Press process was performed in order to toughen HA/PLLA bio-composite material. Mode I fracture properties and bending mechanical properties of forged and unforged HA/PLLA were then evaluated to assess the effect of press process on these properties. FE-SEM was also used to characterize the toughening mechanism due to press. Testing results showed that the bending strength and modulus slightly increased due to press; on the other hand, the bending absorbed energy and the mode I fracture property dramatically improved. FE-SEM observation of the fracture surfaces showed that ductile deformation and fracture of PLLA fibrils were enhanced and in addition, interfacial cohesion strength was improved. These microstructural modifications appear to result in the toughness improvement.

View full abstract

Injection molded bulk-shape Poly (L-lactide) (PLLA) specimens were hydrolyzed in the phosphate buffered solution and the mechanical properties were evaluated after the hydrolysis tests. In order to evaluated the effect of crystallinity on the hydrolysis, specimens were annealed with 70 and 130°C for 24h. Hydrolysis tests were conducted with soaking the specimens in the phosphate buffered solution (pH 7.4) in an incubator where temperature was kept as 37°C. Vickers hardness was not influenced by hydrolysis until 30 days. Tensile tests results, however, indicated the strength reduction with hydrolysis. From the appearance inspection, whitened regions were observed at the inner of the specimen. These results indicated the bulk erosion with accelerated inside erosion occurred.

View full abstract

Polymer blends were fabricated with poly(lactic acid) (PLA) with better mechanical properties and poly(butylene succinate) (PBSU) with better biodegradable properties to clarify the effect of the blending ratio on the biodegradable and mechanical properties of the blends. The specimens were heat treated to improve the reduction in stiffness due to blending. Hydrolysis and soil burial tests up to 16 weeks were conducted to investigate the biodegradation properties. Young's modulus increased with increasing contents of PLA, whereas tensile strength is lowest in the PLA/PBSU (50/50) polymer blend before biodegradation because of the immiscibility of PLA and PBSU. Young's modulus kept constant up 16 weeks both in the hydrolysis and the soil burial tests. On the other hand, tensile strength decreased remarkably in the hydrolysis tests. The observation results of the specimen surface and the fracture surface indicate that the surface and bulk degradation occur in hydrolysis and soil burial tests, respectively

View full abstract

Static experiments are performed to confirm the COD method that is the only method to measure the energy release rate of fast propagating cracks just after bifurcation. The crack opening displacement (COD) of the mother cracks of bifurcated cracks is measured in static condition by Moiré interferometry. Thin notches are used instead of cracks. The bifurcation angle is 13.5 degrees that is the same as the angle of bifurcation of fast propagating cracks in PMMA. The measured CODs are proportional to the square root of the distance from the nominal tip of the mother crack. Stress intensity factor of the bifurcated crack is obtained from the measured CODs through the formula of the COD of single crack. The experimental results say that the stress intensity factor of a bifurcated crack has the same value as that of the single crack whose length is the same as that of the bifurcated crack. This is caused by the small bifurcation angle of 13.5 degrees. It is concluded that the COD method is correct to measure the energy release rate of rapidly bifurcating cracks.

View full abstract

The conical spring washer (CSW) has been considered to prevent the loosening of a bolted joint and has thus been widely employed. However, experimental results obtained in studies focusing on loosening due to transverse loading conducted by Yamamoto et al. and Sakai have shown that a CSW did not prevent loosening in any context. In the present paper, we performed three-dimensional finite element analyses of an M10 bolted joint using a CSW subjected to transverse loading and investigated its loosening resistance performance. Two kinds of axial force were applied: one was low axial force (10kN) under which a CSW is not fully compressed, and the other was high axial force (20kN) under which a CSW is fully compressed. In the case of high axial force, the CSW showed no vivid effect on preventing loosening. On the other hand, in the case of low axial force, the CSW showed two opposite effects. The negative effect was an increase in the loosening rotation angle, while the positive one was the prevention of a decrease in axial force. When complete bearing-surface slip occurs, a CSW can prevent loosening because the positive effect is larger than the negative. However, when small bearing-surface slip occurs, a CSW cannot prevent loosening because the negative effect cancels the positive one. It is supposed that the small stuck region as well as the small equivalent diameter of friction torque leads to large loosening rotation.

View full abstract

In this study, the effect of whisker orientation on monotonic and fatigue strength of aluminium cast alloy locally reinforced by SiC particles and Al₂O₃ whiskers are investigated. The material is monotonically and cyclically deformed to failure at room temperature. A significant effect of reinforcement orientation on the monotonic strength and fatigue strength are observed experimentally and numerically. With respect to the stress direction the whisker orientation gives significant difference in strength of this material. SEM analysis shows almost all whiskers are transversely debonded when whisker direction is perpendicular to the stress direction and almost all whiskers are broken when the whisker direction is parallel to the stress direction. A three-dimensional single whisker unit cell model of cylindrical shape whisker in the periodic boundary condition is conducted using finite element method (FEM) to describe the overall behavior of the composite. The prediction results based on FEM analysis is found to be in reasonable agreement with the experimental observations.

View full abstract

In the numerical simulation of temperature of particles traveling in the jet flow of cold spray, the particle temperature is usually assumed uniform. This assumption is valid for spray materials with larger thermal conductivity. For spray materials with smaller thermal conductivity, however, this assumption may not be valid. This paper numerically clarifies the spray conditions for which the assumption of the temperature uniformity of the particle is no longer valid in the cold spray.

View full abstract

This paper discusses a stochastic response analysis using the perturbation-based homogenization method for a homogenization problem of a fiber reinforced composite material considering microscopic uncertainty. Since an estimated result of the stochastic characteristics such as the expectation and variance using the first order perturbation-based homogenization method includes a large error in some cases, applicability and effectiveness of the second or higher order perturbation method should be investigated. In this paper, at first, the second order perturbation-based homogenization method is formulated. A second order perturbation-based procedure for estimating the stochastic characteristics of equivalent elastic constants of an orthotropic material is also introduced. Next, the second-order perturbation-based stochastic homogenization method is applied to the stochastic analysis of a homogenized elastic property caused by uncertainty in material property of a microstructure. As a numerical example, the stochastic characteristics of the homogenized elastic properties of a unidirectional fiber reinforced plastic caused by the microscopic material uncertainty are estimated using the Monte-Calro simulation, the first and second order perturbation method. Also, a detailed stochastic analysis for a homogeneous isotropic material is performed. From the numerical results, effectiveness and a problem of the second-order perturbation method are illustrated.

View full abstract

The effect of the type of prestrain (tensile or torsional prestrain) on the fatigue limit of carbon steel was investigated. Annealed 0.1%, annealed 0.5%, and quenched and tempered 0.5% carbon steel specimens were subjected to monotonically tension or torsion to produce plastic strain. Fatigue tests were performed using rotating bending fatigue test machines (R=-1). The results of experiments showed that the fatigue limit of the torsional-prestrained specimen was higher than that of the tensile-prestrained specimen and the degree of cyclic softening induced by fatigue loading in the torsional-prestrained specimen was lower than that in the tensile-prestrained specimen. The results mean that the effect of the prestrain type on the fatigue limit is related to the cyclic softening and the cyclic softening depends on the relationship between prestrain type and fatigue-loading type. However, the effect was small when there is precrack formed by prestraining or artificial defect on the specimen. A method of predicting the fatigue limit of prestrained carbon steels was also discussed on the basis of the results.

View full abstract

In order to estimate the material property with sufficient accuracy and acceptable computational time using ab-initio calculation, the reduction of degrees of freedom required to approximate highly oscillated wave function around the nuclei is necessary. The curving-grid mesh which allocates a fine mesh around the nuclei seems effective for the reduction. We investigate the convergence property for ab-initio finite-element calculation with curving-grid mesh in terms of total energy. Through the calculation of an oxygen molecule, we found that the favorable convergence property, i.e. the variational convergence with the uniform rate, of the finite element method with uniform mesh is conserved. The application to bulk rock-salt magnesium oxide including 216 atoms shows the effectiveness of the curving-grid mesh in large-scale ab-initio calculation.

View full abstract

This paper introduces a new all-metal gasket that incorporates strategically located circumferential annular lips that form seal lines with the flanges. This gasket, named Super Seal Gasket, by virtue of its special shape, makes use of the material's spring effect, resulting in sealing performance. This change in approach, from the traditional one based on material development to one of mechanical design, brings with it the need for optimization of the different design parameters for leakage performance. We were able to develop an optimization methodology for this new product using the Taguchi method. As a test case, a 25A sized industrial gasket was optimized using this methodology. In FEM analysis, contact stress and deformation information was used to quantify leaking. Helium leak testing reveals considerable improvement in the sealing performance, hence verifying the applicability of the methodology developed. This work has proved that material development is not the only approach towards the development of new, more effective gaskets and novel gaskets, such as the one introduced in this study, have a useful part to play in the sealing of flanged joints.

View full abstract

The new Θ probe, which is used in eddy current testing, has various advantages compared to previous probes. In the present study, the fundamental characteristics of this probe are first investigated using commercially available aluminum plates. The effects of liftoff and the angle between the detecting coil and the flaw on the flaw signal are then clarified. The effects of the flaw dimensions on the flaw signal are also investigated, and it is shown that the amplitude reaches a constant value for a flaw depth of H>8 mm, a flaw width of W>7 mm, and a flaw length of L>20 mm, within the experimental conditions of the present study. Next, the monitoring of crack initiation under static loading and crack growth under the low cycle fatigue condition in the duralumin plates is attempted by applying the Θ probe combined with the caustic method. It is shown that the signal amplitude can be obtained when fracture is initiated under static loading. In addition, in the A7075 plate, the amplitude is increased at the fatigue crack growth rate of da/dN larger than 0.017 mm/cycle. Consequently, the possibility of crack monitoring using the Θ probe is confirmed.

View full abstract

In this paper, we have proposed an evaluation method for the initiation conditions of heat crack under thermal shock using the tensile test of a notched specimen. In some materials, such as hot rolling rolls and dies for aluminum die-casting, there is a problem of heat crack initiation, and the evaluation of heat crack resistance using test pieces was difficult until now. The thermal stress and stress gradient that occurred in such material were analyzed by the finite element method (FEM). The maximum stress when tensile stress was applied to the material with a notch was compared to the stress gradient of the thermal stress. As a result, the quantitative evaluation during the initiation conditions of a heat crack for materials became possible.

View full abstract

The fatigue ratio of lamellar pearlite steel used for railroad rails is lower than the ratio of general steels. The reason for this has not yet been clarified. It is reported that the direction of the lamellar microstructure in the lamellar pearlite steel affects the fatigue characteristics. In order to clarify the microscopic deformation behavior of the lamellar microstructure considered to affect the fatigue characteristics, static tensile tests of the lamellar pearlite steel were conducted. Using the plastic replica technique, the microscopic deformation behavior i.e. the initiation and propagation of microcracks was observed. Based on the experimental results, transgranular cracks and intergranular cracks were observed. The obtained main results are as follows.
(1) The transgranular crack originated in the discontinuous area in the lamellar pearlite microstructure. The transgranular crack initiates in a 45 degree direction to the axis of tension. The transgranular crack does not break through the grain boundary.
(2) The intergranular crack does not propagate, but opens.
(3) The difference in the material characteristics affected by heat treatment is the total strain of the crack initiation.

View full abstract

In core structural materials of next generation reactors such as a liquid-metal cooled fast breeding reactor and a supercritical-water cooled thermal or first reactor, materials' degradation behavior by neutron irradiation damage and thermal (cyclic) stress should be considered with fair accuracy in design process (including maintenance and repair plans), because the materials are used under higher temperature gradients and higher neutron flux fields than those in the present light water reactors. In the current experiential design rules, service lives of core structural components were determined by the materials degradation such as the increase of ductile-to-brittle transition temperature after post irradiation examination data. However, other materials degradations such as irradiation-assisted stress corrosion cracking (IASCC), which occurs by the degradation synergistically interacting with radiation hardening, local chemical composition change, swelling and radiation creep, should be considered reasonably in the design process of the next generation reactors, because of the anticipation of the beneficial effects by synergy of radiation damage. The radiation hardening and local chemical composition change at grain boundaries due to radiation-induced segregation increased with increasing dose. Above some threshold dose, swelling increased rapidly with increasing dose. Residual stress due to thermal stress and welding procedure decreased with increasing dose. To predict material failure by IASCC with reasonable accuracy, in this study, each material degradation phenomenon with different dose dependence was modeled with consideration of radiation induced stress relaxation. And then the models were integrated to simulate the failure behavior for the duration of reactor operation period. In this paper, the models obtained by ion-irradiation experiments and compared by data from neutron irradiation experiments were presented, and the concept of our new evaluation method and the programming code for the failure simulation were outlined.

View full abstract

By putting photo-electric conversion materials and electron storage materials together on one device, full-time cathodic protection system was expected to function and its coating technique is required to realize the system. So far, Warm Spray, impact deposition of supersonic particle with temperature controllable in a wide range, enabled nano-sized TiO₂ and Fe₂O₃ to be deposited controlling thermal degradation of the particles and it was revealed that each of photo-electric conversion and electron storage were functioned. The rest potential of the coatings as an indicator of overall system performance was not negative sufficiently, which might be related to the less conductive nature. Therefore, metallic Zn was introduced in this paper for shifting the rest potential to more negative direction and increase of the conductivity. The coatings were successfully fabrication with control of thermal degradation of Zn, which worked as a binder to connect the oxide particles. Photo-cathodic protection behaviour indicated that the steady rest potential of the Warm Sprayed coatings of the TiO₂-Fe₂O₃ system was not influenced on the introduction of metallic Zn, implying that it might not be limited by the conductive property of the coatings.

View full abstract

To develop non-destructive evaluation methods for oxidation damage on graphite components in High Temperature Gas-cooled Reactors (HTGRs), the applicability of ultrasonic wave and micro-indentation methods were investigated. Candidate graphites, IG-110 and IG-430, for core components of Very High Temperature Reactor (VHTR) were used in this study. These graphites were oxidized uniformly by air at 500 °C. The following results were obtained from this study. (1) Ultrasonic wave velocities with 1 MHz can be expressed empirically by exponential formulas to burn-off, oxidation weight loss. (2) The porous condition of the oxidized graphite could be evaluated with wave propagation analysis with a wave-pore interaction model. It is important to consider the non-uniformity of oxidized porous condition. (3) Micro-indentation method is expected to determine the local oxidation damage. It is necessary to assess the variation of the test data.

View full abstract

This article has been found to be a duplicate publication of their article on another journal, and thus retracted by the Editor.

View full abstract