In this JSME M&M2016 conference, the author describe about the opinions as for join/attending cost for JSME conference etcetera in first issue. And next, the author describe about the reviewings of the stress invariant tensor for the mechanics/strength of materials with iron and steel materials in study of department of engineering in the graduate school. In the expanding for stress invariant tensor, it almost derive to the results same as references though the each scholarship, researcher and engineer have the each defines refer to the references. And, it different a few from the generally stress and the principal stress notation.

Segregation behavior of Y at a triple junction in a {1011}-{1012} double twin in Mg-Y alloy containing 0.1, 0.2 and 0.3 at.% Y was investigated by Monte Carlo simulations. At the Y high concentration (0.3 at.%), many Y atoms segregated at the triple junction, while few Y atoms segregated at low Y concentrations (<0.2 at.%). The Y segregation behaviors could not be explained by the free volume. The interactions between basal screw partial dislocations and the double twin in Mg-0.3 at.% Y alloy were investigated by molecular dynamics simulations. The leading partial was not absorbed in the twin boundary, while the trailing partial was absorbed and it became a twinning dislocation. No absorption of the leading partial suggests that Y atoms tend to suppress movement of the dislocations.

To understand deformation behavior of non-basal slips in magnesium, tensile tests parallel to basal plane were carried on pure magnesium and Mg-Y alloy single crystals in various temperatures. Pure magnesium single crystals were yielded due to {1122}<1123> second order pyramidal slip under room temperature, while {1011}<1123> first order pyramidal slip were activated above room temperature. In Mg-Y alloy single crystals, first order pyramidal slip was dominant slip mode even at room temperature. {1100}<1120> prismatic slip also observed in Mg-(1.1-1.3)Y alloy single crystals. Mg-Y poly-crystalline sheet specimens were tested in tension to estimate activity of those non-basal slips. In the specimen, those non-basal slips were activated even at yielding in addition to basal slip and frequency of the activity of non-basal slips increase with increasing strain. From the results, we conclude that high ductility of Mg-Y alloy caused from activation of first order pyramidal slip by Y addition.

To understand the cross-slip mechanism of 〈a〉 screw dislocations on the basal and prismatic slip planes, we evaluate the shear-stress dependence of activation energy for kink-pair nucleation (and migration) processes based on atomistic modeling, which are the dominant, rate-controlling steps for screw dislocation motion in Mg. Using the energetics, the critical resolved shear stresses (CRSS) for basal and prismatic slip planes are obtained according to the Orowan's eqation. The estimated CRSS for the basal plane is quite low because of the low activation energy of kink-pair nucleation on the basal plane. The result suggests that the difference in kink-pair nucleation mechanism between the basal and prismatic planes is a key facter to understand the characteristic temperature dependence of CRSS for these slip systems in Mg.

The non-normality effect of polycrystalline pure magnesium is numerically investigated using the crystal plasticity approach. A homogenization-based finite element method is introduced to compute the macroscopic material behaviors with a crystalline scale structure. The intensity of the non-normality depends on the amplitude of the offset strain. It is clarified that the difficulty of switching the dominant slip system when the strain path changes plays an important role and is the principal mechanism behind the strong non-normality effect of hexagonal metal.

In order to investigate effects of the normal stress to the slip plane on deformation modes in magnesium, we carried out molecular dynamics (MD) simulations of shear deformations for the deformation modes under various normal stress applied using an embedded atom method potential. For the basal slip, dislocation were generated from surfaces, irrespective of the applied normal stress. On the other hand, for the prismatic slip, the deformation behavior was affected by the normal stress. When the cell was not relaxed in the normal direction, dislocations on the prism planes were observed in the MD simulation of shear deformation. Under the normal stress, dislocations were not observed but deformation twinning was activated.

To understand mechanisms of non-uniform deformation of α-Ti under cyclic loading, crystal plasticity analysis was conducted employing a tricrystal model, and the relationship between the work hardening and activities of slip systems was investigated. The crystal orientations were arranged so as that the prismatic <a> or basal slip system was the primary one in each crystal grain. The results showed that the activity of basal slip system increased with decreasing that of non-basal one on the condition that the work hardening occurred on the prismatic <a> slip system. The phenomenon indicates that there is a mechanism where the basal slip system can operate under cyclic loading even though the non-basal slip is firstly activated.

In the present study, the deformation behavior upon reverse loading in a commercially pure titanium sheet was studied by means of experiments and crystal plasticity finite-element simulations. When the sheet was subjected to reverse loading from tension to compression, strong Bauschinger effect occurred after the stress reversal, followed by a linear strain-hardening behavior. In the case of r everse loading from compression to tension, strong Bauschinger effect also occurred after the stress reversal as in the former case. Thereafter, in contrast, a small but clear sigmoidal curve was presented. The simulation results were in good agreement with the experimental results. The simulation results showed that this strain-hardening behavior upon co mpression-tension would be b ecause of the activity of { 1012 } detwinning in the beginning of tension.

Plastic slip deformation in the vicinity of the tip of U-shaped notch in α phase titanium single crystal models subjected to cyclic tensile load is analysed by a crystal plasticity finite element method. Crystal orientation is defined so as that the basal plane is parallel to the specimen surface and a prismatic slip system operate as the primary one under tensile load. Results show that slip deformation develop in two thin strip shaped regions extending from the notch root during loading and these plastic shear strain does not diminish upon unloading.

To improve cold formability of AZ31 magnesium alloy sheets were subjected to a continuous repetitive asymmetric roll bending by different roll diameter. The asymmetric roll bending increases the strain amount applied per 1 pass than a conventional symmetric one. It also can apply strain effectively at the center of the thick of the sheet due to change of neutral axis of bending. The Eriksen value was increased for subjected 2 passes asymmetrical roll bending and subsequent 160°C annealing with little decreasing the hardness of the sheet. The presence of deformation twins was observed both just after the asymmetric bending and subsequent 160°C annealing by optical micrograph, the low temperature annealing can suppress evolution of microstructure such as recrystallization or grain growth.

The effect of initial loading direction on fatigue life of AZ 31 magnesium alloy plate was investigated. Axial fatigue test at high stress amplitude were performed at room temperature. We recorded stress-strain hysteresis loops to investigate the cyclic deformation behavior. There were noticeable changes in N-S curves at stress amplitude of 90 MPa and 100 MPa, fatigue lives of initial loading from tension were longer than that of initial loading from compression. On the other hand, at stress amplitude of 110 MPa fatigue life of initial loading from compression was shorter than the initial loading form tension. As the maximum tensile strain at the first cycle became larger, the fatigue life became longer at the same stress amplitude.

Formation of curious deformation bands has been reported as one of the deformation mechanisms occurring in a Mg-based long period stacking ordered (LPSO) phase. The origin of the deformation band is still unknown, and the possibility of the deformation kink band and/or the deformation twin has been discussed. To clarify this, in this study the crystallographic nature of deformation bands formed in the hcp-Zn was first examined in details both by the experimental and computational approaches. And the obtained results were compared to those observed in LPSO phase. As a result, it was found that deformation bands formed in them showed many similar features, such as an ambiguous crystal rotation axis that varied on the [0001] zone axis from band to band, an arbitral crystal rotation angle that was not fixed etc. This suggests that the developing feature of the deformation bands in LPSO phase crystals can be explained by the "deformation kink band formation model".

Compressive creep behavior and dislocation substructures in an Mg-Y based dilute alloy have been investigated in this study. Apparent activation energy for creep in this alloy is about 190 kJ/mol. Many a-dislocations are not only on the basal planes but also the non-basal planes at 480 K and higher temperatures. On the other hand, most of the a-dislocations are on the basal planes at 450 K. The rate controlling mechanism of this alloy is considered to change from the cross-slip controlled dislocation creep to the climb controlled dislocation creep and the estimated transition temperature for the creep mechanism is ranging from 450 to 480 K.

The influence of loading direction on microstructural development of rolled AZ31 magnesium alloy (average grain size: approximately 40μm) was investigated. Tension and compression test specimens were machined from rolled plate; loading axes were parallel (L-specimen) to and perpendicular (S-specimen) to the rolling direction. Mechanical anisotropy of both specimens were observed in stress-strain curves. Pseudoelastic behaviors were also observed in cyclic tension and compression loading-unloading and reloading stress-strain hysteresis curves. The large anelastic strains and work hardening coefficient were observed in compressive stress-strain hysteresis loops of the S-specimen. {10-12} deformation twins were observed uniformly as increasing plastic strain in compressive loading for the L-specimen. In contract, several different types of deformation twins were observed locally as increasing plastic strain in tensile loading for the L-specimen. The slips and deformation twins can play an important role in cyclic pseudoelastic behavior of polycrystalline magnesium alloy.

Plastic slip deformation in textured α phase Ti alloy polycrystal models is studied by a crystal plasticity finite element analysis. Models consist of 57 crystal grains and their crystal orientations are defined in accordance to existing experimental data. Crystal orientation of the texture is called TD-split type where the normal to the basal plane inclines about +/- 40 degree from ND toward TD. Critical resolved shear stress for prismatic slip systems is the lowest and that for basal systems is slightly higher than that for the prismatic ones. Results show that slip deformation is highly non-uniform at the deformation stage of macroscopic yielding point and two prismatic systems operate depending on the inclination of + or - 40 degree to TD. After some amount of deformation, activation of slip systems multiply in crystal grains.

In this study, we carry out dynamic recrystallization (DRX) analysis for Mg/LPSO alloys based on dislocation-based-crystal plasticity and multi-phase-field (MPF) models considering change in environmental temperature to reproduce temperature dependence of the strengthening mechanism: the kink deformation of LPSO phase and the grain refinement of α-Mg phase in the vicinity of LPSO phase due to DRX. We introduce some thermal effects into the critical resolved shear stress (CRSS) and the dislocation mobility in dislocation-based crystal plasticity model, and into the nucleation condition in MPF model. The results obtained here show that the grain refinement and work hardening occur depending on environmental temperature through the interaction between deformation and recrystallization.

Thin metal wires and films have been used for various industrial fields such as automobiles, sports equipments, and medical products. It is therefore important to evaluate their strength, microscopic deformation, and micro cracking behavior. In this paper, thin wires of pure titanium were focused on and the tensile tests were carried out on the stage of the digital holographic microscope. The surface height was measured around the center of grains at each loading step and their change during loading steps were examined. The surface height was scattered for each grain and the change in the height by a unit tensile stress increased with the tensile load. This result suggests that the microscopic inhomogeneity at the latter stage of tension is predictable from that at the early stage of the tension.

Pure Mg single crystals with different orientations were subjected to four-point bending tests, and the bending deformation behavior was investigated. With the basal plane parallel to the neutral plane, A and B specimens deformed due to basal slips. Conversely, with the neutral plane and the axis parallel to (1120) and [1100] , C specimen deformed due to {1012} twinning. However, the specimens fractured after a few percent of displacement. E specimen, whose neutral plane and axis were (1010) and [1210] , also deformed due to {1012} twinning. No crack was observed in E specimen since basal slips were activated within {1012} twins.

FTMP-based model for deformation twin, implemented in crystal plasticity based FEM is extended to 3D conditions, where more realistic boundary conditions are introduced. Demonstrated are the more comprehensive descriptive capabilities for reproducing the stress-strain responses based on a single set of material parameters. Not only mesh dependency but also those of model size and dimensions are systematically examined, where relatively large size/dimension dependencies as well as slight mesh dependency, which is absent in the 2D counterparts, are observed in 3D. These rather complex-looking dependencies are shown to be roughly explained by considering intimate interactions between slip and twin degrees of freedom in the present model, by focusing particularly on the activity/inactivity of the “prismatic slip” system.

Long period stacking ordered type magnesium (LPSO-Mg) alloys have low density, excellent mechanical strength and ignition resistance. LPSO-Mg have a great potential as structural materials of satellites. Lip formation and ejecta size were examined when spherical projectiles strikes thin plates made of LPSO-type magnesium alloy at hypervelocity of 5 km/s. Witness plates were placed in front and behind of each target to determine the scattering area. After impact experiments, ejecta were collected from test chamber and lips near penetration hole was observed. Results of LPSO-type magnesium alloy plates were compared with those of aluminum alloy (A6061-T6).

Nano-crystallization technique can improve the strength of metallic materials without reducing the toughness by refining the grain size up to nano region. In the present study, nano-crystalline nickel films with 10 μm in thickness were produced by electrodeposition which is one of the nano-crystallization techniques. Four types of electrodeposition baths with different concentration of brightener were prepared: 0.0g/L in bath A, 0.4g/L in bath B, 0.8g/L in bath C and 1.6g/L in bath D. In addition, nickel thin films with heat treatment (250°C, 1 h) were also prepared. From the results of tensile tests, the tensile strength increases with increasing the concentration of the brightener. From the results of the fatigue tests, the fatigue limit increases with increasing the concentration of the brightener. High concentration of the brightener is effective for improving the tensile strength, the 0.2 % proof strength and fatigue limit of Ni film.

In the present study, fatigue fracture behavior of commercially pure iron thin wires with the diameter of 700 μm was investigated under cyclic bending by using a bending fatigue testing system developed for thin wires. At bending moment ratio M_min/M_max = －1, －0.8, －0.6 and －0.2, microscopic observation of fracture surface shows striations. Therefore fatigue cracks initiated at both the upper and lower surfaces and propagated toward the neutral axis of the specimen. On the other hand for tension-tension fatigue with the stress ratio R = 0.1, ratcheting failure and necking were occurred and the initiation and propagation of fatigue crack were not observed.

In fatigue crack propagation tests of thin films, crack length has been measured by an optical microscope. In the present study, a fatigue crack propagation testing system was developed for thin film by the automatic measurement for crack length with compliance. Nickel thin film used for this test was prepared by electrodeposition method. Two types of electrodeposited nickel thin films with different grain sizes were prepared by changing the concentration of brightener (0.0 g/L and 0.8 g/L). As results of EBSD analysis, the grain size of nickel thin film tended to decrease with increasing the additive amount of brightener. ΔK-decreasing tests were conducted on nickel thin films under stress the ratio R of 0.3 in a laboratory atmosphere. The automatic measurement of crack length with compliance was effective in fatigue crack propagation tests for nickel thin film, and the threshold stress intensity range, ΔK_th, which obtained in our developed tensing system, showed a tendency to decrease with decreasing grain size.

Using a fatigue testing method by which fatigue cracks can be initiated and propagated in a film adhered to cover an circular through-hole in a base plate subjected to cyclic loads, annealed rolled commercially-pure titanium films with Anisotropy of 30μm thickness were fatigued under constant stress amplitudes with a stress ratio of R=0. In order to discuss correlation between fatigue crack propagation and change of crystal orientation, crystal orientation on the surface of the film materials was measured before and after fatigue testing. The crystallographic information of these films was analyzed using the Electron Back-scatter Diffraction (EBSD) system. As a result, fatigue cracks propagated faster in the specimen loaded to the rolling direction than in that loaded to the transverse direction. It is considered that the fatigue crack propagated by prismatic slips with screw dislocations in the specimen loaded to the rolling direction. On the other hand, the fatigue crack seems to propagate by basal slips with edge dislocations in the specimen loaded to the transverse direction.

In this study, the effects of thickness on the tensile and fatigue properties of Cu thin films were investigated to understand mechanical behavior of Cu thin films. Both the tensile tests and fatigue tests were carried out using Oxygen-free Cu thin films. The thickness of the films was 15μm and 30μm, respectively. It was shown that both the tensile strength and fatigue strength of the 15μm thick film were lower than that of the 30μm thick film. The thickness effect on the fatigue strength was larger than th at on the tensile strength.

Even if metal thin film is the same material, it does not know that different mechanical characteristics form a bulk material are shown, and general fatigue behavior showing a bulk material does not necessarily appear. The present condition is that research of the fatigue behavior in the repetition shear load of a metal thin film is not made compared with the thing of load direction axis and bending. From studies of thin films in the cyclic shear loading to date, a crack with a locally slip in S10C occurs. The cause such crack occurs, the strain in the ferrite by applying the cyclic shear loading occurs, pearlite is considered to crack been concentrated deformed part that is sandwiched between the perlite harder than ferrite because no deformation occurs. So, in this study we used S45C that is high perlite volume fraction and has many of ferrite sandwiched perlite, observed intergranular crack, transgranular crack, and crack with locally slip. As a result, we confirmed difference of crystal orientation near the crack and the crack initiated along slip plane and not reached back face. And we considered that crack was initiated by local concentration of deformation.

Fatigue properties of transformation-induced plasticity (TRIP) maraging steel were investigated by rotating bending fatigue testing. An initial microstructure of the TRIP maraging steel is maraging martensite with metastable austenite films. First, as we expected, precipitation hardening by intermetallic compound realized outstanding improvement of fatigue limit. In addition, compared to TRIP steel containing metastable austenite and pearlitic steel containing nano-laminate, the TRIP maraging steel showed superior fatigue limit and fatigue life at the same stress amplitude normalized by tensile strength. The higher fatigue limit is attributed to suppression of crack initiation owing to TRIP effect and subsequent transformation-induced crack closure (TICC). The higher fatigue life stems from roughness-induced crack closure (RICC). More specifically, the TRIP and TICC originates from the metastability, and the RICC was caused by the nano-laminate structure of the TRIP maraging steel.

The material often would be subjected to cyclic load in corrosive circumstance as living body. The better fatigue strength is rather needed in such condition. In our previous paper, it was found that there was an appreciable degradation in fatigue life time of yttria doped tetragonal zirconia polycrystals (TZP). In this study, the materials were annealed for 100 h at the respective temperatures of 200, 400 and 600°C in a low vacuum atmosphere to improve the decrease in the fatigue life. On using those annealed specimens both static and cyclic fatigue tests were conducted under static four-point bending and under cyclic reversed plane bending respectively. The obtained test results indicated that there was an appreciable increase in lifetime in the case of cyclic fatigue by low applied load for the specimens annealed at the low temperatures of 200 and 400°C. These results shows that the occurrences of fine microcracenlargement of process zone in wake field of crack on cycling loading which contribute to the ks accompanied with phase transformation phase cause the increase of crack propagation resistance.

Lead free solder materials have been used wildly in electronic apparatuses. Solder joints are subjected to fatigue damage under thermal cyclic load. Many studies on fatigue damage have been done, and low-cycle fatigue tests were carried out at strain rate of 1×10^-4/s～1×10^-1/s by authors. On the other hand, it was known that creep damage is induced sometimes in solder joints due to the elastic follow-up phenomena and the interaction between fatigue damage and creep damage shortens the life of the solder joint. In this study, low-cycle fatigue tests were carried out at strain rate of asymmetric slow-fast wave. The fatigue mechanize was discussed from the view point of the creep-fatigue interaction or creep damage recovery.

In this study, the fatigue behavior of TiNi Super Elastic Alloy under hydrogen environments were investigated by using thin wires with the diameter of 0.7 mm. Fatigue tests with stress ratio of R = 0.1 were conducted in two environments: in air and NaOH solution with hydrogen charging (current density J = 127 mA/mm²).Frequency were 2Hz and 10Hz. Fatigue life decreased in hydrogen environments than that in air. Frequency does not effect of the fatigue life in air, while fatigue life in hydrogen environment are time deperdent in the region of low stress amplitude. In microscopic observation, brittle surface layer was formed and surface spalling occurred only for J = 127 mA/mm².

The temperature effect on the fatigue crack propagation behavior was studied with center-notched specimens which were cut from injection-molded plate of short fiber reinforced plastics, PPS, at three angles of the loading axis relative to the molding flow direction. The crack propagation behavior was investigated at four temperatures : room temperature (RT=298 K), 343K, 373 K and 403 K. In the relation between crack propagation rate, da/dN, and the stress intensity factor range, ΔK, the propagation rate of fatigue cracks was slowest for the zero angle, MD, and increased with increasing fiber angle at all temperatures. For each orientation, da/dN was nearly the same at RT and 345K, and increased greatly at temperatures of 373 K and 403K above T_g (=363 K). When da/dN was correlated to the J-integral range, ΔJ, the relations for different orientations became closer, and also the influence of temperature on da/dN was decreased. The inelastic deformation and the decrease of elastic constants are responsible for crack acceleration seen in da/dN vs ΔK relation at high temperatures.

In this study, we conducted a low-cycle fatigue testing in a Fe-28Mn-6Si-5Cr-0.5NbC (FMS) alloy and a SUS304 steel. The fatigue tests were made using a servo hydraulic fatigue testing machine of capacity 50kN, at maximum total strain amplitudes (ε_ta) of 2.0%, 1.4%, 0.9%, 0.6%. In the SUS304 steel, with decrease in applied strain amplitude, stress-strain hysteresis loop is reduced. On the other hand, the stress response of the FMS alloy is almost unchanged, irrespective of the applied strain amplitude. The life to failure of the FMS alloy is 4 times higher at ε_ta=0.6%, and twice at ε_ta=0.9% and at ε_ta=1.4% than the SUS304 steel. In addition, the stress amplitude of the FMS alloy is 2 times higher at ε_ta =0.6 %, and 1.5 times higher at ε_ta =0.6% than the SUS304 steel. The prolonged fatigue life of the FMS alloy is attributable to the reversible deformation associated with the transformation pseudo-elasticity that can reduce the accumulated strain.

Rotary cantilever bending fatigue tests were conducted using β titanium alloy, Ti-22V-4Al, specimens which were treated by the ultrasonic shot peening (USP). SEM observations have revealed that the USPed specimens exhibited a subsurface crack initiation with a fish-eye in the high cycle fatigue life over 10⁵ cycles. The crack initiation site was flat and no inclusion was observed. The phases of crystal lattice were analyzed using the electron backscatter diffraction (EBSD). EBSD analyses have revealed that the fractions of the α phase and the β phase in the base material were 5 % and 95 %, respectively while the α phase was concentrated locally. The subsurface crack initiated from the site where the α phase was concentrated.

In order to investigate the effect of a surface treatment by anodic oxidation treatment generating an anodic porous alumina layer on the surface of the aluminum alloy on the joint of the aluminum alloy and the CFRP welded by the Friction Stir Spot Welding, the microscopic observation of the welding area, the fatigue tests were carried out, and the mechanism of the fracture has been investigated. The result of the microscopic observation of the welding area shows that a number of pores generated in the CFRP, and according to the result of the fatigue test, the fatigue strength was improved by the application of this treatment. Also, the welding area and the stability of these area were increased by the application of this treatment. In addition, the proportion of the adhesion area in the welding area of the specimen with treatment was higher than the proportion of the specimen without treatment. Moreover, for the fracture mechanism, for specimen without the treatment, the interfacial failure is regarded as the main reason of the fracture while the cohesive failure is regarded as the main reason of the fracture of the specimen with the treatment.

In order to improve the fatigue properties of similar butt friction stir welded joint of A2024 Aluminum alloys, laser peening which was laser peening treatment with femtosecond pulse laser equipment was applied. This femtosecond laser peening does not need water in the peeing processing. And fatigue tests were conducted at stress ratio R= -1, road frequency 23Hz with plane bending fatigue condition. As the results, fatigue property was improved by femtosecond laser peening. Residual stress distribution, Vickers hardness distribution and surface roughness were investigated and the relation between fracture position of specimen and these were investigated.

In this study, fundamental theory to predict fatigue crack path propagating in disturbed stress field was proposed. In order to show the validity of the theory proposed here, the basic problem of the interaction between a circular hole and crack tip was discussed. As a result, it was shown that the predicted crack path was almost coincided with the crack path obtained by FEM.

In order to investigate the effect of specimen thickness on near threshold crack propagation without oxide induced crack closure, ΔK decreasing tests under the stress ratio 0.1 and the loading frequency 5 Hz, specimens thickness 12.7 mm and 6 mm conditions were carried out. In case of Cr-Mo Steel, no clear oxide films were observed on macroscopic fracture surface on both specimens. EDX results showed that no oxides were detected on the near threshold fracture surface of both specimens. The crack propagation showed almost the same behavior in both high ΔK and near threshold areas. The threshold values were 4.6 on both specimens. These results showed that there were no effects by specimen thickness on both high ΔK and near threshold range.

High strength steel shows the unique fatigue fracture called Fish eye failure, that's internal originated fatigue failure, in very long cycle region with the stress below a fatigue limit. Therefore, in order to build the lifetime method evaluation in a very high cycle region, an elucidation of the mechanism of internal fatigue failure is desired. It's reported internal fatigue crack in high strength steel is propagating extremely slowly. However, it is not clear whether this extremely slow fatigue propagation is a phenomenon peculiar to high strength steel. In this study, the materials which were changed hardness of high strength steel SNCM439 were used, and the crack propagation by ΔK decrease fatigue test in vacuum were observed. And it was inquired whether the extremely slow crack propagation would be very peculiar to high strength steel.

This paper discusses the effect of stress path on fatigue life of type 316 stainless steel. Load and pressure controlled fatigue tests at room temperature ware carried out using a hollow cylinder specimen under multiaxial loading by cyclic inner pressure and cyclic tension loading. Loading conditions were 5 types; they are cyclic pull loading, cyclic inner pressure loading, cyclic biaxial loading and two combination loadings by cyclic inner pressure and cyclic pull loading. Fatigue lives of the latter two combination loadings were reduced due to increase in cyclic ratchetting deformation, although that of the former three loadings could be evaluated by Mises’ equivalent stress.

Shear dominant fatigue crack propagation in a single crystal Ni-base superalloy, NKH-304 were experimentally investigated at room temperature. A series of experiments revealed that the shear dominant cracking was influenced by both primary and secondary crystal orientation, and crack growth rates were correlated with macroscopic deformation measured by strain gauge on the back surface of the CT specimen. In order to numerically assess the effect of elastic anisotropy and the crack geometry, 3-D FEM for the CT specimen was developed. It was found from fracture mechanics analysis that geometries of 3-D inclined crack played an essential role to determine the driving force of the shear dominant crack propagation in the superalloys.

Fatigue tests of ultrafine-grained pure Ti plates fabricated by multi-directional forging (MDFing) were conducted in laboratory air and physiological saline. The plate with 1 mm in thickness was developed aiming at dental implant application. The fatigue strengths of MDFed pure Ti plates in laboratory air were higher than those of rolled pure Ti plates. It could be attributed to the much finer grains evolved by MDFing. In the very high cycle fatigue (VHCF) regime, sub-surface crack initiation was observed in the MDFed pure Ti, and the sub-surface crack initiation mechanism was related to the inhomogeneity of microstructure. The corrosion fatigue strengths in physiological saline were comparable to those in laboratory air, indicating high corrosion resistance of MDFed pure Ti.

In the present study, slip band formation and fatigue crack initiation in magnesium alloy (AZ31) were examined by means of atomic force microscopy (AFM). Plane bending fatigue tests were performed using computer-controlled electro-dynamic fatigue testing machine under a stress ratio of -1 in ambient laboratory atmosphere. Based on the replica method, replicas of the specimen surface were observed to examine the fatigue crack propagation. The crack initiation site on the specimen surface was identified by comparing the crack propagation and observing fracture surface. Result of fatigue tests, fatigue strength at 107 cycles of magnesium alloy was found to be about 92 MPa. It was seen many slip lines with various angles. In any cross-sectional shape, extrusion of slip bands in the vicinity of the crack was observed. Main crack shape during 6000 and 8000 cycles was curved, then the crack straightly propagated. It was found that crack propagation behavior depended on the stress intensity factor range ΔK. The main crack propagated even at very low stress intensity factor range.

Titanium alloy (Ti-6Al-4V) with bimodal “harmonic structure”, which consists of a coarse-grained structure surrounded by a network structure of fine-grains, was produced by sintering mechanically milled powder to achieve high strength and good plasticity. In this study, we examine the behavior of small fatigue crack initiation and propagation of Ti-6Al-4V alloy with harmonic structure. In the material with harmonic structure, fatigue crack growth rate (da/dN) was high and threshold stress intensity range (ΔK_th) was low compared to the material with coarse acicular structure. On the other hand, the material with harmonic structure showed higher fatigue limit. This is because the fine-grained microstructure, which shows high fatigue strength, is formed in the harmonic structure. However, their fatigue crack growth resistance was lower compared to the coarse-grained microstructure.

In this study, the application of cumulative damage rule under variable amplitude stressing in very high cycle fatigue regime was investigated. Ultrasonic fatigue tests were conducted using Ti-6Al-4V alloy under constant amplitude loading and variable amplitude loading. Under constant amplitude loading, both internal fatigue failure and surface failure are observed. Under variable amplitude containing LCF and HCF loading with surface origin and VHCF loading with internal origin, the application of cumulative damage rule was not confirmed. Additional test results to confirm the application and study of fracture mechanism under variable amplitude stressing in the very high cycle fatigue regime are needed.

The initial process of internal fracture in Ti-6Al-4V was non-destructively observed by synchrotron radiation μCT imaging to clarify the crack initiation and initial growth behavior in very high cycle fatigue. As a result, a number of cracks were detected all over the observation volume. The internal crack firstly initiated at around 5.0 × 10⁶ cycles, however, the initiation lives of these cracks varied widely. The just initiated crack propagated microstructure-sensitively at significantly low growth rate. Some cracks initiated so early and propagated so slowly, but others initiated so lately and propagated so rapidly. As above, the crack initiation sites and lives widely varied, and the crack growth rate just after the initiation has a large scatter. These results let us to conclude that the local microstructural conditions and the effect of environment around the crack are indispensable factors to fully understand the initial process of internal fracture in very high cycle fatigue.

Small fatigue crack closure is observed as a thing with a possibility that the influence of change of the stress level and a stress ratio, etc. to the crack growth behavior can be explained, and until now various researches have been done. Although there is a method using the strain gage as the dynamic measurement method required for the elucidation of a minute fatigue crack, in respect of generating of minute fatigue cracks being directly contacted to a measuring object thing, and there is a problem in durability or measurement of the crack growth behavior, it may not necessarily be effective. Then, its attention was paid to the magnetostriction effect which is one of the magnetic actions of a ferromagnetic substance. Measurement of the opening stress of the minute fatigue crack under dynamic load and crack length performed by measuring the magnetic field change by the magnetostriction effect, without needing the contact to an object with simple equipment.

In the soundness and safety assessment based on fracture mechanics of equipment and a structure, the nondestructive evaluation of a crack is indispensable. In recent years, various the evaluation method is also developed and studied with remarkable progress of a sensor. An inspection method magnetic also in nondestructive evaluation is an effective inspection method from being safe and being able to automate. In this research, a possibility that the position of crack can be measured was considered by against a rotating test piece of measuring the magnetic flux leakage using the hole sensor. As a result, it is possible to measure position of crack of 2.6mm or more by two-dimensional distribution of magnetic flux leakage.

To substitute for a traditional replication technique, an in-situ fatigue crack observing system for 4-point bending fatigue has been newly developed. This system consists of (1) 4-point bending fatigue jig having reflector, (2) stroboscope synchronized with test frequency, (3) long working distant type microscope, (4) computer controlled XYZ automatic stages and (5) hi-definition digital video camera. For verifying performance of this observing system, fatigue tests were carried out by using several type of specimens. It is proved that this system can be detect a small fatigue crack and its propagation behavior.

This paper presents a newly developed multiaxial high cycle fatigue testing machine to carry out the fatigue tests with high frequency under a non-proportional loading condition in which directions of principal stress and principal strain are changed in a cycle. The non-proportional loading can be practicable by a combination cyclic bending and reversed torsion. The maximum frequency is 50Hz which is approximately 25 times higher than that of the electrical servo controlled hydraulic multiaxial fatigue testing machine. An applicability of the developed testing machine is also discussed based on the obtained test results.