Brazing of cemented carbide with silver filler metal is widely used in manufacturing Polycrystalline Diamond Tools. A cemented carbide chip onto which Polycrystalline Diamond is sintered is then brazed onto a larger cemented carbide base. Sometimes the chip and the base are different widths creating an extended lip. In this case, the strength of the brazed joint is decreased as the brazing material gets thinner and thinner. Previously, the authors developed a method to ensure the optimal brazing thickness using metal spacing powder. Using this method, the static strength of the brazing joint was increased by 50%. The conventional thinking has been that while using a powder can improve static strength, the powder will eventually cause cracking and decrease strength over time. So, in this paper the bending fatigue test was carried out as the next step. Firstly, a bending fatigue tester was developed where the bending load was generated by an electric solenoid actuator. Next, Cantilever bending specimens with a cross section of 4 mm by 4 mm were tested. The specimens were made of cemented carbide. Two brazing methods were tested: the normal furnace brazing method, and the newly developed brazing method using Titanium powder. The bending fatigue strength of the newly developed brazing method was double that of the normal brazing method.

Application of weld-joint to steel and aluminum plates is still limited due to uncertainty of the fatigue strength because of stress concentration at the edge and formation of intermetallic compounds along the interface. This study investigated the key factor that determines the fatigue strength of steel-aluminum brazing joint by considering the variation of strength along interface. The welded part of single lap joint was partially removed by a wire-cut electric discharge machine to investigate the influence of length and location of interface on fatigue strength. When the applied load was relatively low it was found that the numbers of cycles to failure were found to be similar, even though the lengths of interface were different due to partial removal. Therefore, we concluded that the number of cycles to failure was mainly consumed at the early stage.

Interfacial strength of dissimilar materials laser welded joint between SUS304 and PET was evaluated with indentation test. In the test, a wedge shape indenter was inserted into a groove machined at the interface. Initiation and propagation behavior of interfacial crack was observed during indentation test. Interfacial crack was successfully introduced with indentation test by using a wedge shape indenter. The crack length increased with increase in indentation load. Distribution of interfacial strength in welded region might be continuously measured in one time by the indentation test. Fracture toughness was calculated as interfacial strength.

In recent years, connecting techniques using conductive nanomaterials imitating gecko's feet, for example, metal nanowires, carbon nanotubes and conductive polymer materials as a surface fastener have been reported. These connecting techniques utilize van der Waals force generated between the surfaces of nanomaterials. Recently, we have developed the copper nanowire surface fasteners (Cu NSF) as a conductive connector. Since copper has high electrical conductivity and is inexpensive, the Cu NSF can be expected to be practically used as a new surface mounting technique. Moreover, Cu NSF can be connected at room temperature utilizing van der Waals force, so that it is possible to avoid the damage to electronic parts during solder reflow process. However, the adhesion strength of current Cu NSF is still small as compared with traditional solders. The reason is considered that the contact areas of the nanowires were not sufficient because the nanowires collided and collapsed during connecting. In this research, we attempted to increase the contact area of nanowires for improving adhesion strength of Cu NSF. The adhesion strength of the Cu NSFs having nanowires with 100 nm and 400 nm diameter reached the largest value, and increasing with the increase of preload. This investigation showed that the contact areas of nanowires increased and mechanical entanglement of nanowires occurred because of increasing the preload of connecting.

Singular stress and electric displacement fields occur at the vertex of interface in piezoelectric joints under an external loading. It can be expected that concentrated electric displacements causes large electric fields in an adjacent space and electric potential will be induced on the surface of electrode when the electrode approaches to the electric field. In the present paper, characteristics in electric field and electric displacement field near a singular point which locates at the vertex in a cross piezoelectric joint are investigated using a commercial program coupling displacement and electric potential. In the model for analysis, the cross piezoelectric joint is formed by joining four blocks of piezoelectric material using epoxy resin. A compressive stress of 1MPa in the x-direction is applied to a side surface of the joint. Distributions of electric filed and electric displacement field near the vertex follow a power law of the distance from the singular point. In particular, distributions for electric filed and electric displacement field near the vertex in the joint and in air have singularity. When the thickness of adhesive layer increase, the strength of the singular field is larger. This can be said for the distributions in air. Distributions of electric field and electric displacement for the distance from the origin locating at the center of adhesive layer in the bottom surface of the joint approach to a constant value as the observation point approaches to the origin.

Our previous study showed that the adhesive strength can be expressed as a constant value of the intensity of singular stress field (ISSF) by using two-dimensional modelling. By considering the three-dimensional (3D) geometry, the mesh-independent technique proposed in our previous study is applied to evaluate the ISSF distributions along the side of butt joint interface. The results show that the critical ISSF distributions are almost the same independent of the adhesive thickness. For 3D butt joint, different singularity appears at the corner, so it is difficult to obtain the ISSF at the corner. In this study, it is shown that the ISSF at the corner can be easily obtained by considering a model with rounded corners close to the actual test specimen. Also, the influence of corner radius on the ISSF at the corner is discussed. It is found that the ISSF at the corner decreases with increasing corner radius and becomes smaller than ISSF at the center. However, as the corner radius further increases, the ISSF at the corner increases.

We verified the compatibility of the singular stress fields around an interfacical corner obtained by the three-dimensional finite element analysis and that obtained by the two dimensional quasi-analytical method. We proposed normalization method that the eigen vectors obtained by the three dimensional finite element method correspond with the eigen vectors obtained by the Stroh formalism. Obtained scalar parameters by the three dimensional finite element method are compatible with the two-dimensional corners and cracks. We proposed universal definition of fracture parameters for three dimensional corners, two dimensional corners and cracks.

In this paper, singular indexes at the vertex and side on interface in three dimensional (3D) bonded body are computed under arbitrary material combinations by the eigen analysis based on the finite element method (FEM) and compared with those at interface end in two dimensional (2D) bonded plate. It is shown that the singular index at the side in 3D bonded body equals that at interface end in 2D bonded plate under plane strain condition and is governed by Dundurs' parameter (α, β). However, the singular index at the vertex is different from that in 2D bonded plate. A simple discriminant to judge the good, equal or bad pair is discussed.

We analyzed the stress distribution along an interface between Ge and Si that beween SiGe and Si using the molecular statics with the MEAM potentials. The lattice constants of Ge and Si are different of 4%, the lattice constants of SiGe and Si are different of 0.95%, and these differences of lattice constants caused misfit dislocations along the interface. We also analyzed the stress distribution using the anisotropic linear elastic theory superposing the coherent stress field and the stress around the dislocation. Stress distributions obtained by the molecular simulation and the anisotropic elasticity are in good agreement.

Cyclic deformation behavior of copper single crystals with a single slip orientation has been investigated intensively. However, geometrical features of dislocation structures in multiple slip orientations have not been satisfactorily explained so far. In this study, therefore, fatigue tests of copper single crystals with [001] and [111] multiple slip orientations were conducted under constant plastic shear strain amplitudes ranging from 1.7 x 10^-4 to 1.7 x 10^-2 at room temperature. Both high-voltage scanning transmission electron microscopy and electron channeling contrast imaging technique in scanning electron microscopy were used to characterize dislocation structures in fatigued specimens. Characteristic dislocation structures, such as wall, labyrinth and cell, were developed during cyclic deformation and were highly dependent on the stress axis and the applied strain amplitudes. Vein-like structure elongated along the [010] direction was formed at low strain amplitude less than 3 x 10^-3. Typical labyrinth structure constituting of the (100) and (001) dislocation walls was evolved in the [001] oriented specimen fatigued at strain amplitude higher than 3 x 10^-3. Contrarily, a single set of (111) dislocation walls and cell structure were formed in the [111] oriented specimen. It was concluded that the dislocation wall plane is perpendicular to the direction of the sum of the Burgers vectors for all the dislocations in the active slip systems.

Fatigue crack initiation occurs through dislocation structure formation around stress concentration sources near surfaces. The crack propagation is attributed to plastic deformation at the clack tips. Plastic deformation is caused by the dislocation glide under loading condition. Therefore, direct observation of the dislocation behavior under the loading condition gives important information for fatigue mechanisms. The formation of lattice defects is also key to understanding the fatigue mechanisms because dislocation glide is governed by the presence of other lattice defects: dislocations, stacking faults, grain boundaries. In addition, residual stress around the stress concentration sources has large effect on fatigue behavior. Electron channeling contrast imaging in a scanning electron microscopy is a powerful technique, which enables us to detect lattice defects and elastic strain fields in bulk specimens. A bulk austenitic steel specimen was pre-deformed up to 2% tensile strain. Then, the specimen was reloaded in the elastic regime and subsequently kept the displacement in a secondary electron microscopy. We observed the microstructure around a circular hole (a stress concentration source) during the displacement holding by electron channeling contrast imaging. In-situ electron channeling contrast observation allowed us to visualize elastic relaxation of residual stress, dislocation motions and the formation of plane defects.

Density evolution of atomic vacancies in α-titanium single crystals subjected to cyclic loading is numerically evaluated by using a crystal plasticity software code. Slip deformation is evaluated by dislocation density-based models and generation rate of atomic vacancies is given by the model proposed by Essmann and Mughrabi (1979). Parameters for dislocation movement, accumulation and annihilation are set so as that they mimic the dislocations' property in PSBs. Cyclic stress-strain curve, evolutions of SS dislocation density and atomic vacancy density are shown. It is obtained that atomic vacancy densities after 15 cycles of tensile/compressive loading of +/- 3 % reach 10²⁵ to 10²⁶ m^-3 , depending of the dislocation mean free path.

To study anelastic effect in steel, we observed residual elongation of a compression steel coil spring after unloading compression force, which was previously applied to the specimen for various periods. Total residual elongation and saturation time are increased with the compression period. Since pure share stress is applied to the spring wire during compression, major share elastic strain is explained from bow-like expansion of dislocations between strong pinning points. The above phenomena indicate small partial expanded dislocations are weakly pinned at certain sites such as magnetic domain walls and the pinning cites are increased with the compression time.

The influences of cross slip activity on the low-cycle fatigue behavior and dislocation structure of pure Al single crystals were systematically investigated. Plastic-strain amplitude controlled fatigue tests were conducted at room temperature (RT) and 77 K using pure Al single crystals bearing a single-slip orientation but having a different value of the Schmidt factor (SF) of 0, 0.14 or 0.24 for a cross-slip system. Low-cycle fatigue behavior at RT varied greatly depending on the values of SF. The specimens with SF = 0.14 and 0.24 exhibited initial hardening, softening and then secondary hardening. While no softening behavior was observed in the specimen with SF = 0. Microstructural observations revealed that cell structure was developed in all the specimens. The high density of dislocations on the crossslip planes was confirmed within the cells formed in the specimens bearing non-zero SF. On the other hand, dislocations on the cross slip panes were rarely visible in the specimen with SF = 0. Shear stress amplitudes showed cyclic hardening to saturation at 77 K, regardless of SF values. After the stress saturation, well-developed persistent slip bands (PSBs) and matrix vein structure coexisted in all the specimens. The dislocation walls were regularly aligned perpendicular to a primary slip direction in PSBs, and their average spacing was about 0.4 μm. It can be concluded that the SF-value dependent fatigue behavior of specimens at RT is attributed to the activity of cross slip system.

Lattice rotation near a fracture surface of fatigued copper single crystals was investigated by EBSD analysis. Two copper single crystals having [372] and [010] longitudinal axes were grown by the Bridgman method. The single crystals were shaped to a plate shape of 50×20×3 mm³ dimension. The single crystal plates were preliminarily hardened by cyclic loading and then a small notch was introduced at each center of the single crystal plates. Fatigue crack growth tests were carried out on the notched samples. After the surface was polished again, lattice rotation near a fracture surface was analyzed by EBSD system. In the single-slip-oriented single crystal having the [372] tensile axis, a primary fatigue crack was grown almost along (111) primary slip plane in a shear decohesion manner. Near the notch root at which the stress intensity factor is low, there existed almost no lattice rotation even in the close vicinity of the fracture surface. However, at a long crack length near which slip bands of secondary system were visible on a surface, lattice rotation was clearly detected. The maximum lattice rotation angle at an intermediate crack length was about 20°. In the multiple-slip-oriented single crystal having the [010] tensile axis, the crack growth direction was basically towards the notch direction. At a longer crack, the maximum lattice rotation angle amounted to about 30°, which exceeded the maximum angle at the [372] single crystal. However, the surface area where lattice rotation is detectable was small at the [010] single crystal than that at the [372] single crystal at an equal crack length.

High Strength Steel shows internal origin type fatigue failure as called Fish eye failure in low stress and high cycle region. In this failure, ODA is formed on the fracture surface. Although the explication of the mechanism of ODA formation is needed for the explication of the Fish eye failure mechanism, the explication is not yet accomplished. It’s reported that internal fatigue crack in high strength steel is propagating extremely slowly by recent investigations. And crack propagation rate was suggested that series of the low alloy steel depend on the maximum stress and ΔK. In this study, fatigue test of the same material was in vacuum environment. As a result, ΔK≤4 MPa√m as for crack propagation rate, it suggested that the test on condition of long initial crack was faster than the test on condition of short initial crack. Therefore crack progress speed depend on the test conditions as well as the maximum stress.

High entropy alloys(HEAs), including a large amount of solid solution elements, have not only high capability of solid solution strength but great strength-ductility balance and high fracture resistance. Although mechanical properties of HEAs except for fatigue are reported, there are few reports on fatigue properties. Because fatigue fracture accounts for about 80% of accidents in structural materials, understanding of the fatigue properties of HEAs is the most important task for practical use. In this research, fatigue crack growth characteristics of an Fe20Cr20Ni20Mn20Co (at.%) high entropy alloy were investigated by ΔK increasing compact tension testing in comparison with a SUS316L. Fatigue crack growth rate of the HEA was lower than that of the SUS316L, irrespective of ΔK. The main crack growth path of both alloys was grain interior and a difference between two alloys was found to be in the roughness of the crack growth path. The roughness of the fatigue crack growth path of the HEA was larger than that of the SUS316L. This fact indicates that the HEA has larger roughness-induced crack closure effect than the SUS316L, which causes low fatigue crack growth rate of HEA. Another key is a non-crystallographic transgranular crack growth mechanism. The main crack path of the SUS316L was straight so the SUS316L shows crack growth via crack blunting/re-sharpening, while the HEA shows transgranular crack growth associated with dislocation substructure alignment, resulting in the relatively rough.

Low-cycle fatigue tests were carried out on pure nickel samples having a wide range of grain sizes. Nickel bullets having 4×4×40 mm³ shape were processed by the equal-channel angular pressing (ECAP) technique for four passes. Then, the ECAPed samples were annealed for 12 hours at various temperatures ranging from 423K to 1073K. The average grain sizes of the samples annealed at 423K and 1073K were 0.8 μm and 105 μm, respectively: the ECAP process and the subsequent annealing treatment enabled us to prepare the samples that differed in grain size by a factor of one hundred. The annealed samples were shaped to strip samples having a gauge shape of 1.5×1.8×2.0 mm³. The low-cycle fatigue tests were conducted under a constant plastic strain amplitude of ε_pl=10^-3. The specimen annealed at 423K showed cyclic softening from beginning of the fatigue test. At an intermediate annealing temperature (753K), the cyclic softening became moderated, while the samples annealed at 773K and 1073K showed cyclic hardening. From SEM observations after the fatigue tests, shear bands generated across many grains were detected at the sample annealed at 423K, while slip bands were developed inside individual grains in the sample annealed at 773K. Hence, it is likely that the cyclic softening and hardening are associated with the shear banding and the slip band development, respectively.

Most of the vehicle components have a variety of materials, such as fine- and/or coarse-grained polycrystalline alloys, and resins. Since they and their jointed components are often surrendered under severe fatigue conditions, we would like to evaluate their integrity, hopefully non-destructively. From such points of view we have developed and introduced some techniques on three-dimensional non-destructive mechanical analysis methods with at SPring-8:BL33XU (Toyota beamline). The first one is a conventional diffraction for depth resolved stress and strain measurement but with area detector and our-developed rotating spiral slit. This technique enables us to measure space-resolved strain distribution of arbitrary materials. The next one is a scanning three-dimensional x-ray diffraction (scanning 3DXRD) microscopy with a high-energy x-ray microbeam. By this technique we can measure inter- and intra-granular distribution of crystal orientation. We also tried the corresponding crystal plasticity finite element (CPFE) analysis to understand the plastic behaviour for polycrystalline iron and interaction between the grains. The third one is a synchrotron laminography for three-dimensional material analysis. The digital volume correlation is also applied to visualise not only the internal morphology but also its strain distribution non-destructively. These three individual or integrated methods are helpful for developing and analysing electric-powered mobilities’.

Crack growth under the elastic-plastic fracture is an important issue of the structural integrity, because seismic wave causes low cycle fatigue in the engineering structure. We developed modeling technique for CT specimen with shear-lip surface including crack front lines. The validity of the FE model was evaluated by several fundamental analysis. The relationship between total reaction force and load point displacement and crack propagation rate and ΔJ relationship are computed for a validity of the simulation. ΔJ computed by the numerical simulation agrees with the experimental one. However, crack propagation rate and ΔJ shows linear relationship in logarithm scales like Paris’ law, however its fluctuation is very large. In order to obtain smoothed physical quantities along crack front, an application phase analysis should be conducted using approximate crack propagation rate.

In order to evaluate the effect of solute Cu atoms on the high-cycle fatigue properties, Al-Cu binary alloys subjected to different tempering conditions are tested at room temperature. The S-N diagram of the naturally aged (NA) alloy shows a distinct fatigue limit while the artificially aged (AA) alloy does not. The fatigue life of the NA alloy is longer than the AA alloy, which is attributed to the higher resistance against crack growth.

Carbon concentration dependence of fatigue limit in Fe-33Mn-xC (mass%) steels was investigated using rotating bending fatigue test at ambient temperature. The hardness increased with increasing carbon concentration, accordingly, the fatigue limit also increased. A factor increasing the fatigue limit by carbon is solution hardening. In addition, I noted the carbon concentration dependence of the fatigue limit normalized by hardness. The fatigue limit normalized by hardness increased with increasing carbon concentration up to 0.6%, but did not change with further increase of carbon content from 0.6% to 1.1%. The extraordinary increase in fatigue limit normalized by hardness in the Fe-33Mn-0.3C and Fe-33Mn-0.6C steels cannot be explained by hardness increase due to solution hardening. This fact implies that dynamic strain aging that occurs in the alloy system contributes to increase the fatigue limit. More interestingly, the effect of dynamic strain aging is not monotonically enhanced by increasing carbon concentration, namely, the optimal carbon concentration in the alloy system is 0.6% regarding fatigue limit improvement by the dynamic strain aging.

In carbon steel, the fatigue limit is determined by non-propagation limit of crack and this non-propagation limit depends on plasticity-induced crack closure and hardness near the crack tip. Especially, in ferritic steel, dynamic strain aging (DSA) is important in terms of hardness near the crack tip. This phenomenon, DSA, may be enhanced by adding Mn in ferritic steel through occurrence of Mn-C coupling. Also, the coaxing effect has been recognized as a phenomenon related to fatigue limit. This phenomenon enhances the fatigue resistance through work hardening and stain-age hardening. To examine the effect of Mn on fatigue behavior of ferritic steels, rotating bending fatigue tests including stepwise stress increases every 10⁷ cycles were carried out in ferrite-cementite Fe-Mn-C steel. The steel showed a higher fatigue limit when arranged in Vickers hardness that contains effect of solid solution strengthening by Mn and C and showed a significant coaxing effect in spite of low solute carbon content. In addition, the crack propagation behavior changed in response to stress amplitude: at high stress amplitudes, the cracks propagated along grain boundaries; at low stress amplitudes, the cracks propagated in the grain interior. These behavior is different from Fe-C ferritic steel. Also, fracture in the coaxing effect test was not resulted by the cracks that initiated in the early stages of the test but caused by the crack that initiated in the final stage of the test. This feature is also different from that of Fe-C ferritic steel and annealed S10C.

Three Mg single crystalline round-bar specimens with different crystal orientations were prepared and then subjected to uniaxial tension-compression fatigue tests so as to investigate crystal orientation dependence on fatigue fracture behavior. The lording direction of AD, BC and EF specimens was [1120], [1100] and [0001] respectively. At stress amplitude (σ_a) over 60 MPa, fatigue life of BC specimen was longest and fatigue life of EF specimen was shortest. When the stress amplitude was 20 MPa, fatigue lives of all specimens were almost the same. Fatigue life in Mg single crystals showed high dependence of crystal orientation and stress.

In this study, to clarify the orientation dependence of fatigue strength and fatigue life in magnesium, fatigue test of pure magnesium single crystals with different orientations have been performed by originally developed test method for small specimen. Moreover, Mg-Y alloy single crystals were tested to understand alloying effect to fatigue behavior of magnesium. In the A- and the B-specimen of pure magnesium, whose crack direction is normal to [0001] and crack plane is normal to (0001), a fatigue crack propagated [1100]. The cracks would be propagated due to alternating shear of secondary pyramidal slip. In the C and the D-specimen, whose crack direction is parallel to [0001], a fatigue crack propagated parallel to [0001] with activation of basal slip at the crack tip. In E-specimen, whose crack plane is parallel to (0001), a fatigue crack propagates (0001) and {1012} twin was formed around the crack tip. S-N plots showed strong orientation dependence. Fatigue life of C and D-specimen was longest and fatigue life of E-specimen was shortest among these five specimens. Fatigue crack in pure magnesium does not prefer to propagate twin interface so much. In the B-specimen of Mg - 0.75at.%Y alloy, a fatigue crack propagated [1100]. The crack behavior is similar to the case of pure magnesium. Fatigue file and fatigue limit were increased by yttrium addition.

Low-cycle fatigue tests were conducted on ZK60A magnesium alloy that were processed by equal-channel angular pressing (ECAP) method. The ECAP processing was carried out on a hot-extruded ZK60A alloy at a die temperature of 473K. Total pass number was four and the processing route was Route Bc. One of the ECAPed sample was annealed at 573K to induce grain coarsening. The low-cycle fatigue tests were conducted under a constant plastic strain amplitude of ε_pl=2×10^-3, in air at room temperature. A hysteresis loop shape during the low-cycle fatigue tests depended on the sample: a tension-compression asymmetric shape was recorded in the as-drawn specimen having the tensile axis parallel to the extruction direction, while a symetric shape was seen in case of the tensile axis perpendicular to the extrusion direction. Deformation twinning during fatigue was most probably attribute to the asymmetric hysteresis loop shape. Because the asymmetric hysteresis loop was absent in the ECAPed specimen, it is possible that grain refinement suppressed the deformation twinning in the fatigue test. In the as-drawin specimen having the tensile axis parallel to the extrusion direction, a small crack was observed near a deformation twin. By contrast, a fatigue crack in the as-ECAPed specimen was generated along a shear band that was inclined to the tensile axis at 45°. After the ECAPed specimen was annealed at 573K, an intense shear banding and a subsequent crack nucleation were moderated. Accordingly, it can be said that the low-temperature annealing on the ECAPed ZK60A magnesium alloy reduced the fatigue crack nucleation sites.

The present study aims at investigating the fatigue behavior of AZ31 Mg alloys through crystal plasticity finite element simulations of statistically representative polycrystalline aggregates. A crystal plasticity model including twinning, detwinning and slip in the twinned region is presented and calibrated against cyclic experiments conducted on rolled AZ31 Mg alloys. Fatigue experiments under load-controlled conditions are conducted with in situ optical microscopy observation in order to identify the appearance of fatigue cracks with regards to the number of applied cycles. Fatigue simulations of two-dimensional periodic microstructures are then performed with similar loading conditions than in experiments. Mesoscopic non-local fatigue criteria for slip and twin-induced crack initiation are presented, investigated and compared to experimental data. The results indicate that basal slip and deformation twinning are the main mechanisms occurring during fatigue conditions as the low applied stress prevents the onset of prismatic and pyramidal slip. In addition these deformation modes appear to take place in different grains. The analysis of the fatigue criteria shows that the twin-induced crack initiation criterion exhibits a better trends in terms of mean behavior, scattering and sensitivity against loading conditions compared with the slip-induced criterion. It suggests that under fully-reversed conditions, the appearance of a fatigue crack is intimately related to the presence of twins within the microstructure.

Surface nanostructured S45C, Si+B added S45C (S45C(Si+B)) and SCM440H steels were prepared by surface nanostructured wearing (SNW) and subsequent induction heating and quenching (IHQ). The Vickers hardness values at the top surface of these steels were around Hv 8.5 GPa, and also the thicknesses of quenched surface layer (Hv 7 GPa) were around 800 μm. The rolling contact fatigue lives of these steels in a roller pitting test were longer than those of the S45C, S45C(Si+B) and SCM440H steels without the surface nanostructure prepared by only IHQ. This is because the degree of the softening during the roller pitting test was smaller in the SNW&IHQed steels comparison with the only IHQed steels.

Ti-6Al-4V alloy is widely used as aero engine parts such as low pressure compressor disks and blades. For these applications, fatigue properties are very important. It is well known that fatigue properties of titanium alloys are severely affected by their microstructures. In this study, Ti-6Al-4V samples with bi-modal microstructure (combination of equiaxed α phase and lamellar α/β phase) were produced through upset forging, and the effects of microstructure on high cycle fatigue properties were investigated. 100 mm diameter and 200 mm height Ti-6Al-4V cylinder shape samples were isothermally forged to 50 mm height pan-cake shape samples at two different temperature of 1173 K or 1073 K using NIMS 1500 t forging simulator. To obtain bi-modal microstructure, forged samples were heat treated at 1223 K, α/β phase region, followed by aging at 1023 K. High cycle fatigue specimens were machined along the hoop direction of heat treated samples. Microstructural observation by optical microscopy and scanning electron microscopy, and crystal orientation analysis by electron backscattered diffraction (EBSD) were conducted and compared with the results of high cycle fatigue tests. Material forged at lower temperature of 1073 K exhibited longer fatigue life compared to that of material forged at higher temperature of 1173 K. Materials forged at 1173 K has strong texture (high directivity of crystal orientation) of α phase compared to materials forged at 1073 K, and it seems to affect their fatigue life.

The deformation mode of some titanium alloys differs from that of pure Ti because of the presence of a secondary phase and alloying elements in the α-phase. We investigated the effect of Al and V solutes as typical additive elements on the dislocation motion in Ti alloys using density functional theory calculations. To clarify the energy differences between various dislocation cores, we evaluated possible core structures and motion in various slip planes. The energy difference between most stable pyramidal and prismatic cores is very small, whereas that between the prismatic and basal cores is larger, thereby preventing dislocation motion in the basal plane in pure Ti. However, the Peierls barrier for motion in the basal plane is not as high if the dislocation exists in the basal core. Direct calculations for the dislocation core around solutes revealed that Al solute facilitate dislocation motion in the basal plane by reducing the energy difference between the core structures while these solutes have a reverse trend for the interaction energy with the dislocation core.

Observations of internal small fatigue crack growth in Ti-6Al-4V were conducted using synchrotron radiation μCT imaging at SPring-8, and the just-initiated internal cracks and their growth processes were successively observed. The internal small cracks propagated very slowly with the growth rate of less than 10^-10 m/cycle. The crack growth rate had a lot variability in the smaller crack length regime. In order to discuss the reason for the small growth rate of internal crack in terms of the effect of environment inside the crack, the internal crack growth rate was compared with the surface crack growth rate in air and high vacuum. As a result, the internal crack growth rate was lower than the surface crack growth rate in air; and matched well with that in high vacuum. The vacuum-like environment at the tip of internal crack is considered to be a reasonable cause for the long fatigue life of internal fractures in gigacycle regime.

The activity of slip deformation in titanium single crystal at a U-shaped notch was numerically analyzed by using a crystal plasticity finite element method. The critical resolved shear stress (CRSS) for slip system is given by the extended Bailey-Hirsch model which includes the effects of lattice friction, work hardening, and the size of microstructure. As the lattice friction stress of each slip system was assumed to be the same values in the high-temperatures, all slip systems were activated at the notch tip with the loading condition of 0.1% in nominal tensile strain. In the region of elastic deformation, strain rate at the notch tip is about 3 times larger than the macroscopic strain rate. In the reason of elastoplastic deformation, the strain rate at the notch tip is about 10 times larger than that in the macroscopic scale. The increase in the strain rate contributes to the increase in the lattice friction stress for the slip deformation. Therefore, in the present study, the plastic deformation at a notch tip was investigated by changing the CRSS for each slip systems, representing the increase in the strain rate.

Titanium alloys have been used for jet engine parts because of their excellent strength-to-weight ratio. However, the phenomenon of dwell sensitive fatigue in titanium alloys have been a concern to the aviation industries for several decades. A significant amount of research has been conducted during the intervening years to understand the fundamental deformation mechanisms controlling dwell behavior. In this work, we performed the fatigue test under load control with a triangular waveform, which includes 10-min displacement holding at the peak load for only one cycle at ΔK = 25, 30, 40, 50 MPa√m, namely stress holding of four times. From the observation of fracture surface with a focus on striation, we proposed that the dwell behavior is controlled by two mechanisms: (1) increase of crack growth rate during displacement holding (2) increase of crack growth rate in several cycles after displacement holding. In terms of mechanism (1), we obtained digital image correlation-based strain map of in-situ fatigue test to investigate the relationship of crack growth and crack tip deformation. We suggested that mechanism (1) originates from the blunting of crack tip during displacement holding. Moreover, we observed a planar array of dislocations in the failed specimen by electron channeling contrast imaging. This dislocation structure is important to understand why titanium alloys are sensitive for dwell fatigue. In terms of mechanism (2), we assumed that the reduction of crack closure effect or softening of the material causes the accelerated crack growth in several cycles after displacement holding.

In order to develop reliability of Environmental Barrier Coatings (EBCs) for ceramic matrix composite, effects of high temperature aging on pore structure and thermo-mechanical properties of an atmospheric plasma sprayed BSAS (BaO-SrO-Al₂O₃-2SiO₂) coating and Mullite (3Al₂O₃-SiO₂) coating were investigated. EBCs have an important role to protect substrate from environmental attacks. Free-standing coating specimens of BSAS and Mullite were prepared. The cross-sectional SEM observation was carried out on EBC coatings, and the microstructure of the coating was identified. The elastic modulus of the coating was measured by micro-indentation testing. The measurement of thermal expansion coefficient of the coating was also carried out in this study.

Fretting fatigue has lower fatigue strength than that for smooth specimen, and it is thought that fretting fatigue has no fatigue limit. However, there is little knowledge of the influence of variable loading on very high cycle fretting fatigue because very high cycle fatigue tests using a conventional fatigue testing machine are impractically time consuming. In this study, very high cycle fretting fatigue tests under repeated two-step variable amplitude loading were conducted by using an ultrasonic fatigue testing machine for the sake of accelerating fretting fatigue testing. The applicability of the modified Minor's rule was discussed, and it would seem to show that stress amplitudes lower than fretting fatigue strength at 108 cycles does not contribute to the fatigue life.

In order to investigate the strength characteristics and buckling behavior of carbon nanotubes (CNTs) incorporating another molecules, we have performed molecular dynamics simulation on CNTs including C₆₀. Buckling occurs from the layer near C₆₀ in a CNT have a large cross section. Then, we investigated the buckling behavior on CNTs including several C₆₀. Although the strength characteristics of the CNT including four C₆₀ increased, we cannot find an obvious reason of the increase in buckling strength due to the number of C₆₀.

A 3-D stacking structure of silicon chips has been introduced to satisfy the needs of higher performance and multi functionality of electronic devices. The miniaturization and high densification of interconnections and semiconductor devices, however, would cause a fluctuation of the residual stress during the fabrication process and under operation. Since high residual stress degrades the performance of electronic devices and also long term reliability, it is important to monitor and control the local residual stress. In this study, a strain sensor that is embedded in a silicon chip and can measure the residual stress nondestructively was developed by applying piezoresistance effect of single crystal silicon. The stress sensitivity at room temperature was 1.3 MPa/Ω, which can measure the change of the residual stress sufficiently. Then, this sensor was applied to the measurement of the local residual stress between Cu interconnections and the high residual stress around the interconnections was verified.

In this study, we have studied deformation and fracture behavior of composite materials composed of polyethylene (PE) and carbon nanotubes (CNTs) by using molecular dynamics method. First, compressive analysis is performed in the radial direction of two CNTs placed in the composite model. Then, multiaxial loading analysis is performed by tension or compression in the longitudinal direction of either CNT under the constant compressive stress in the radial direction. As a result, we revealed that compressive stresses to the radial direction of CNTs change the fracture strength under tension or compression to the longitudinal direction in composite models.

In order to discuss the interlayer interaction in the multi-walled carbon nanotubes, one of the layers in five-walled CNT is deformed in torsion or rotation by molecular dynamics method. In the torsional simulation, the relationship between the torque and the specific twist angle in each layer shows the increase of torque in only deformed layer. On the other hand, a decrease in torsional velocity raises the influence on the inner layer on the post buckling. In the rotation simulation, it is observed that adjacent layer to the deformed layer follow the rotation deformation.

In automobiles, an impact absorbing member called a crash box is provided to absorb impact 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. However, in the conventional crash box such as a rectangular-shaped tube type and a hat-shaped type, there is a problem that the load in the initial stage of collision increases rapidly. In order to solve this problem, we focused on the energy absorption characteristics of the cell structure such as the wood structure with multiple large and small holes. To propose the crash box with the cell structure, we studied the effects of the arrangement of holes and size of holes in the cell structure on crash behavior using finite element analysis software LS-DYNA^®. First, three types of models with different arrangement of holes were analyzed. From the analysis results, it was found that stress concentrates on cell walls between holes in all models. Next, from the above results, we analyzed models in which the thickness of cell walls and the ratio of the diameter of large holes and that of small holes were changed variously. As a result, it was found that the load-displacement curves have the same shape if the ratio is the same in spite of the wall thickness of the cell.

The conventional riveting method requires drilling process of sheets. In order to solve this problem, the punching rivet method has been proposed. In the punching rivet method, drilling is unnecessary in advance, and since the punching and joining of sheets are carried out at the same time, the production efficiency is good. In this study, an Aluminum alloy sheet and a GFRP sheet were joined using the punching rivet method. After joining, the cross sections of the joints were observed, and the deformed conditions of the rivets were examined. For the riveted and bolted joints, tensile tests were carried out to compare their strength. From experimental results, the strength of riveted joint was almost same as strength of bolted joint because of the seating pressure working on the sheet surface of the riveted joint. Though the shape of rivet and maximum load in joining were changed to examine the effect of them, strength of riveted joints were almost same. The cause due to the fact that the deformed shapes of the rivet shafts was almost same, when the cross sections of joints were observed.

Evaluation test of high-pressure hydrogen gas should be carried out more simplified. As an alternative approach in high-pressure hydrogen gas test, to simulate the hydrogen gas environment SSRT was performed with cathodically charged. Specimen was used a low-alloy steel, SCM435 with the tensile strength of 1000 MPa. The tensile properties was evaluated with cathodic charging with 3%NaCl solution and the current density of 400 A/m2. The results of the tensile properties of the pre-charge only were almost consistent with the test results in the room temperature atmospheric pressure, the effects of hydrogen was not confirmed. On the other hand, test results were charged continuously to the tensile strength after the pre-charge, decrease in ductility due to hydrogen was observed. From the above results, hydrogen affects embrittlement are those charged in the elastic range.

Advanced-Ultra Super Critical(A-USC) thermal power plant has been developed for improving the thermal efficiency and reducing the emission of CO₂. Ni-base superalloy such as Alloy 617 is used for components in the A-USC because of its high strength at elevated temperatures. Since this alloy has higher coefficient of thermal expansion than conventional ferritic steels, however, the increase of thermal stress is concerned. In addition, frequent output change is demanded for thermal power plants because of the instability of the output of renewable energies. Therefore, it is necessary to consider the effect of creep-fatigue damage of this alloy on its lifetime. It was found, however, that the effective lifetime of this alloy decreased drastically under creep-fatigue loadings at elevated temperatures. The conventional linear damage rule could not explain this drastic decrease of the lifetime. In this study, Creep-fatigue test was conducted and the change of crystallinity of the broken specimen was observed by EBSD analysis. Then, the local strength of a grain and a grain boundary was measured by micro tensile test system. As a result, the fracture mode of the bicrystal sample around a grain boundary was changed from ductile fracture to brittle fracture. The main reason for the change was analyzed by EBSD method and it was found that the accumulation of dislocations around the fractured grain boundary caused the change of the fracture mode and the effective strength of the alloy.

Dissimilar material joining is widely used for many applications such as automobiles, electric devices, and so on. Adhesive strength is one of the most important factors in joining. There are various theories for adhesion of dissimilar materials such as anchor effect, molecular interaction and chemical bonding, however, adhesive mechanism is difficult to understand, because some theories working at same time, and make complex interaction each other.

In this study, the influence of interface property (effect of micro groove) on adhesive strength of SUS 304 / epoxy resin bonded body was investigated. Higher adhesive strength was indicated on specimen with higher angle interface grooves to the shear loading direction. Delamination crack path analysis was also carried out in this study. The crack path was strongly dependent on the angle of the interface grooves, and specimen that, indicated higher adhesive strength shown crack propagation near the interface.

Shear deformation is known as one of the mechanical factors which applies on the skin surface and causes a pressure ulcer in soft tissues in bony prominence regions. We conducted a shear test on the skin surface of soft tissues which was obtained from porcine hind feet and examined the mechanical properties and deformation behavior of subcutaneous tissue. We also examined the microscopic structure of the soft tissues including subcutaneous tissue using techniques of Masson trichrome stain and X-ray computed tomography. In addition, we applied a standard linear solid model, one of one-dimensional linear viscoelastic models, to the results of the shear loading test on the subcutaneous tissue, and identified the parameters of elasticity and viscosity. Using a microscopy for the stained specimen, we observed thin layers of collagen fibers and a large amount of adipocytes in the subcutaneous tissue. In the shear loading test, we found “sliding deformation” or a large amount of shear deformation in a thin layer in the subcutaneous tissue and the regions between the subcutaneous tissue and the adjacent tissues. We speculated that the layers of loosely-bonded collagen fibers with elastic fibers which were embedded in the adipose tissue may be the cause of such deformation.

Perfect crystal of graphene which is two-dimentional nano material consists of six-membered rings. Curved surface is formed spontaneously by interaction of arranged 5-7 defects composed of a five-membered ring and a seven-membered ring which show intrinsic curvaturte. We utilize this phenomenon to control the buckling mode and the shape of bending of graphite under compression. Our purpose of this study is to obtain the fundamental information about controlling the out-of-plane deformation of graphene and its layered structure with lattice defects. We report the result of simulation of graphite which is controlled the arrangement of lattice defects under compression using molecular dynamics. The result of simulation shows that delamination occurs in the graphite with 5-7 defects under compression. The delamination mode become remarkable as increasing the number of 5-7 defects in graphite. In addition, we can also observe that such delamination occurs near the 5-7 defects. Stress-strain curve of the simulation shows that the compressive stress of graphite which has perfect lattice(no lattice defects) is higher than that of graphite with lattice deffects. This curve also shows that compressive stress becomes relatively higher as increasing the number of 5-7 defects.

Plain bearings of industrial equipment are damaged by the wear, and it is main causes in the failure phenomena. Thus, inhibition of wear is significant for preventing the failure of machinery, but there are two subjects. The first is to elucidate the wear progress mechanism and to grasp the wear phenomena and the sign before machinery breakdown. The second is to perceive the damage type of equipment and to give the sufficient performance against machinery failure for the bearing. To satisfy these subjects, new tribometer is required, which can reproduce the sliding condition of actual machines. In this study, the authors have developed new tribometer, and carried out three types wear tests. In the initial friction test, there were two wear modes. In seizure test, wear particle was generated as the load increased, and they were buried in the Al alloy bearing.

The fatigue limit estimation based on the dissipated energy measurement has been developed and the applicability of this technique for some steels has been investigated. However, there have been no reports on high-strength materials such as Ni-base high-strength alloys. Establishment of a simple and rapid fatigue limit estimation method for materials used under high load environment like this material is expected to greatly contribute to improvement of machine safety and reliability. In this study, we investigated the applicability of fatigue limit estimation method based on dissipated energy measurement for Ni-based alloy Inconel718.

When the oxide film is damaged by stress, the electrochemical properties change and the material may corrode. The polarization curve of the aluminum alloy under stress loading in the NaCl solution was measured and the boundary element electric field analysis of the damaged surface was carried out. From this analysis, the electrochemical properties of the new surface exposed from the damaged oxide film were identified.