The Tersoff potential is one of the most widely used interatomic potentials for silicon. However, its poor description of the elastic constants and melting point of diamond silicon is well known. In this research, a new bond-order type interatomic potential has been developed that can reproduce the elastic constants and melting point of diamond silicon as well as the cohesive energies and equilibrium bond lengths of polytypes of silicon. We improved the original Tersoff potential function through the introduction of a flexible angular dependent term. In order to increase the robustness of the potential, systems that include a wide range of local atomic environments are employed for fitting. Optimized potential parameters were found using a genetic algorithm. The elastic constants and melting point of diamond silicon calculated using the developed potential turned out to be C11 = 166.4GPa, C12 = 65.3GPa, C44 = 77.1GPa and Tm = 1681K. It was also found that only elastic constants can be reproduced using the original Tersoff potential function, and that our proposed angular dependent term is a key to reproducing the melting point.
In this paper, an analytical solution for infinite medium containing elliptical inclusions under uniform out-of-plane normal stress is presented. It is assumed that each layer consists of isotropic elastic material, and that all elliptic boundaries have common focal point. Introducing such assumptions, prescribed solution is obtained to apply the concept of two-dimensional elastic theory. Because only a normal stress whose direction is parallel to a generator of elliptical inclusion affects a matrix, it is noted that antiplane shear stresses do not occur on every points of analytical model. Some numerical examples are shown in graphic and tabular representations.
Several authors have formulated the couple stress theory and the micropolar theory of two-dimensional Cosserat body, and obtained several relations between stress and displacement in complex variable forms. In this paper, we show the unified treatment of the problems of three-dimensional Cosserat body with a circular hole, circular solid inclusion or circular ring of solid inclusion by using the new micropolar constants K, G*, K* and ν*. And we give the solutions of micropolar elastic body with a circular hole, circular solid inclusion or a circular ring of solid inclusion under uniform stress at infinity. Lastly, several numerical results are shown by graphical representation.
The purpose of this research is considering how reusing used wood. Used wood was obtained when an old temple which passed about 200 years or more after construction was disassembled. Finger-jointed (FJ) laminae were manufactured from those material, and the tensile strength performance was investigated. This used wood is Japanese red pine currently used as a roof member. The form and the size of a finger are the general one which are adopted in case FJ laminae for structural use is manufactured in present Japan. It was joined using resorcinol resin adhesive. As a result of tensile test of the FJ laminae, the tensile strength and Young's modulus showed significant positive correlation. About this, a statistical examination was tried for the tensile strength of FJ laminae (eight factories, 4 wood species) by the JAS factory production which is circulating in the market in Japan. That is, the tensile strength of factory production FJ laminae which has a Young's modulus distribution equivalent to FJ laminae made from used wood by the Monte-Carlo simulation method was presumed. And KS official approval compared both FJ laminae made from used wood, and FJ laminae by JAS factory production. Consequently, FJ laminae made from used wood became clear having the tensile strength superior to FJ laminae by JAS factory production. The above result has suggested the possibility of reusing to the glued laminated lumber for structures using the use wood obtained when an old building is disassembled.
Excessive cutter wear was observed in tunneling with a TBM through the sandstone layers of the Mesozoic era in the Shimanto Belt. Generally, it causes not only the additional cost of cutter discs but also the additional time to replace them. The authors investigated the mechanical properties of rocks, quartz contents and shape of chips through the unconfined compression test, point load test and polarizing microscopic observation in order to clarify the relationships among the cutter wear and the geological properties. In conclusion, the excessive cutter wear was caused by the sandstone with high quartz contents and hard matrix. Additionally, it was found that the chip shapes were comparatively thin and wide in chippings through the hard rock which excessively wore cutter discs and the flatness was also higher with quartz contents.
Equal channel angular pressing (ECAP) invented by Segal was applied successfully to metal process of aluminum and magnesium alloys for increasing their strength by fining the grain size. But a little attention has been paid to its application to polymers. In this paper, ECAP process of crystalline polymer, polypropylene (PP), was studied using optical microscope, differential scanning calorimetry (DSC), specific gravity measuring system and small tensile testing machine. It was shown that 1. ECAP was effective at a temperature slightly lower than the crystallization temperature and at a low pressing speed, 2. the shear deformation was very intense at the interior of the specimen, while the surfaces was not sheared and 3. the yield stress of PP along the extrusion direction increased with anincrease in the repeated numbers of ECAP process.
In order to study the formation process of surface damage of ultrafine grain copper, rotating bending fatigue tests of smooth specimens have been carried out. After a total of 8 passes of ECA pressing with Bc route, grains with about 250 - 300nm diameter were formed. Specimens were fatigued at three constant stress amplitudes; σα = 240, 120 and 80MPa (corresponding fatigue lives were Nƒ ≈ 2 × 105, 4 × 106 and 5 × 107cycles, respectively). Significant differences in morphological feature in post-fatigued surfaces between high and low cyclic stress amplitudes were observed. To clarify the formation process of surface damage, morphological changes in surface damage caused by cyclic stresses were monitored successively by an optical microscope. Putting those OM observations and SEM observations of post-fatigued surfaces together, the formation processes of surface damage were discussed. In addition to those experiments, SEM observation of fracture surface and measurement of surface hardness during fatigue tests have been made.
In this study, the fatigue behavior of friction stir welded (FSW) joints of 6061-T6 aluminum alloy was investigated. 6061-T6 plates were joined with the welding speeds of 100 and 200 mm/min and the rotation speeds of tool of 1200 and 1800 rpm. The microstructure of the weld zone was classified into three regions : stir zone (SZ), thermo-mechanically affected zone (TMAZ) and heat affected zone (HAZ). In the SZ, fine equiaxed grains were observed resulting from dynamic recrystallization, and TMAZ was recognized as the microstructural transition zone between SZ and HAZ. Vickers hardness measurement revealed softening inside the weld zone, which was attributed to the dissolution of precipitates due to temperature rise during the FSW process. Fatigue tests were conducted at stress ratio R = -1 under axial loading. It was found that the fatigue strength of FSW joints was lower than that of the parent metal and the location of fatigue fracture was dependent on stress level. In the high stress region, fracture occurred at the TMAZ, while at the HAZ in the low stress region. Macroscopic observation revealed that the localized plastic deformation at the TMAZ was responsible for the fatigue fracture at the TMAZ. Such deformation at the TMAZ was not recognized when the applied stress was low and fracture occurred at the HAZ. Based on the hardness measurements before and after fatigue tests, the fatigue fracture at the HAZ was attributed to both the grain refinement at the SZ and the dynamic aging at the SZ and TMAZ by cyclic loading. The microstructures, hardness profiles, tensile strength and fatigue strength were hardly affected by the welding condition.
This paper describes the microstructure and fatigue behaviour of a newly developed magnesium (Mg) alloy that was produced by solid-state synthesis using Mg alloy (AZ31) powder and SiO2 powder as starting materials. Two different powder sizes of AZ31 were evaluated. SiO2 reacted with AZ31 to form Mg2Si when heated at 753K and subsequently extrusion was applied to produce bars of 24mm diameter at the same temperature. It was found that the microstructure of the produced alloys, Mg2Si-dispersed Mg alloys, consisted of particles such as Mg2Si or MgO dispersed within the equiaxed-grain matrix structure and large particles tended to be present in the material using coarse powder. The mechanical properties of Mg2Si-dispersed Mg alloys decreased compared with a conventional extruded AZ31 alloy, in which the material using fine powder had slightly higher tensile strength and ductility than the counterpart. Mg2Si-dispersed Mg alloys also showed lower fatigue strength than the extruded AZ31 alloy and the fatigue strength of the material using coarse powder decreased significantly. Fatigue cracks always initiated from large particles immediately after cyclic loading was applied and subsequent small crack growth was faster than the extruded AZ31 alloy. Therefore, it was concluded that the decrease of the fatigue strength of Mg2Si-dispersed Mg alloys was attributed to the premature crack initiation from particles and faster small crack growth, and the observed powder size dependence of fatigue strength was due to difference in the size of the particle from which the crack initiated.
The previous papers showed that the type of yielding at a constant strain rate shearing was quite different between as-received (AR) and quenched (Q) states of amorphous polymers. In AR samples, many slip lines and a couple of shear bands were nucleated, while in Q samples, vague shear bands appeared with no visible slip lines. The aim of this paper is to investigate the effect of this difference of yielding on fatigue crack growth in a serial research on the effect of quenching on the yielding of amorphous polymers. The tests were performed at tensile fatigue loading using single edge notched specimens of PC, PVC and PET. It was shown that (1) the plastic zone ahead of the crack tip was involved with slip lines for AR samples, but with craze like flaws for Q samples, (2) the fatigue crack growth rates were not so different between AR and Q samples at the same stress intensity factor ranges and (3) the striation geometry was more straight and sharper for Q samples than for AR ones.
The formation behavior of factory-roof in circumferential notched specimen was examined. To achieve an observation of cracks in internal of material, two plastics were used for testing. One was acryl and the other was polycarbonate. Cyclic torsional tests were performed with and without application of static tension. With the polycarbonate, a factory-roof was not formed in fracture surface. However, using acryl, that was observed when cyclic torsion tests were conducted with static tension. Therefore, the discussion was focused at the results in acryl. When the factory-roof was formed, many small cracks were initiated and grew by shear mode, followed by coalescence and branching of them. After that, the crack growth mode was changed from shear mode to mode I. During mode I crack growth, the formation of the factory-roof was observed.
In order to examine the influence of impurity content on the characteristics to prevent the recession of yttria stabilized zirconia in combustion gas flow, sintered yttria stabilized zirconia (ZrO2) specimens with different impurity (Al2O3 or SiO2) contents were prepared and exposed under a combustion gas flow condition (T = 1500°C, P = 0.3MPa, V = 150m/s, PH2O = 35kPa, t = 5 - 20h). As a result, the significant weight loss of high purity ZrO2 was not found, and it was shown that high purity ZrO2 has superior characteristics to prevent the recession and is promising candidate materials. In addition, the impurity content influences on the recession characteristics, and the weight loss of ZrO2 with more impurity increases as the impurity content increases. Thus, in order to prevent the recession, the increase of purity is required. Furthermore, impurity contained in ZrO2 disappeared selectively from the surface under the combustion gas flow. The disappearance of impurity was also observed under the surface, and the thickness of the layer without impurity under the surface increased according to the exposure time.
Silica-sol solution was prepared using tetraethoxysilane as a starting material. A silica-gel film was formed on both silica glass substrates and hot-dip galvanized steel plates by single-step dip coating and drying at room temperature. X-ray diffraction (XRD) analysis was performed on the films formed on the silica glasses and the steel plates; it revealed that the film formed on the former consisted of H2Si2O5 and both Zn2SiO4 and ZnNO3(OH) · H2O were detected in the film on the latter. From these results, it was cleared that the as-prepared silica-sol itself was H2Si2O5, however the silica-sol was changed to Zn2SiO4 when coated on the hot-dip galvanized steel plates resulting from a reaction with Zn on the surface of plates. The thickness of tripe-layered film formed by three-times dipping was estimated to be about 1.3μm by direct SEM observation. Using a salt spray test and a stack test, corrosion resistance of the hot-dip galvanized steel plates coated with the silica-sol was examined, in comparison with a chromate-coated and a naked hot-dip galvanized steel plates. The silica-sol coated plates exhibited superior corrosion resistance to both former and latter samples in each test. This might be explained in terms of the formation of Zn2SiO4 and ZnNO3(OH) · H2O. The coating materials, insoluble in water, can protect the steel plates from corrosive oxygen and electrolyte ions etc.
The effect of oxygen plasma treatment under various treatment power and time on the interfacial strength properties between PBO fiber and epoxy resin was examined. Microdroplet test was conducted to measure the interfacial shear strength (IFSS) between the fiber and the resin. To investigate how the physical and chemical effect of the treatment on the increase in IFSS would change with change of the treatment conditions, the microdroplet test on the aging specimens left more than 20days was also performed. Furthermore, the interlaminar shear test employing short beam shear method and the static tensile test were conducted in order to investigate the effect of the plasma treatment on the mechanical properties of PBO fiber reinforced plastics (PBO-FRP). The experimental results show the main conclusions as follows. (1) Oxygen plasma treatment increased the IFSS with an increase of the treatment power and time. (2) It was clarified that the physical effects of oxygen plasma treatment caused by the etching such as an increase in the surface area and the anchor effect on the treated fiber mainly improved the IFSS. (3) Oxygen plasma treatment increased the interlaminar shear strength by up to about 76% compared with non-treatment specimens. (4) Oxygen plasma treatment improved Young's modulus of PBO-FRP by up to about 25% while tensile strength of PBO-FRP was slightly decreased due to the damage of PBO fiber caused by the etching.
The carbon nanofiber (CNF)/unsaturated polyester resin (UPR) nanocomposites are fabricated, and their electrical, dynamic mechanical properties are investigated. The electrical conductivity is measured as a function of fiber content to understand the percolation phenomenon. The nanocomposites show the electrical conductivity with low percolation threshold between 2 and 3 vol.%. The mechanical damping and storage modulus of the composites were measured using a dynamic mechanical analyzer (DMA). Damping factor decrease with an increasing in the fiber volume fraction. Nanocomposites having long nanofiber should have higher damping and lower storage modulus than those having short fiber. For the analytical model, the elastic-viscoelastic correspondence principle is used to transform the elastic Halpin-Tsai equations to complex form. The model is then validated for the prediction of the damping and storage modulus of CNF/UPR nanocomposites having a randomly orientation of nanofiber. Good agreement between measured and predicted for storage modulus and damping factor of nanocomposites was recognized.
The scanning electron-induced acoustic microscopy (SEAM) has been developed as a new tool for non-destructive observations of the internal microstructures of materials. It consists of the electric chopper to pulse the high current electron beam and the detector of the longitudinal acoustic waves, being attached to the commercial scanning electron microscopy (SEM). The SEAM gives the best performance for observing the internal defects like the local delamination of the interface in the sub-surface of the semiconductor, because it senses the local difference of thermal properties in the sample. The acoustic waves transmitted from the thermal waves generated near to the surface, in principle, carry the information on such internal defects. In the present paper, the outline of our improvements to the system for getting higher resolution is first stated with emphasis on electric chopper way, sample holder design and image data processing method. Some typical samples of a soldered copper piece, Fe-3%Si and TRIP steels are supplied to ascertain the detectability of the internal microstructures. And then, the effects of the pulsing frequency of the electron beam and the accelerating voltage to the resolution of the electron acoustic images are investigated. The higher resolution can be achieved as the higher pulsing frequency, as suggested by the phenomenological considerations.
When a gauge volume sank below a specimen surface, the diffraction angle shifts. Thus, it is required to correct the surface aberration. For the annealed specimen of S45C, the shift in the diffraction angle was investigated using a strain scanning method with Ge (111) analyzer. This phenomenon was caused by the difference in the centroid between the geometric and the instrumental gauge volumes. This difference is explained by the following factors; 1) the change in the gauge volume by the divergence of the analyzer, 2) the X-ray penetration depth, 3) the gap of the centre line between the double receiving slits due to mis-setting the analyzer. As a result, the correcting method considered into these factors was proposed. For the shot-peened specimens of S45C, the diffraction angles were measured and corrected by our method. The distribution of the residual stress agreed with that obtained by the removal method.
Split Hopkinson Pressure Bar (SHPB) is known as a simple method to investigate the deformation of materials at high strain rate. To heat the specimen upto the melting point of the material of elastic bars, the specimen must have a finite length. In such case, the result is inaccurate because of the assumption of existing evaluation method that the specimen has infinitely short length. This paper introduces a new evaluation method utilizing Fourier analysis which enables accurate investigation of viscoelastisity of materials considering the stress and the temperature distribution in a long specimen. By the use of this method, The attenuation of silicon nitride, at high temperature from 900°C to 1320°C and at high strain rate in frequency around 4kHz, was obtained.
Two-component mortar paste consisting of cement and silica fume was cured in water or steam, and its strength development, cement hydration capacity and silica fume reaction rate were analyzed to identify the role of the admixture, determine the optimum curing condition for RPC containing silica powder and estimate its high strength development mechanism. The total silica fume reaction amount calculated by multiplying the added amount by the reaction rate increased with more silica fume added. Ca/Si ratio of generated C-S-H was about 0.7 at the lowest, suggesting a mixed phase of C-S-H and silica gel. RPC is known that inclusion of silica powder of intermediate sizes increases particle compaction. The low Ca/Si C-S-H with silica gel was considered to further bridge voids between particles firmly, thereby resulting in ultra high strength of RPC.