Various models are proposed to composite materials including clusters of reinforcements. In such models, the concentration of reinforcements in the cluster is generally very high compared to that of the outside region of the cluster. However, in realistic composite materials, the difference of concentrations inside and outside of the cluster is relatively small. In the present study, the so-called low-level cluster model constituted by misoriented reinforcements is analyzed by the modified mixed double inclusion method proposed by authors and stresses and strains induced inside and outside of the cluster are expressed explicitly. The total internal stresses obtained are good agreement with results obtained by Taya et.al (JSME A,43-1(2000), pp.46-52.) in the high-level clustering region.

Equivalent expression is derived for a mixed double inclusion which is a misoriented inhomogeneous inclusion embedded in an inclusion and it is developed to the composite material containing many clusters consistent of many misoriented inhomogeneous inclusions. Then the macroscopic electric-fields and the die-electric constants of such composite materials are analyzed by the resultant equivalent expression. As a result, the magnitude of total internal electric-field occurring in the composite material is good agreement with that obtained by Taya et.al (JSME A,43-1(2000), pp.46-52.) under high-level clustering condition.

Composite materials containing many clusters of reinforcements are analyzed by using the double inclusion method presented by one of the authors and the solutions of stress and strain are expressed explicitly in the previous report. Stresses occurred in such a composite calculated from the solution are good agreement with the results obtained by Taya et.al (JSME A,43-1(2000), pp.46-52.) in the case of the condense cluster in which the volume fraction of reinforcements is larger than that in its surroundings. However, no results have been reported in the case of the dilute cluster with smaller reinforcements so far. Therefore, FEM analysis may be performed to the dilute cluster problem. In the present report, the procedure of elements subdivision for FEM analysis is examined by taking the Dirichlet tessellation for the dispersed reinforcements into consideration.

Micromechanical analysis is performed to the chopped-strand fiber reinforced composite material containing many clusters constituted by misoriented fibers. Equivalent expression for the fiber can be derived by using assumptions of both flat-ellipsoidal cluster and axisymmetric type of distribution of fibers. Then strain and stress fields induced in the composite are expressed explicitly. Moreover, by introducing the stereological concept into the model, the probability density function of profiles of fiber appeared on the side of plate can be expressed to the model.

Off-axis compressive creep recovery behavior of a unidirectional thermoplastic carbon/polyamide composite has been examined under different load histories. Creep recovery tests after loading up to a certain level of stress are performed on off-axis specimens of different fiber orientations. Creep recovery after prior creep is also examined. Maximum stress levels of prior off-axis loading are determined in such a way that they are associated with an equal effective stress based on the Sun-Chen one-parameter plasticity model. The creep recovery tests showed that recovery strain tends to increase with increasing total strain due to prior loading. In case prior creep deformation involved, the subsequent recovery strain and non-recovery strain turn to be larger than those without prior creep history. It is suggested that off-axis creep deformation of UD CFRTP contains recoverable and non-recoverable strain components.

The statistical distributions of fatigue life data of stampable CF/PA composite at different stress ratios are identified, and a procedure for predicting the S-N relationships for any stress ratios that consider the influence of probability of failure P is proposed. Constant amplitude fatigue tests are first performed at different stress ratios R = 0.1, 10, χ (≡ σ_UCS/σ_UTS), respectively. To identify the statistical nature of fatigue life, a lognormal distribution and a Weibull distribution are applied to the fatigue data obtained, respectively. The anisomorphic constant fatigue life (CFL) diagrams for different probabilities of failure (P = 10%, 50% and 90%) are constructed using the P-S-N curves for the critical stress ratio. It is shown that the P-S-N curves for different stress ratios can successfully be predicted using the probabilistic methodology proposed in this study.

A method for predicting fracture toughness of a given laminate of any stacking sequence on the basis of the fracture toughness values of the constituent unidirectional plies is proposed. The laminate toughness prediction method is combined with a finite fracture mechanics (FFM) model to predict the notched strengths of two kinds of quasi-isotropic laminates. Static tension tests are first carried out on specimens with a circular open hole for four different values of hole-diameter to specimen-width ratios, respectively. The quasi-isotropic laminates of different stacking sequences exhibited similar notch sensitivity. Slightly smaller values of notched strength were predicted using the FFM model predicted with the ply-based laminate fracture toughness prediction method, regardless of notch size.

Because mechanical properties of composites reinforced by fiber significantly owe to fiber orientation, fiber waviness plays an important role. In this study detailed measurement of fiber stress along fiber in unidirectional composites was carried out by using micro-Raman spectroscopy. Micro-Raman measurement was done along waved fiber in mm spatial resolution to clarify local load-shearing. As the result, fiber stresses on longitudinal and transverse plain are almost uniform in case of without waviness. On the other hand, in a specimen with waviness fiber stresses along fiber are smaller in a waved fiber than in a straight fiber. They show almost uniform distribution in low tensile load but become un-uniform with increase in loading.

Ropes made of ultrahigh molecular weight polyethylene fibers (one of synthetic fibers) are used for tendon driven robots because the synthetic fibers have both high strength and flexibility. In general tendon driven robots, the rope is wound around a grooved pulley. Although tension is applied to the rope by frictional force on the groove surface, the relation between the groove shape and the frictional force for the synthetic fibers is unknown. Since the rope is made of a lot of fibers, it requires a huge calculation cost to finely model and analyze the fiber. In this study, we propose a method to analyze frictional force between rope and pulley. We treat the rope as a homogeneous elastic body to reduce the calculation cost. By comparing with experimental results, we examined the effectiveness of the method. The tendency of analytical results agreed with experimental results.

Currently 3D printing technology has attracted much attention as new molding methods for carbon fiber reinforced thermoplastics (CFRTP). The purpose of this study is to investigate in situ resin impregnation during 3D printing of continuous carbon fiber reinforced plastics. The specimen was manufactured by fused deposition molding (FDM) using carbon fiber yarn including 1000 mono-filaments and polylactic acid (PLA) filament. Resin impregnation ratio measurement and tensile tests were conducted on the molded specimens in order to investigate the effect of process parameters during 3D printing on mechanical properties. As a result, good resin impregnation and mechanical properties were obtained for specimen with molding temperature more than 200 °C.

Carbon fiber reinforced plastics (CFRP) has high strength and light weight properties. However, for impact load, an improvement has been required since resin matrix is weak. For CFRP manufacturing, the authors have developed the electro-activated deposition resin molding (EDRM) method to impregnate the resin, and the strength of CFRP for the static load was increased by the application of CNF without hydrophobic treatment. In this study, applying EDRM method, the impact strength of CFRP was improved by using cellulose nanofiber (CNF) without hydrophobic treatment. The electro-activated deposition solution contains polymer with epoxy group. The method was to apply the mixture of CNF dispersion and electro-activated deposition solution to the prepreg. After thermal curing, the CFRP which has CNF layer on the surface was obtained. The three-point bending test was carried out to confirm the static strength, and the impact strength was also confirmed by the Charpy impact testing.

We reported an experimental study in which we investigated the nominal tensile strength of the thin MWCNTs prepared by thermal chemical vapor deposition method, and to provide insight required to enable the development of multi-walled carbon nanotubes (MWCNTs) with superior mechanical properties, we investigate the nominal (engineering) tensile strength–fracture strength–fracture cross-section ratio relationship of MWCNTs by conducting uniaxial tensile tests. The average outer and inner diameters of the MWCNTs are 7 nm and 5 nm, respectively. The fracture strength and nominal tensile strength of the MWCNTs are 13.9 ± 9.6 and 10.8 ± 6.9 GPa, respectively, which is markedly higher than those of previously reported CVD-grown MWCNTs. The MWCNTs have fractured in a clean break manner (i.e. large fracture cross-section ratio), and exhibited the higher fracture strength, which led to the higher nominal tensile strength. This result suggests that the fracture morphology and nominal tensile strength of CNTs are affected by the degree of inter-wall crosslinking, and there is an optimal crosslinking density for increasing the nominal tensile strength; not too low but also not too high, to permit an adequate load transfer between the nanotube walls. Our finding can help in developing better strategies for CNT composites and yarns that effectively exploit the superior mechanical performance of CNTs.

Poly(lactic acid) (PLA) has attracted much attention as typical bioabsorbable materials. In this study, the effect of higher order structure before drawing on mechanical properties of PLA screws were investigated. PLA screws were prepared through a series of routes including compression-molding or casting, extrusion and forging. Crystallinity and orientation function as the higher order structures, and shear strength as mechanical properties were measured. As a result, the shear strengths of the screws of casted billets were higher than that of the screws of compression-molded billets. The orientation function of casted billet was higher than that of the compression-molded billets for low extrusion ratio. The orientation function of screws from extruded billets showed opposite tendency to that of billets before extruding. On the other hand, crystallinity of screw was constant irrespective of the extrusion ratio and molding method. Those results suggested that shear strength of screw mainly depend on molecular orientation. Crystallinity of rod which molded by casting and extruded in extrusion ratio 1 was about 37 % while that of compression-molded rod was about 20 %. And, orientation function of casted billet decreased from 0.5 to 0.35 by extrusion in extrusion ratio 1.3 while that of compression-molded billet increased from 0.1 to 0.6. These results might be due to a difference in crystal size induced during cooling in the molding of billets.

In this study, we performed the equilibrium free swelling and subsequent uniaxial tensile tests of polyacrylamide hydrogels to investigate the interactions between swelling and elasticity. The experimental data were reproduced by adjusting two scaling exponents of the extended Frory-Rehner model. We discussed the tendency of a set of the obtained material parameters.

A bimodal structure material, in which coarse grain region is surrounded by fine crystal grain region, shows excellent mechanical properties of both high strength and high ductility. However, the mechanism has not been fully understood yet. In this study, we perform tensile deformation tests of bimodal structure models with different finer grain networks via molecular dynamics simulations to investigate the influence of the network on the formation of shear bands that strongly govern the strength and ductility of bimodal materials. Three types of the network are considered: the shapes of the network are squire lattice, triangle lattice, and honeycomb structure. Initial plastic deformation occurs in fine grain regions because of the stress concentration due to the bimodal structure. As plastic deformation progresses, the crystal slips in each finer grain gradually combine to form a shear band in the finer grain network regions when the coarse grain regions are not an extension of the propagation direction of the shear band. Therefore, it is found that the shape of the finer grain network is important factor to design excellent mechanical properties of bimodal structure materials.

Excellent mechanical properties can be obtained by composing different nature materials. In this study, we investigate the mechanical properties of composite materials of crystal and amorphous phases through molecular dynamics simulations. Composite materials consist of two kinds of atoms with different radii, atom S and atom L. To obtain various composite structures, the mixing rate of atom S and the interatomic force between S and L are changed. All atomic models are classified into the single phase of crystal structure or amorphous structure and the multi phases of crystal and amorphous structures. Tensile deformation tests reveal that the strength of the multi phases shows smaller than that of the single phases; hence, the heterointerface between crystal and amorphous regions possibly weakens the strength of the composite materials.

Two-dimensional (2D) materials have attracted attentions as unique functional materials. Among them, graphene is well-known as a fundamental structure of 2D materials of nano-carbon. In 2D materials, lattice defects, such as dislocations and disclinations, cause out-of-plane deformation. In this study, we focus on the fundamental mechanism which can explain how the shape of 2D materials with defects is determined. Typical four structure models of GS with defects are studied, i.e. positive perfect wedge disclination, negative perfect wedge disclination, positive partial wedge disclination, and negative partial wedge disclination. The partial wedge disclinations are implemented by the array of edge dislocations in which the local structure consists of pentagon-heptagon atomic bonds. The equilibrium configuration is calculated by using large-scale atomic/molecular massively parallel simulator. After a detail examination, we found the site potential energy is proportional to the square of curvature. The fundamental knowledge obtained would be applicable to desgin/control the shape of 2D materials.

In this paper, a phase-field method for brittle fracture is presented. In this method, the diffuse fracture is supposed. Here, the order parameter of the phase-field model takes values between zero and one, which correspond broken and unbroken regions, respectively. The elastic strain energy field and the fracture field are coupled with each other and solved separately. This model are does not need any predefined cracks and it can simulate complex fracture paths, branching and crack coalescence. In this paper, the simple example is presented for the purpose of the explanation of the formulation of this model and the benchmark of the model.

In this study, temperature dependence of elastic-viscoplastic behavior of plain-woven glass fiber-reinforced plastic (GFRP) laminates is experimentally observed, and the behavior is analyzed using a triple-scale homogenization analysis method. In the triple-scale homogenization method, first, an analysis model is defined, in which a plain-woven laminate is regarded as a macro structure, plain fabrics and a matrix as a meso structure, fibers and a matrix in fiber bundle as a micro structure. Tensile tests of a plain-woven glass fiber/epoxy GFRP laminate are conducted under several temperature and strain rate conditions, showing its strong dependence on temperature. Based on the experimental results, temperature-dependent elastic-viscoplastic parameters of the epoxy are identified. Using the obtained parameters and the triple-scale homogenization method, elastic-viscoplastic behavior of the plain-woven GFRP laminate subjected to on-axis (0°, 90°) and off-axis(15°, 30°, 45°) loading is analyzed at several temperature conditions. It is found that the elastic-viscoplastic properties of the plain-woven GFRP laminate significantly depends on the temperature conditions and loading directions. It is also shown that the analysis results are in good agreement with the experimental data in various temperature conditions and loading directions.

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. On the specimens rolled as 20% and 40% reduction rate the axis of some crystals oriented to one of [001], [2√2 0 1], [√2 √6 1], [√2 √6 1] and the other of [001], [2√2 0 1], [√2 √6 1], [√2 √6 1]. The elastic recovery ratio improved from 27% to 36% 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 50% in the RR 20% and 40% specimens. Due to the cold rolling the yield stress raising and martensite phase expanding, which enhance the pseudoelastic effect, induced the increase of elastic recovery ratio. On the other hand there are two reasons for improving the SME, the texture formation gains the martensitic transformation and the austenite phase, which easily transforms martensite phase, generates.

Distributions of the leaked magnetic flux vectors are measured at the side of specimens before and after Charpy impact test with or without magnetic field from permanent magnets. Although specimens were demagnetized before the test, neighbors of the notch and the crack line of the specimen were magnetized after the impact test under magnetic field. These phenomena were explained form magnetomechanical effect by Pittman, which makes residual magnetization of ferromagnet metal approach into anhysteresis curve after plastic deformation. Since partial asymmetric residual magnetizations were observed near the crack line, we can estimate intense and area of plastic deformation during the impact test in this anhysteresis residual magnetization method.

Steel materials typified by carbon steel are one of the important materials of social infrastructure. When handling these materials in numerical simulation, they are generally modeled as homogeneous continuum. This is because microscopic structures having various orientations are randomly combined, and as macroscopically can be considered as an isotropic body as a result. On the other hand, each such microscopic structure has a complicated structure. The microscopic structure of steel, in particular the kinematic behavior of cementite, is closely related to the ductile fracture of steel materials. That is, cementite in the steel material cracks during loading, and the void formed in the cracked region spreads, eventually leading to ductile fracture. When considering this series of phenomena as the progress of damage inside the material, it is interesting to see how the damage progresses. In the concept of continuum damage mechanics, not only general variables used for description of deformation, but also new internal variables with unique relationship with internal damage process are introduced to express damage inside the material. Alternatively, using multi-scale analysis based on homogenization method, it is possible to reflect the mechanical response of the microscopic structure as the material property of the macroscopic structure. By using this method, we can express internal damage of the material without preparing new variables representing damage. In this research, we focus on the growth behavior of the voids in the microstructure of the carbon steel after voids are generated, and propose a simple model that characterizes the growth phenomenon. Then, using this model, we perform micro and macro structure coupled analysis based on homogenization method.

Steel is one of the most important materials because of its wide strength level by using heat treatment and processing. Martensitic transformation can strengthen steel highly because shear deformation introduce a lot of dislocations during the transformation. Dislocations can be inherited into martensite or austenite through forward or reverse transformations. So we can generate strengthened austenite by the reverse transformation at an unrecrystallization temperature in order to keep dislocations in austenite. In addition, by the transformation to the martensite again, the martensite with dislocations in the austenite is generated, resulting in strengthened martensite. In this work, we performed microstructure observation and tensile properties of the lath martensite formed from unrecrystaliized austenite in an Fe-24Ni alloy, and compared the results with those from recrystallized austenite. Before the tensile tests, we characterized blocks in the lath martensite by color etching. Characterized blocks enable us to acquire digital image correlation based strain mapping corresponding to blocks. Electron back scattered diffraction analysis showed higher density of high angle boundaries in the martensite formed from unrecystallized austenite. The tensile strength was higher in the marteniste formed from unrecrystallized austenite. The deformation behavior was discussed with the strain maps.

Conventionally, anodizing treatments have been performed on aluminum alloy combustion chamber of internalcombustion 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 (ROM) to evaluate the tensile properties 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 stress-strain relation of the alumite layer had peculiar shape.

Much attention has been paid to composites made of biocompatible ceramics and metals in medical field because the composites are expected to exhibit high biocompatibility and mechanical performance. The properties of an interface made of these ceramics and metals should be investigate to evaluate the mechanical properties of the composites. In this study, we fabricated square pillar-shaped specimens with a macroscopic interface made of the combinations of SUS316L, pure titanium, alumina, and partially stabilized zirconia (PSZ) by spark plasma sintering. Then, their strength and fracture toughness were evaluated by bending test and indentation test. Note that no specimen with the interface made of aluminaPSZ and SUS316L-PSZ could be fabricated due to some flaws which occurred during sintering. As for the influence of material combination on bending strength, the bending strength of SUS316L-Ti was the highest. Similarly, the fracture toughness of SUS316L-Ti was also the highest. However, the fracture toughness of SUS316L-Ti was around 8 MPa √m , and that was much lower than that of monolithic SUS316L and Ti.

Stress corrosion cracking (SCC) is caused by a specific combination of applied stress, environment and material. This study deals with the nucleation of SCC in sensitized type 304 austenitic stainless steel based on a strain-based approach. Prior to testing, strain distributions on a specimen surface subjected to an applied strain of 1% were measured by the digital image correlation technique. Then, SCC testing was conducted in a tetrathionate solution under a constant load corresponding to the initial strain of 1%. After testing, crystal orientation on the specimen surface was measured by the electron backscattered diffraction technique, and the relationship between grain boundary character and strains at cracked and uncracked grain boundaries were investigated. As a result, stress corrosion cracks tended to preferentially initiate at the random boundaries where normal strain, shear strain or both strains were high.

The intermetallic compounds (IMCs) layer consisting of Cu3Sn and Cu6Sn5 is generated at the interface between solder and copper wiring during a soldering process. The strength reliability of fine solder joints must be evaluated by conducting a finite element analysis (FEA) considering the presence of the Cu/Sn IMCs layer. To conduct such FEA accurately, the tensile stress-strain curves of the Cu/Sn IMCs are needed. Therefore, we proposed a method to estimate the stress-strain curve of the IMCs by conducting the tensile test using the miniature composite solder specimen with IMCs layer. The stress-strain curve estimated by the proposed method suggested that the both IMCs showed material nonlinearity. This finding should be verified in some way, because the Cu/Sn IMCs have been conventionally regarded as brittle materials which does not show plastic deformation. In this study, the shear tests using a copper-solder joint specimen, in which the solder joint has the IMCs layer, were conducted both experimentally and numerically. The numerical tests were conducted as FEAs, and the FEAs employed two different constitutive models of elastic model and elasto-plastic model to describe the deformation behavior of the Cu/Sn IMCs. Based on the experimental results and the results of FEAs, we discussed the presence or absence of the material nonlinearity of the Cu/Sn IMCs.

In past studies, the small punch (SP) test using SUS304 at different deformation rate is investigated and rate sensitivity of its fracture behavior is reported. The microstructure including strain-induced martensitic transformation greatly influences the fracture mode. However, the influence on the rate sensitivity of the fracture behavior is still unclear. To make clear the rate sensitivity, the previous result must be verified. Considering the actual application of SUS304, the SP test with deflection rate over 103/s is necessary to be investigated. In this study, the impact SP test based on the split Hopkinson pressure bar method is performed and the fractographic study using the scanning electron microscope is conducted. For a comparison, the SP test under a quasi-static condition is also carried out. From the above tests, the rate sensitivity on fracture behavior in SUS304 is investigated. As the result, negative rate sensitivity of maximum force as well as deflection and positive rate sensitivity of work-hardening can be captured in the force-deflection curves. In the fractographic study, a fracture surface due to a glide plane decohesion which cannot be seen in impact SP test is confirmed in the quasi-static SP test while elongated dimples can be observed in both conditions.

This study aims to design optimum partial reinforcement with curvilinear fibers to maximize tensile strength of laminate composites with an open-hole. The authors compared difference of strain distributions around the hole of CFRP plates with or without curvilinearly shaped reinforcement by numerical and experimental studies. In the numerical study, optimum shape of reinforcement fiber is determined by using Distributed Genetic Algorithm. Obtained fiber shape reduce maximum principal strain by 36 % compared with without reinforcement. The CFRP plate specimens based on by the numerical optimization are fabricated by the tailored fiber placement machine and VaRTM method, and strain concentration around hole is measured by the tensile test. The experimental result showed reinforcement with curvilinear fiber successfully reduced strain concentration around the hole Thus, it was revealed that the reinforcement with curvilinear fibers is effective to improve strength of open-hole laminate composites.

In discontinuously-reinforced composites, their mechanical properties are affected by cracking or debonding damage and size of reinforcements such as particles or short-fibers. This presentation deals with a constitutive model of discontinuously-reinforced composites which can describe the matrix plasticity, evolution of cracking or debonding damage of reinforcements, and reinforcement size effects on deformation and damage. The model is developed based on the Eshelby equivalent inclusion method and Mori-Tanaka mean field concept, and can be applied to particulate- or short-fiber-reinforced composites with progressive cracking or debonding damage. Influence of progressive damage and size of reinforcements on the stress-strain relations of the composites are demonstrated based on the numerical results by the model. Furthermore, a finite element method (FEM) is developed based on the model for particulate-reinforced composites with progressive debonding damage. As an example, FEM analysis is carried out on a crack-tip field in the composites, and the influence of particle size and debonding damage on an elastic-plastic singular field around a crack-tip is discussed based on the numerical results.

Sub-surface fractures are known as a characteristic fatigue mode of high strength steel in the very high cycle region. To reveal the mechanism of sub-surface fractures, ΔK-decreasing tests were conducted in high vacuum and air environments based on the idea that sub-surface cracks are exposed to a vacuum-like environment. As a result, fatigue crack growth rates in high vacuum were slower than those in air. In high vacuum, fatigue crack growth still took place at a considerably low rate around 10^-13 m/cycle, and the effective threshold stress intensity factor rage (ΔK_eff) in high vacuum lower to that in air. Unlike in air environment, negligible effects of oxide-induced crack closure in vacuum was considered as the reason for the results. The behavior of fatigue crack growth properties in high vacuum well explained the characteristics of sub-surface crack growths in very high cycle regime.

In recent years, it has been revealed that in high strength steel, fracture occurs from non-metallic inclusions inside of the material in the very high cycle region. However, because of the difficulty in directly observing the internal fatigue process, its mechanism is largely unknown. In this study, we try to clarify the internal fatigue process on the basis that the environment around an internal crack is like a vacuum environment. Also, it has been reported that the effect of vacuum environment is more prominent in the initial stages of crack propagation. This suggests that targeting the crack propagation process of small cracks, like that with the same size as a small defect, would shed light on the controlling factors in the internal fatigue process. Thus, we conducted ΔK-increasing tests in air and in vacuum using specimens with a small artificial defect on its surface. As a result, fatigue crack propagation rates in vacuum were lower than that in air. Also, when compared with the results of long cracks, the crack propagation rates in both environments were much higher. However, after eliminating the effect of crack closure for the results of long crack, the crack propagation rates in both environments matched.

The purpose of this study is to investigate the effects of surface oxide layer on fatigue crack propagation properties of copper (Cu) nano-films. Fatigue crack propagation experiments in about 500-nm-thick Cu nano-films with native and thermal surface oxide layer were conducted under in situ field emission scanning electron microscopy (FESEM) observation. Specimens with native surface oxide layer with a thickness of about one nm, and with thermal surface oxide layer with a thickness of about six nm were fabricated. Experimental results showed that the fatigue damage area around crack in Cu nano-films with thermal surface oxide layer was narrower than in Cu nano-films with native surface oxide layer. Fatigue crack propagation rate (da/dN) is roughly the same in Cu nano-films with thermal surface oxide layer and with native surface oxide layer in lower stress intensity factor range (ΔK). However, da/dN of Cu nano-films with thermal surface oxide layer is slightly faster than that of Cu nano-films with native surface oxide layer in higher ΔK.

Two types of nickel thin films with grain sizes of 384nm(UFG : Ultra Fine Grain) and 17nm(NCG : Nano Crystalline Grain) were created by electrodeposition using a nickel sulfamate bath. Nickel thin film of 10μm thickness was adhered to a through elliptical hole in a base plate and was fatigued in accordance with the displacement constraint along the hole circumference in the base plate subjected to a cyclic stress. Commonly, a stress intensity factor range was increased with a crack propagation under a constant stress amplitude but was decreased toward the hole edge because of the difference in thickness between the film and the base plate in this ΔK-decreasing test method. As a result, the figure crack propagation rate da/dN, is higher for NCG than for UFG in the relationship between the da/dN and stress intensity factor range, ΔK. The crack opening stress was evaluated by DIC(Digital Image Correlation) method and effective stress intensity factor, ΔK_eff was estimated. Relationships between da/dN and ΔK_eff are almost equal UFG and NCG.

The cold-rolled pure titanium film has a rolling texture in which the c-axis is inclined by 30 degrees with respect to the rolling perpendicular direction. The film was adhered to a through elliptical hole in a base plate and was fatigued in accordance with the displacement constraint along the hole circumference in the base plate subjected to a cyclic stress. Commonly, a stress intensity factor range was increased with a crack propagation under a constant stress amplitude but was decreased toward the hole edge because of the difference in thickness between the film and the base plate in this ΔK-decreasing test method. After a fatigue test, an active slip plane is identified by using the Electron Back-scatter Diffraction (EBSD) system. As a result, the fatigue crack propagation rate decreased with decrease in the angle between the loading direction and the rolling direction. The fatigue crack propagated by prismatic slips in the film loaded to the rolling direction and by basal slips in that loaded to the rolling perpendicular direction. The ratio of prismatic planes to basal planes decreased with decrease in the angle between the loading direction and the rolling direction.

Using the compact tension (CT) specimen made of the short carbon fiber reinforced plastic (sCFRP), the fatigue crack propagation and crack closure behavior was observed in this study. The fatigue crack propagation rate, da/dN, was lower for the specimen loaded to the molding direction than that loaded to the transverse direction in the relationship between da/dN and stress intensity factor range, ΔK. For both MD specimen and TD specimen, da/dN is higher in the specimen with high stress ratio, R(=P_min/P_max), under the same maximum load, P_max. The displacement on the edge of the notch was measured by a clip-gage at the same frequency of fatigue test. Then, crack opening load, P_op, and closure load P_cl were measured from the load-displacement diagrams by unloading elastic compliance method. P_cl was higher in MD specimen than in TD specimen. The effective fractions, U=(P_max-P_op)/(P_max-P_min) or U=(P_max-P_cl)/(P_max-P_min), were calculated and the effective stress intensity factor range, ΔK_eff(=UΔK), and ΔK_eff/E, in which E is Young's modulus, were evaluated. The relationship between da/dN- ΔK_eff/E evaluated from P_cl was relatively identical for the different specimens with the various R.

Fatigue crack initiation from the notch hole in the short carbon fiber reinforced plastic (sCFRP) plate is studied 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 fatigue life decreased for the notched specimen with compared to the smooth specimen and the crack generated faster in the specimen with the notch hole of 2mm than that with the notch hole of 0.5mm under the same stress amplitude. The crack propagationrate immediately after the crackinitiation was reduced in the MD specimen with a notch hole of 2mm, so it was considered that the fiber oriented to the molding direction decelerated the crack propagation at the bottom of the notch.

Zr₅₅Cu₃₀Ni₃₀Al₁₀ bulk metallic glass (BMG) is expected to be applied in a wide range of fields due to its high strength and high corrosion resistance. In this study, 4-point bending fatigue tests have been conducted in air and in distilled water to investigate the conditions of corrosion fatigue crack initiation focusing on the behavior of corrosion pits formed during fatigue tests. The fracture surface and the tensile surface were observed using a laser microscope and a scanning electron microscope (SEM). Experimental results show a decrease of fatigue strength in distilled water compared with air. It is considered that fatigue cracks initiated earlier than in air due to stress concentration at corrosion pits. In addition, the stress intensity factor range at the crack initiation ΔK_ci was calculated from the fractured surface observation using √area method. ΔK_ci fell within the range of 2.3 ~ 3.6 MPa√m for low stress level range tests. However, since ΔK_ci increases with increasing stress, it is suggested that ΔK_ci has stress dependency. Finally, the aspect ratio α of the corrosion pit, where the fatigue crack initiated, was about 1.4 regardless of the stress level. It suggests that it is necessary to consider the shape of the corrosion pit in fatigue crack initiation condition.

Naturally initiated small fatigue crack and its growth behavior of several simulated HAZ microstructures were successively observed using automatic microscope system. Simulated HAZ microstructure specimen were prepared by heat treatment with controlling maximum temperature and cooling rate. In addition, crack initiation and early stage crack opening-closing behaviors were evaluated by Digital Image Correlation (DIC) technique. As a results, fatigue life of coarse grained microstructure was shorter than that of finer grained one due to the difference from not crack initiation life but crack growth life. Since initiated crack size, depending on microstructural unit size, of the coarse grained specimen was larger than the other, remained fatigue crack growth life became shorter. In addition, early stage crack opening stress of naturally initiated small fatigue crack were successfully measured by combination of automatic microscope system and DIC technique. This result showed that small fatigue crack just after initiation opens near the minimum load, under the compressive stress, and it was increased depending on crack length. However, crack opening stress of coarse grained specimen increased more slowly than that of finer grained specimen. This behavior also affected to fatigue crack growth life. Furthermore, early stage small crack growth life became comparable to long crack data by using effective J-integral range evaluated from measured crack opening point. Therefore, it was indicated that initial crack size and early stage crack closure model depending on microstructure are needed to calculate fatigue life.

This study investigated fatigue crack propagation behavior and the number of cycles to fatigue crack arrest of cast Ni based superalloy 246 below its fatigue limit. Fatigue tests were performed under two-step variable amplitude stresses above and below the fatigue limit under load controlled condition. Low level stresses σ_Low below the fatigue limit are set as 91.4 MPa and 68.6 MPa. As the result of the observation of the micro cracks and they progresses under σ_Low = 91.4 MPa. Also in the case of σ_Low = 68.6 MPa, the micro crack progresses. In comparison between the number of cycles to stop fatigue crack propagation under σ_Low = 91.4 MPa and 68.6 MPa, the number of cycles under σ_Low = 68.6 MPa is less than σ_Low = 91.4 MPa.

A new test equipment was developed so that high cycle non-stationary thermal fatigue loading can be superimposed on stationary low cycle thermo-mechanical fatigue (TMF) loading. By means of the testing equipment, the early growth of small cracks around the cooling holes was investigated under the superimposed condition in a directionally solidified Ni-base superalloy where some cooling holes were artificially introduced for a simulation. The experimental works clearly demonstrated that the crack growth rate under the superimposed condition was significantly accelerated, compared with that under the stationary low cycle TMF loading. This behavior was discussed from the viewpoint multiaxiality of thermal stress induced by nonuniform temperature field around cooling holes.

Although low-cycle fatigue properties of solder materials have long been studied, high-cycle fatigue properties have rarely been investigated. In particular, there is not much research evaluating high-cycle fatigue properties using the strain control method due to difficulties such as the control of small strain. In this study, the low-cycle and high-cycle fatigue properties of Sn-Ag-Cu solder were investigated by torsional fatigue test with strain control. Experimental results show that when fatigue life was evaluated in the inelastic strain range, the fatigue ductility exponent differed between the low-cycle region and highcycle region, and was large in the low-cycle region and small in the high-cycle region. However, when fatigue life was evaluated using inelastic strain energy density, a power law was found over a wide range from the low-cycle region to the high-cycle region. Thus, when cumulative damage in the high-cycle fatigue region is evaluated by using only a MansonCoffin plot in the conventional low-cycle region, the results overestimate the cumulative damage. To predict the fatigue life accurately, it is necessary to consider changes in the fatigue ductility exponent of the Manson-Coffin model or to use inelastic strain energy density.

Hydraulic excavators and cranes are major heavy equipment with numerous one-sided welded joints. Although fillet welded joints are easily implemented and economical, the unwelded portion frequently initiates crack propagation. Because of this problem, high evaluation method is needed for the unwelded area. In this study, the fatigue properties and the fracture mechanism of the load carrying type fillet joints with one-sided welding were investigated experimentally to evaluate its fatigue damage with high accuracy based on the experimental and FEM analysis results. As the results, we confirm multiple initial cracks occur in 5% of lifetime. And we modeled initial crack of FEM according as 3-dimentional observation of fatigue crack propagation and we improve analysis accuracy.

Lead free solder materials have been widely used for electric circuit boards. Solder joints subject to fatigue damage due to the temperature change and creep damage due to long term load derived by elastic follow-up phenomena. Strain is particularly concentrated on the solder joint and there is a possibility of disconnect by low cycle fatigue. In our previous, fatigue tests were carried out under different strain ratio for Sn-3.0Ag-0.5Cu. In this study, low cycle fatigue tests were carried out under different strain ratio and strain waveform and we discussed the effect of strain rate and strain waveform on fatigue life.

Self-lubricating system using oil impregnated sintered material is widely used in journal bearing. The system has been transplanted to highly loaded gear contact and proofed its functionality. Lubrication is an important factor for fatigue of gear. However, conventional lubricating system have power loss because oil is shook up in gear box. The new lubrication system has merit that the loss can be reduced as it lubricates contact by bleeding oil from inside gears. Because now it still has problem in stability, it is required to understand its behavior by theoretical calculation. Therefore, this study aims to establish a simulation model to calculate lubricant flow under highly loaded contact, considering compressibility of fluid and deformable porous material. Darcy's law is used as governing equation for homogenized flow in porous material. Biot's theory, that describe interaction between lubricant flow and deformation of matrix of porous material, was extended to use in the condition that porous matrix is moving as gears contact is sliding each other. Those equations are implemented in commercial finite element method (FEM) software and solved. Result shows that deformation effect is not large but positive on the system, and when contact pressure is applied both on solid and fluid, the amount of oil that flow into porous material from oil exceeds the amount that can be supplied from oil film. It implies the interaction between porous flow and fluid flow in the oil film is not negligible.

Thermal fatigue cracking may initiate at a tee pipe where high and low temperature fluids flow in. The mixing flow causes thermal stress fluctuation at pipe inner surface. The change in fatigue life due to variable strain around hot spot, which was heated by the hot flow from the branch pipe, were investigated experimentally. The variable strain at the pipe inner surface was obtained by thermal hydraulics tests and finite element analysis. Strain-controlled fatigue tests were conducted using stainless steel specimen and the obtained strain. The minimum accumulated fatigue damage calculated by Miner's rule and rain-flow counting method was 0.18. It showed that the Miner's rule did not always evaluate the fatigue life conservatively. Since the strain around hot spot had the periodic large fluctuations, the loading sequence of periodic overload reduced the fatigue life. The accumulated fatigue damage of strain inside the hot spot was larger than those outside the hot spot and at the position of maximum stress fluctuation, because the strain inside the hot spot had the load sequence of the periodic tensile overload.

In recent years, high power output of marine diesel engine is being considered, because improving fuel consumption of a ship is required from the viewpoint of energy saving, CO₂ reduction, etc. Therefore, a crankshaft which is a main component of the engine is expected to be higher strength and fatigue strength than ever. Since bending and torsional stresses occur on a fillet of the crankshaft under operating condition, applying surface treatment to the fillet portion is effective to improve fatigue strength of the crankshaft. Fatigue tests under high compressive mean stresses are demanded to confirm fatigue strength of surface treated parts. However, it is difficult to carry out the fatigue test in compressive plastic domain because the test specimens are buckled. Therefore, to conduct the fatigue tests under high compressive mean stresses without buckling, we designed the specimens shape with a notch in the parallel portion using plastic buckling analysis. Finally fatigue tests under high compressive mean stresses could be carried out using the designed fatigue test pieces, and the validity of the design method about fatigue test piece with notch was confirmed.

Existence of grain boundary has generally considered to influence on fatigue strengths of materials. To study the influence on a Ni-based superalloy (CMSX-4) at 650°C, fatigue tests were carried out for bicrystal specimens with a low-angle grain boundary. Fatigue failure criterion based on the maximum shear stress range on the octahedral slip systems was used, to evaluate influence of fatigue strengths of grain boundary. It was suggested that influence of a low-angel grain boundary was small for fatigue strength.

Fatigue crack propagation in a polycrystalline Ni-base superalloy was experimentally investigated. Special focus was put on the effect of grains crystal orientation and grain boundary on fatigue crack propagation, employing bi-crystal, coarse grain, fine grain CT specimens extracted from a directional solidified superalloy, MGA-1400. A series of experiments revealed that trans-granular crack propagation was affected by the primary and secondary crystal orientation of grains, and fundamental mechanism of Stage I cracking along slip planes could be interpreted by the experimental and analytical findings in the single crystal material. It was also found that the grain boundary in the bi-crystal specimen caused the retardation of crack propagation depending on crack propagation modes just after the grain boundary. In case the crack propagated in Stage I manner after the grain boundary, crack retardation became more pronounced due to the Stage I + II cracking.

In large rotating machines such as generators and electric motors, demands for more high power and reducing environmental load are becoming severe. As a result, the conductor which is a part of the rotor receives more pulsation change, and the long-life region with low stress which conventionally had no need to think must be investigated. In the field of steel material, researches about the effect of Mean stress and environment temperature on fatigue strength in the case of long-life region are actively conducted. However, in the field of copper which is a conductor material, there are few researches in the case of long-life region, and the effects of Mean stress and environment temperature on time strength are not clear. Therefore, in this study, the influence of the Mean stress and environment temperature in the high cycle area of the copper material was clarified by using the servo hydraulic load frame.