underground resources such as oil, natural gas, and metal deposits, and is used for our wealth and comfort. Geophysical exploration can be thought of as an inspection technology for the subsurface. In this paper, geophysical exploration for the exploration of subsurface structures and resources is reviewed, and its relevance to nondestructive testing for material mechanics is explained. This paper provides an overview of non-destructive testing and geophysical exploration concerning the mechanics of materials. While non-destructive testing focuses only on flaws within a material, geophysical exploration focuses on the physical properties of the material as a whole, which may be useful for investigating material heterogeneity. I hope this paper has stimulated your interest in non-destructive testing and geophysical exploration.

Geothermal energy is a renewable energy which can contribute to reduce atmospheric emission of carbon dioxide (CO₂) by stable electric power generation and hot water supply for district heating. Recently, Japanese government has strongly supported geothermal development. Geothermal power plants of a total capacity of 593 MW in Japan and 15,948 MW in the world have been installed in 2020. It is still difficult to survey and develop natural geothermal reservoirs even at shallow depths, because the underground structure is complicated and invisible. Therefore, new technologies called EGS (Enhanced Geothermal Systems) which inject surface water into natural reservoirs, improve natural reservoirs, create artificial reservoirs, and extract heat from supercritical geothermal resources are being developed to reduce developing risks and extend developing areas. Other new innovative technologies which use CO₂ as a heat extraction fluid instead of water are also being studied. For these new technologies, basic studies on creation of artificial fractures, evaluation of natural and artificial fractures, and fluid flow in fractures are needed.

We proposed a novel deformation process in the tensile stress-strain behavior of semicrystalline polymer solids. At the yield point, the stacked lamellar clusters are fragmented into cubic cluster blocks of single chain size; the rearrangement of these blocks under uniaxial tension produces the texture of necking. The initiation of necking is explained by a first-order catastrophic phase transition analogous to the model of a van der Waals gas. Thus, the catastrophic arrangement of the cluster blocks results in a sudden emergence of a locally close-packed layer structure. The negative molecular dependence of the natural draw ratio at the necked portion can be explained by the catastrophic rearrangement of the cluster blocks, the size of which is proportional to the square root of the molecular weight.

316FR steel is a candidate material for the main components of the fast reactor, and its design allowable stress is specified in Japan Society of Mechanical Engineers fast reactor standard. The reactor will be larger in size. For the design, welding is also required at a part to be a high-temperature and high-stress. Therefore, allowable stress for the weld joint is required. In the evaluation of welded joints, it is necessary to consider the characteristics of the weld metal and heat affected zone in addition to the base material. In this paper, by comparing the strength properties of 316FR steel base metal and weld joint, it was examined whether it is necessary to specify the allowable stress of the weld joint in the restriction of the primary stress of the JSME fast reactor standard.

In the structural design of fast reactor equipment, structural strength is evaluated using elastic follow-up coefficient considered stress relaxation by creep. JSME standard for fast reactor limits the strain to prevent excessive inelastic deformation and the creep-fatigue damage factor to prevent failure due to fatigue and creep-fatigue. In case of the long-term primary stress is low, elastic follow-up strain, creep damage factor and strain range used to these limits is calculated according to elastic follow-up coefficient: q=3. Therefore, in order to adopt more reasonable design, JSME standard will be proposed that can use elastic follow-up coefficient: q^r calculated from the results of inelastic analysis instead of q=3. In this paper, it’s shown concretely how to calculate three kinds of elastic follow-up coefficient described above, and shown elastic follow-up coefficient calculated from repeated behavior with respect to strain range is smaller than others.

In current practices, stress analyses of Class 1 vessels with internal pressure of nuclear power plants have been carried out using the minimum thickness of the component. In this paper, effects of thickness on stresses have been investigated by comparing stress analysis results for two kinds of 3D-FEA models of a typical nozzle attached to a cylindrical vessel with minimum thickness and nominal thickness. Since variations of stresses between minimum and nominal thickness models are small, nominal thickness can be employed in the 3D-FE analysis. However, in case of evaluations for primary stresses, a correction method to multiply primary membrane stress by the thickness ratio and primary bending stress by the thickness ratio squared should be used in order to conduct conservative evaluation.

Finite element analyses were conducted to improve the elasto-plastic response analysis of piping systems with tees, taking into account the actual geometry and material properties, and comparing the results with the design analysis. The tee geometry was converted from point cloud data to CAD data and applied to vibration mode analysis and elasto-plastic response analysis. Values obtained from tensile test specimens cut from actual tee pipes were used to consider the effect on mechanical properties during forming. In piping systems where the tee section is the damaged location, the stiffness of the tee section has a significant effect on the modal deformation, and it was confirmed that the tee section with small elongation requires appropriate damage evaluation.

Beam elements are generally used for response analysis of structures using shaped steel. However, the beam elements cannot accurately simulate the shape of the welded joints, and the length of the shaped steels is shortened because it is modeled to be joined at the centroid. Therefore, there is a concern that the natural frequency may differ from that of actual structures. So that, we compared the vibration characteristics of both the case of using the beam element and the case of using the solid element that simulated the actual weld structure as much as possible by FEM analysis. Although the vibration characteristics of both are almost the same, the beam element is slightly lower, confirming that it is appropriate to apply the beam element to the determination of the rigid structure.

Elbow piping is characterized by complicated stress conditions and easy wall thinning. Therefore, a highly accurate fatigue life prediction method is required to accurately evaluate the soundness of elbow piping. In previous studies, the revised universal slope method was proposed as a life prediction formula for elbow pipes, and the relationship between strain range and fatigue life was obtained. However, with the conventional analysis method, the strain range is overestimated compared to the actual behavior, resulting in a significantly conservative prediction result. In this study, we will improve the prediction accuracy of the strain range by using a more detailed stress-strain diagram than before in the finite element analysis and examine the further improvement of the fatigue life prediction.

The Rules on Fitness-For-Service (the Rules on FFS) of the Japan Society of Mechanical Engineers (JSME) limits the circumferential crack angle to 60 degrees or less for Class-1 piping. This limitation was based on a deterministic evaluation because of the concern about fracture that would occur if the measurement error of the crack size was taken into consideration. However, ASME Section XI, which is a standard similar to the Rules on FFS, has no limitation on the circumferential crack angle, and the crack angle limitation is unique to the Rules on FFS. After the first edition of the Rules on FFS was published in 2000, some research on crack size measurement were performed. The number of inspections according to the measurement error that satisfy the target fracture frequency regardless of the crack angle can be evaluated by using probabilistic evaluation. In this study, the number of measurements of the crack size required to satisfy the target fracture frequency is determined based on the thickness of pipes, crack size measurement error, the fracture frequency according to the number of crack size measurements, and the conditional core damage probability due to pipe fracture. Based on this result, the requirement for inspection and evaluation of the Rules on FFS was proposed.

In the JSME fitness-for service code, fracture evaluation methodology based on linear elastic fracture mechanics is provided for reactor pressure vessel (RPV) components by considering brittle fracture due to neutron irradiation in the RPV beltline region. However, in order to make a rational fracture evaluation for the RPV component such as bottom head component of boiling water reactor (BWR), where irradiation embrittlement is not concerned, it is necessary to consider the applicability of the ductile fracture evaluation methodology consistent with material fracture behavior. In this study, the applicability of the damage mechanics analysis method using Gurson-Tvergaard-Needleman (GTN) model, which is a numerical analysis method for ductile failure, was investigated for RPV bottom head component. Mechanical properties tests and fracture tests with plate specimens were performed using RPV law alloy steel (LAS) material. GTN model parameters of RPV LAS material that can reproduce stress-strain curve and load-displacement curve were determined. Fracture analyses of the plate specimens were performed using determined parameters as well. The load-Crack Mouth Opening Displacement (CMOD) curves and fracture behavior of the fracture tests were compared with the analyses results. The analyses predicted the maximum loads of the tests within an error of 4% and the fracture behavior between the tests and analyses was consistent. The results in this study clearly supported the applicability of the GTN model to RPV bottom head ring component.

This paper evaluated a closed formed solutions of stress intensity factor K and J-integral for a flawed elbow of French Code, RSE-M, in order to introduce a flaw evaluation for aged elbows into the JSME Rules on Fitness-for-Service (JSME Rules), which have flaw evaluation procedures using K and J for only straight pipes and cylinder-shaped vessels in the current version. The stress distribution of an elbow is more complex than straight pipe and depends on not only flaw shape, flaw direction and radius/thickness but also bending radius, which may result in smaller allowable flaw size than that of a straight pipe. The authors performed K and J-integral calculation using FE analysis (FEA) focusing on an aged primary coolant pipe of PWR plant in Japan and compared with the solutions of the RSE-M code. As a result, it was confirmed that K and J solutions by the RSE-M code gave generally good result with conservativeness. Some cases of K showed large unconservative predictions by the RSE-M code and it was found that those were for the elbows with geometries out of the application range of the RSE-M code. The authors will develop a flaw evaluation for elbows with larger application range than that of the RSE-M code.

This paper summarizes the formulations and some of the applications of the three-dimensional J-integral and J-integral range ΔJ that were proposed by the authors. The proposed crack parameters are evaluated by the domain integral method. Their values are guaranteed to be unconditionally independent of size and shape of integral domain by their formulation. The numerical examples demonstrate that the proposed crack parameters can characterize the severity of the crack tip deformation field. Hence, they can characterize the crack propagation behavior.

In order to provide a stress intensity factor (K)-solution for a nozzle corner flaw subjected to internal pressure for the Japan Society of Mechanical Engineers (JSME) Rules on Fitness for Service (FFS) for nuclear power plant, four candidates, which are the Paris and Sih based solution, the Fife’s solution, the Kobayashi’s solution and the solution of a flat plate with a semi-elliptical surface flaw in the JSME Rules on FFS, were compared with the Ks by Finite Element Method. As the result, the flat plate solution was confirmed to be the most adequate for K calculation of a nozzle corner flaw in the JSME Rules on FFS.

Stress corrosion cracking (SCC) has been detected in the welded components of nuclear power plants. The components of nuclear power plants consist of highly ductile materials such as austenitic stainless steels, and their failure mode is expected to be plastic collapse due to ductile fracture. The limit load analysis, which is applied to fracture assessment for ductile materials, is employed in the fitness-for-service (FFS) codes such as JSME Code “Rules on Fitness-for-Service for Nuclear Power Plants”. The FFS codes provide flaw characterization rules for multiple flaws, and the technical basis seems to be based on interaction of stress intensity factors which are linear elastic fracture mechanics parameters based on small scale yielding. Thus, the applicability of current flaw characterization rules for multiple flaws to limit load analysis is unclear. The limit load analysis for non-aligned multiple flaws was developed based on the net-section approach and the test results of flat plates with two flaws in the past studies. The test results can be represented by using the defined net-section of non-aligned multiple flaws. Finite element analyses were conducted to interpolate the test conditions. The effect of flaw positions on collapse load was estimated by the test and analysis results.

Tensile tests were conducted at room and high temperature to clarify the fracture strength and crack coalescence behavior of multiple cracks in SUS304 flat plates. The shape of the specimen is a plate, and electrical discharge machined (EDM) notches were introduced on both sides of the central smooth part. Then, a fatigue pre-crack was generated at the tip of the notch. The position of the crack was expressed by the distance between the crack surfaces and the distance between the crack tips. As a result of the tensile test at room temperature, when the distance between the crack surfaces was shorter than about 8 mm, the cracks coalesced. On the other hand, when the distance between the crack surfaces was about 10 mm, no crack coalescence was observed, and there was a change point in the crack coalescence behavior. As a result of the high temperature test, crack coalescence was observed when the distance between the crack planes was about 8 mm, but the change point of the crack coalescence behavior is unknown because there is no data. It was found that the change points of the coalescence behavior of the cracks obtained in the experiment satisfy codes for nuclear power generation facilities (JSME S NA1-2019).

In the next-generation fast reactor, 316FR steel is expected as a structural material of the reactor vessel, and in the design of a large tank reactor, it is inevitable to place the weld bead on the high stress part. Therefore, a creep-fatigue evaluation method that can appropriately evaluate weld joint is required. The welded joint is composed of a base metal, weld metal and heat-affected zone. In this study, finite element analysis considering these material property differences was performed on the test piece of the welded joint. And the creep fatigue life was evaluated based on the stress redistribution during the test. Our evaluation method was able to predict the number of failures in the fatigue test and creep fatigue test within the factor of 10.

In the next-generation fast reactor, Mod. 9Cr-1Mo steel is expected as a structural material of the steam generator, and in the design of a large tank reactor, it is inevitable to place the weld bead on the high stress part. Therefore, a creep-fatigue evaluation method that can appropriately evaluate weld joint is required. The welded joint is composed of a base metal, weld metal and heat-affected zone. In this study, finite element analysis considering these material property differences was performed on the test piece of the welded joint. And the creep fatigue life was evaluated based on the stress redistribution during the test. Our evaluation method was able to predict the number of failures in the fatigue test and creep fatigue test within the factor of 10.

The present paper proposes new creep life prediction methods that can be used in situ as an in-service inspection tool for the condition-based monitoring of high-temperature structure components of 316FR stainless steel that has been developed in Japan for fast breeder reactors. The methods were based on two types of the modified theta methods and the combination of the modified theta and modified omega methods and were applied to creep deformation data during creep tests made at different applied stress levels at 823K and 923 K in air. Creep lives were predicted by the proposed three methods at arbitrary time in the continuous measurement of deformation during the creep tests. The results show that the combination of the modified and modified omega methods gives the most accurate prediction of creep rupture time at the two levels of test temperature for different applied stress levels.

A creep test was conducted to clarify the coalescence behavior of cracks in a SUS304 flat plate with multiple cracks on different planes. The test temperature is 600 degrees. The shape of the specimen is a plate, and electrical discharge machined (EDM) notches were introduced on both sides of the central smooth part. Then, a fatigue pre-crack was generated at the tip of the notch. The position of the crack was expressed by the distance between the crack surfaces and the distance between the crack tips. As a result of the experiment, the cracks coalesced when the distance between the crack planes was shorter than about 3 mm. On the other hand, when the distance between the crack surfaces was about 7 mm, no crack coalescence was observed, and there was a change point in the crack coalescence behavior. It was found that the change point of the coalescence behavior of the crack obtained in the creep test satisfied codes for nuclear power generation facilities (JSME S NA1-2019). As a result of fracture surface observation, it was found that the crack growth from the fatigue pre-crack due to creep is due to grain boundary rupture regardless of the presence or absence of crack coalescence.

Application of heavy-duty anticorrosion coating is a typical method to prevent corrosion in steel bridges. It is required to repaint before coating deterioration becomes prominent because the anticorrosive performance of the coating decreases over a long term. Thus, it is important to develop a remote and nondestructive inspection method for the deterioration of coating of entire bridges at an early stage. This study proposed a new inspection method for the deterioration of the top coat of the heavy-duty anticorrosion coating by short-wave (SW) infrared spectral characteristics on the absorption and reflection. The remaining thickness of the coatings can be quantitatively evaluated by measuring the infrared reflected energy from the top coat because the absorption of the coating depends on the thickness. We measured the infrared reflected energy from the specimen consisting of various thicknesses of the top coat. Moreover, the infrared reflected energy for the in-service bridge cannot be ignored the effect of the reflections from the sea surface and the shadows of bridge members. Thus, we developed the measurement method independent of the lighting condition that combines the active infrared measurement with a halogen lamp and the lock-in processing that can extract only the fluctuating component of infrared intensity, thereby reducing the influence of disturbances. It was found that the remaining thickness of the top coat for the actual bridge in-service can be quantitatively evaluated based on active lock-in infrared measurement even in the artificially reproduced disturbance condition.

Gas turbine (GT) blades are exposed to high-temperature combustion gas that exceed the thermal resistance temperature of Ni-based superalloy. Therefore, the blade surface is coated by thermal barrier coating (TBC), which consist of two layers: a ceramic top coat (TC) and a bond coat (BC). The cyclic thermal loading in GT operation causes damage to the TC layer, such as cracking and delamination. Therefore, GT blades utilized in thermal power generation are periodically recoated to prevent such damage. However, recoating costs constantly, and it is required to extend the recoating period to reduce it. For this reason, advanced technology that enable nondestructive and simple deterioration diagnosis of damage to TBCs has been actively developed globally. In this study, we focused on the rebound-type hardness test, in which microspheres are impacted on the TBC surface, and attempted to relate the hardness value (Leeb hardness) to the damage of the TBC.

In recent years, there has been a concern about the occurrence of gas leakage and fire accidents in plant facilities due to damage caused by aging deterioration of gas piping. However, conventional gas leakage source estimation methods require prior teacher data or calculated data of gas clouds to be compared, which requires large computation time and may not be applicable under unexpected circumstances. Therefore, this study proposes a method to estimate the source of gas leakage by visualizing the leaking gas cloud using an infrared camera and performing an inverse problem analysis based on the concentration information without the need for prior teacher data or calculation data. It is found that the leakage source can be estimated by retrospective search from the gas concentration distribution to the location where the infrared absorption is large. In addition, the leak source estimation is achieved with high accuracy by removing the initial search position data and using only the position data near the leak source in the retrospective search.

To rationalize the evaluation process of temper bead repair welding, a numerical model that can predict hardness during repair welding was developed by combining a coupled bead shape-heat conduction analysis model with a hardness prediction model based on many experimental results. To confirm the usefulness of the developed model, bead-on-plate welding was performed experimentally and numerically to compare the measured and calculated distribution of hardness. As a result, measured and calculated hardness were in good agreement. In addition, the developed model was applied to temper bead repair process for a bottom-mounted instrumentation, and the appropriate welding conditions for reducing the hardness of the repair welded zone were discussed. Numerical results revealed buttering and small heat input could be effective to reduction of hardness for temper bead welding. Since these results was consistent with previous findings, the developed model was found to be applicable to the temper bead welding and it enables quantitative evaluation of hardness according to temper bead repair welding conditions.

In this study, basic research was conducted on a new lubricant analysis technique to constantly monitor the deterioration state of rotating equipment and other components in plant facilities. The research method used terahertz electromagnetic waves, which are electromagnetic waves in the frequency range of 0.1 to 10 THz, the boundary region between light and radio waves. As a measurement method, we focused on the fact that terahertz electromagnetic waves are safe for the human body yet have excellent transmittance, and conducted experiments using terahertz time-domain spectroscopy (THz-TDS). In this study, THz-TDS measurements were performed on artificially degraded lubricating oil to verify the possibility of detecting degradation factors and separately identifying each degradation factor. As a result, it was possible to detect water and iron powder and to identify them separately using the transmittance of THz electromagnetic waves obtained from the obtained time waveforms and the standard deviation ratio of the peak intensity values of THz waveforms. In addition, the detection and separation of iron powder, water, and antioxidants were possible using the attenuation of terahertz electromagnetic waves obtained from the frequency waveforms obtained by Fourier transforming the time waveforms. These results suggest that terahertz electromagnetic waves can be used to detect iron powder, water, and antioxidants, which are factors involved in lubricant degradation, and that each degradation factor can be separated and identified by using terahertz electromagnetic waves.

Residual stress evaluation in Solid Oxide Reactors (SORs) is essential issue to ensure mechanical durability and reliability. However, it requires specialized knowledge and skill in experimental techniques, and computing. In this study, two diagrams consist of dimensionless numbers based on thermomechanical properties: Young’s modulus, Poisson’s ratio and thermal expansion coefficient, and geometrical parameters: thickness of each layer, were drawn for electrode-supported-type configurations. These diagrams provided researchers and engineers in the field of SORs the way to estimate thermal stress.

For solid oxide fuel cells, the following cell configurations have been developed: electrolyte-supported cell (ESC), anode-supported cell (ASC), and metal-supported cell (MSC). In this study, to compare the mechanical load tolerance corresponding to each of the abovementioned cell configurations, the curvature and residual stress were calculated using the Euler–Bernoulli beam theory. The results indicated that when the cells were flattened under the application of an external bending moment, the stress gradient in each layer of the cell disappeared, and the maximum stress in the cell was shown to decrease. Eventually, the failure probability of the cell substrate was demonstrated to take the smallest value. The allowable bending moments for the cell substrates were as follows: −0.17 to 0.08 Nm for ESC, −0.18 to 0.11 Nm for ASC, and −0.29 to 0.40 Nm for MSC at room temperature. Thus, MSC was found to possess the highest bending moment tolerance.

In this paper, we incorporate the surrogate model and substitution approach into electrochemo-mechanical analysis of plate shaped solid oxide fuel cell (SOFC). A local structure which is called in-plane unit structure (IPUS) is extracted from the plate shaped SOFC. The oxygen potentials of the IPUS as training data are obtained from the electrochemical potential analysis. A surrogate model using proper orthogonal decomposition is constructed from the oxygen potential of the IPUS obtained from the electrochemical potential analysis, and the distribution of oxygen potential in plate shaped SOFC is predicted by the surrogate model. The reduced expansion strain is calculated from the predicted oxygen potential. Then, a substitution approach using numerical plate testing is applied to analyze macro deformation considering mechanical behavior and thermal and reduced expansion.

In order to improve the performance of all-solid-state lithium ion batteries, understanding the diffusion properties of lithium ions in solid electrolyte is necessary. However, because the diffusion properties of lithium ions are nanoscale phenomenon, and polycrystalline materials with grain boundaries are often used in the experiments, it is difficult to evaluate the physical properties of materials in ideal bulk crystal states. In this study, a molecular potential models are constructed and simulations by molecular dynamics method are performed to analyze the diffusion properties of lithium ions in the solid electrolytes.

The purpose of this study is to investigate effect of machine processing scratches on fatigue strength of a notched specimen of aluminum alloy. In this study, strain-controlled fatigue tests were carried out for two types of notched specimens with machine processing scratches. The fatigue strengths of notched specimens decreased by more than the elastic stress concentration factor due to the notch. To discuss the fatigue strength reduction, the specimen and fracture surfaces were observed carefully. The observations showed that the fatigue cracks initiated from the machine processing scratches and they are aligned around the notch root due to the stress concentration. As a result, the fatigue strength reduction factor became greater than elastic stress concentration factor.

Metal additive manufacturing technology can easily produce metal parts with complex shapes and is being considered for application in a variety of fields. However, it is known that the fatigue strength of products manufactured using this technology is low due to internal porosity and surface roughness generated during fabrication. In this study, the effect of stress concentration sources on the rotating bending fatigue strength of aluminum alloys, AlSi10Mg alloys, which is expected to be applied to lightweight components, it was examined by changing the hatch size. The results of tests using a quadruple rotating bending fatigue tester showed that surface treatment to remove stress concentration sources significantly improved fatigue strength. Furthermore, it was considered that the variation in the size of near-surface defects in the smoothed specimens may affect the variation in fatigue life.

In order to investigate the three-dimensional fatigue properties of resistance spot welded joints made of high strength steel, this study was carried out observed three-dimensional fatigue cracks and calculated stress intensity factors using fem analysis. In addition, this study was conducted under shear mode and pure shear mode to investigate the influence of loading mode during fatigue testing. As a result, it was found that a new crack growth behavior branched in the welding circumferential and width direction under the low loading force range. However, all the cracks leading to final fracture were of the same type, and it was confirmed that the fatigue strength was not affected by a new crack growth behavior. Moreover, the results of observed initial fatigue crack growth angles and the calculated stress intensity factor, K_Iθ, were in close agreement, which proved that the fatigue crack growth behavior was strongly influenced by the fracture mechanics.

Rotating bending fatigue tests were carried out to investigate the effect of microstructure in subsurface on the fatigue properties of a shot peened maraging steel. The steel had a fine-grained microstructure formed at the surface layer with 50-70 μm in depth. Especially a thin surface layer showed a nano-size structure. A surface fracture occurred at high stress levels, while the combined fracture occurred due to coalescence of a surface crack and an internal one initiated independently with decreasing in stress level until 108 cycles. In all of the macroscopic fracture surfaces in the shot-peened specimen, a ring-shaped flat area was observed at the surface layer irrespective of the fracture mode and the depth of the area nearly corresponded to the one of the fine-grained microstructure. The formation mechanism of the ring-shaped area was explained from the initiation and propagation properties of a surface crack in the surface layer in connection with the microstructure.

For greater fuel efficiency and increased safety, shifts to design of car body using multi-material such as ultra-high strength steel, aluminum alloy and carbon-fiber-reinforced plastic (CFRP) in addition to conventional steel are accelerated. Therefore, effective joint methods for dissimilar materials are required. Adhesive bonding has attracted attention from this point of view. The strength of adhesive bonding joints is affected by the surface conditions of the adherend. Therefore, in this study, the effects of abrasive blasting and atmospheric plasma treatment on the static and fatigue strength of SPCC (steel plate cold commercial) adhesive joint were evaluated. It was found that abrasive blasting increased the surface roughness and waviness due to the collision of particles and improved both static and fatigue strengths of the joints due to anchor effect. On the other hand, atmospheric plasma treatment created hydrophilic surface of SPCC, however, only fatigue lives of the joints increased and the static strength was not improved.

The effect of high temperature tension hold on fatigue crack growth in Inconel 718 is investigated. A single tension hold at maximum load was applied during cyclic loading at 650 °C, and the effect of load level and hold time are examined. In all experiments, a rapid acceleration of fatigue crack growth is observed immediately after tension hold. A retardation is observed after the initial acceleration at lower stress intensity factors, while only an acceleration occurs at higher intensity factors. These results are discussed by considering two competed mechanisms: grain boundary damage related to oxygen in front of the crack tip, and stress relaxation due to creep deformation.

In general, high-strength steel is known to occur an internally initiated fatigue fracture called "Fish-eye failure" in the very long life region. In addition, the fatigue life of a fish-eye failure is dominated by the ODA (Optically Dark Area) that forms around the fracture origin. Therefore, it is necessary to elucidate the formative mechanism of ODA in order to predict the fatigue life of fish eye failure. Besides, it has been reported that small surface cracks propagate at extremely low speeds below the lattice spacing in a vacuum simulating the interior of high-strength steel. However, it has not been confirmed that the growth of these small cracks forms ODA. In this study, fatigue tests were conducted from a very short crack to a long crack, which corresponds to internal failure in the very long life region, instead of the fatigue tests with a relatively long crack of around 150 μm that have been conducted so far. The results of intermittent observation of the growth behavior and the relationship between the crack growth rate, stress intensity factor, and maximum stress suggest that small cracks grow and form ODAs.

The growth direction and growth rate of fatigue cracks initiated from the surface of A7075 aluminum alloy plate were investigated under 6 sets of nonproportional loadings produced by the combined axial-torsion loading. The digital image correlation method was simultaneously applied to measure the crack opening displacements around the crack tip during the fatigue tests. Fatigue crack tended to propagate orthogonally to the direction in which the resultant normal stress in the nonproportional loading cycle was maximized, suggesting that the maximum resultant normal stress dominate crack growth under nonproportional loadings. The growth lifetime until the surface crack penetrates the plate could be estimated with an accuracy of about 20% based on the modified Paris-type equation through numerical integration.

In recent years, the application of thermal barrier coatings to the components used in gas turbines for thermal power generation has become indispensable as higher inlet temperatures for requirement to further improve the efficiency of thermal power generation. Thermal barrier coatings, generally referred to as TBCs, consist of a top coat, which is a thermal barrier layer, and a bond coat, which improves the adhesion strength between the TC and the base material. As the development of new thermal barrier materials is progressed, it is also demanded to clarify the stress and damage behavior of TBCs under the complex thermal fatigue loading conditions in actual gas turbine operation. In this study, comprehensive finite element analysis is performed on a TBC system under cyclic thermal fatigue loading conditions with temperature and stress variations to clarify the deformation behavior and crack initiation life of the TBC, and to develop a fatigue life evaluation method for TBCs.

This study focuses on discussing fatigue life evaluation method of SUS316 and SUS430 steels under wide ranged proportional multiaxial loading conditions. Fatigue tests under 7 types of loading path which are combined axial stress and inner pressure were carried out using hollow cylinder specimens at room temperature. It has been considered that the proportional loading does not cause additional hardening, however this study shows that the complex loading path causes additional hardening without principal direction change of stress and strain in a cycle. The Itoh-Sakane model which evaluates loading and lives under non-proportional multiaxial fatigue by only considering the maximum and the minimum principal stress or strain and their principal directions. However, this model is also needed to consider middle principal stress because change of the middle principal stress influence fatigue life under current loading paths. Therefore, the Itoh-Sakane model was modified to evaluation fatigue life of SUS316 and SUS430 under combined internal pressure and axial loading conditions.