Porous aluminum is a multifunctional metal material that is lightweight and has excellent shock absorption. Larger porous aluminum is considered to be necessary for its industrial use. In our previous studies, it has been found that it is possible to joint two porous aluminums by press forming during precursor foaming to increase in size of the porous aluminum by jointing. In this study, porous aluminum with uniform dimensions was fabricated using a mold made of ceramics. And non-destructive internal observation of the fabricated porous aluminum was performed by X-ray CT imaging. Thereafter, the bending strength was examined. First, a rectangular parallelepiped mold was fabricated by combining ceramic plates. Two precursors were placed in the mold and heated by light heating. The precursor then expands by foaming. When the expansion stopped, heating of the precursor was finished, and then press forming was performed to give shape and joint. The fabricated porous aluminum had almost constant dimensions. It was also found that the internal porous structure was sufficiently maintained. Next, a four-point bending test was performed on the fabricated porous aluminum. The case where the fracture occurred from the porous aluminum joint and the case where the fracture occurred from other than the joint showed almost the same bending strength. The bending strength was about 40% that of base porous aluminum.

When using porous aluminum (Al) as a machinery member, it is necessary to shape it according to the purpose. We used precursor foaming process in this research. In our previous research, a pin screen mold was prepared by a stainless steel pin having heat resistance and ceramics honeycomb board, and the shape was applied only from below of the porous Al. In this research, fabrication of three-dimensional shape porous Al was attempted by applying the pin screen mold with three-dimensional pin direction. Two pin screen molds were placed in contact with each other for the horizontal direction, and then heating them with a precursor sandwiched between them in the space of the pins for foaming. A precursor was fabricated using a friction stir welding method. Heating was performed with optical heating apparatus using a halogen lamp. The pin screen mold was made by making a hole by pushing the shape of the model to be shaped into the aluminum foil, and then piercing the pin through a ceramic honeycomb board. The pore structures of the obtained porous Al were evaluated using X-ray computed tomography (CT) images. It was found that porous Al with three dimensional can be fabricated by the pin screen mold.

A technique to observe three-dimensional grain mapping of polycrystals, called X-ray diffraction contrast tomography (DCT) has been proposed and developed in recent years. It is possible to evaluate fatigue damage by total misorientation β of each grain which is calculated from diffraction spread angle measured in DCT measurement. In this study, Fatigue test was conducted on the beam-line, and DCT measurement was performed without removing the specimen from the fatigue testing machine. The change of total misorientationβ in each grain was investigated during high cycle fatigue test for untreated SUS304.

This study is concerned with observation and evaluation of the creep damage progress in the fine-grained heat affected zone (HAZ) of modified 9Cr-1Mo steels under multi-axial stress conditions. Creep rupture and interrupted tests have been carried out using a miniature creep test specimen. X-ray micro-tomography at SPring-8 has been applied to observe the creep damage process. Quantitative evaluation has been performed to identify the nucleation and growth of the creep voids. The increasing tendency of the void number density in the vicinity of notch root was observed by in situ quantitative analysis. Furthermore, macroscopic stress has been calculated in order to evaluate its correlation with the development of creep damage.

Aluminum foam composite of aluminum alloy die casting ADC12 core coated by pure aluminum A1050 was fabricated combining foaming of an ADC12 precursor by optical heating and foaming of an A1050 precursor by an electric furnace. In this study, the pore structures of the fabricated aluminum foam composite were examined using X-ray computed tomography (CT) inspection. It was shown that, by controlling optical heating conditions (voltage of halogen lamp and heating time), the aluminum foam composite with good pore structures of ADC12 part and good bondability between A1050 and ADC12 parts could be fabricated. Moreover, drop weight impact tests of the aluminum foam composite was carried out. Based on the test results, it was shown that the plateau stress could be evaluated within ±20% error by using mean compressive stress on a maximum-porosity cross section perpendicular to the loading direction.

A porous aluminum (Al) has light weight and high energy absorption characteristics. However, the tensile strength and bending strength of porous Al is lower than those of dense Al materials. It is promised that a sandwich structure consisting of a porous Al core and dense metallic face sheets is effective to use as structural elements. In this study, sandwich structures of a functionally graded (FG) porous Al core of ADC12 and A6061 with two face sheets of A1050 were fabricated using friction stir welding (FSW) route. The fabricated sandwich structures of FG porous Al core were observed by X-ray computed tomography (CT). Through the observations of the pore structures and the thickness of ADC12 layer or A6061 layer in FG porous Al core, foaming conditions for sandwich structure of FG porous Al core were examined. It was found that sandwich structure with each layer of ADC12 and A6061 of good pore structures and almost same thickness was successfully fabricated by controlling holding time and thickness of ADC12 and A6061 precursors.

The behavior of the deformation-induced martensitic transformation (DIMT) of ferritic(α)/austenitic(γ) steel is affected by various factors. Moreover, each γ grain has the different behavior of the DIMT. In this study, the behavior of the DIMT each γ grain was investigated with using fine focused X-ray diffraction during in situ uniaxial tensile loading. The DIMT has already occurred to a lot of γ grains immediately after the yielding point. The bigger γ grain transformed early to martensite. The γ grains transformed immediately after the yielding point, plastic strained before the yielding point. It is considered that the transformation mechanism in this material is strain-induced transformation.

The fracture behavior of ferritic/austenitic steel is closely related the deformation-induced transformation of retained austenite phase to martensite. In this study, it is conducted that visualization of deformation-induced transformation and fracture behavior of ferritic/austenitic steel with 3D/4D imaging in SPring-8. While austenite grains mostly transformed to martensite in a strain range up to 6.8%. there are also untransformed grains in a same strain range. Therefore, it was found that the transformation behavior is inhomogeneous. Some austenite grain has complex geometric shape tends to transform to martensite, others do not transform. It is considered that the deformation-induced transformation behavior is dominated by several factors including the grain shape.

There have been many previous studies on dislocation structures developed in copper single crystals during cyclic deformation. Formation and development of deformation bands have been reported in copper single crystals with the [111] multiple slip orientation. The study of deformation bands is important because they become crack initiation sites; however, their formation mechanism has not been well investigated. In this study, therefore, push-pull fatigue tests of the [111] copper single crystals were conducted under constant plastic shear strain amplitudes between 2.0 × 10⁻⁴ and 1.0 × 10⁻² at room temperature. The deformation bands and dislocation structures in the fatigued specimens were observed by the electron-channeling contrast imaging technique of scanning electron microscopy and high-voltage scanning transmission electron microscopy. Bands of secondary slip (BSSs) and kink bands (KBs), which are parallel and perpendicular to the primary slip plane, respectively, were observed in cyclically softened specimens. Within the BSS and KB, cell bands with crystallographic misorientation were developed parallel to the {112} planes, which are perpendicular to the cross-slip plane and parallel to the primary slip direction. Cell boundaries in KBs consisted of three types of dislocations, including sessile dislocations.

Two-step low-cycle fatigue tests of pure Al single crystals with single-slip orientation were performed in the pull-push mode under a constant plastic strain amplitude. In the first-step tests, the plastic strain amplitude within the plateau region of the cyclic stress-strain curve was applied at 77 K. For the second step, the same plastic strain amplitudes were further applied at RT. In the first step, the stress amplitude showed cyclic hardening to saturation. On the other hand, stress amplitude in the second step gradually decreased with increasing cumulative plastic shear strain and transformation of the dislocation structure occurred. Well-developed matrix-vein and PSB-ladder structures after the first-step tests changed into a cell structure. The ladder structure first transitioned to the cell structure the early stages of the second-step test, the volume fraction of the cell structure gradually increased with increasing cumulative shear strain, and then the whole specimen was covered with the cell structure.

Plastic deformation mechanisms in hexagonal close-packed metals are characterized by dislocation slip and twinning. Therefore, direct observation of dislocation motions and nucleation/growth of deformation twins gives important information for the plasticity mechanisms in hexagonal close-packed metals. From a spatial resolution perspective, transmission electron microscopy is an effective technique that enables direct observation of microstructures. However, regarding in situ deformation experiments, the special boundary conditions in a thin foil type specimen may be problematic, e.g., effects of image force. To overcome this problem, we applied electron channeling contrast imaging (ECCI) to in situ observation, which enables the detection of the lattice defect motions and the evolution of elastic strain fields in bulk specimens. In this work, a Co single crystalline compact tension (CT) specimen of which the thickness was approximately 1 mm was deformed in a field emission scanning electron microscope. We observed the microstructure around the notch during deformation using in situ ECCI. The following in situ ECCI-visualized plasticity-related crucial phenomena were observed: (1) dislocation motions in the parent phase during deformation, (2) nucleation/growth of deformation twins via loading to a higher stress, and (3) crack formation along the matrix/twin boundaries.

In metals cyclically deformed, the formation of ladder-like dislocation structure brings about the extrusion/intrusion on the surface, which results in fatigue cracking. It is widely known that the pattern size of the ladder-like structure is around a few microns or smaller independently of specimen size. This suggests that low dimensional metals with micro or nanometer scale possess characteristic fatigue behavior because they have no space to form the ladder-like dislocation structure. The purpose of this work is to investigate the cyclic response of a micro-sized copper single crystal specimen which possesses a single slip orientation. In order to apply a tension-compression fully-reversed load to the specimen, we developed a special device, which could grip the specimen end and adjusted the loading axis. The specimen was deformed without cyclic work hardening and a slip localized region, which penetrates the specimen, was formed. The region resulted in intrusions/extrusions on the surface. The TEM observation of the specimen after fatigue showed that the ladder-like dislocation structure does not exist there.

Bauschinger effect is closely related to plastic anisotropy as pointed out in the past studies. In the present study, underlying mechanisms of Bauschinger effect in magnesium (Mg) alloys, where basal slip system is predominantly activated, were studied by experimental and numerical approaches. Experimentally, uniaxial loading tests with stress reversal were performed on the alloys with different plastic anisotropy. The plastic anisotropy of the alloy was controlled by changing the volume fraction of LPSO phase which shows more significant plastic anisotropy compared with α-Mg matrix. Numerically, the influences of inter-granular and intra-granular heterogeneities on Bauschinger effect were evaluated by a crystal plasticity finite element analysis.

Typical fatigue failure mode is primarily triggered by surface crack initiation processes, associated with formations of persistent slip bands (PSBs) during cyclic straining, where the PSBs consist of a well-organized dislocation substructure accompanied by the alternating ladder-channel patterns. FTMP-based finite element simulations have been demonstrated to be able to reproduce not only the ladder-structured PSBs but also the attendant surface groove formation and its growth. To advance the simulation framework toward the crack initiation stage, we attempt here to extend it to the corresponding restart analyses based on Di-CAP concept, which allows us “cut-&-paste” operations. Some preliminary results including the cut-away and restart processes are demonstrated.

The plastic slip deformation and evolution of atomic vacancy density in Cu polycrystal subjected to cyclic loading was numerically analyzed by using crystal plasticity analysis. Slip deformation and strain hardening are evaluated by dislocation density-based models. Generation rate of atomic vacancies is given by the model proposed by Essmann and Mughrabi (1979). A cyclic loading condition is 5 cycle from 0 to 0.5 % nominal strain. The work hardening rate of cyclic loading deformation was lower than monotonic loading deformation. The lower work hardening rate of cyclic load deformation caused plastic deformation localization. In addition, the accumulated atomic vacancy density was higher than monotonic load deformation.

I here present recent examples of crack tip microstructure observations. In particular, special emphasis is placed on dislocation microstructure evolution near a small fatigue crack tip, observed by electron backscatter diffraction measurements and electron channeling contrast imaging. Both of the techniques can show microstructures at a specific location such as plastic zone at a crack tip in a bulk specimen. The first example is a case of metastable austenitic steels which initial microstructures transform to body-centered cubic or hexagonal close-packed phase. The second and third examples are given in Fe-Mn-C and Fe-Cr-N stable austenitic steels. With these examples, effects of transformation, strain aging, and dislocation planarity on small crack growth resistance are revisited. Moreover, I present successful examples of in situ electron channeling contrast imaging of dislocation motion at a small fatigue crack tip in the Fe-Cr-N austenitic steel. The in situ characterization results implied significant contribution of dislocation stress shielding and planar slip-induced damage evolution on resistance to Mode I and II small fatigue crack growth.

To investigate the effect of free surface on lattice rotation near fatigue crack, fatigue crack propagation test was performed on a compact tension (CT) specimen of pure iron polycrystal. After a fatigue crack propagation test on the CT specimen, the free surface was repolished to measure the lattice rotation at top surface by electron backscatter diffraction (EBSD) technique. We analyzed change in misorientation angle from a reference matrix along a line perpendicular to fracture surface. It is confirmed that the misorientation angle from a reference matrix orientation increasing with decreasing distance from fracture surface at grains neighboring fracture surface. This local lattice rotation can be caused by dislocations emitted from a tip of a propagating fatigue crack. In addition to the misorientation analysis, we estimated the direction of a rotation axis. In the grains faced to free surface, the lattice rotations were often occurred not around the free surface normal but around the axis lying in the free surface plane. This rotation axis can be explained by screw dislocation emission from Mode-III type crack. By contrast, in grains locating at through-thickness center, the lattices rotated around various axes including the free surface normal. It is suggested that edge dislocation emission from Mode-I crack became relatively dominant at the though-thickness center.

High-cycle fatigue crack initiation and propagation in an equiatomic CrMnFeCoNi high-entropy alloy (HEA) were investigated using smooth specimens. The microstructural deformation characteristics, i.e., planar dislocation slip, significantly affected the fatigue crack initiation and small fatigue crack propagation. Multiple crack initiation was observed. In the grain where a crack initiated, the fatigue crack plane at the initiation site was along the slip plane, but the planar dislocation array was aligned on another slip plane, which suggests slip deformation highly localized on a single {111} atomistic layer where the crack initiated. High dislocation planarity of HEA caused the localization of slip deformation, which led to multiple crack initiation. Dislocation planar array was observed near the fatigue crack of HEA. In the same line, the planar dislocation motion associated with a low SFE increases the dislocation density on specific slip planes and stress arising from the nearest dislocation to the crack tip, which strongly shields crack tip stress during loading. This effect is referred to as dislocation stress shielding. Dislocation-driven stress shielding contributed to the decrease of fatigue crack growth rate. Here, considering the crack propagation mechanism, we recognize two modes: (A) Mode I type driven by dislocation emission from a crack tip, and (B) Mode II type associated with dislocation accumulation on limited slip planes ahead of crack tip. In HEA, fatigue crack propagation along slip lines was frequency observed, which suggested the promotion of Mode II crack propagation. Mode II crack propagation was promoted because Mode I crack propagation was suppressed by dislocation-driven stress shielding and instead shear-induced damage was localized along a slip plane near the crack tip.

Transformation-induced plasticity (TRIP) maraging steel has a high fatigue resistance because of its laminated structure and metastable austenite. In this study, we present the crack length dependence in fatigue crack growth on two types of TRIP-maraging steels with different annealing times. ΔK increasing compact tension tests were carried out on the TRIP-maraging steels. In the case of rotating fatigue bending test (short crack), it is reported that 1h heat-treated TRIP-maraging steel has better fatigue limit than 8h heat-treated TRIP-maraging steel. However, in the case of ΔK increasing compact tension test (long crack), the 1h heat-treated TRIP-maraging steel showed larger fatigue crack growth rates than the 8h heat-treated TRIP-maraging steel. To consider the crack length dependence, we focused on plasticity-induced crack closure (PICC), roughness-induced crack closure (RICC) and transformation-induced crack closure (TICC). The most dominant factor for the crack length dependency for fatigue property is TICC.

In this study，a method is developed to predict the burst pressure of spherical pressure vessels with non-uniform thickness. The commercial 6061-T6 aluminum alloy which has wide applications as a structural material in the transport machines and construction industries is selected for this study．At this study we focus on the spherical pressure vessels which have periodic thickness changes and is subjected to internal pressure loads．The elastic-plastic and damage properties of the material are estimated inversely from the uniaxial tensile properties，and the stress，strain and damage state of the spherical pressure vessel with non-uniform thickness is analyzed by the finite element method．The Quantitative analysis of the effects of thickness heterogeneity on burst pressure is conducted in two aspects: its amplitude and wavelength．The greater the amplitude of the thickness change， the lower the burst pressure，but the effect of wavelength on the burst pressure is less severe．Through the analysis of the simulation results，it is shown that Hill's material instability theory is effective as a bursting prediction method for spherical pressure vessels with non-uniform thickness．The degree of change in radial strain is also positively related to the instability of the material．When the local radial strain change drastically，it is often considered that the instability of the material occurs．

A round bar with a deep circumferential crack is proposed as one of specimens of fracture toughness test. A bar specimen has larger strain constraint for its size than a CT specimen. However a ligament of a bar specimen is so narrow that the specimen is easily fractured without enough stable crack growth area. The specimens in this research are annealed or quenched steel with 0.45 % carbon. These specimens with b/R = 0.3-0.4 (ligament/radius) and R = 5 mm showed pre-crack blunt and the following growth crack blunt, and total stretch zone size were 0.6mm of quenched one and 0.9 mm of annealed one. The corresponding J integral are 17 kJ/m² and 31 kJ/m², respectively. Although J-R curves of a CT and a bar show not big difference each other, the corresponding J of a bar as mentioned above is much different scale from critical J of CT specimen. On the other hand, the relative comparison of the annealed and the quenched steel, the ratio 1.8, is near with that obtained by CT specimen, 1.5.

FTMP concept is applied to computationally fabricate complex microstructured samples to be further utilized in various deformation analyses based on, e.g., FEM. Here, we focus on the process of modeling single lath-block structures provided the corresponding eigenstrain distributions based on the Bain lattice correspondence is initially introduced. One of the keys for the lath-block modeling is the development of misorientation across the lath boundaries, together with the attendant internal stress fields. FTMP-based approach exhibits spontaneous evolution of such misorientation when substantial contribution of the incompatibility tensor is introduced in the hardening law. And it was found that the dislocation density term has a function of suppressing misorientation in the hardening law.

Fatigue failure is considered to involve the accumulation of atomic vacancies, but its behavior is still unknown. In this study, we investigate the accumulation of atomic vacancies in α-titanium alloy single crystal specimens with microscopic cracks during fatigue deformation by using a crystal plasticity analysis code. Results show that slip in the forward and backward direction take place almost constantly and repeatedly in narrow regions near the crack tip. This activity causes the accumulation of atomic vacancies at the crack tip.

Dwell sensitive fatigue is a problematic issue when titanium alloys are used as aero engine parts. It has been found that displacement/load holding accelerates local strain evolution, crack initiation, and crack growth, which causes deterioration of fatigue resistance. The present study focused on elucidating the crack growth acceleration mechanism during dwell fatigue. The dwell fatigue test was performed under load control with a triangular waveform, which included 10-min displacement holding at the peak load for only one cycle at ΔK = 25, 30, 40, 50 MPa√m in the Ti-6Al-4V alloy with a bimodal microstructure. Digital image correlation-based strain maps of the in-situ fatigue test showed the strain evolution near the crack tip during 1-cycle dwell. Moreover, locally extended striations corresponding to 1-cycle dwell were observed on the fracture surface. From the two results, it was proposed that the crack propagation during dwell fatigue occurs with crack tip blunting that is originated from dislocation emission assisted by thermally activated process during the stress holding. Another important finding was that the substructure beneath the fracture surface consisted of planar dislocations bundles and a locally thick bundle was observed in the region corresponding to 1-cycle dwell.

In order to evaluate the effect of solute Cu atoms on the high-cycle fatigue properties, a naturally aged (T4) Al-Cu binary alloy is tested under different temperature conditions. The S-N diagram of the alloy tested in room temperature shows a distinct fatigue limit while those evaluated in high temperature do not. An eminent increase of crack growth resistance is observed particularly at lower stress amplitude and higher temperature, which is different from the room temperature behavior. In order to analyse the microscopic aspect of the temperature effect, the crack tip region is investigated by TEM observation.

It is well known that dislocation structures are formed during cyclic deformation in metals. However, the formation mechanism of the dislocation structures in bcc metal is not yet elucidated. In this study, the dislocation structures in a polycrystalline Fe－3 mass%Si alloy are investigated using high voltage electron microscopy (HVEM), and the formation mechanisms of the dislocation walls in bcc metals are discussed. Push-pull fatigue tests were conducted with a constant total strain amplitude of 1 × 10^-2 at a strain rate of 4 × 10^-3 /s. Plastic strain amplitude during the fatigue test slightly decreased from 8.4 × 10^-3 to 7.9 × 10^-3. The fatigue test was stopped at 30 cycles, and thin foils parallel to the (001) plane and the (111) plane were extracted. The dislocation structures in the foils were observed using the HVEM equipped scanning transmission electron microscopy (STEM) detector in Nagoya University. The dislocation walls parallel to (110) were found to be formed during the first few tens cycles of fatigue in both of the (001) foil and the (111) foil. In order to determine the Burgers vectors of the dislocations, the (111) foil was observed. The Burgers vectors were identified by the invisibility criterion as a/2 [111] and a/2 [111]. Slip planes were also investigated by trace analysis and stereo observation. From these analysis, the two types of dislocations were characterized as (211)[111] and (112)[111]. It seems to be reasonable that the (110) walls were lie in the direction bisecting the angles between the Burgers vectors of a/2 [111] and a/2[111].

Dynamic strain aging improves the non-propagation limit of a fatigue crack in ferritic iron alloys containing supersaturated carbon. However, upon increasing the test temperature, the non-propagation limit of the fatigue crack decreases owing to carbide precipitation. In this study, we present a guideline to improve high-temperature fatigue resistance in ferritic steels containing supersaturated carbon via Si addition that suppresses carbide formation. Compared with a Fe-0.017C binary steel, at 293 and 433 K, the Si addition increased the fatigue limit at both temperatures as compared with that of the Fe–C binary steel. The higher fatigue limit than that of the binary alloy at 293 K originated from the solid solution strengthening of Si, whereas the improved fatigue limit in Fe-0.016C-1.0Si at 433 K was attributed not only to the solution hardening, but also to the suppression of carbide formation at 433 K. These results indicated that the robustness against temperature can be improved by the addition of Si. However, at 433 K, as time passed, carbides precipitated and the Vickers hardness decreased finally even in the Fe-0.016C-1.0Si steel. Also, fatigue crack did not stop propagating at 293 and 433 K. It is necessary to investigate this by observing the dislocation structure around the fatigue crack tip and fracture surface.

Solute carbon plays an important role in fatigue phenomena of Fe–C ferritic steels. For example, strain aging owing to solute carbon hardens the crack tip regions, resulting in a higher non-propagation limit of the fatigue crack. However, the low solute carbon content regions are formed near grain boundaries during quenching process from ferrite single phase region because of the segregation of carbon at grain boundaries. As a result, the relatively low resistance to plastic deformation in the weak regions is a primary factor causing intergranular cracks. When these cracks coalesce and lengthen, these cracks continue to propagate until fracture. That is, in the Fe–C ferritic steels, a suppression of crack initiations is also required. In this study, we try to improve the fatigue properties of ferritic steel from a viewpoint of the suppression of crack initiations. We used a 1.9 mass% Mn-added Fe–C–Mn steel with a ferrite-cementite structure based on a consideration for the fact that the carbon diffusion coefficient is reduced by Mn addition. Rotating bending fatigue test was performed at ambient temperature and at 50 Hz, and a replica method was used to observe the fatigue crack initiation behaviors. The Mn-added steel showed the comparable fatigue limit as Fe–C ferritic steels when compared at the same Vickers hardness in spite of low solute carbon content (0.0035 mass%). Also, the crack initiation sites near fatigue limit were different from Fe–C ferritic steels, namely, transgranular cracks were observed in Fe–C–Mn steel and intergranular cracks were observed in Fe–C ferritic steels.

In recent years, the material strength of metals used in automotive manufacturing has steadily increased to meet improved crashworthiness and weight reduction requirements. This increase in material strength has led to several difficulties in traditional press forming methods. Failure of the press-forming die is one such difficulty. To prevent failure, improved die design is necessary, which requires a detailed understanding of the mechanical properties of die materials. This paper identified the work-hardening parameters of a hardened tool steel (SKD11). Due to the brittle behavior of SKD11 in tensile tests, which were fractured at the 1.2 % equivalent strain, a shear punching test was used as the low stress-triaxiality suppresses material fracture. Furthermore, a minute punch-die clearance of 5.0 μm was used to increase the compression stress due to material deformation. SKD11 steel sheets of 1.0 mm thickness were successfully deformed with up to 30% strain, and the punching force and stroke were recorded for inverse analysis using the finite element analysis (FEA). Subsequently, FEA of the shear punching test was repeated to optimize the parameters of Swift’s work-hardening law. Consequently, the Swift parameters were derived to ensure that the measured values of the punching force and stroke correspond with the numerically simulated curve.

A new approach of nano-indentation measurements at elevated temperatures has been developed to evaluate a temperature-dependent mechanical behavior of heat resistant alloys on a submicron scale, where the measurement is conducted in an inert atmosphere to prevent the surface oxidation of a sample and heat an indenter passively. The developed approach enables us to perform nano-indentation testing with a diamond indenter in the temperature range of 23~800 °C. The developed approach has been applied to gamma single-phase single-crystal of a nickel-based superalloy. Then the degradations of the sample surface and the indenter tip have been discussed in this case.

Identification of creep exponent n and creep coefficient k in Norton’s law is very important to characterize creep deformation of a high-temperature material. We have previously proposed the high-temperature indentation creep test with a spherical ball for the estimation of the n and k as an alternative method to the uniaxial creep test. However, this creep test is needed to be interrupted every 100 h to measure impression radius. To prevent this problem, more practical estimation method based on the high-temperature indentation creep test with a cylindrical indenter was developed. This method allows us to estimate the n and k from the relationship between impression stress and indentation rate continuously measured during testing. So far, it is confirmed that n and k are properly estimated by results of cylinder indentation test which is conducted for each loading condition. In this study, continuous test was conducted, which can obtain all data needed for estimation of n and k at once. In order to validate this method, the high temperature indentation test was conducted with a cylindrical alumina indenter to estimate the n and k of pure aluminum A1050 plates. As a result, it was demonstrated that the n and k estimated from the developed method coincide with those estimated from the conventional methods. Furthermore, it was found that the developed method needs shorter time for the estimation as compared with the conventional methods.

A new method of creep life assessment was developed to consider heat-to-heat variations of welded joints of materials used in power plants. In the method, creep properties of the welded joints are related to those of each base metal because the heat-to-heat variations of welded joints strongly depend on the creep properties of the corresponding base metals. To estimate the creep properties of each base metal of the target pipe, microstructure analyses and small punch creep tests were conducted using small samples cut from the base metals in service, and evaluations were done on the basis of material data base obtained using standard test samples of long-term service exposed pipes. Validity of the assessment method was verified through comparisons of estimation results and experimental ones for welded joints of Grade 91 steel used for long-term at USC plants. Then, the assessment method was applied to piping on site to estimate remaining life of welded joints of the material in service.

The small punch (SP) creep testing technique was applied to the long-term service-exposed 2.25Cr-1Mo steel, which had been actually used as a boiler piping in the fossil power plant for long periods of time, to investigate the applicability of this technique to the remaining-life assessment. The SP creep tests were carried out at the temperatures of 580 and 650°C and under the load range from 150 to 400 N using small disk-type specimens removed from the piping. In order to compare the results obtained from the SP creep test with those from the uniaxial creep test, the SP load (F) was converted to the stress (σ) with the load/stress conversion coefficient (F/σ) determined by the high temperature SP test, that is, the central displacement to the maximum load. The experimental results revealed that the SP creep rupture life was slightly shorter in the outer surface than the inner surface. Additionally, the F/σ were determined to be around 2.9 irrespective of temperature, and the SP creep rapture data were well correlated with the uniaxial ones using this F/σ.

It is required to evaluate the high-temperature strength of turbine blade for a jet engine with a miniaturized specimen, because it is too small to investigate the strength distribution with a standard specimen. In this study, the small punch (SP) testing technique was applied to the wrought TiAl alloy (Ti-43Al-5V-4Nb) for the creep property assessment. The SP creep tests were carried out at the temperatures of 760 and 800°C and under the load range from 300 to 600 N using a small disk-type specimen (8 mm in diameter, 0.5 mm in thickness). In order to compare the results obtained from the SP creep test with those from the uniaxial creep test, the SP load (F) was converted to the stress (σ) with the load/stress conversion coefficient (F/σ), which was determined based on the high-temperature SP test results. The experimental results revealed the SP fracture characteristic was significantly different depending on the testing temperature, that is, between 760℃ and 800°C. It was also found that the SP creep rapture data were well correlated with the uniaxial ones using the obtained F/σ.

CMAS attack is known to be occurred owing to the deposition of volcanic ash on the surface of thermal barrier coating (TBC) at a high-temperature environment. The serious problem is TBC spallation resulting from the penetration of molten volcanic ash into the porous microstructure of TBC. The penetration induces inner stress and phase transformation, which leads to TBC spallation. In this study, the diffusional equation for expressing the infiltrating process of the molten ash into the porous structure of TBC and the associated constitutive equation involving phase transformation are formulated. The simulation results are compared with the cross-sectional SEM observation for the volcanic-ash-deposited TBC sample exposed at a high-temperature. As a result, it was found that tensile stress was widely distributed in penetration area of molten volcanic ash in TBC. Through the comparison with experimental result, it can be stated that the crack in TBC was initiated and propagated due to this tensile stress.

In this study, honeycomb structure having a hexagonal periodic microstructures were prepared by using a 3D printer. A four-point support bending test of the honeycomb structure beams with different sizes was performed, and the effect of microscopic structures on the macroscopic mechanical properties of the structures was evaluated. Digital Image Correlation was used to evaluate the local strain generated in the specimen during the bending. The relationship between the bending load and deflection obtained from the bending test showed that the bending stiffness decreased as the size increased. Result of DIC, revealed that the magnitude of the strain on the hexagonal side increases in the pure bending region of the specimen. The strain on the upper and lower ends of the beam was the largest.

In past studies, the small punch (SP) test using SUS304 is investigated at various deformation rate both experimentally and computationally. The negative rate sensitivity of maximum force and deflection is reported. According to observing fracture surfaces of the specimens by SEM, the failure changes from tensile to shear mode with the increase in deflection rate, and the fracture initiates the outside of the specimens at any deformation rate. However, according to the computational result, the initiation of fracture changes from inside to outside with the increase in deflection rate. To solve the contradiction, it is necessary to get back the boundary conditions, which is precisely same as experiments, in simulation and discuss the damage evolution during the deformation process. In this study, the finite element simulation, in which the constitutive equation and fracture criterion is introduced, is performed for SP tests at various deflection rate. For comparisons, the same deflection rate with experiment is imposed as boundary conditions. From the force-deflection curves obtained by these two methods SUS304 shows negative rate sensitivity in maximum force and deflection as well as positive rate sensitivity in work hardening rate·

In this study, we investigate the linear elastic properties of gel membranes constrained on a rigid substrate. To this end, the Flory–Rehner framework for polymeric gels is considered, which is extended by introducing an extended Gent model. The linear perturbation analysis demonstrates the dependence of limiting chain extensibility and the second strain invariant on the in-plane stiffness of gel membranes constrained on a rigid substrate.

In the recent wider applications of fiber reinforced plastic composite materials, there is a growing need to replace the huge number of coupon tests by numerical simulation. Since the requirement for the huge number of tests is partially due to the variability, there is a growing need for stochastic simulation. The author has developed the first-order perturbation based stochastic homogenization (FPSH) method that considers random physical parameters for the constituent materials and can solve damage propagation at the microscale. In this paper, the geometrical variability was also considered in the nonlinear simulation of short fiber reinforced composite material. Variety of probable damage patterns and scenarios were predicted, which implied that the fiber arrangement was more influential on the stochastic results rather than the fiber orientation.

Variable stiffness composites induced by curvilinear reinforcing fibers have become possible to be manufactured due to improving fiber orientation technology. Since variable stiffness composites have high degree of freedom in design, it has excellent mechanical properties. In this study, In general, the metaheuristics has been used to design curved fiber shapes. However, it is extremely computationally expensive, and a simple method for designing curved fiber shapes by referring to the principal stress direction has also been proposed. In this study, the method of referring to the principal stress direction was used to improve buckling loads of fiber reinforced composites, and the performance was compared with that of an optimization method. In the present method, the forced displacement, imitating the buckling mode shape, is first applied to the composites to obtain the direction of principal stress. Then, the curved fiber shapes are determined referring to the direction of principal stresses. As a result, it was shown that the best curved shape designed by the present method has almost the same performance with those by the optimization method. In addition, the present method is less computationally expensive than the optimization method, thus is useful for efficiently improving the buckling load.

Equivalent expression is derived for a mixed double inclusion which is a misoriented inhomogeneous inclusion embedded in an inclusion and it is developed to the composite material containing many clusters consistent of many misoriented inhomogeneous inclusions. Then the macroscopic dielectric flux and electric-field of such composite materials are analyzed by the resultant equivalent expression. As a result, the magnitude of total internal dielectric flux occurring in the composite material is good agreement with that obtained by Taya et.al (JSME A,43-1(2000), pp.46-52.) under high-level clustering condition. Moreover, macroscopic dielectric flux and electric-field induced in the composite material containing various clusters are calculated. Multiplicative effects of distribution of orientation and clustering of reinforcements on thermo-electromagnetic properties of composite materials are examined.

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

In this study, numerical analysis of mechanical properties of textile composites was performed using GMCsFEM. Three-dimensional woven composites can suppress delamination, which is a major issue of CFRP, by weaving fiber bundles in the through-the-thickness direction. However, in finite element analysis of textile composites, the complex fiber shape causes a high cost of time required for element division and a high cost of calculation cost due to a large number of elements. So far, an attempt has been made to apply mesh superposition method to analyzing textile composites that allow numerical analysis by dividing the resin and fiber bundles meshes independently and overlaying them. However, there is an issue that accurate strain cannot be obtained around the material interface. Therefore, in this research, we constructed a new method combining the mesh superposition method with Generalized Mathod of Cells (GMC) that can be considered micro inhomogeneities, applied it to textile composites, and verified its validity.

Indentation tests are effective methods to evaluate mechanical properties of materials. One of problems of indentation tests, especially, indentation creep tests, is a definition of reference area used to obtain indentation stresses. Recently, the authors proposed a new definition for the reference area of indentation creep tests. The definition is based on the surface area normal to the maximum principal stress during the indentation creep tests. The new definition of the reference area was verified by a series of numerical indentation creep tests. In this paper, using an indentation creep testing machine, indentation creep tests are carried out using a solder alloy of Sn-3.0Ag-0.5Cu and the new definition of reference area is verified. The effect of crystal size on the steady state creep is also discussed by the indentation creep tests.

Creep and creep recovery behaviors of a thermoplastic resin have been examined for different load histories in order to establish a model for the inelastic behavior of its short fiber reinforced composites. Creep recovery tests are carried out for different creep stresses and creep times, respectively. The creep recovery test results for the resin show that recovery strain tends to increase with increasing prior creep stress as well as with increasing prior creep time. The creep and creep recovery behaviors are assumed to be a superposition of viscoelastic and viscoplastic behaviors, and they are described by means of the Schapery models and Bailey-Norton law. The predictions uisng these models are found to be in good agreement with the experimental results observed not only for a single loading-unloading history but also for a cyclic loading-unloading history.

Flexural creep deformation behavior of an injection-molded short fiber composite material has been investigated with an emphasis on its anisotropy, stress dependence and temperature dependence. Three-point bending creep tests are conducted on test specimens with different orientations oblique from injection direction at room and high temperatures, respectively. It is shown that the creep deformation at room temperature develops more markedly with the increase in off-axis angle from 0° to 90°. The creep deformation increases as creep stress increases, regardless of the off-axis angle of specimen. It is also observed that creep deformation turns to be more significant at high temperature compared with that at room temperature. A phenomenological constitutive model has been developed to describe the orientation dependence, stress dependence and temperature dependences of the observed creep deformation behavior. It is demonstrated that the flexural creep deformation behaviors of short fiber composites at different temperatures can adequately be described using the proposed creep model. The creep model established allows identifying the flexural creep rupture stresses at any temperature in a range by means of a strain-based failure criterion.

The evaluation method of adhesion strength of PSA (pressure sensitive adhesive) tapes has been extensively investigated because of their demand especially in electronics field. In this study, we focus on separation energy of PSA tapes measured from conventional peel test and probe tack test. Assuming separation force is consumed for making new surface, we can simply calculate the separation energy from both tests. In peel test, separation energy was calculated from the energy conservation between the work of external force and surface energy generated on peeled area. In probe tack test, it was calculated by subtracting strain energy of PET substrate from the work of external separation force. The separation energies obtained from these tests were compared in terms of strain rates of PSA layers during separation process from movies taken by high speed microscope. PSA tapes of different visco-elastic properties were prepared by changing the amount of the cure agent in order to also investigate the influence on the separation energy. As a result, it was found that the separation energies obtained from these tests are identically determined despite different separation process, but further investigation is required for calculation of strain rate for large elongation of PSA layer.

Carbon fiber reinforce plastics have been applied to structural materials of blades of wind turbines and aircraft turbine engines. These blades may suffer from very high cycle fatigue up to 108–1010 cycles. However, conventional fatigue testing methods consume longer period in excess of 1 year for obtaining only 1 fatigue data in the very high cycle region. Recently, several researchers have reported that an ultrasonic fatigue testing method enables us to conduct accelerated bending and axial fatigue testing. In this study, an ultrasonic fatigue testing method was applied to accelerate axial fatigue testing of CFRP laminates in the very high cycle regime. In order to discuss the feasibility, a specimen shape for ultrasonic fatigue testing was designed to achieve resonance at 20kHz. Then, fatigue testing was conducted up to 109 cycles, and the S-N property was compared with fatigue testing results by using a conventional serve-hydraulic fatigue testing machine.

This study evaluated the nucleation of stress corrosion cracking (SCC) on a smooth surface of sensitized austenitic stainless steel SUS304 from viewpoints of microscopic strain and GB structure. Before SCC testing, microscopic strains were measured on a surface of a specimen subjected to an initial tensile strain of 1% by digital image correlation (DIC) technique. SCC testing was conducted under a constant load corresponding to the initial strain in a tetrathionate solution. After testing, crystal orientations on the cracked surface were measured by electron backscattered diffraction (EBSD) technique. The site and structure of each GB were identified by EBSD, and the distributions of normal and shear strains at the GBs were calculated by DIC. From the microstructural viewpoint, stress corrosion cracks tended to be mainly initiated at large-angle random GBs, and no cracks were initiated at low-angle GBs and Σ3 boundaries. From the viewpoint of microscopic strain at GBs, the cracks occurred at a part of GB where the normal strain was high, and then grew along the whole GB. On the other hand, the relationship between shear strain at the GBs and SCC nucleation was not found. As a result, crack initiation was characterized using the maximum normal strain along GBs.

The miniature specimens made from Sn-3.0Ag-0.5Cu (SAC) solder tend to show larger variation in the tensile strength than the ordinary size specimen. This means that the comparison of the internal structure between high-strength specimens and low-strength specimens will give valuable information to propose a method for giving high strength to solder joints in electronic equipment. Therefore, in this study, the tensile tests applying 10 % strain to miniature SAC solder specimens made by casting were conducted. The shape and distribution morphology of primary Tin crystal in the specimen after the tensile test were also investigated, and they were compared between high-strength specimens and low-strength specimens. The shape of each Tin crystal was represented as an ellipse by employing an image processing method. The length and the angle to the loading direction of the major axis of an approximate ellipse were used for representing the size and the distribution morphology of a tin crystal, respectively. Tin crystals the size of which was in the range from 10 μm to 15μm were most frequently observed irrespective of the strength of the specimen. Meanwhile, the orientations of the tin crystals in the size range were different depending on the strength. Namely, tin crystals the orientation of which made 0-10° and 80-90° to the loading direction were observed frequently in the high-strength specimens, but 30-40° were frequent in the low-strength specimens.