Notched fatigue behavior a quasi-isotropic woven fabric CFRP laminate has been studied. Static and fatigue tests are performed on unnotched and center-hole coupon specimens for different stress ratios. The notched fatigue life of the plain woven CFRP laminates is predicted using the notched fatigue life prediction method that was developed in an earlier study. The values of fatigue life predicted using the model are compared with the fatigue lives observed in this study. The notched fatigue life of the plain woven CFRP laminates under the assumption of fatigue notch insensitivity agrees well with the experimental fatigue life observed, regardless of stress ratio, demonstrating the potential usefulness of the notched fatigue life prediction method for notched fatigue life prediction for the composite.

Conventionally, anodizing treatments have been performed on aluminum alloy combustion chamber of internal-combustion engine to suppress occurrence of heat cracks. Recently, it has been found that the alumite layer formed by the treatments is effective in improving the thermal efficiency of internal-combustion engines. Therefore, some anodizing methods have been proposed to form thick alumite layers on aluminum alloy parts in internal combustion engines, and the parts made by the methods have already been put into practical use. To evaluate the strength reliability of those anodized parts, it is essential to understand the tensile characteristics of the alumite. In this study, we proposed a method based on the rule of mixtures to evaluate the tensile deformation behavior of the alumite. Then, tensile tests were conducted using anodizing treated aluminum specimens and non-treated aluminum specimens. Before the tests, the thickness of the alumite layer in the anodizing treated aluminum specimen was measured by using X-ray CT, and it was used to estimate the area ratio of the layer in the cross-section of the specimen. By using the stress-strain relations obtained from the tensile tests with the estimated area ratios, the tensile stress-strain relations of the alumite layers were estimated and they were used to discuss the tensile characteristics of the alumite layers. These results showed that the alumite was fractured for brittleness without regard to thickness of the alumite layer on the specimens.

We investigate the occurrence and growth behavior of the semicircular cracks caused by shear (hereinafter referred to as "shear cracks"), which is the first stage of tensile fracture of hole steel plates subjected to punch process (hereinafter referred to as "punched materials"). Punched material is widely used for automobile because of its low cost. When a punched material is subjected to a tensile load, a shear affected zone (SAZ) by the punch process exists within a certain depth range near the punched hole edge, and make the strength characteristics of the punched materials be worse than those holed materials by drilling and hole inner surface polishing (hereinafter referred to as "ideal materials"); particularly, punched materials of some steel types can be brittlely fractured. And then, different from the ideal material, no matter the fracture types be a ductile failure or be a brittle fracture, in the initial stage of the fracture of the punched material, there is a semi-elliptical crack occurred by shearing at an inclination with the tensile direction, which is called "shear crack." In this research, we used the punched material which was reported to be brittle fracture at the static strain rate, the results of both ductility and brittleness were obtained by tensile tests at several strain rates. Shear cracks in both the ductile and the brittle fracture surfaces occurs in multiple stages. The timing and driving force of these several stages were considered, the fracture process of both ductility and brittleness was clarified, and the factors that influence the process were also clarified.

This study aims at evaluating damage process of torsion fretting fatigue behavior of carbon steel using acoustic emission. Acoustic emission (AE) technique was applied to observe the growth of damage in carbon steel specimens under torsion fatigue and torsion fretting fatigue loadings. AE frequency spectrum was clustered into groups. The indicated groups were associated with damage types - crack initiation and propagation, and the accumulation process of clustered AE signals were observed.

The essence of rolling contact fatigue is so-called “Mode II fatigue crack propagation.” However, its understanding has not progressed as much as that of Mode I. When cyclic Mode II loading is applied, plastic strain localizes in a region depending on the crystal structure ahead of the crack tip, crack initiates repeatedly and crack propagates by its coalescence. This will be termed “damage accumulation mode fatigue crack propagation.” In this study, localization and progress of plastic strain were successively observed during damage accumulation type fatigue crack propagation, and the relationship between plastic strain and microstructure was observed under cyclic Mode II loading by using digital image correlation (DIC) method. We used perlite steel as a test material. As a result of the DIC analysis, plastic strain localization was observed ahead of the crack tip by cyclic Mode II loading. The plastic strain was localized at the interface of the microstructure. Crack propagation was observed from the location where plastic strain is localized. In pearlite steel, when cyclic Mode II loading is applied, strain concentrates due to the discontinuity of deformation at the interface of the microstructures, and crack initiation and coalescence along the interface causes damage accumulation mode fatigue crack propagation.

Mode I and Mode II are defined as terms of fracture mechanics which express the deformation mode of a crack under some loading. However, the terminology which expresses the fatigue crack propagation mode under a mode or mixed-mode loading is not clearly defined until now. For example, it is not clear what is called the mode of fatigue crack propagation in which the initial crack under the Mode II loading propagates along the maximum shear plane in macro-scale and propagates along the maximum principal stress plane in micro-scale; that is zig-zag shape fatigue crack. It is considered that the unclear classification such as “Tensile mode” and “Shear mode” prevents the development of this field. In this paper, we pointed out the definition problem of shear mode fatigue crack propagation to predict the propagation behavior of fatigue crack. Focusing on the phenomenon, not the loading, we propose the terms “Plastic deformation mode” and “Damage accumulation mode” fatigue crack propagation. The revealed fatigue crack propagation phenomenon under cyclic pure Mode II loading is the achievement of the author’s previous study. Furthermore, we propose a method for predicting fatigue crack propagation behavior in each mode and a method for classifying existing fatigue cracks.

The latest advance in the studies conducted into the growth behaviours and the near-threshold conditions of a shear-mode, small, fatigue-crack in bearing steel is overviewed. Also, the relevant challenges for the future are discussed.

In order to study the determination factors of the torsional fatigue limit of bainitic steels, torsional fatigue tests were carried out, using two materials; CG (HV = 329) and FG (HV = 314) having different microstructure morphologies. Electron backscattered diffraction (EBSD) observation revealed that coarse microstructure and fine microstructure were co-existed in both the materials. The area fraction of fine microstructure in FG was about 15% higher than that of CG. The torsional fatigue limit of FG was about 80 MPa higher than that of CG, even though they had almost the same Vickers hardness. Shear-mode cracks in these steels initiated and propagated transgranularly in coarse grain region. In addition, some cracks stopped propagating at the grain boundaries. The statistics of extremes predicted that CG and FG have almost the same maximum grain size. This result inferred that the size of crack initiation units in these materials was nearly equivalent to each other. Based on the series of experimental results and observations, the effect of microstructure in bainitic steels on the torsional fatigue limit is discussed.

In order to establish a comprehensive understanding of the fatigue strength of precipitation-strengthened materials, fatigue tests were carried out under pure axial and torsional cyclic loading conditions respectively. The materials investigated were two types of Alloy718; one was solution-treated and another was precipitation-strengthened. The shear-mode fatigue propagation in the principal shear stress direction was observed despite loading conditions. The threshold stress intensity factor ranges for shear-mode crack, ΔK_τth, were investigated for each testing condition. Based on the results, the effect of the normal stress on the critical plane to the value of ΔK_τth was discussed.

In rolling element bearing, premature delamination failure, called white structure flaking (WSF) or white etching crack (WEC), often occurs. This premature flaking decreases the durability and reliability of bearing significantly. It is known that a flaking failure is attributed to shear-mode fatigue crack growth under heavy compression. In addition, some previous studies point out that WSF is related to hydrogen that is derived from decomposition of lubricant and penetrates into the bearing steel in operation.

In this study, in order to establish the safety of bearing, the effect of hydrogen on shear-mode fatigue threshold was investigated using a continuous hydrogen charging method.

Short carbon fiber reinforced thermoplastic (SCFRTP) is promising for use of the automobile members because of its high specific strength and moldability. An accurate prediction of its strength is necessary for practical use. Due to the uncertainty of carbon fiber distribution, it is difficult to obtain the stochastic property of strength of members without numerical investigation, to which complexities solved by high performance computing is associated. In this study, we develop a stochastic macro strength model to predict minimum strength of SCFRTP specimen in terms of fiber volume fraction and fiber direction tensor regarded as spatial stochastic variables. As a first step, we have obtained a three-dimensional full model of specimen, where carbon fiber and resin are clearly separated, via X-ray computed tomography. We divided this full model into small volumes, and fracture simulations for both models to obtain macro stress-strain curves. We investigate methodology to constitute macro strength model through the comparison of both simulations in regard to stochastic homogeneity.

One method of manufacturing MEMS is the strereolithography which uses UV photopolymer. Microelements, however, fabricated by stereolithography have poorer strength than metal components usually used to machine. So that reason, it is applied in limited fields. Mechanical properties of micromaterials are different from those of bulk materials and characterization of mechanical properties for micro specimens is needed to design reliable micromachines and MEMS.

In this study, we tried to improve the mechanical properties by adding cellulose nanofibers (CNF) in one of the reinforcement to UV photopolymer. Moreover we created micro specimens by stereolithography, and evaluated the mechanical properties by tensile test.

Since CNF has Hydrophilic Groups, it is difficult to disperse by adding CNF to general hydrophobic photopolymer. In previous studies, When composites were made using freeze-dried CNF and water-dispersed CNF, we were able to make test specimens with addition rate of 1.5 mass% and increase the Young's modulus of the specimen. However, as the addition rate increased, CNF aggregated in the resin to form clusters due to decrease in dispersibility. The tensile strength and the elongation also decreased greatly. For these reasons, we needed to improve dispersibility of CNF in UV photopolymer.

Therefore, we examined the effect of improving dispersibility of CNF in the photopolymer by using a method of Replacing water of CNF slurry in acetone by using centrifugation and a method of chemically aminating a hydrophilic group of CNF. As a result of compositing with acetone-dispersed CNF and aminated CNF, dispersibility was improved. With an addition rate of acetone-dispersed CNF 4.0 mass% and aminated CNF of 5.0 mass% , we could create test spesimens. As compared with the composite of water-dispersed CNF, the Young's modulus and the tensile strength increased, and the decrease in strain was suppressed.

Carbon fiber reinforced plastics (CFRPs) have the properties of high strength and lightweight, and it has been applied for aircrafts or automobiles. Recently, cellulose nanofibers (CNFs) have been found to be superior in strength, thus, researches are being conducted to enhance the strength of CFRP by applying CNF. However, since CNF is hydrophilic, a hydrophobic treatment is inevitable. Previously, authors have reported that the electrodeposition resin molding method to manufacture CFRPs using the electrodeposition solution (EDS) containing a polymer with epoxy group, and the strength enhancement by CNFs could be possible. In this study, to improve the strength of CFRP conventionally manufactured by prepregs, a method was developed to disperse CNFs between prepregs using EDS. Carrying out the three-point bending test, the effects of the dispersed CNF on the bending stiffness and strength were clarified. As a result, the bending stiffness per weight (specific bending stiffness) increased in proportion to the weight fraction of CNF, and was approximately 2.9 times that of the CFRP specimen without CNF when the CNF weight fraction was 17 %. In the same manner, the bending strength per weight (specific bending strength) was approximately 1.6 times that of the CFRP specimen without CNF when the CNF weight fraction was 22 %.

The nonlinear mechanical behavior of the thermosetting polymer is represented based on the molecular chain network model. In this model, the crosslink of the network is divided into the physical and chemical crosslinks, and the physical bond is allowed to separate and recombine during the deformation. To predict the effect of the ambient temperature on the mechanical response of the thermosetting polymer, the proposed model is further extended by introducing the temperature-dependent development of the physical bond. The FEM simulation based on the extended model could demonstrate the strain rate- and temperature-dependent mechanical response of the epoxy.

In this study, cellulose nanofiber (CNF)-dispersed resin with an addition of metal chloride were prepared for the purpose of repairing aged steel structures and extending their life. The resin is applied to steel sheets and corrosion tests were carried out. In corrosion tests, rust was grown under repeated wet/dry condition up to 30 cycles in accordance with SAEJ2334. The test pieces were used following three types, No.B (Bare steel plate), No.R (Applying only water-based epoxy resin) and No.C^5wt% (Applying resin containing 5 wt % CNF in epoxy resin). Al³⁺ and Ni²⁺ were used as metal chloride to be added. During the transition from drying to wetting process, the test pieces were respectively impregnated with 1M aqueous solution of Al₂(SO₄)₃ and NiSO₄ for 1 minute. Appearance observation and XRD analysis were performed on these test pieces. The findings are as follows. From the appearance observation results, it was confirmed that rust was generated at No.C^5wt%, because CNF supplied water and oxygen to the steel surface. In addition, the color of the rust produced was different between the test pieces coated with CNF-dispersed resin and the test pieces coated with nothing, and there was a difference in the rate of rust formation depending on the amount of CNF added. As a result of XRD observation, there was no significant difference between the specimens with Al³⁺ and Ni²⁺ added under each test condition, and the peak of α-FeOOH was observed at No.B, No.C^5wt%.

For widespread use of fuel cell vehicles, reductions of cost and weight of hydrogen tank has been stringently required. The most effective method for the reduction is concerned with strength evaluation of Carbo Fiber Reinforce Plastic (CFRP). The tank is manufactured by filament winding method. Carbon fiber bundle cross-overs caused by helical filament winding easily raise stress concentration, which cannot be evaluated by conventional model based on continuum anisotropic body formulated by rule of mixture. A method to represent CFRP by meso-scale mode, where fiber bundle and resin is separately handled, has been proposed. An applicability of embedded elements method for the meso-scale modeling by shell element for fiber bundle is investigated in this study.

A methodology evaluating the fatigue strength of Carbon Fiber Reinforced Plastic (CFRP) has been investigated in terms of Interfacial Normal Stress (INS), which is evaluated through the micro-scale finite element analysis handling carbon fibers and resin separately. Fibers array uniformly in identical orientation in the model. So that, a unit cell is defined and the boundary conditions are given easily. The INS is defined as normal stress on the plane that is normal to the minimum length segment connecting central axes of conterminous two carbon fibers. Results of fatigue life tests changing fiber orientation angle in terms of the maximum nominal stress are modified to enhance rationality via INS, and the applicability of proposed methodology is discussed.

There is a risk that FW (Filament Wound) composite pressure vessel is exposed to a high temperature by a fire accident of Fuel cell Vehicles. This study aimed to develop a method to predict the relationship between the burst pressure and temperature when a Type-IIIFW/CFRP-Al composite pressure vessel was exposed to high temperatures. After exposure to 493 K to 573 K in the atmosphere for 60 minutes at high temperature, burst tests and tensile tests of the aluminum liner material cut out from the composite pressure vessel were performed at room temperature. The burst pressure decreased by about 15% due to the effect of thermal exposure. And the yield stress decreased by about 65 %.

In this study, the failure behavior of CFRP was analyzed in consideration of inelastic properties of an epoxy resin and fiber/matrix interfacial failure to investigate a micromechanical failure process. For this, an elasto-viscoplastic constitutive equation was employed to consider the inelastic properties of the matrix. The fiber/matrix interfacial failure behavior was modeled by the mixed mode cohesive elements. The failure behavior of a unidirectional CFRP subjected to a uniaxial tensile stress in the transverse fiber direction was analyzed using the finite element method. In the numerical demonstrations, three types of the interfacial strength models were analyzed to investigate the difference of failure processes for CFRP due to variation of interfacial strength of the cohesive elements. From the numerical results, it was confirmed that the macroscopic stress value, the microscopic stress distribution, and the failure process strongly depended on interfacial strength. A major failure mode is fiber/matrix interfacial damage in a low interfacial strength case and localized matrix failure by stress concentration between the fibers in a high interfacial strength case.

In this study, impact damage simulation of composite laminates was performed by the discrete element method (DEM). In the discrete element method, a structure is modeled by a large number of particles and the equation of motion is solved for each particle to track deformation behavior. This means that there is no need to generate a mesh when building a model and updating the motion of each particle, therefore it can reduce the costs of the numerical simulation of impact damage problems. In order to analyze the impact behavior of CFRP laminates by DEM, the extended discrete element method was introduced to express the behavior as the continuum and the spring parameters were adjusted to express anisotropy. In addition, springs are inserted in each interface between layers and the failure criterion based on the energy release rate was also introduced for representing the interface failure so that the simulation can consider the interlayer failure. In the numerical demonstration, a model of the cross-ply laminate was used and the out-of-plane impact load was applied by collision of a spherical object with an initial velocity. As a result, the progress of the transverse cracks and interlayer cracks were predicted in the simulation results.

A carbon nanotube (CNT) sheet with almost unidirectional orientation was fabricated from a CNT forest (three-dimension structure) that was produced by chloride-mediated CVD. The CNT sheet was laminated with unidirectionally carbon-fiber-reinforced plastic (CFRP) prepregs to produce a CFRP laminate with an interlaminar layer made of unidirectionally aligned CNTs. Tensile tests and mode II interlaminar fracture toughness tests (i.e., end-notched flexure tests) were conducted, and the effects of the interlaminar CNT layer on the tensile strength and fracture toughness were investigated.In the tensile test, there was no effect of inserting a CNT sheet having 1.25 μm thickness. The Young’s modulus of the CFRP with CNT sheet insertion were almost equivalent to that of normal unidirectional laminates. This is because the volume fraction of CNTs is much less than that of carbon fibers.The mode II fracture toughness decreased by the interlaminar CNT layer parallel to the carbon fiber. However, in the case of the CNTs inserted in the direction perpendicular to the carbon fibers, a significant increase in the mode II fracture toughness was obtain. Observation of the fracture surface revealed that the delamination crack extended in a zigzag manner through the CNT layer.

To clarify the influence of fine structure of metal surface on joining strength between carbon fiber reinforced plastic (CFRTP) and metal, CFRTP and Al with the nanometer scale (Type Nano) or the micrometer scale dimple (Type Micro) were joined by hot pressing, and joining strength was evaluated by single-lap tensile shear test and cross-lap tensile test. Additionally, to verify the influence of joining temperature on joining strength, joining plates were made under 230℃ and 240℃ conditions of joining temperature. It was obtained that shear strength of that the joining temperature was 230℃ was higher than 240℃ due to the decomposition of resin. But in the cross-lap tensile test, joining strength exceeds the strength of CFRTP at both joining temperatures of 230℃ and 240℃. The reason is that the weak phase containing the γ-crystal of PA6 was formed in the vicinity of the joint interface during hot pressing. Furthermore, Type Micro has higher shear strength than Type Nano because the anchoring effect of Type Micro is exerted more strongly than Type Nano in the single-lap shear test. Conversely, Type Nano has higher cross-lap tensile strength than Type Micro because Type Nano has stronger chemical joining force than Type Micro due to larger wet area of nano than micro.

The purpose of this study is to develop the dismantlable adhesive for improving recycling performance of cars and plains. For this purpose, we made the adhesive containing 10vol% of silicon carbide (SiC) particles in a thermoplastic resin, and irradiated microwaves to selectively heat only the adhesive, aiming at strength reduction. As a result of irradiation for 240 seconds, there was a temperature difference of nearly 100 degrees between the adhesive with SiC and the adhesive without SiC. Furthermore, as a result of the tensile test, it was confirmed that the strength of the adhesive without SiC was hardly lowered, however, the strength of the adhesive with SiC decreased 43%. From the above results, it was suggested that the disassembly of the adherent becomes easier by applying adhesive with SiC and microwave irradiation.

Piezo-impedance behevior of a unidirectional (UD) carbon fiber reinforced plastic (CFRP) laminate toughened with interlayers was investigated. The impedance and its phase angle of a UD CFRP laminate consisting of two prepreg sheets were measured during tensile loading. The electrodes were formed on both sides so that the measured impedance includes not only in-plane but also through-the-thickness components. A simple equivalent electrical circuit model was then proposed for qualitatively explaining the piezo-impedance behavior. The impedance and piezoimpedance exhibit large nonlinearity since the impedance strongly depends on the interlayer conductive fibers.

The effect of layer structure on the fatigue crack propagation behavior was studied by using center-notched specimens which were cut from injection-molded plates (IMP) of short carbon-fiber reinforced polyphenylene sulfide (PPS) at two fiber angles relative to the molding flow direction (MFD), i. e. θ = 0 deg. (MD), 90 deg. (TD). The short carbon-fiber reinforced plastics (sCFRP) plates have three-layer structure where the fiber orientation is parallel to MFD in the shell layer and is nearly perpendicular in the core layer. The thickness of IMP was 1 and 2mm. The fraction of the core layer increases with increase in the plate thickness. In the relation between the crack propagation rate, da/dN, and the stress intensity factor, ΔK, da/dN increases with increasing MD specimen thickness. Conversely, da/dN decreases for TD specimen. The relationship between crack propagation rate da/dN and crack tip opening radius, Δρ, converged compared with the relationship between da/dN and ΔK. By thinning the plate after fatigue testing, the crack length in the core layer was estimated. In the MD specimen, the crack length at the center of the core layer is longer than the crack length at the center of the shell layer. On the other hand, the crack propagated hardly in the core layer for TD specimen. The crack length distribution was predicted so that the crack tip opening radius at the crack front would be equal along the thickness. The prediction showed atendency close to the actual crack length distribution.

The effect of fiber orientation on fatigue crack initiation in injection molded plate of short carbon fiber reinforced plastics was studied by using hourglass type specimens which were cut from plates at two fiber angles relative to the molding flow direction (MFD), i. e. θ=0 deg. (MD), 90 deg. (TD). For comparison, a specimen without carbon fiber PPS (Polyphenylene sulfide) was also investigated. The short carbon-fiber reinforced plastics (sCFRP) plates have three-layer structure where the fiber orientation is parallel to MFD in the shell layer and is nearly perpendicular in the core layer. By thinning the plate to 0.4mm, a shell layer specimen was prepared in this study. From the S-N curve, MD specimen has higher fatigue strength than TD specimen. From the predicted crack growth curve, it is considered that assumed exist in PPS phase defects of about 50 μm. In the case of the high stress amplitude of the composite material, it seems that a large crack initiates at the beginning of the cycle. When a large crack initiates at the beginning of the cycle, the stiffness decreases rapidly, while when the crack propagated slowly, change in stiffness is gradual.

Fatigue crack initiation from the notch hole in the short carbon fiber reinforced plastic (sCFRP) plate is investigated in this study. The sCFRP was processed by injection molding and it is constituted from surface layers in which the fiber is parallel to the molding direction and an inner layer in which the fiber is perpendicular to the molding direction. The specimen with a notch hole at the center of the plate was fatigued by loaded to the molding direction (MD specimen) and the transverse direction (TD specimen). Diameters of the notch hole is 0.5mm and 2mm. As a result, the crack generated slightly faster in the specimen with the notch hole of 2mm than that with the notch hole of 0.5mm under the same notch bottom stress. In addition, the fatigue life decreases as the hole size increases. The crack propagation rate, da/dN, is equal regardless of notch hole size in both MD and TD specimen in the relationship between da/dN and stress intensity factor range, ΔK. Comparing the predicted and experimental results of the crack propagation curve, in MD specimen, regardless of the size of the hole, cracks initiated later than expected under the high stress amplitude and gradually propagated along the predicted crack propagation curve. In TD specimen, crack initiated and propagated along the predicted crack propagation curve.

Carbon fiber reinforced thermoplastics (CFRTP) are used for automobile structures, recently, due to their high performance and mass productivities. In this study, tensile properties of the press-formed discontinuous carbon fiber reinforced thermoplastics (CFRTP) plate were evaluated considering fiber orientation distribution. The fiber orientation was inspected nondestructively by using a Talbot-Lau interferometer. The result showed there was a large fiber orientation distribution across the whole area of the forming plates due to high compaction flow molding. Tensile test results showed two types fracture modes which characterized tensile strength of discontinuous CFRTP. Fracture planes of broken specimens were almost corresponded with the low fiber orientation of the fiber against the loading direction. The relationship between fiber orientation and tensile fracture in press-forming plate was revealed.

In this study, a small sphere impact test was carried out using a short fiber reinforced composite material to investigate damage due to impact. After impact test, the bending test was carried out and the residual strength was investigated. As the results, when the impact speed of the sphere is low, only indentations are formed on the surface of the specimen. It was found that radial cracks occurred in addition to indentations as the impact speed of the sphere increased. As a result of the four point bending test, the residual strength tended to decrease as the impact speed increased. FEM analysis of small sphere impact test was performed. It was found that when the small sphere impacts with the test specimen, compressive stress is generated inside the specimen just below the impact surface and plastic deformation occurs. After that, the small sphere bounced back and the indentation remained on the specimen.

Ti-Ni alloys show the shape memory effect (SME) and super-elasticity (SE) with the thermoelastic martensitic transformation, and are used in a wide range of fields such as automobiles, home appliances and medical devices. Their property depend on Ni contents whereas the Ni-rich Ti-51at%-Ni alloy having SE at shape recovery temperature (Af) ≤ 37 °C are used for interventional Radiology (IVR) devices such as stents and guide wires (GW). Ti-50Ni alloys with Af > 37 °C do not generally exhibit SE at body temperature. The commercially available Ti-51Ni alloy is subjected to 500 °C treatment for straightening. Since this treatment temperature is near the recrystallization temperature of Ti-Ni alloys, the stiffness of the stent is decreased. The Ni-rich precipitates such as Ti3Ni4 phase, which are considered a breakage factor, are inevitably decomposed in the parent phase matrix of the Ti-51Ni alloy. The purpose of this research is to obtain high-stiffness SE at body temperature by developing new processing technology of Ti-50Ni tubing, and its clinical application. In addition, this developed technology is applied to Ti-51Ni alloy tubes on the market.

The shape memory effect (SME) in Fe-Mn-Si based alloy is associated with γ ⇄ ε martensitic transformation. It is necessary to suppress the sliding deformation and preferentially generate the martensitic transformation in order to improve the SME. In this study, each specimen was rolled and/or annealed and then the elastic and shape recovery ratios of them were quantified by means of the bending mechanical test. Crystal structures also were inspected by electron backscatter diffraction (EBSD) for obtaining the phase distribution maps and then pole figures were determined. The elastic recovery ratio improved from 27% to 34% at the reduction ratio (RR) as 20% and to 40% at the RR 40% and the shape recovery did no change. After the 600°C annealing the shape recovery improved from 20% to 37% in the RR 20% specimens. Annealing at 800℃ decreased both elastic recovery and shape recovery. Due to the cold rolling the yield stress raising by bending test, which enhance the pseudoelastic effect, induced the increase of elastic recovery ratio. On the other hands, there are two reasons for improving the SME, the texture formation gains the martensitic transformation and the presence of dislocations and stacking faults suppresses slip deformation and becomes the origin of martensitic transformation.

Recently, Ti-Ni shape memory alloys (SMA) are used as temperature sensors and actuators. However, the driving temperature range of the shape recovery behavior of Ti-Ni SMA has been limited in the temperature range below 100 ° C. Therefore, more recently, Zr-Cu SMA has attracting attention as a high temperature SMA, which is driven in the temperature range over 100 ° C. However, because of the difficulty of microstructure control of Zr-Cu binary alloy, it is difficult to control the martenstic transformation behavior. Therefore, relatively little is known about the martensitic transformation behavior, shape memory effect and mechanical properties of ZrCu-based SMA. The objective of this study is to clear the effect of additional third element on microstructure, martenstic transformation behavior and mechanical properties of ZrCu based SMA. In this study, Zr-Cu-Al alloys with different alloy compositions were prepared by arc melting method. The alloy composition of the fabricated sample is Zr- (50-x / 2) Cu-xAl (x = 0, 2, 4, 6, 8 at.%). XRD tests were performed to investigate the effect of Al concentration on the microstructure. Vickers hardness tests were performed to investigate the effect of Al concentration on the hardness. As the results it was found that the Al concentration affects the microstructure and mechanical properties of ZrCu-based SMA.

In the process of downsizing and/or energy saving of actuators and devices, the application of functional materials has attracted attention in addition to the optimizing of design and system. Functional materials are the materials that have some additional responsiveness to electricity, magnetic field, light ,heat, etc. in addition to the function of the structural material. Shape memory materials are the materials that memorize a certain shape and returns to the original shape under certain conditions after deformation. In recent years, new shape memory alloys which have a different properties from the conventional types, such as higher performance, quick response and high durability etc., are developed, and applied research has been actively conducted on shape memory alloys due to their practicality. For improving the characteristics of shape memory alloys and taking advantage of these characteristics, a technique for adding some processing to the materials has been proposed in this paper. Such material modification aims to expand the application area of shape memory alloys and improve the characteristics of the actuator and device using shape memory alloys. The coating technique aiming at biomedical applications using carbon-based thin films, and the powder molding technique by severe plastic deformation aiming at improvement of material strength and magnetic property are reported.

In this research, we developed a shape-memory polymer synthesized via multiblock copolymerization. We have studied biodegradable shape-memory polymers (SMPs) for the medical applications targeting polymer stents and drug delivery systems. Biodegradable SMPs that were composed of crystalline polylactide and random copolymers of _D,L-lactide and ε-caprolactone (PLCL) were prepared (PLCL/PLA blends). By controlling the CL content ratio, the PLCL/PLA blends showed shape-memory behavior at around body temperature (~39.0° C). But the deterioration of the mechanical properties occurred through the microphase separation of PLCL and PLA. We, therefore, attempted to improve the mechanical properties of the biodegradable shape-memory polymers by the multiblock copolymerization of poly(L-lacticde) (PLLA) and PLCL through urethane bonds (PU(PLCL/PLLA)).

Differential scanning calorimetry (DSC) showed that the PU(PLCL/PLLA) sample had its glass transition temperature (T_g) at 41.5°C and its melting temperature (T_m) at 165°C. The existence of PLLA crystal structures was also confirmed from the results of the melting temperatures. Due to the crosslinking formed through the PLLA crystals, the PU(PLCL/PLLA) sample showed rubber elasticity above T_g. By the tensile testing at 80°C, it was revealed that the fracture strain and that the maximum stress were both increased to 4.00 and ~4200 kPa, respectively, which were 12.9 and 7 times higher than those of our previous samples (PLCL/PLA blend). By the elasticity recovery test, the recovery behavior was improved by the multiblock copolymerization of PLCL and PLLA.

Fe-based shape memory alloy is attempted to be applied to many engineering fields under different working environments loaded at various deformation rates. In the past, the rate sensitivity of forward stress-induced martensitic transformation behavior in Fe-SMA during quasi-static and impact tensile loading processes has been revealed successfully by measuring volume resistivity. However, the rate sensitivity of shape memory effect including reverse stress-induced martensitic transformation behavior during unloading and heating processes has been studied rarely after tensile loading. In this study, the rate sensitivity of volume resistivity in the Fe-SMA is experimentally estimated under unloading and heating processes after tensile tests at various strain rates by using the modified circuit based on Kelvin double bridge from the previous study.

Shape memory alloys are attracting attention as an intelligent material. In this study, we propose a functionally-graded TiNi shape memory alloy fabricated using combination of powder metallurgy and secondary working. This advanced material can be utilized as a medical guidewire exhibiting a bending rigidity varying from low to high along the axial direction and a driving element that performs the functions of both a temperature sensor and an actuator. The powder metallurgy process can easily control the composition ratio of Ti and Ni. The materials fabricated using the powder metallurgy process are generally not used severe deformation conditions because of their poor ductility. However, the materials that can withstand such deformation conditions must be considered, especially for developing a medical guidewire from sintered TiNi shape memory alloy. This study aims to clarify the relation between the secondary working conditions and fatigue properties of the sintered TiNi shape memory alloy. Further, we present the effects of secondary working on the fatigue properties of the alternating-plane mode of the material. The fatigue lives of all the sintered material were shorter than that of the wrought material, however, those of the materials subjected to the secondary working under appropriate conditions are improved. This may be mainly caused by an increase in the density because of the rolling.

This study is concerned with extremely large extension of helical coils when a densely wound coil is extended to roughly wound one. When shape memory alloy is used as an actuator, a helical coil is useful for getting large expansion resulting in large stroke. When a helical coil is deformed, the pitch angle increases to change from dense winding to sparse winding, and the coil diameter decreases simultaneously. It is noted that, since the deformation is accompanied by rotation, it is necessary to consider the rotation angle of the coil. In order to treat these geometrical changes correctly, elastic-plastic deformation analysis should be developed in place of using well-known formula in elasticity. First of all, we obtained the load-elongation characteristics in the elastic region. Next, an equation that gives the elastic deformation and shape memory strain boundary of the wire cross section that moves with increasing the elongation was derived on the basis of the maximum shear stress theory. The load-elongation characteristics at large deformation was calculated from the balance between the force acting on the wire cross section and the bending moment and torque of the external force. Numerical calculation was made to deal with mathematical expressions that cannot be expressed by elementary functions. The experimental results and the analysis results showed good agreement. Further study may be required for getting higher accuracy.

In recent years, polyurethane-shape memory polymer（PU-SMP）has been practically used as shape memory polymer materials in the world. PU-SMP has unique deformation properties such as shape fixity and shape recovery based on the properties of molecular motion differ above and below the grass transition temperature（T_g）. In PU-SMP, it is possible that T_g is set at around the room temperature. The tanδ of PU-SMP is close to the human body at around T_g. In this study, the indentation properties of PU-SMP foam were investigated for applying to the nursing-care robot. The biological soft and hard tissues are described by several densities and thicknesses of PU-SMP foam for simulating the human body. The model equation and the tendency of its coefficients were for the simulating. Then elements of the Functionally-graded shape memory polymer foam（FGSMPF）were calculated using the model equation. The indentation test was carried out with fabricated FGSMPF. The deformation properties of the FGSMPF and human finger were compered. The results obtained are summarized as follows. （1）In the indentation test, the maximum indentation depth of fabricated FGSMPF is close to that of the human finger. （2）It is possible to make a pseudo-finger surface for the nursing-care robot by FGSMPF using the constructed model equation.

A TiNi shape memory alloy is a representative of the intelligent materials, and it is applied in the medical and the mechanical engineering fields, etc. In this study, in order to expand the field of application of the TiNi shape memory alloy, we propose a functionally-graded TiNi shape memory alloy. We focused on the composition dependency of the transformation temperatures and changed the transformation temperatures in the material in a stepwise manner by the powder metallurgy process. The Ni composition was changed by 0.3% from 49.8 at% to 51.0 at% in the longitudinal direction of the TiNi shape memory alloy. The material fabricated by the spark plasma sintering was subjected to hot and cold rolling processes and shape memory heat treatment. We confirmed that the sintered material subjected to hot and cold rolling has a functionally-graded property of the transformation temperatures, and the cold rolling has effect on narrowing the transformation temperature range. It means that the material can transform completely even in small temperature change. Furthermore, from the digital image correlation during a tensile test including a shape recovery process, we found that the material fabricated by both of the hot and the cold rolling partially shows the functionally-graded properties of the deformation properties.

TiNi shape memory alloys have been using in medical devices such as self-expanding stents which expand narrow parts of blood vessels. However, if Ni ions in the TiNi shape memory alloy dissolve in the blood through a corrosion reaction, this may cause an allergy. Furthermore, fatigue fracture is expected to occur in the early stage because stents are placed in blood vessels for a prolonged amount of time. This study aims to improve the corrosion resistance and corrosion fatigue life by surface modification on TiNi shape memory alloy wires. To improve the corrosion resistance of the TiNi shape memory alloy wires, a passive layer was generated to its surface by thermal nitridation treatment. And then, we revealed that the corrosion resistance and corrosion fatigue life of the surface-modified TiNi shape memory alloy wire by an anodic polarization test and a rotating bending fatigue test in NaCl water solution. The results obtained in this study are summarized as follows: (1) The corrosion resistance of the thermal nitrided material is much better than those of the normal heat treated and the polished material. (2) The electrolytic polishing before the thermal nitridation treatment has little contribution to the corrosion fatigue life. (3) The corrosion fatigue life of the electrolytically-polished material is much higher than those of the any other materials prepared in this study.

Our study group devised and fabricated the shape memory alloy (SMA) actuator using weft-yarn SMA element, which is consisted of a long SMA wire woven into the weft pattern for the increase of the output of SMA actuator. The increase of cooling efficiency of SMA is important to increase the responsiveness of this SMA actuator. In previous research, warp-direction alumite wires were installed in the weft yarn SMA element for the increase of the cooling efficiency. However, this alumite wires were not completely fixed in the previous actuator. Thus, new fixing system were devised and fabricated. In this study, the effects of installed the number of warp-direction alumite wires on dynamic characteristics of weft yarn-type shape memory actuator is investigated. As a result, the operating velocity from initial position to desired position decreases with the increase of number of alumite wires regardless of the weight condition. On the other hand, the operating velocity from desired position to initial position is almost constant regardless of the weight condition and the number of warp alumite wires. These tendencies are thought to be caused by the variation of the cooling efficiency and the friction between SMA wire and almite wires with the variation of the number of alumite wires.

The development and application of smart materials and structures have attracted worldwide attention. Shape memory alloy (SMA) has activated the research on smart materials and structures due to a unique characteristics of shape memory effect and superelasticity based on the martensitic and the reverse transformation. The TiNi SMAs are among the best SMAs since corrosion resistance is higher, the fatigue life is longer and cyclic deformation property is better than other SMAs. In this study, transformation-induced creep recovery (TICR) of TiNi SNA under constant stress in subloop is considered for the practical application. In particular, the behavior of a transformation band that appeared on specimen surface in TICR after loading until 8% strain followed by unloading to the strain of 6% was investigated. The TICR occurs due to small temperature change and reverse transformation band is generated after unloading followed by keeping constant stress, then the strain recovers to 1%.

The straight-shaped Ti-Ni shape memory alloy (SMA) element shows negative stiffness during post-buckling deformation, and the deformation is recovered by the removing of external force (super-elastic effect). Negative stiffness is applied to the vibration-free system using zero-stiffness structure. We devised and fabricated the vibration-free system using the negative stiffness of tape-shaped SMA element, and the vibration characteristic of this system was investigated. As the results, this vibration-free system shows the excellent vibration characteristic. However, the post-buckling behavior of SMA is varied by the variation of fabrication condition, use conditions and the dimension of SMA element. In previous studies, it is cleared that the stiffness of SMA element during post-buckling deformation is affected by the Yung’s modulus of Austenitic-phase and Martensitic-phase. In this study, effects of material thickness on the buckling properties of SMA element are investigated by finite element method (FEM) analyses. The negative tangential stiffness of SMA element during post-buckling deformation increases with increase of the thickness of SMA element. Furthermore, the decreasing of volume faction of Martensitic-phase during buckling deformation leads to the increase of negative tangential stiffness of SMA element during post-buckling deformation. From these results, it is thought that the increase of negative tangential stiffness of SMA element during post-buckling deformation is due to the decrease of volume faction of Martensitic-phase during buckling deformation.

In this study, we applied a new surface modification technique, scanning cyclic press (SCP) to machined specimens of low carbon steel S25C. In the SCP process, a vibrating indenter scans the metal surface and cyclically applies a variable low compressive load to modify the microstructure at the surface. During applying the SCP, the processing parameters, such as the scanning speed, the amount of compressive load and the number of cyclic loading, were changed to investigate the effects on the microstructure and the fatigue property. After applying the SCP, the surface roughness of specimens became smaller than that before. To clarify the changes in the microstructure beneath the surface of the SCP-treated specimens, the cross-section was created using focused ion beam technique. Scanning ion microscopic observations showed the microstructure refinement at the surface layer. The microstructure refinement was promoted with increasing the scanning speed, and the surface hardness also increased. Additionally, at the higher compressive load and the larger number of cyclic loading, finer microstructure was observed at the surface layer above the modified microstructure. This result suggested that the microstructure refinement is promoted by increasing the amount and the number of compressive loading. Fatigue test results showed that the fatigue lives of the SCP-treated specimens were 2 to 50 times longer than that of non-treated specimens.

It is known that fatigue strength of fretting fatigue is lower than that for the smooth specimens, and fretting fatigue limit is considered to disappear. In order to improve fretting fatigue strength, there is a solution providing a stress relief groove at a contact end for railway axles.(1) The purpose of this study is to examine the feasibility of the improvement in very high cycle fretting fatigue strength using stress relief groove. Fretting fatigue tests were conducted using an ultrasonic fatigue testing machine for specimens without or with stress relief groove. As a result, it was found that fatigue life was much improved by providing the appropriate stress relief grooves when stress concentration factor was appropriate.

This study presents investigation of analytical technique using the smoothed particle hydrodynamics (SPH) method and the finite element method (FEM) for micro drilling by water jet technology. Water jet is used for processing various materials, but simulation method of water jet has not been established. In this study, motion of water was analyzed by SPH method and deformation of solid body was analyzed by FEM. In order to increase the quantity of water that can be handled, the analytical technique was also examined for the case where both water and the work material were expressed as the SPH model. Validity of analysis methods was clarified.

In this study, the punching rivet method was applied to joining of a CFRP sheet and an aluminium alloy sheet. The conventional riveting method requires drilling process of sheets. In the punching rivet method, punching and joining of two sheets can be carried out at the same time, so it is not necessary to drill in advance. Therefore, production efficiency is excellent. A tensile test was conducted to examine the strength. All joints fractured from the CFRP sheet side in tensile testing. From the experimental results, ( )1 The strength of the rivet with a hole up to the head was lower than that of the hollow rivet, ( )2 The strength was increased by eliminating the chamfered part of the rivet holder. A cross-section of the joints was observed to examine the fastening condition. In both cases, delamination was observed in the CFRP sheet around the rivet shaft. This damage may cause reduction in the joint strength. Therefore, it is necessary to examine the countermeasures for suppressing the delamination.

In automobiles, the crash box is provided to absorb impact energy in the traffic accident. The impact energy is absorbed by deforming the crash box when a collision accident occurs, and the safety of the occupant is protected. Crash boxes are required to reduce sudden changes in acceleration transmitted to passengers by the low initial impact load, and to absorb a great deal of impact energy. Currently, a simple square tube structure is used for the crash box. However, the simple tube structure has some problems such as insufficient strength and buckling. To solve this problem, this study proposed a model in which the inside space of the thin polygonal tube is reinforced by a honeycomb structure. By analyzing the model using finite element analysis software LS-DYNA , we examined^® changes in load, displacement and deformation which are caused by reinforcement with a honeycomb structure. From these analysis results, effects of the honeycomb structures inserted to the inside space of the thin polygonal tube on the energy absorption characteristics were demonstrated.

Recently, gold is expected as a new material for micro bumps in 3D integration structures because its Young’s modulus and Yield stress are lower than that of conventionally used Cu, and its CTE (Coefficient of Thermal Expansion) is closer to that of Si. In addition, since gold has high ductility, it’s appropriate for the flexible bumps. However, the micro structure of gold thin films was found to vary drastically depending on their manufacturing method and thermal history, and the mechanical properties show wide variation. Therefore, it is important to control the micro structure of gold thin films to assure the reliability of the micro bumps in semiconductor devices. Thus, in this study, uniaxial tensile test was applied to the as-electroplated and annealed copper thin films and the cross-sectional texture of the films was observed by SIM (Scanning Ion Microscope), to clarify the micro texture dependence of their mechanical properties. It was found that mechanical properties of the electroplated gold thin films varied from brittle to ductile by annealing at 400℃ for 30 minutes. However, from the observation of the cross-sectional texture, no significant change of micro texture was confirmed. Therefore, crystallinity and crystal orientation based on the order of atomic arrangement were important factors of the variation of their mechanical properties.

Two-dimensional (2D) materials have attracted attentions as unique functional materials. Among them, graphene sheet(GS) is well-known as a fundamental structure of 2D materials of nano-carbon. In this study, we focus on the geometrical properties and the fundamental mechanism which can explain the different shape of out-of-plane deformation in 2D carbon nano-materials, due to the different types of arrangements of lattice defects. Equilibrium configuration by using molecular dynamic method. According to the origami and kirigami approach, we build atomic model of GS with hierarchical lattice defects, we discuss the relationship between site potential energy and curvature in details. The fundamental knowledge obtained would be applicable to desgin and control the deformation of 2D materials.