This paper discusses evaluation of fatigue life of low carbon steel, type 400 steel under proportional and non-proportional multiaxial random loading. Fatigue tests were carried out using a hollow cylinder specimen at room temperature and an electrical servo controlled hydraulic fatigue testing machine for push-pull and reversed torsion loading, of which maximum push-pull loading and torque are r50 kN and r500 Nm, was employed as testing machine. In the test, three types of stress paths, a push-pull, a reversed torsion and a circle were employed. The circle loading is a cyclic loading combing the push-pull and the reversed torsion loading in which axial and shear stress waveforms have 90 degrees phase difference. In addition, these fatigue tests were carried out by using random waveform which simulate actual using situation of construction machine. From the obtained test results, evaluation method combined IS (Itoh-Sakane) method, rain flow method and modified Miner's rule was effective. In Miner's rule, by considering reduction in fatigue limit due to non-proportional loading, the evaluation method was also effective. From observations of specimen surface afterthe constant amplitude tests around the fatigue limit, the validity of these evaluation method under non-proportional multiaxial random loading was clarified.

In this study, it was investigated the fatigue strength characteristic of the hot work die steels. Test material was used the newly development hot work die steel which improved JIS SKD61. “N” sample which improved SKD61, “A” sample to which the production technique of “N” sample was changed and “B” sample which were reduce oxide and a carbide of “N” sample were used. As a result of static bending tests, the “A”, “B” and “N” showed the same strength. Result of four-point bending fatigue tests, fatigue strength of “A” and “B” samples in the low cycle region showed the same value as “N” sample. However fatigue strength of “B” sample in the high cycle region was higher than “A” and “N” samples. As a result of fracture surface observation, the fracture origin was aluminum or oxygen inclusion mainly. As a result of EBSD analysis, the size of the mass having the same crystal orientation in the “B” sample was smaller than that of “N” and ”A” samples.

In recent years, MEMS technology has been applied to sensor and electronic components. Nickel thin films have been used for main structural materials for MEMS devices made by LIGA process. Nanocrystallization of thin films will improve the fatigue strength, while the fatigue strength may become more sensitive to defects or stress concentration. In this paper, the influence of micro-notches on the fatigue strength was studied using electrodeposited nickel thin film with ultra fine grain (UFG) of 384 nm in grain size. Micro-notches were introduced to thin films with FIB. Micro-notches have four lengths ranging from 8 to 120 μm. The notch-tip shapes are two types: square type (MN type) and round type with radius of 1 μm (MNR type). The fatigue strength decreased with increasing notch depth. The fatigue strength is nearly equivalent between MN and MNR specimens in short-fatigue-life regime, while near the threshold, the fatigue strength is lower in MN than in MNR specimens because of smaller notch-tip radius. Below the fatigue limit, there is no non-propagation crack observed. The fatigue limit is controlled by the initiation of fatigue cracks. The influence of notch length on the fatigue limit of both type of specimens are predicted by the fictitious-crack model.

In general, the platinum alloy is used in glass production lines, due to an excellent corrosion resistance to melted glass at high temperature. On the other hand, the development of high quality glass is needed, because of wide use of glass application in recent years. So, Oxide-Dispersion-Strengthened Platinum alloy is developed which had better fatigue properties than the conventional one. However, there are few researches about the fatigue properties in a practical environment with axial force and bending stress. Therefore, in order to clarify fatigue properties and fracture mechanism of Oxide-Dispersion-Strengthened Platinum-10% rhodium with dispersed zirconium oxide particles, bending fatigue test at 1400℃, fracture surface observation and internal cross-section observation were performed. As a result of the fatigue test, it was clarified that the tendency of fatigue life with axial force depends on the times to failure rather than number of cycles to failure. Also, it was clarified by fracture surface observation, fatigue fracture was mainly under the condition of axial force 0MPa, and the intergranular fracture due to creep was mainly under the condition of axial force 23.4MPa. Furthermore, it was clarified that both wedge type and round type fracture were generated by bending fatigue test under the condition of axial force 23.4MPa.

In this study, the effects of surface roughness in additive manufactured Ti-6Al-4V alloy on fatigue strength were investigated. Fatigue specimens with as-build surface (as-build specimens) were manufactured using ARCAM Q20 Electron Beam Melting Machine. Fatigue tests and observations of fracture surfaces on HIP'd (Hot isostatic pressed) and Non-HIP'd as-build specimens were conducted. Fatigue lives of as-build specimens were shorter than those of reference data for specimens with machined surface. There were almost no differences between fatigue lives of HIP'd and Non-HIP'd as-build specimens. Porosities were observed on fracture surfaces of Non-HIP'd as-build specimens. On the other hand, there were no porosities on fracture surfaces of HIP'd specimens. Regardless of presence of porosities, fatigue cracks initiated from surfaces of the HIP'd and Non-HIP'd as-build specimens. Therefore, the surface roughness was more sensitive to the fatigue crack initiation than porosities and HIP did not improve the fatigue life of as-build specimen.

Aluminum alloy is used for the compressed hydrogen gas container of the fuel cell electric vehicle. Since aluminum alloys do not have fatigue limit, very high cycle fatigue property is important to obtain by experiment for safe design. Aluminum liners of the container is subjected to compressive mean stress by autofrettage process. The purpose of this study is to evaluate the effect of compressive mean stress on the very high cycle fatigue strength and to clarify the fracture mechanism for the aluminum alloys, 6061 and 6066. The evaluated fatigue strength is independent of the compressive mean stress and alloys of 6061 and 6066.However, the crack initiation and growth behaviors aredifferent. Under fully compressive cyclic loading, many stage I cracks are initiated, but they do not form a main crack. While tensile component in the cyclic loading contributes to the initiation of stage II cracks which grows by opening mode to be a main crack. Detailed observations are performed on the surface and the section of the specimens in order to clarify the mechanisms of crack initiation and growth.

The difference of the effects of residual stress and stress ratio on fatigue properties of S45C steel was examined. Two types of specimens were prepared, one is polished to mirror finish (P series) and the other is treated by fine particle peening (FPP series). The residual stress of the specimens was measured by X-ray diffraction residual stress measurement system. The surface roughness of the specimens was measured by a laser microscope. FPP series exhibited higher compressive residual stress and higher arithmetic average roughness than those of the P series specimens. Fatigue test were performed under axial loading and with different stress ratio of R =-1 and 0. Equivalent stress amplitude calculated by Smith-Watson-Topper (SWT) method was used to correct for the effect of stress ratio. Fatigue lives of the P series and FPP series were comparable because of the generation of compressive residual stress and increase of surface roughness. In the case of P series specimen, there were difference in fatigue lives between stress ratio R = -1 and 0. Modified SWT equation was proposed with considering the effect of residual stress.

A new method for strengthening of magnesium (Mg) alloys, Deformation Restricted Forging (DRF) processing, has been invented by Miura et al. High tensile strength over 400 MPa can be attained without any rare earth additions nor special heat treatments. However, the fatigue characteristics of Mg alloys fabricated by DRF processing have not been investigated. For this purpose, rotating bending fatigue tests, static tensile and compression tests were carried out using two types of Mg alloys, i.e., commercial AZ80Mg and DRF-AZ80Mg alloys. As a result, tensile strength and compressive proof strength of DRF-Mg were improved by 80 MPa and 85 MPa respectively compared to those of commercial AZ80Mg alloy. Fatigue characteristics were, however, hardly improved. Generally, in steel materials, it is believed that there is a close correlation between fatigue limit and static strength, where fatigue limit increases with increasing static strength. The relationship between static strength and fatigue strength of the commercial AZ80Mg was proportional and followed the equations statistically of σ_w =0.87σ_0.2c. obtained from the past researches. The characteristics of DRF-Mg alloy, however, did not. Accordingly, fatigue characteristics of DRF-Mg alloy has been found to exhibit entirely different from conventional commercial AZ type Mg alloys.

Aiming at an enhanced fatigue strength of quenched and tempered SCM435, surface treatment called cutting and rubbing process was developed. In the processed specimens, the compressive residual stress on the surface decreased at the beginning of the rotating bending fatigue tests but remained at a high value of -1800 MPa or more even after 10⁷ cycles at the fatigue limit stress. Consequently, fatigue limit stress of the processed specimens was enhanced to approximately 1.5 times of a smooth specimen finished with #1000 emery paper, and most of the fractures were fisheye fractures. Then, fatigue tests were carried out on the processed specimens from which residual stress was removed by heat treatment. As a result, it was found that the effect of compressive residual stress is larger than that of the processing layer for enhancing fatigue strength. In order to investigate the effect of residual stress on the internal fatigue crack of the processed specimen, the fatigue test was carried out to 5×10⁷ cycles at the fatigue limit stress. The internal crack was not obtained by electrolytic polishing and replica methods. However, the actual fatigue limit is much higher than the calculation result obtained from the fatigue limit prediction equation, which implies that the compressive residual stress near the surface layer promotes non-propagating internal fatigue cracks.

The elastoplastic constitutive equation with the damage is formulated incorporating the subloading surface model. Further, the simple method for the incorporation of the unilateral damage phenomenon is further proposed by proposing the relevant unilateral damage tensor in this article.

Research on “Mode II fatigue crack propagation” is being conducted by many researchers. However, its understanding has not progressed as much as that of Mode I. We think that this cause is the name: “Mode II fatigue crack propagation.” Originally, Modes I and II represented the loading modes for still cracks in fracture mechanics, and not the fatigue crack propagation mechanism. There are many cases where the knowledge of fatigue crack propagation under Mode I loading is applied to that under Mode II loading without consideration. The rolling contact fatigue is a typical example of fatigue crack propagation under cyclic Mode II loading. In the region where the fatigue crack propagates, a large plastic deformation is caused by the rolling contact load. Therefore, it is necessary for the test method to reproduce the effects of an actual machine to test materials that exhibit large plastic deformation. Therefore, in this study, we aim to classify the fatigue crack propagation phenomena, regardless of Mode I and II loadings, and examine the mechanisms. To that end, we developed a novel test method that enables pure cyclic Mode II loading. We used a micro-thin film disc as a specimen, making it possible to cut out and test a part subjected to a large plastic deformation from the actual machine. By successive observation of the fatigue crack propagation behavior, we propose a crack propagation mechanism, namely, damage accumulation type fatigue crack propagation under Mode II loading, which is different from the opening type fatigue crack propagation.

This paper describes application of singular integral equation method to crack moving near an inclusion in two-dimensional infinite plate which is subjected to a tensile loading at infinity. The theoretical formulation is based upon the superposition of the problem of a continuous distribution of edge dislocations spread along the crack locus in an infinite plate with single inclusion and the problem for the same geometry without crack which is subjected to a tensile loading. The superposition leads to simultaneous singular integral equation, and they relate the surface zero-tractions to the dislocation densities along the curved crack locus. The stress intensity factors are derived directly from the crack-tip stress field. Then, the crack tip is automatically moved to a direction as satisfying the restriction that the stress intensity factor K_II is zero, which was developed in this study. The direction search is conducted by rotating the tip of virtually incremented crack around the origin crack tip. The search and extension processes are repeated in sequence. In this numerical calculation, the influence of the initial crack locations on crack moving path is discussed.

The purpose of this study was to investigate the fatigue properties of friction stir welded (FSW) joints of flame-retardant magnesium alloy (AZX 611) and to examine the effect of improving fatigue properties by zirconia shot peening treatment. The specimens were taken from the FSW joints plates so that the welding center and specimen center coincided. After sampling, the tool marks by FSW were removed by machining. The SP treatment was carried out with a zirconia shot (B 120) having a particle diameter of 0.12 mm. Fully reversed (R = -1) plane bending fatigue test were carried out using the base metal (BM) specimen, surface finished FSW joints (FSW-f) specimen and SP treated specimen. Surprisingly, the fatigue strength at 2×10⁷ cycles of the FSW-f was improved by about 15 MPa over of the BM specimen, and the fatigue life was also extended. The factor of fatigue property improvement of the FSW-f specimen was work hardening on the surface of the specimen when machining at the time of removal of tool marks. With this machining, high level compressive residual stress and surface hardening were induced to specimen. And then, fatigue property of BM specimen and FSW-f specimen were improved by the SP treatment. For the BM specimen, SP treatment could improve the fatigue strength and fatigue life property by peening effect. On the other hand, for the FSW-f specimen, only fatigue strength at 2×10⁷ cycles was improved.

Lap joints between an A6061 Al alloy plate and a CFRP plate coated with a stainless steel wire mesh were fabricated using the friction stir spot welding (FSSW) technique. Three kinds of meshes, an SUS316L single layer mesh (316SL), an SUS316L double layer mesh (316DL) and an SUS329J1 single layer mesh (329SL), were used. Fatigue tests were conducted using the fabricated Al/CFRP joints and fatigue behavior was investigated. All of the three joints exhibited comparable fatigue strengths in the low cycle fatigue region, while the fatigue strength of Al/CFRP316SL was lower than those of Al/CFRP316LDL and Al/CFRP329SL. This indicates the fatigue strength of the joint is dependent on the strength of the mesh that coats the CFRP.

In the automobile industry, multi-material structure of car body is progressing with the aim of reducing CO₂ emission and reducing fuel consumption. As a joining technique, adhesive bonding joint is possible to bond dissimilar materials, and since it is surface joining, it is considered to be useful. However, adhesive bonding is a complementary technique for other joining approaches due to its instability of reliability and durability, and improvement of strength of the joint itself. Thus, it is expected that the interfacial strength will be improved by removing the weak boundary layer on the surface of the adherend and using the anchor effect and the increasing in surface area produced due to the uneven shape. Therefore, in this study, in order to improve the interface strength between the adherend and the adhesive, the laser patterning treatment was applied as the surface treatment, fatigue properties of the adhesive bonding joint were evaluated and were compared to the untreated specimens. As a result, the static and fatigue strength of the joint were improved by the surface treatment. In particular, the joint strength was further improved by removing the weak boundary layer on the entire surface as the laser irradiation. Therefore, processing conditions have been established based on usefulness of surface treatment in fatigue strength of joint with high strength and high reliability.

In the automobile industry, the thinned materials were applied to reduce the weight of the car body. Thus, the application of high-strength steel and resin material are currently expanding. At the same time, it is nessesary to develop a new welding method. The laser welding has attracted attention to weld high speed and low heat input to high-strength steel, So, the fatigue characteristics and a fracture mechanism of laser welded joints to high-strength steels are important from the viewpoint of reliability and safety. However, there researches of the joints in the automobile bodies are limitted, and it is required to expand the application range of laser welding in the future. Therefore, in this study, the fatigue properties of single lap laser welded joints were evaluated. Moreover, this study was focused on the loading mode of the lap joint, the pure tension-shear type in which the jig is restrained in the welded area was created and compared to the conventional lap joint. As a result, the static and fatigue strength of the joint was improved by the pure tension-shear type restraining the jig. In addition, the fatigue life of each joint was not uniformaly evaluated by the equivalent stress intensity factor ΔK_eq.

Steam leak accidents occur frequently at weld zone involved in pipes in aged thermal power plants. It is known that the weld zone has heterogeneous microstructure and causes cracking due to thermal stress distribution. Therefore, the material property associated with such a microstructure must be clarified to evaluate precisely crack initiation. Indentation test method is an effective technique for measuring the localized material property. Thus, our group has been established evaluation scheme of the material property by the indentation test. However, the evaluation scheme does not take into account the influence of the indentation rate. In this study, instrumented high-temperature indentation test machine, which can control the indentation load speed by an electric actuator, was developed. Using this machine, the high temperature indentation test was conducted to estimate the yield stress of a heat-resistance alloy Ka-SUS410J3. In addition, the influence of indentation rate on the yield stress was investigated. As the results, it was confirmed that the yield stress was significantly influenced by the indentation rate and the yield stress can be appropriately estimated within the indentation rate of 0.2 - 0.5 mm/min in comparison with a tensile test.

The dual indentation technique has been proposed in order to evaluate stress-strain curve, and it is very effective for tiny specimen. For a reliable establishment of this technique, it is very important to define the applicable range. This is because the evaluation error is not small enough for stainless steel. Thus the authors proposed a critical strain which shows the effective limit of stress-strain curve obtained by this technique. First, a sharp indenter penetrated into the original power-law material and several materials which assigned perfect plastic behavior after a bifurcation strain, respectively. When the error of the loading curvature between the original material and the modified material became less than 1%, the bifurcation strain of that modified material was defined as the critical strain. If two materials that have different stress-strain relationship in the larger strain range than the critical strain are penetrated, those load-displacement curves show the same behavior. In other words, those materials can not be distinguished by the dual indenter technique. According to the parametric study with the finite element simulations, the critical strain depends on the indenter angle, the work-hardening exponent and the ratio of the Young's modulus and the representative stress. Finally, the authors proposed the equation which gives the critical strain for a wide range of the engineering materials.

Sensitivity analysis of an indentation test was performed to confirm the effectiveness of inverse estimation using a single indent. Mechanical properties such as a stress-strain curve play an important role in product design and evaluation. Many studies have been addressed to develop inverse estimation techniques to obtain mechanical properties using indentation tests instead of a tensile test, most of which require plural load-displacement curves using indenters of different apex angles, that is, plural indentation tests due to the uniqueness problem. However, plural tests are time-consuming and does not give local properties on a specific point of interest. In this work, the authors investigated the dependency of indentation results on the elastoplastic properties, in particular, on yield stress and strain hardening exponent. Finite element analyses were performed to simulate an indentation test with a Berkovich indenter, with changing yield stress and strain hardening exponent. Two parameters of a coefficient of a loading curve, C and a maximum pile-up height, Z_max/h_max showed different dependencies on the elastoplastic properties, which indicates that the yield stress and strain hardening exponent are uniquely determined from C and Z_max/h_max obtained from a single indentation test. This technique can estimate the elastoplastic properties with higher reliability at lower strain hardening exponent, because Z_max/h_max had higher sensitivity in the region.

This paper provides effects of removal amount of surface with crack introduced by indenter on the fracture toughness for the single crystal silicon wafer. Removing the residual stress should be required in CSF method for evaluation of the fracture toughness in brittle materials. It attempts to clarify the effects on the fracture toughness by using ion shower which can control the removal amount of the surface with micro crack.

Fracture toughness of cleavage plane of single crystal silicon was evaluated by CSF method. Effects of removal amount of the surface and crack shapes on the fracture toughness were evaluated experimentally by using ion shower. The validity of the fracture toughness value for each indentation load was considered by comparison of both results of CSF and IF methods. An appropriate amount of surface removed was proposed for evaluating the fracture toughness.

Hydrogen embrittlement (HE) in steel has been recognized as a problem due to the remarkable reduction of mechanical properties. Regarding the mechanism of hydrogen embrittlement, the influence of solute hydrogen on the behavior of lattice defects has drawn attention, it can not be said that the whole picture has been elucidated yet. Therefore, in this study, the influence of hydrogen concentration on plastic deformation process in polycrystalline S25C ferrite phase was evaluated using nanoindentation method. The depth of indentation was set to 50 nm. Hydrogen charge was carried out using FIP bath. In order to evaluate the effect of hydrogen concentration, we change the exposing time to the atmospheric environment from 0 to 9 days (day 0, 1, 2, 9) after by hydrogen charging. And the specimen without hydrogen charging was also prepared as comparison. The obtained load - indentation depth curve represents the influence by hydrogen, it hardened on the day 0 to the uncharged material, softened on the day 2 and finally the hardness recovered to the original condition on the day 9. Furthermore, hydrogen induced work amount per unit volume after 50 nm indentation was calculated. As a result, a similar tendency was confirmed. These results indicate that the ferrite phase in S25C hardened when the hydrogen concentration is high, softened when the hydrogen concentration is low, and returns to the original condition when the hydrogen concentration is extremely low with the indentation depth of about 50 nm.

This paper presents the creep damage evaluation of Mod.9Cr-1Mo steel using miniature specimens. Creep damaged materials were produced by interrupting creep tests at 600°C performed by using standard specimens. Uniaxial creep tests were conducted using miniature specimens with 1mm diameter taken from the gage portion of standard specimens at 600°C, to assess the creep damage of the standard specimen. It was found that times to creep rupture of miniature specimens were longer than those of standard specimens.

This paper studies the creep-fatigue life evaluation of Sn-3.0Ag-0.5Cu, Sn-58Bi and Sn-5Sb using miniature specimen under push-pull loading using fast-fast, slow-slow, fast-slow, slow-fast and strain-hold strain waves. Several creep-fatigue data were generated using these strain waves and the effect of strain waveform on the creep-fatigue life of each lead-free solder miniature specimen was discussed. Stress-strain relationship and failure life of Sn-3.0Ag-0.5Cu miniature specimen were compared with those of Sn-3.0Ag-0.5Cu bulk specimen which were quoted from the database of the Japan Society of Material Science. The applicability of two creep-fatigue damage rules, the linear damage rule and the grain boundary sliding model for Sn-3.0Ag-0.5Cu miniature and bulk specimens, was examined. The grain boundary sliding model was originally developed for Sn-37Pb bulk specimen and assumed that the specimen failed when the sum of the grain boundary sliding in each strain wave reached to a critical value. The linear damage rule correlated the creep-fatigue lives of Sn-3.0Ag-0.5Cu miniature and bulk specimens with a large scatter. By contrast, the grain boundary sliding model correlated the lives of both specimens within a factor of 2 scatter band.

In recent years, materials used in automobile engines are required to be used in harsh environments, and there is concern that the material strength will be lowered due to corrosion caused by deteriorated fuel and increase in pressure. Therefore, this study conducts the fatigue test in the same environment as actual condition. The test uses type 440C martensitic stainless steel under corrosive atmospheric environment which is used as a part of automobile engine. However, this type of testing machine had not been available in past. Therefore, a small sized tension and compression fatigue testing machine for fatigue tests under corrosive and air environments were developed. In addition, this study established tension and compression fatigue testing method. This machine can also carry out the tests using a small sized specimen in flammable liquid, and allows testing at high frequencies. Then, the fatigue strengths in air, regular petrol, and deteriorated petrol conditions were compared to examine the effect of corrosion. The fatigue life in the regular petrol was almost equivalent to that in the air environment regardless of the stress amplitude. However, in the deteriorated petrol environment, the fatigue life was largely reduced compared to former two conditions which will be discussed from the fracture surface morphology.

This study focuses on ductile fracture in a low-stress triaxiality. Fracture in a low-stress triaxiality does not occur in a common tensile test, because necking induces a high-stress triaxiality. Therefore, for fracture strain evaluations in a low-stress triaxiality, a variety of sheet-type specimens are employed, so that their shapes induce a low-stress triaxiality on the ductile fracture site. Such analyses provided fracture strains in a low-stress triaxiality through numerical simulations, showing irregular inflection points in the fracture strain locus. However, these irregular inflection points are not consistent with the fracture strain locus of bulk metals evaluated in a Bridgman's round bar tensile test, i.e., tensile test in a high-pressure fluid. Therefore, the validity of fracture strain measurements with a low-stress triaxiality must be discussed. The compact tensile machine developed in Osaka university enables tensile testing of a small round bar under hydrostatic pressures of up to 700 MPa. We used this machine in this study. Fracture strains under hydrostatic pressures of 200 MPa and 400 MPa were evaluated in two types of 590 MPa-class steel sheets with 1.2 mm thickness. The small round bar specimens with minimum diameters of 1.0 mm were cut from these thin steel sheets. These tensile tests revealed a monotonic increase of fracture strains with respect to the imposed hydrostatic pressures. The results are consistent with those in Bridgman's tensile tests. Conversely, they are not consistent with the fracture strains evaluated by using sheettype specimens.

Surveillance tests (Charpy impact tests) are employed to evaluate material degradation of Reactor Pressure Vessel (RPV) by neutron irradiation. According to the new regulation standard concerning the extension of nuclear reactor operation, at least two additional surveillance tests will be required. Application of embrittlement evaluation technology of RPV using sub-size Charpy impacte test specimen is under verification. In applying the sub-size Charpy test specimen to the RPV embrittlement evaluation, it is necessary to confirm the compatibility with the data acquired by full-size Charpy test specimen. In this paper, conversion equation based on area of fracture surface is examined and proposed.

Small punch tests were conducted for carbon steel specimens prepared with and without cold working. A digital image correlation technique was applied to measure displacement. Finite element analyses using the true stress-strain curves including post-necking strain could reproduce the load-displacement (LD) curve. Use of the nominal stress-strain curve obtained by the conventional tensile test was not enough to simulate the maximum load. It was found that the maximum load did not depend on the ultimate strength. Therefore, the ultimate strength prediction using the maximum load in the LD curve may not be valid for the general case. On the other hand, the yield strength correlated well with the deflection point in the LD curve. A new equation was proposed to estimate the yield strength that takes into consideration the influence of the specimen thickness.

Shear punch test specimen, whose dimensions are small as 8 mm-diameter × 0.5 mm thickness, is recently closed up as one of the alternative for tensile specimen. In this study, a Japanese reactor pressure vessel (RPV) steel was subjected to the shear punch test at temperatures from –120°C to R.T. using a shear punch test frame newly developed and the capability of the test technique for uniaxial tensile property evaluation was investigated. Shear punch test specimens with a dimension of 8 mm-diameter and 0.5 mm of a thickness were machined from the Japanese RPV steel; SQV2A plate. A puncher with a diameter of 3.0 mm gives compressive load to center of the specimen and then the shear stress is generated in the specimen. The tests were continued in stroke control mode (0.25 mm/min) until the load remarkably drops at the specimen failure. Stress – strain curves were successfully obtained at temperatures from –120°C to R.T. and the shear punch yield stress could be evaluated from the stress – strain curve. The shear punch yield stress increased with decreasing test temperature and this temperature dependence is similar with that of uniaxial yield stress. From this result, the capability of the shear punch test for tensile property evaluation was demonstrated.

An influence of miniature sample scoop on creep life of ASME Grade 91 steel piping was experimentally assessed in this work. Internal pressure tests were conducted on tubular type specimens having defects on outer surface of the materials, which simulate the sampling scoop. When a depth ratio of the defect to a wall thickness of the specimen is about 17%, creep life decreased to 2/3 of that of no defect specimen. On the other hand, no creep life reduction was observed on the specimen of the depth ratio 5%, while stress concentration around the defect was revealed by FE analysis. The result suggested that the miniature sample up to 5% of the thickness direction of pipings scarcely has the influence on the creep life of the Grade 91 steel pipings.

In the previous study, the small punch (SP) creep testing technique was applied to five heats of Gr.91 steel, which had been actually used for boiler pipings in the different USC power plants for long periods of time. As a result, it was found that the difference in creep strength between the heats, that is, the heat-to-heat variation, was well reflected in the SP creep property also. In this study, the SP test was carried out at room temperature and 650°C to obtain the load/stress conversion coefficient (F/σ) which is necessary for the conversion of SP creep rupture test results to the uniaxial ones. The F/σ for each heat was estimated by the displacement to maximum load (u_m) using the correlation F/σ = u_m h₀/320. The experimental results revealed that the F/σ showed no significant change and it was 2.4-2.5 irrespective of heats. Additionally, it was also found that the SP creep rupture data were relatively well converted to the uniaxial one with those coefficients.

Creep damage in structural components under elevated temperatures leads to a severe failure and / or fracture. In order to prevent these accidents and to improve the integrity of components, more reliable methodologies have been required. Small Punch (SP) creep test has been proposed as a semi destructive testing method which employs small disc specimen. Polycrystalline materials were mainly examined in the SP creep test so far, and there are few reports where the anisotropic material was employed. A series of the SP creep test were conducted at 720oC using very small disc specimens of a Ni-base directionally solidified (DS) superalloy; Inconel 713C. The disc-type SP creep specimen has 5 mm in diameter and 0.25 mm in thick, and was cut out along longitudinal (LD) and transverse (TD) directions of the DS ingot. Experimental test results show that the creep rupture lives cut along the TD are one and a half times or twice as long as those cut out from the LD due to the hardness of breakage across the directionally crystalline. It was also found that the rupture life of the specimen of the TD depends on Dendrite Arm Spacing, i.e., finer the DAS is, longer the rupture lives are. Then, the SP creep test should be performed attentively checking the DAS if the anisotropic material is employed.

The small bulge fatigue (SBF) test equipment using a small disk-type specimen with flat and concave surfaces has been developed by the author’s group. In this study, the crack initiation/growth behavior during the SBF test and its strain range dependence were investigated for establishing this method as a fatigue test. The experimental results revealed that the crack growth behavior in the SBF test was strongly dependent on the strain range, and its rate was faster than that in uniaxial fatigue test due to the effect of biaxial stress/strain. However, the crack growth behavior was well correlated with the normalized number of cycles, which was normalized by the fatigue life, and it was found that the number of cycles to the crack penetration corresponded to 35% of fatigue life irrespective of strain range.

PZT piezoelectric ceramic has been widely used in our society, such as buzzers, actuators and sensors. Furthermore, in recent years, PZT piezoelectric ceramic has been employed as the energy harvesting system. However, piezoelectric ceramic has technical issue that has low piezoelectric properties compared to other energy harvesting system. Especially, low efficiency of electric power generation is problem. Thus, in the present work, the electric power generation was analyzed by changing the size of loading fixture (changing the contact area) on the PZT piezoelectric ceramic to improve power electric generation efficiency. From the present work, the electric power generation was basically decreased with increasing the contact area. However, the high electrical properties were obtained as loaded with the surface contact of less than φ5 mm loading fixture. Namely, the electrical generation properties for φ5 mm fixture were similar to that for the point contact. Such high electrical properties were explained by the deflection, bending strain, and total strain of the PZT ceramic. On the basis of the experimental results, suitable loading conditions to obtain high electric voltage and long material life were proposed considering the energy harvesting system.

Metal-core piezoelectric fiber/aluminum composite is expected as a candidate of sensor used for structural health monitoring (SHM). In SHM, even if a large load is applied to a structure due to a disaster such as earthquake, the sensors are required to function. This composite material with high mechanical properties is thought to be suitable for its use, however, the characteristics in such a large strain region are not clear. In this study, the output of this composite when applied tensile vibration was evaluated material and the output characteristics of this composite in the large strain region were clarified. As a result, it was found that this composite works even at extremely high strain of about 20%. In addition, it was found that it can be classified into the first stage, which decreases with increasing strain added by the output, and the second stage, which is almost constant output regardless of the strain increase / decrease. Finite element analysis revealed that the decrease in the output of the first stage is caused by the stress reduction occurring in the piezoelectric ceramics due to the fracture of the piezoelectric ceramics. And it is considered that the constant output in the second stage is caused by the stress transmitted from the matrix becomes constant due to fragmentation of the piezoelectric ceramics.

Piezoelectric thin films, which are widely used for actuators and sensors in various devices, generally have a laminated structure consisting of a substrate, buffer and piezoelectric layers. If the piezoelectric layer is fabricated at high temperature, some residual stresses occur due to difference of thermal expansion among layers. In case of functional materials, the residual stresses, which often reduce strength of structural materials, have the possibility to enhance piezoelectric performance by coupling effect. In this paper, the residual stresses in piezoelectric thin films were analyzed and their influence on piezoelectric performance was computationally revealed. Additionally porous ceramics were introduced into the buffer layer, and then elastic modulus and thermal expansion coefficient of the buffer layer were changed by porosity. A multiscale simulation based on homogenization theory was employed to evaluate material properties of porous buffer layer, and finite element simulation was applied to estimate the performance of piezoelectric thin film. The influence of porosity on material properties and the performance of piezoelectric thin film on porous buffer layer and substrate were investigated and the effective condition was discussed.

In this paper, the fracture problem of a functionally graded piezoelectric material strip (FGPM strip) containing a crack perpendicular to the interface between an FGPM strip and a homogeneous layer is considered. The problem is solved for the laminate that is suddenly heated from the surface of the FGPM strip. The surface of the homogeneous layer is maintained at the initial temperature. The crack faces are supposed to be completely insulated. Material properties are assumed to be exponentially dependent on the distance from the interface. By using the Laplace and Fourier transforms, the thermo-electro-mechanical fracture problem is reduced to a singular integral equation, which is solved numerically. The stress intensity factors are computed and presented as a function of the normalized time, the nonhomogeneous and geometric parameters.

In order to make functionally graded piezoelectric materials, a forming process using electrophoretic deposition (EPD) was developed. Two types of PNN-PZT calcined powder were used as materials. The piezoelectric ceramics could be deposited on copper electrode immersed in the ethanol suspension with calcined PNN-PZT powder. The 3-point bending tests was carried out on sintered bilayer material samples formed by EPD. It was found that the strength characteristics of the bilayer material was similar to that of the weaker single layer material. Multilayer piezoelectric ceramics were formed by EDP using several suspensions in which mixing ratios of two kinds of ceramics were changed. On the multilayer material, it was confirmed that the mixing ratio of the two piezoelectric ceramics gradually changed by XRD. The cantilever type monomorph actuator with multilayer piezoelectric ceramics was made by using EPD and confirmed its driving under alternating voltage.

This study deals with a one-dimensional dynamic thermoelastic problem in a functionally graded piezoelectric thin film. It is assumed that all response quantities are initially zero. The top surface of the thin film is kept at the reference temperature and the bottom surface is subjected to a sudden temperature rise. For the elastic field, the top surface is fixed to a flat rigid body and the bottom surface is free of stress. For electric field, a voltage is applied between the top and bottom surfaces. The variation of material properties is assumed to be expressed as exponential functions of the space-variable. First, the transient heat conduction equation is solved under the thermal initial and boundary conditions. Solving the equation of electrodynamics under the electrical initial and boundary conditions, the dielectric flux density is derived. The elastic constitutive equation and the equation of motion which govern the particle velocity and thermal stress are solved by employing the method of characteristics. Numerical results are obtained when the material at the top surface of the thin film is taken to be PZT-4. Time histories of thermal stresses at the middle of the thin film are illustrated graphically. The effect of the applied voltage on the thermal stress is discussed.

In this study, a one-dimensional dynamic thermoelastic problem of a functionally graded piezoelectric thin film is analyzed mathematically. It is then assumed that the top surface is fixed to a flat rigid body, while the bottom surface is subjected to thermal shock. The voltage between the top and bottom surfaces is prescribed. Assuming that the variation of material properties is given as exponential functions of the space-variable, an analytical solution to the dynamic thermoelastic problem is derived by means of the space-variable transform as well as of the Laplace transform with respect to the time-variable. Numerical results for the dynamic stress are obtained when the material at the top surface of the thin film is taken to be PZT-4. Finally, the time history of the dynamic stress is illustrated graphically.

This paper describes the results of our numerical study of the electromechanical and thermoelectric responses in functionally graded piezocomposites (FGPCs) under mechanical load and temperature change. A finite element analysis (FEA) is employed to simulate the bending behavior by concentrated and cyclic loads and the deformation by temperature change of FGPCs. A phenomenological model considering the temperature dependence of piezoelectric constants and permittivity is used in the calculation, and the effects of property gradation on the deformation, internal stress and induced voltage of the FGPCs are examined.

With the appearance of the wearable devices and Internet of Things (IoT) sensors, energy harvesting devices are becoming more and more important. Energy harvesting is a process that captures unused ambient energy. We present the results of output voltage measurements in lead-free piezoelectric particle/polymer composites due to air-flow or temperature change. Barium titanate (BTO)/epoxy composite and potassium sodium niobate (KNN)/epoxy composite sheets were prepared. The composite specimens were polarized using either a parallel plate contact or contactless (corona) polarization technique. The output voltage was then measured for the composites due to air-flow and temperature changes. As the same time, the effect of polarization condition on the performance was discussed.

In previous studies, PVDF piezoelectric film was fabricated and the output voltage was measured under impact force. In this study, we will improve the method for fabricating PVDF membrane and annealing temperature. It was found that the higher the annealing temperature, the higher the output voltage. In addition, the stability of the PVDF film produced by this method is improved.

In this study multiscale finite element simulation was applied to multiferroic composite materials consisting of ferroelectric (FE) and ferromagnetic (FM) phases to design the material combination and the microstructure. An asymptotic homogenization theory was employed for scale bridging to analyze macroscopic homogenized physical properties and microscopic response against external loads. While barium titanate and PZT were picked up for FE phase, cobalt ferrite was selected for FM phase. Three-type microstructures, a random polycrystalline structure, a layered polycrystalline structure, and a layered single-crystal structure were used for computation. Firstly the relation between the content of FE phase and macroscopic homogenized physical properties was investigated for every condition to find the optimum content for maximizing macroscopic magnetoelectric effect. The influence of material combination and microstructure on magnetoelectric (ME) effect was compared. The effective microstructure and its advantage were discussed to emphasize macroscopic ME effect.

Energy harvesting devices from ambient energy (e.g. shock, vibration, thermal energy) have drawn a lot of attentions due to Internet of Things (IoT). The IoT comprises everyday objects, lightness, stiffness and energy conversion efficiency are required for the devices. In this study, we fabricated as-drawn or annealed magnetostrictive FeCo wires dispersed epoxy resin matrix (FeCo_w/Epoxy) composites and investigated their output voltage density by compression tests. The compression tests were carried out using five cyclic programs with some constant velocities. In addition, the output voltages were predicted by simple nonlinear magnetomechanical coupling model of the FeCo_w/Epoxy composites. The measured output voltage densities of FeCo_w/Epoxy composite with tensile residual stress was consistent with the predicted values. It seems that our model is available to design novel magnetostrictive wire/polymer composites and predict their capability as a sensor and an energy harvester. Moreover, the output voltage density of annealed FeCo_w/Epoxy composite was larger than that of as-drawn one. This result clearly showed that the inverse magnetostrictive characteristics of FeCo wire was improved by the heat treatment. In addition, the magnetostrictive composite design utilizing the characteristics of FeCo wires enable to develop energy harvesting devices. This study indicates that FeCo is expected to be use in sensors and actuators in severe environment because it can withstand high pressure and temperature.

We evaluated the inverse magnetostrictive response characteristics of carbon fiber reinforced plastics (CFRP) composite laminates with FeCo fibers, which have different diameters. The results of static bending test revealed that the diameter difference of FeCo fibers affected the magnetic flux density change of FeCo fibers inserted CFRPs. Therefore, it seems that the microstructure of FeCo fibers dispersed CFRPs dominates their magnetostrictive properties and the microstructure optimization of FeCo fibers dispersed CFRPs improves their capability as an energy harvesting material.

The aim of present study is to develop a novel energy harvesting device and investigate its properties. First, magnetostrictive Fe-Co wire/Al-Si matrix composites are fabricated by casting method. Then, using the composite, coil and magnets, a device is designed for vibration energy harvesting. Second, an original impact loading machine is developed by improving the Izod impact test machine. Finally, the induced voltage is measured for the energy harvesting device under impact loading by original impact loading machine.

Fe-Co alloy has recently attracted attention becase the alloy has high Curie temperature, high saturation magnetization and high strength. Fe-Co has a small amount of magnetostriction compared with conventional magnetostrictive materials, but it has been reported that high magnetostriction is expressed by appropriate heat treatment. In this study, we investigated the characteristics of the magnetostrictive actuators by using the laminated Fe-Co alloy composites. The raw materials were melted in a furnace, and the melted metals were forged into original ingots by hot and cold rolling for utilizing a thermal diffusion generated in the bonding surface under the melting temperature of the matrix. We revealed that the deflection of the laminated Fe-Co/Ni composite is larger than that of the laminated Fe-Co/Fe composite when an electric current is applied to the laminated composite. In the case of the laminated Fe-Co/Ni composite, deflection of about 1 mm was obtained at about 14 A.

An unsteady thermal stress analysis of an inhomogeneous rectangular plate in which material properties are given in a form of power of a coordinate in the thickness direction has been developed. Analytical solution of temperature change in the plate with arbitrary inhomogeneous material properties has been derived when the plate is subjected to sinusoidally time varying temperature change on the upper surface, and is kept at zero temperature change on the bottom surface. Thermal stresses and deflection of the plate due to the temperature change are developed theoretically under the simply supported condition at the whole circumference. Performing numerical calculation, the effect of inhomogeneity in specific heat capacity on the thermoelastic behavior of the rectangular plate is discussed.

In this paper, vibration and sound absorption properties of poroelastic lattice structure are investigated through numerical simulation and measurement experiment. The lattice structures discussed in this paper are composed of slender beams only (BCC) and beams and shells (BCC-S), and can be fabricated by using the selective laser metal melting machine. In our numerical investigation, the homogenization method is used to calculate a macroscopic elastic tensor for a solid phase, an equivalent density and a bulk modulus for a fluid phase. Also, the finite element calculation is used to obtain the natural vibration frequency of the lattice structure. The obtained numerical result of the sound absorption coefficient agrees fairly with the experimental result.