In recent years, the development of car body structures using lightweight materials such as an aluminum alloy has progressed by taking into consideration environmental issues. Laser welding of aluminum alloys has been widely applied due to the commercialization of high-power fiber lasers. The application of welding techniques in actual structures requires a high level of safety and reliability. Thus, it is necessary to evaluate the fatigue fracture behaviors of the welds, which account for the majority of fractures. This study focused on the fatigue limit estimation method based on dissipated energy. The dissipated energy is a small heat generation caused by local plastic deformation. In recent years, the estimation method of fatigue limit by the dissipated energy is being considered for application in the industrial fields through the development of infrared sensors of high sensitivity and lock-in processing. This paper discussed the applicability of the estimation methods of the fatigue limit for the laser welds of aluminum alloys based on the dissipated energy measured using infrared thermography. The laser welds of aluminum alloys were fractured at the boundary between the weld metal and heat-affected zone. The fatigue limit of the laser welds was lower than that of the base metal specimen. In the measurement results of dissipated energy, the estimated fatigue limits of the laser welds and the base metal specimens were consistent with the fatigue limit by the fatigue tests. Thus, the fatigue limit of laser welds of aluminum alloys could be estimated by the dissipated energy.

In the same materials, film and bulk materials show different mechanical properties. In fatigue testing of metallic thin film, several studies have been reported for axial and bending directions, however, there are few studies for shear direction. In addition, multiple parallel slips in the same grain are considered out-plane direction, a single linear slip in a grain is considered in-plane direction. Therefore, in this study we conducted fatigue tests under cyclic in-plane pure shear loading to S10C and S45C.We focused on the effect of pearlite volume fraction on slip generation and the relationship between cracks and slips. As a result, about 90% of the slip that occurred in S10C was out-plane direction. About 70% of the slip that occurred in S45C was in-plane slip. This is considered to be caused by the concentration of strain due to pearlite. In addition, more cracks were observed in S45C than in S10C, Consequently, cracks in the carbon steel film are considered to have grown from in-plane slip.

Rolling contact fatigue cracks propagate in mode II or mixed mode of I and II owing to the cyclic shear stress under multiaxial compressive stress states at the subsurface. No standardized evaluation method for shear mode crack propagation has been established. A method to evaluate the rates and behaviors of rolling contact fatigue crack propagation was studied using twin disc type rolling contact fatigue tests on railway wheel steels with artificial defects and finite element analyses (FEA) simulating the tests. The rolling contact fatigue test results indicated that the cracks initiated from the leading side of defects propagated in the depth direction, whereas those initiated from the trailing side propagated in the surface direction. Calculation results of the stress intensity factor (SIF) indicated that the cracks initiated from the leading side of defects propagated in the mixed mode, whereas those initiated from the trailing side propagated in only mode II. In addition, the crack propagation rates in the trailing side of defects accelerated with increasing number of cycles. This was because of the increase in the shear mode equivalent SIF during the crack propagation. Thus, the results of FEA corresponded to those of rolling contact fatigue tests. Therefore, the relationship between the rolling contact fatigue crack propagation rate and the shear mode SIF range could be evaluated by combining twin disc type rolling contact fatigue tests using test specimen with artificial defects and FEA.

In the present study, a practical methodology for creating and formulating a master diagram which is utilized for fatigue strength estimation of machine parts and structural elements is discussed. The consideration proceeds based on the correlation parameter between fatigue strength of smooth and notched specimens “Equivalent cyclic stress ratio R_EQ” that is followed by “Size-dependent magnification factor F_Sk^M1”. The master diagrams originally represent the axial load fatigue strength diagrams of smooth specimens and therefore they are determined as material-specific diagrams without depending on the shape and dimensions of the cross-section. However, they are mainly derived from axial load fatigue strength data of laboratory small notched specimens rather than smooth specimens and the derivation is done by the reverse direction procedure against the method of estimating the fatigue strength of the notched specimen from that of the smooth specimen. From a methodological point of view, it is more effective on the contrary. First, the notch fatigue strength estimation method by previous study is reviewed and then next improvements and new idea for the derivation of master diagrams are proposed. Finally, some examples of master diagrams are shown.

Recently, the small punch creep (SPC) test, one of the small sample methods, has attracted much attention. However, since the SPC test is a new test, there is a great demand for the accumulation of data to compare with the uniaxial creep test. In this study, SPC tests of A5052 alloy at around 0.7T_m were performed under various loads, and load dependence of displacement velocity were evaluated. The microstructures before and after fracture were observed by EBSD to investigate the microstructural changes and deformation behavior during the tests. The results showed that the displacement velocity and load followed a power law relationship and behaved similarly to the uniaxial creep test.

This paper presents the development of a creep testing machine with a maximum temperature 1000°C for utilizing miniature specimens and evaluation of creep strength for IN713C-MIM. The remaining life assessment of actual devices using the miniature specimens are expected as the better method because it can decrease the damage to them by sampling a specimen, so the test dates about miniature specimens are required. This study shows the developed testing machine and verification test results as well as some experimental results using IN713C-MIM. However, the MIM-materials creep strength is lower than the casting materials, although the experimental result are very few. This research aims at meeting the above needs and creating the superior heat-resistant alloy. The creep rupture time of IN713C-MIM and improvement of strength have been done by authors, but more detailed evaluating of the strength is also necessary. For this study, a suitable testing was required to evaluate creep strength more precisely.

Amorphous alloys without a crystalline structure exhibit excellent soft magnetic properties but are difficult to machine due to their tough mechanical properties. The authors have proposed a new machining method that improves machinability by crystallizing only the local regions involved in machining, thereby reducing local strength and toughness. To realize this method, it is necessary to evaluate the crystallinity of the local area nondestructively, but it is difficult to detect a local area of crystals precipitated in amorphous materials by the conventional X-ray diffraction method (sin²ψ method). In this paper, we attempted to investigate the crystallinity of the local area using a two-dimensional X-ray diffraction method (2D method) with a two-dimensional detector. Fe-based amorphous alloys were locally crystallized using an ultrashort pulsed laser with a beam diameter of about 66 μm. Then, it was revealed that the 2D method can evaluate even a small area of crystallization within several tens of micrometers.

Most cells are significantly affected by the intensity and duration of the dynamic field stimulus due to the culture environment. Therefore, it is important to understand the mechanical environment of an elastic substrate. Therefore, the purpose of this study is to clarify the mechanical environment when the elastic culture substrate is stretched. The displacement field when the elastic culture substrate was stretched was calculated using the digital image correlation (DIC) method. In addition, the strain field was calculated by numerically differentiating the result of this displacement field. From the results, we showed several regions of the maximum strain for the vertical strain components ε_x and ε_y. The strain was roughly uniformed in this range.

It is a well-known fact that there is significant difference in magnitude of flow stresses and work hardening rates of polymeric materials between in tension and compression. There is a possibility that hydrostatic pressure is involved in this asymmetry to the loading axis, in which case volume change occurs in plastic deformation. And also, the propagation of localized deformations is often observed as a deformation behavior of polymeric materials. In such cases, it has been confirmed that the propagating portion of the crack must be hardened, but the deformed portion is softened. The aim of this study is to measure the variation of plastic Poisson’s ratio by means of Digital Image Correlation method to clarify the existence of the plastic compressibility of Naylon66 and Polycarbonate. Another aim is to elucidate the mechanism of softening at the neck. The authors conducted tensile tests of thin strip specimens and measured the axial and lateral strains by DIC method, then, plastic Poisson's ratio was derived. The result showed that plastic Poisson’s ratio of Naylon66 and Polycarbonate was smaller than the value of 0.5, that result in the existence of plastic compressibility in these materials. The Micro-Vickers hardness measurement for a gauge length was also performed to clarify the strength distribution of a specimen, and the results showed that the necking part of the materials was softened.

The use of super pressure balloon (SPB) is currently attracting attention as an inexpensive high-altitude observation technique. Through a flight test of a newly developed SPB, it was found that the balloon envelope made of polyethylene film is damaged at the time of balloon launch to yield hundreds of slits with creep bands around the edges. In this study, we investigated a possibility if the damage is caused by tiny deficits on the balloon envelope film. Static tensile tests of specimen with micro wrinkles or scratches were conducted. It was found that the specimen with microscopic wrinkles showed no difference, however, those with micro scratches showed 30% and 15 % smaller values in breaking strengths and breaking lengths, respectively. Since the creep of the broken specimen were not limited to their broken edges, we consider that the deficits on the balloon envelope are not directly related to the slits found in the flight test of the SPB.

The present study evaluates the mechanical characteristic of polybutylene terephthalate based on nonlinear viscoelasticity and non-isochoric viscoplasticity by creep recovery test and uniaxial test. A series-connected model of Schapery nonlinear viscoelasticity and Perzyna viscoplasticity is used to express the mechanical behavior. The nonlinear viscoelastic parameters and the viscoplastic parameters are identified by creep recovery test and by uniaxial tensile test, respectively. As the results, the mechanical behavior in creep recovery test and uniaxial tensile test can be reproduced by using the series-connected model. However, the identified initial yield stress is larger than the loading stress in creep recovery test. This fact means that further consideration is needed the viscoplastic model with physically meaningful parameters.

Tensile and compression tests at 850℃ were conducted by using 6 types of specimen for ductile fracture evaluation of Ni-based superalloy. The digital image correlation method was applied to measure the strain during the tensile tests. Fracture strain, Lode angle parameter and stress triaxiality were obtained by the finite element analysis of the tensile and compression tests. These parameters were used to calibrate the material parameters of the modified Mohr-Coulomb (MMC) ductile fracture model.

Static tensile and creep tests were conducted on Ti-64 (α+β) and β single-phase Ti-15-3 to confirm the effect of the β phase for Ti alloys. Both tests were conducted in a scanning electron microscope (SEM), and strain distribution within the observation area was measured by a digital image correlation (DIC) method. With respect to Ti-64 (α+β), the influence of β phase was not clearly observed in the tensile and creep tests. The crystallographic orientation distribution of α phase strongly affected the deformation behavior and strain distribution. For the deformation behavior of Ti alloys with α+β phases, it is sufficient to consider only the crystallographic orientation distribution of the α phase. On the other hand, Ti-15-3 (β single-phase) formed slip zones along the direction of 45° to the loading line in both tensile and creep tests, which coincided with the macroscopic shear zones. This is primarily because β phase (BCC) has multiple slip systems and little plastic anisotropy. For the deformation behavior of β single-phase Ti alloys, there is little need to focus on the plastic anisotropy caused by the crystallographic orientation.

Estimation of the creep constitutive equation requires uniaxial creep tests under various stress conditions, which is costly and time consuming. In this study, a rapid estimation method of a creep constitutive equation is investigated based on an inverse analysis using digital image correlation (DIC) and FEM. First, a steady-state creep strain field that develops non-uniformly near the stress raiser was measured by DIC. Next, an inverse analysis was performed to minimize the difference between the strain measured by DIC and the strain calculated by FEM, and the creep constitutive equation in the steady creep region was estimated. The strain rate calculated from the estimated creep constitutive equation were consistent with the results obtained from uniaxial creep tests at high stress side, though the strain rates were overestimated at lower stress condition, where the duration of transient creep is relatively longer.

Cracks in mechanical structures lead to serious destruction accidents. High precision detection of cracks and evaluation of mechanical parameter of crack are important issues in order to prevent destruction accidents. In this study, we developed a crack detection method by digital correlation method which can evaluate the strain distribution from the surface image. Using this method, the stress intensity factors for mode I and mode II can be evaluated from sample surface. It also can be used to evaluate the inclination angle of the cracks.

Aluminum alloys used i internal combustion engines have been coated with an alumite layer. Conventionally, the alumite layer has been provided to suppress thermal cracking. Recently, it has been found that the alumite layer is effective in improving the thermal efficiency of internal combustion engines. To evaluate the strength reliability of those anodized parts, it is necessary to understand the tensile and thermal properties of anodized aluminum. In this study, we proposed a method based on the rule of mixtures to evaluate the tensile deformation behavior and the linear coefficient of expansion of the alumite. Al alloy specimens with and without anodized treatment were used to conduct tensile tests at R.T., 473 K, 573 K, 623 K, and linear expansion coefficient measurement tests. By using the stress-strain relations obtained from the tensile tests, the tensile stress-strain relations of the alumite layers were estimated and they were used to discuss the tensile characteristics of the alumite layers. As a result, the stress-strain relation and linear expansion coefficient of the alumite could be estimated. It was also found that the alumite has higher strength than Al alloys at RT., 473 K, 573 K, and 623 K.

In order to improve fatigue properties of metallic materials, mechanical surface modification such as shot peening is one of powerful methods. Major key parameters of the material properties on the improvement are work hardening and introduction of compressive residual stress. In the case of measurement by using an indentation test, hardness which relates resistance on plasticity such as yield stress is evaluated, and residual stress is also affected by residual stress. Namely, elastic properties such as compressive residual stress can be measured by the indention test as well as material flow stress characteristic of the transition from elastic to plastic deformation. In the present paper, an evaluation method of elastic and plastic properties of mechanical surface modified layer by the indentation test was proposed. In view point of practical application, as the indentation test which was similar to Rockwell superficial hardness was useful, the ratio of preliminary minor load and additional major load was investigated.

The casing of a rotating machine is required to contain the fragments when the rotating body is broken. The purpose of this study is to clarify the material model for the fracture simulation, which can be applied to the evaluation of containment capability of casing made of cast iron with low ductility. The dynamic penetration test of the FCD 450 plate showed that the fracture mode changed, and the absorbed energy increased as the bullet velocity increased. As a result of the reproduction analysis using Modified Johnson-Cook law considering the stress triaxiality in the fracture strain, it was found that the dependence of the fracture mode and the absorbed energy on the bullet velocity could be explained and the absorbed energy could be reproduced within an accuracy of ± 20%.

In this study, a simple method to evaluate the local pipe wall thinning by using the DC potential difference method was examined. The pipe has outer diameter D and thickness t₀. The local inner-wall thinning is modeled as a back wall slot having arc-shape cross section of width 2w and depth b. The constant DC current is applied to the pipe through two probes, and the distribution of potential difference between current input and output probes is measured. The potential differences measured before and after thinning are represented as V₀ and V₁, respectively. The thinning depth b/t₀ is evaluated from the maximum value of V₁/V₀. Three-dimensional finite element method was used to analyze the relation between thinning size and potential difference for various cases. As a result, it was shown that the relation between b/t₀ and V₁/V₀ is affected by w/D. The calibration equation between b/t₀ and V₁/V₀ was obtained by approximating FEM results, and its validity was shown by numerical experiments.

The authors have been conducting studies on standard specimens for the acoustic emission (AE) method that realize the quantitative discussion about the performance of measurement systems, including both instruments and engineers’ skills, in the environment equivalent to the practical situation. In this study, the applicability to standard specimens was examined for those fabricated by the fused deposition modeling type 3D printer. The behavior of AE generation was monitored during the four-point bending test using a desktop-type loading instrument for the specimen laminated toward the longitudinal direction and having a conical surface defect. It was shown that multiple delaminations associating AE signals occur on the extensional line of the cone tip. This fact implies that the intended actual AE events are generated intermittently at specific locations. Hence, it was found that the specimen has potential as standard specimens for the AE method with low cost and convenience of use.

A research project is underway to apply the Acoustic Emission (AE) method to establish a non-destructive, non-opening, in-service inspection of high-pressure hydrogen Type 2 accumulators for hydrogen stations. The objective is to prevent hydrogen leakage due to fatigue crack initiation, propagation, and penetration in the steel liner of the accumulator caused by cyclic pressure loading. In this study, the relationship between fatigue crack initiation/growth and AE wave generation behavior is evaluated using a plate-type specimen that simulates the low-alloy steel liner of a pressure accumulator. As a result, AE generation is considered to capture the transition of a crack from Stage I to Stage II, and the usefulness of applying the AE method for in-service inspection of high-pressure hydrogen Type 2 accumulators is demonstrated.

We investigated the effect of load frequency using thermoelastic finite element (FE) analysis on a gusset welded joint specimen commonly used in welded structures. To simulate the thermoelastic effect, we developed a stress field-temperature field thermoelastic FE analysis technique that calculates the heat transfer of heat generation and heat absorption according to compression and tensile stress. The calculated stress distribution showed a good agreement with thermoelastic stress distribution measured using an infrared ray method. The result implies that the developed technique is effective in reproducing the thermoelastic effect. In the case of a load frequency of 1 Hz, a small gradient of temperature distribution appeared. Therefore, when applying the thermoelastic method to the stress concentration field, it is important to consider the influence of the load frequency.

A relationship between microstructure and strain induced by different plastic working is investigated in AZ31 magnesium alloy. AZ31 magnesium alloy was processed by different plastic workings (forging, rolling, extrusion, equal-channel angular pressing (ECAP), caliber rolling) at 573 K with equivalent strain controlled to the similar value. The strain induced in the processes was computed by using the finite element method (FEM). The deformation texture of forging and rolling were intensive basal texture, but the texture of extrusion, ECAP and caliber rolling were relatively weak. The FEM calculation and deformed texture showed that in all five processes, the basal plane is oriented to the direction perpendicular to compression strain, and tendency that the more compression strain is induced, the more intensive basal texture is formed.

The creep tests were performed at 473 K in the range of 40-80 MPa on the AZ31 alloy extruded rod in which microstructures are controlled by pre-compression and heat treatment. The microstructures were characterized by EBSD. As a result, fraction of grains in which basal planes were almost perpendicular to extruded direction were evaluated after microstructure control by pre-compression and heat treatment. In addition, remarkable coarsening grains were confirmed. In the creep test, it was found that the minimum true strain rate and the rupture life were improved by microstructure control. The coarsened grains and texture control are considered to be the factors for improving the creep resistance.

Some components which constitute a part of the boundary in nuclear plants are very important from the viewpoint of protection of radioactive material propagation. Especially, it is a serious matter if the function as the boundary is maintained or not in the case of severe accident (SA). In this paper the characteristics of FLD (Forming Limit Diagram) as the failure criteria is studied compared with the rule of ASME Code and the applicability to the boundaries of structures is discussed. Furthermore, the prospect of the application of FLSC (Forming Limit Stress Curve) to structural components including loading hysteresis is referred.

A new model of critical resolved shear stress with the effect of geometrical relationship between slip plane and microstructure interface, was investigated using a crystal plasticity analysis with Cu polycrystals. The extended expression of the Bailey-Hirsch type model was used for the critical resolved shear stress. The effect of geometrical relationship between slip plane and microstructure interface was incorporated into the values for microstructural size parameter that express size effects in the extended expression of the Bailey-Hirsch type model l. The distribution of equivalent plastic strain and plastic shear strain on slip systems were investigated. At a nominal strain of 0.03, almost the same distributions of equivalent plastic strain were obtained with the simple grain size model and the new model where the effect of geometrical relationship between slip plane and microstructure interface were taken into account. On the other hand, the distributions of plastic shear strain on slip systems were different in the two models.

By using molecular dynamics (MD) simulation, the mechanism of nanometer-sized wiredrawing is studied. The possibility of nano-sized plastic working process and predictive microstructures are investigated. As a drawn material, pure iron (α-Fe) and magnesium (Mg) single crystals are compared. The MD models are constructed by utilizing many-body-type (FS or EAM) interatomic potentials with adequate modification for lubrication between wire and die. Dislocation slip is a principal mechanism of plastic deformation in the case of α-Fe in any drawing direction of single crystal. Besides, dislocation densities increases with increase of the wire diameter, where nucleation, annihilation and mutual behavior of dislocations are observed in detail. On the other hand, in Mg single crystal, mechanism of plastic deformation depends strongly on crystal orientation as to drawing direction. Twinning and twin boundaries can be identified by detecting a network of interface dislocations. When the c-axis of hcp lattice is inclined by 45 degrees from the drawing direction, dislocations on basal plane occur, but no twin, and therefore the minimum dislocation density is obtained.

Thin sheets of pure titanium have been used for medical and dental products and their downsizing seems to be accelerated more and more. On the other hand, it is known that mechanical properties of polycrystalline thin sheet are strongly influenced by microstructures such as the shape and size of grains, crystal orientation, and grain boundaries. In this study, tensile tests were conducted using notched thin sheet specimens of polycrystalline pure titanium and the microscopic inhomogeneous deformation was observed. Tensile deformation mainly occurred by crystallographic slip and the condition of slip activation was examined by Schmidt factor, critical resolved shear stress, and macroscopic stress distribution around the notch.

Micro-tensile tests using single-colony specimens of Ti-6Al-4V alloy with a fine lamellar microstructure were employed in the temperature range between 298 and 423 K to understand the effects of geometrical orientation of β lath on <a> slip behaviour. The critical resolved shear stress (CRSS) for the basal slips was lower than that of the prismatic slips, in addition, the temperature dependence of the slip behaviour was greater in the basal slips than the prismatic slips. In the case of single α phase excluding β lath, the CRSS and temperature dependence are independent of the slip modes. In the lamellar microstructure, however, the basal slip propagates simultaneously through all the lamellar layers (divider type) and the prismatic slip propagates sequentially from one layer to another (arrester type). Therefore, considering that dislocation motion is restricted through the pinning and pile-up mechanisms in divider and arrester orientations, respectively, the CRSS and temperature dependence in the basal and prismatic slips can be explained by the geometrical orientation of β lath included in lamellae. This finding suggests that the geometric configuration of β lath dominates the <a> slip in single-colony lamellar structure of the Ti-6Al-4V alloy.

Microtensile tests were performed to elucidate the mechanical properties using micro-scale specimens extracted from single packet (SP) and single block (SB) structures of lath martensite in steels with low- and medium-carbon contents. In addition, micro-Vickers hardness and nanoindentation tests were conducted to measure the hardness corresponding to single substructural units. A habit-plane-dependent-yielding occurred in the SP structures, and the anisotropic yielding behaviour was also observed in the SB structures excluding the block boundaries, independent of the carbon content. While the low-angle grain boundaries are included in the SB structure, they are difficult to hinder the dislocation motion. Therefore, this indicates that the plastic anisotropy on microstructure of lath martensite is attributed to the dislocation cell structures formed during the early stage of deformation. The tensile strength of the SP and SB structures were correlated with the micro-Vickers hardness and nanohardness, respectively. The average correlation coefficient for the SP structures coincided with the Tabor’s constant, whereas the average coefficient for the SB structures was half lower than that of the Tabor relation.

Compression creep tests were conducted in order to investigate the creep behavior of high entropy alloy Cantor at high temperatures. It was found that the creep behavior of Alloy-type was observed in low stress region and that of Metal-type was observed in high stress region. In addition, the activation energy of creep increased with increasing stress. This suggests that a kind of rate controlling diffusion atoms is changing depending on the stress. Microstructures after creep were observed using the Electron Back Scatter Diffraction method. The generation of dynamic recrystallized grains and formation of sub-boundaries were observed in high stress region. This is known as a typical deformation structure of Metal-type creep. In addition, a lot of low-angle grain boundaries were observed. These characteristic structures are similar to that observed in FCC metals in which stacking fault energy is relatively low.

The uniaxial compression test using the micrometer-sized specimen is employed to evaluate elastic-plastic deformation behavior of the selective single crystal from polycrystalline materials. Through reflecting these properties into the crystal plasticity finite element method (CPFEM), elastic-plastic analyses of the polycrystalline materials can be performed based on the single crystalline properties of real materials. In this study, the micro-pillar compression tests were conducted in the some single crystals of recrystallized polycrystalline pure aluminum. A method identifying CRSS, work hardening parameters and anisotropic elastic constants was proposed. The predicted stress-strain curves of single crystals obtained by CPFEM agree well with the original experimental data.

Steel containing the metastable austenitic phase shows excellent mechanical properties such as strength, ductility and toughness. The transformation-induced plasticity (TRIP) in the steel can optimize these mechanical properties. Since strain-induced martensitic transformation (SIMT) causes TRIP, deep understanding of the continuous development of SIMT is very important to control the mechanical properties by TRIP. In addition, a dislocation attributes to the strain hardening so it is very important to clearly explain its complexity. It is necessary to construct a model on the dislocation deeply related to size effects. In this study, a hardening model and yield conditions for monocrystalline austenitic steel are established based on the notion of microforce. By incorporating the finite element crystal plasticity method, the two-dimensional unit cells of polycrystalline austenitic steel are obtained by Voronoi tessellation, and the deformation behavior and microstructure are simulated computationally.

In recent years, Zn can potentially be applied as biomaterials owing to its excellent biocompatibility and biocorrosion resistance. However, Zn has poor strength and large plastic anisotropy due to its hexagonal close-packed (HCP) structure. In this study, to increase the strength and plastic isotropy, fine-grained Zn polycrystal was produced by spark plasma sintering from Zn fine particles prepared by arc plasma method. Yield strength and the plastic anisotropy of the sintered Zn were evaluated by compression tests and Vickers indentation. The yield strength of the sintered Zn was significantly higher than the as-cast Zn. Moreover, the plastic anisotropy of the sintered Zn was small because of its fine grain and random texture.

An elongated Shape memory alloys (SMAs) elements which shows superelastic properties can exhibit the quasi-zero or negative stiffness during post-buckling deformation. Since the buckling deformation is recovered by the superelastic effect upon unloading, the quasi-zero or negative stiffness can be used continuously. Our research group has devised, studied and reported on passive vibration isolator devices that utilize this property, but this property can be applied not only to passive vibration isolator but also to various other possible applications. This paper reports on the characteristics of a force-limiting device using the buckling properties of shape memory alloy elements, which was devised and prototyped by our research group. We also report on the newly devised convex-tape shape memory alloy element and the movement characteristics of a rehabilitation mechanism using its buckling characteristics.

To develop a vibration isolator for the vertical direction using a post-buckled shape memory alloy (SMA), it is important to understand the restoring force and the mechanism of the negative stiffness characteristics of a post-buckled SMA beam. In this study, we investigated the relationship between the restoring force characteristics and the phase transformation of a post-buckled thin plate SMA beam. The Finite Element Analysis was used for the investigation. As a result, we confirmed a qualitative correlation between the restoring force characteristics and the volume fraction of the phase transformed SMA material.

The fatigue life of a high-dilatation stent fabricated from TiNi shape memory alloy tube with a high reduction of area was investigated in the radial and uniaxial compression mode. In this study, fatigue properties of the stent were discussed focusing on morphology of the fatigue fracture surface. In addition, the local strain during compressive deformation was examined by observation of the stent subjected to the compression. The main points obtained in the study are as follows (1) Fatigue fracture occurs at base of a strut of the stent specimen. (2) Fracture surface appears at an angle to the strut axis. This means torsional deformation occurs at base of the strut of the stent specimen. (3) The fatigue crack initiation point appears on the outside of the strut part because of the maximum positive bending strain.

TiNi shape memory alloys, which are representative of functional materials, have been applied in various fields such as medical and aerospace. To expand the application of TiNi shape memory alloy, the authors fabricated a functionally-graded TiNi shape memory alloy with a stepwise change in Ni composition in the material. Since the functionally-graded TiNi shape memory alloy is fabricated by powder metallurgy, the composition ratio of Ni can be controlled accurately and near-net-shape processing will be achieved. However, in general, strength and ductility of a sintered material are low due to the poor density. Therefore, the authors attempted to produce a sintered compact with a high density by increasing the sintering temperature. In addition, differential scanning calorimetry and tensile tests were conducted on the prepared functionally-graded TiNi shape memory alloy to clarify the transformation and mechanical properties. This study shows that a higher sintering temperature is an effective means of improving the strength and ductility of the TiNi shape memory alloy by increasing their density.

The effect of ultrasonic shot peening on corrosion fatigue properties of a TiNi shape memory alloy was investigated. In this study, the surface condition of the peened specimens was optically observed and the arithmetic mean roughness of the specimen surface was measured. The main points of the results obtained in this study are as follows: (1) The surface roughness of as-peened specimen decreases with increasing the peening coverage because of smoothing effect by shot media. (2) Surface roughing derived from adhesion of Fe in the shot media was observed on the as-peened specimen, especially under high coverage conditions. (3) Mechanical polishing after the peening reduces the surface roughness to the level before peening. (4) No change was observed on the specimen surface even if the peened and then polished materials were heated to induce the reverse transformation.

Shape memory polymers (SMPs) have unique properties and can be formed into arbitrary shapes using an FDM 3D printer. In this study, SMP lattice structures simulating foam materials were fabricated using an FDM 3D printer, with the goal of developing SMP lattice structures with recovery equivalent to that of foam materials. The main results obtained are summarized as follows. (1) Specimens at a nozzle temperature of 473 K were more susceptible to significant deformation by heat treatment than by shape recovery testing, and in addition, specimens prepared with an infill rate of 20% were deformed by more than 10% by heat treatment. (2) Increasing the nozzle temperature increases the crystallinity and reduces the deformation rate during heat treatment. However, since molding is insufficient and recovery is small, it is necessary to increase the crystallinity by another method.

The negative stiffness exhibited by shape memory alloys (SMA) during buckling can be applied to passive vibration isolators and force-limiting mechanisms. However, to apply the negative stiffness of SMA, it is necessary to calculate the negative tangential stiffness value and buckling load of the shape memory alloy after buckling by design. In our previous study, we have shown that it is possible to increase the negative tangential stiffness and buckling load after buckling by increasing the curvature of a plate-like SMA material in the form of a convex tape with a given cross-sectional curvature. In this study, it is considered that the cause of the negative tangential stiffness after buckling is the change in martensitic phase transformation behavior during buckling deformation due to the addition of curvature to the cross-section. The results showed that the phase transformation behavior of a flat SMA element and a convex tape SMA element with curvature in its cross section changed. As a result, it was found that the flat SMA element does not exhibit negative stiffness immediately after buckling because martensitic phase transformation does not occur in the center of the specimen immediately after buckling, while the convex-tape SMA element tends to exhibit negative stiffness from immediately after buckling due to martensitic phase transformation in the center of the specimen.

Shape memory alloys (SMAs) are functional materials with high shape memory and super elasticity effects that are used in a wide range of fields, including medical and engineering fields. Our research group has focused on the negative stiffness characteristics of plate-shaped SMA during buckling deformation and has attempted to apply them to vibration-isolation devices. The buckling properties of Cu-Al-Mn SMA, which exhibits almost the same super elastic properties as Ti-Ni SMA, were evaluated in comparison with those of Ti-Ni SMA. The results showed that Cu-Al-Mn SMA is more suitable for vibration isolators because of its smaller change in post buckling properties due to temperature change and smaller hysteresis compared with Ti-Ni SMA. In addition, the buckling fatigue and functional degradation properties of Cu-Al-Mn SMA were studied. As a result, it was clarified that fatigue failure is basically caused by the propagation of cracks originating from grain boundaries, and that the increase of these cracks is the cause of the decrease in buckling load. In this study, the buckling fatigue properties of Cu-Al-Mn SMA material in which bainite phase is formed at the grain boundary by changing the heat treatment conditions were investigated, and the effects of the differences in internal microstructures on the buckling fatigue and functional degradation properties of plate-like Cu-Al-Mn SMA with different internal microstructures were compared with those of conventional Cu-Al-Mn material. The effects of different internal microstructures on buckling fatigue and functional degradation properties of plate Cu-Al-Mn SMA with different internal structures were investigated. The results show that the specimens with bainite phase have superior fatigue properties, but in terms of functional degradation properties, they increase the rate of decrease in Pcr and increase the rate of increase in tangential stiffness after buckling.

We have been investigated one-coil torsion springs using shape memory alloy (SMA) wire for application to welfare equipment (for example, lower limb orthoses), because one-coil torsion springs using SMA wire have small size and large displacement. It is known that the spring properties of a torsion spring depend on the shape of the spring, and design calculation method for springs made of normal metals have already been established. However, SMA has different characteristics from normal metals, and it is difficult to apply conventional design calculation methods. Therefore, we fabricated several SMA torsion springs with different the angle between the spring arm and investigated the effects of shape of the spring on the spring characteristics of SMA torsion springs by spring characteristic tests and fatigue tests. The experimental results show that increasing the angle between the spring arm improves the fatigue life of the SMA torsion springs but the spring generating force decreased. On the other hand, the functional degradation characteristics are independent of the angle between the spring arms.

The authors have proposed a functionally-graded shape memory alloy with varying Ni composition by combination of sintering and thermomechanical treatment. A powder metallurgy process has advantage of the easy adjustment of composition ratios. However, in general, strength, ductility, and fatigue life of a sintered material are low due to the poor density. To improve this problem, the authors employed secondary working into the fabrication process such as hot rolling for consolidation and ultrasonic shot peening for inducing compressive residual stress. In this study, the mechanical properties of the sintered shape memory alloy fabricated by above-mentioned process were revealed.

Functional fatigue in shape memory alloy is known as the degradation of superelasticity after the repetition for large number of cycles, which would result in the accumulation of residual strain. Present study found that cold-drawn wires of Titanium 50.6 at.% Ni alloy exhibit superelasticity even after the heat treatment for a few seconds at 500℃. We shall call this flash heating “short time aging”. It was also found that the short time aged sample showed smaller amount of residual strain in function fatigue. The amount of residual strain decreased with decreasing the period of aging. The stress-strain curves of superelasticity in specimens with different aging time were compared.

Reversible deflection of coil spring due to shape memory effect was examined. Coil springs were prepared by winding the wire of Nickel -50.at.% Titanium alloy followed by the annealing at 800 C for 1h to set the coil shape. The Mf and Af temperatures were 30 C and 87 C and the spring was deformed in the fully martensite state at room temperature. A tough pitched spring with the coil diameter of 6.6 mm and the element diameter of 0.6 mm showed shape memory effect after extension larger than 1,000 %. The load-deflection curves measured showed nonlinear relationship with rapid increase of load at a certain extension. It was found that the shape memory strain of coil spring is not limited by that of element wire but by the geometry of coil spring against the extension. Noted that and the deflection was accompanied with the rotation around the coil axis. Elastic plastic deflection analysis was performed, and the prediction of the theory was compared with the experimental curves.

Nickel Titanium shape memory alloy was proposed for heat storage material. When the shape memory alloy is in contact with a heat source, the heat flow from the heat source to the alloy was enhanced by the latent heat associated with the reverse martensitic transformation of shape memory alloy, and vice versa. This study performed both theoretical and computational analyses. First, the enthalpy curve in the temperature range of martensitic transformation was determined from the DSC curves obtained in experiment, which can describe the emission/absorption of latent heat during the martensitic transformation as the function of temperature. Next, a metal matrix composite of NiTi shape memory wires embedded in aluminum block was designed and the heat transfer with a heat source was observed by three-dimension finite element heat conduction analysis with ANSYS. We focused on the effects of the configuration of SMA wires and the heat resistance between the wire and aluminum matrix on the performance of the heat storage.

When a wet materials dries up, cracks are generated due to the volumetric shrinkage, and curious complicated pattern is generated. The formation mechanism is difficult to completely understand because various factors are related, for example, the circumstantial conditions such as humidity and temperature, mechanical conditions as static or dynamic, and materials condition like the uniformity and homogeneity. The simulation of the crack-pattern formation is consequently not straightforward, and many kinds of difficulty is raised. In this study, therefore, we are trying to construct a simple and widely applicable simulation model for the phenomena. As a first step, the prototype was constructed, and the basic process was successfully regenerated. In this model, volumetric shrinkage generates a single crack and the crack progresses straightly to the boundary. This process occurs successively, and various patterns were obtained depending on the difference in the parameters corresponding to the drying condition.

Using a bonding material of metal and ceramics for heat-resistant materials, large thermal stress is caused by a difference of the coefficient of thermal expansion between metal and ceramics. The functionally graded materials, FGMs could relax the thermal stress. Heat resistance testing for FGMs has been conducted, but a tensile test has not been done. We use powder injection molding, PIM method to fabricate 9-layer Ni/PSZ system FGMs. The purpose of this study; 1) Fabrication of FGMs without warping. 2) Evaluation of tensile properties of FGMs and single sintered body. The conclusions of this study are as follows; 1) The warping of FGMs decreases with increasing load applied to the specimen during sintering. However, cracks occur on the Ni-0 surface. 2) Tensile stress of FGMs decreases rapidly because of breaking the PSZ rich layer.