In the aim of tissue regeneration of an alveolar bone, we developed three-dimensional fabric structural composite scaffolds using a bioabsorbable polymer. This scaffold consists of a polylactic acid (PLLA) resin fiber and a 75/25 poly L-lactide-co-glycolide (PLGA) copolymer resin coat. Scaffold is woven on a new-type of three-dimensional loom, has high porosity (89%) and continuous hole. The compressive rigidity and collapse strength of scaffold are increased due to the resin bonding between fiber intersections. The strength of scaffold that did a dip to phosphoric acid buffer solution (PBS) decreased in half due to the hydrolysis in six weeks. Mouse osteoblast-like cells (MC3T3-E1) were seeded onto the scaffolds and cultured in vitro for six weeks. The cells proliferated during in culture and formed a space-filling tissue between polymer fibers. Bone regenerative messenger ALP/DNA levels remained high compared with those one of culture dish. Mineralization of the deposited collagen on scaffold was initially observed at four weeks. Culture of cell on scaffold constructs for six weeks led to formation of a bone tissue.
Ultra High Molecular Weight Polyethylene (UHMWPE) has been used for the bearing materials of artificial knee joints owing to its superior mechanical properties and chemical resistance. In vivo, however, delamination fracture occurred, because of wear and fatigue of UHMWPE components. Although γ-irradiation and following aging was reported to accelerate the delamination fracture, the effects on the fatigue crack growth behavior have not been revealed yet. On the other hand, the addition of Vitamin-E (α-Tocopherol) was reported to prevent the delamination wear, but the prevention mechanism has not been clarified yet. In this study, in order to understand the influence of γ-irradiation and accelerated aging, and the addition of Vitamin-E on the fatigue crack growth properties of UHMWPE, tensile tests and fatigue crack growth tests of UHMWPE were carried out. After the γ-irradiation and accelerated aging, the specimen surface was oxidized and its crystallinity was increased. However the addition of Vitamin-E reduced the oxidization of the specimen and the increase of its crystallinity. For the tensile tests, the yield stress was increased and the tensile strength was decreased by γ-irradiation and accelerated aging. For the fatigue crack growth tests, the addition of Vitamin-E reduced the decrease of ΔJth by γ-irradiation and accelerated aging. Although the fibrillation and the brittle fracture were observed on the fracture surface of γ-aged specimen, they were not observed on that of Vitamin-E added γ-aged specimen.
The effects of low-gamma irradiation on the impact compressive properties of ultra high molecular weight polyethylene (UHMWPE) were investigated. Gamma irradiation was performed at 30kGy in nitrogen or at 29kGy in air without post-irradiation treatment. Impact compressive tests using the split-Hopkinson pressure-bar (SHPB) technique were performed to measure stress–strain relations up to a true strain of 8% at a strain rate of 260s-1. Gamma irradiation in air significantly increased the Young's modulus and the 0.5% yield stress, and gamma irradiation in N2 significantly increased the 0.5% yield stress, as determined by impact compressive testing. The impact stress-strain behaviors of both unirradiated and gamma irradiated UHMWPE specimens were compared by the elasto-viscoplastic model proposed by Bergström [Biomaterials Vol.23 p.2329 (2002)]. We found that the elasto-viscoplastic model had a potential to predict the observed impact stress-strain responses.
In this study, the effects of a surface modified layer formed by ELID (Electrolytic In-Process Dressing) grinding on chemical properties and biocompatibility were examined. An applicability of ELID grinding to create a high bone compatible surface was also discussed. Three types of specimens, which were processed with different surface finishing methods were prepared. These surfaces were then characterized by electrochemical corrosion tests and biocompatibility tests in in-vitro. The surface finished by ELID grinding showed higher biocompatibility and corrosion resistance compared to that of the polished surface. This was because formation of a thick and stable surface-modified oxide layer on the ELID ground surfaces, prevented corrosion reaction. The ELID ground surfaces had high biocompatibility with lower cytotoxic reaction. The ground surfaces by ELID grinding using a NaOH-based grinding fluid showed bio-activeness ; the surface were covered by hydroxyapatite after soaking in simulated body fluid (SBF) for seven days.
Relationships between fatigue properties and microstructures of hot-forged Ti-29Nb-13Ta-4.6Zr alloy (TNTZ) in under aged, peak aged, and over aged conditions at aging temperatures of 673K and 723K are investigated. The changes in the fatigue properties of TNTZ subjected to thermomechanical treatments, which includes an aging treatment at 673K or 723K for 259.2ks after a severe deformation process by cold rolling, are also investigated. At an aging temperature of 673K, ω phase precipitates at the early stage of aging, but α phase precipitates at relatively longer aging time. The precipitation site of α phase changes from intra-grain to grain boundary at around peak aging time when TNTZ is aged at 673K and 723K. The elastic modulus of TNTZ increases simply with increasing aging time at both 673K and 723K. The fatigue strength of TNTZ increases considerably when α phase precipitates compared with when ω + α phases co-exist. The fatigue strength of TNTZ decreases slightly due to the coarsening of α phase precipitated in β grain and its grain boundary. TNTZ aged at 723K for 259.2ks after cold rolling exhibits the highest fatigue strength in both the low- and high-cycle fatigue life regions. Furthermore, the fatigue limitof about 770MPa (fatigue raito : 0.71) is nearly equal to that of hot-forged Ti-6Al-4V ELI alloy subjected to aging after solution treatment with equiaxed α structure.
Biocompatible piezoelectric materials are becoming increasingly important for actuators and sensors in medical devices. In this paper, we highlighted on some perovskite-type oxides MgSiO3, CaSiO3 and CaTiO3 of biocompatible piezoelectric materials discovered by first-principles calculation in our previous studies. In order to verify their biocompatibility, the cytotoxicity of similar oxides with the same components was examined as comparing with typical perovskite-type oxides Pb(Zr, Ti)O3 and BaTiO3. The fibroblast (L929) cells were cultured during 7days and the effect of materials was evaluated by relative proliferation ratio and doubling time. As a result, it was recognized that MgSiO3 and CaTiO3 has the higher biocompatibility. On the other hand, CaSiO3 and Pb(Zr, Ti)O3 shows the strong toxicity and they can't be applied to medical devices.
In order to investigate the influence of the tempering temperature on a S-N characteristic in very high cycle fatigue regime, cantilever-type rotary bending fatigue tests up to 109 cycles were performed in air at room temperature using hour-glass shaped specimens heat treated at four kinds of tempering temperature in low-alloy steel, JIS SNCM439. Specimens tempered at low temperature of 433K and 573K showed a step-wise S-N curve, and were ruptured by internal fracture mode at high cycle fatigue regime above 106cycles. On the other hand, surface fracture mode only appeared on the specimens tempered at high temperature of 773K and 893K, and the fatigue limit existed clearly. Granular bright facet (GBF) area was observed in the vicinity of a non-metallic inclusion which was origin of an internal fracture. Properties of the GBF area were verified from the experimental facts that the roughness of the GBF area was larger than that outside the GBF in a fish-eye, the size and the distribution of convex particles on the GBF area were same as those of spherical carbide particles in the matrix, and a high density of carbon was detected on the GBF area by EPMA. It was suggested that a decohesion of spherical carbide from the matrix for the formation of the GBF area was affected by the size of the carbide which became large with decreasing the tempering temperature.
This paper compares mechanical properties of two types of cast aluminum components made in sand mold and cast iron mold, respectively. In addition, this study attempts to establish the correlation between the shear and the tensile ductility and the characteristic size of defects for cast aluminum alloy A356. For each type of the castings, the fracture tests are performed under a wide range of stress states including tensile tests on notched and unnotched round bars and biaxial loading tests on the butterfly specimens. Using a combined experimental-numerical approach, the plasticity and fracture properties are characterized in terms of the true stress-strain curve and the ductile fracture locus. It is found that the sand-molding component is of higher yield resistance and lower ductility than the metal-molding one. Meanwhile, the fractographic study reveals that there exist two competing failure mechanisms: the internal necking of the matrix at high positive stress triaxialities and void sheeting due to shear at negative stress triaxialities. The transition of the failure modes occurs in the intermediate range. In the specimens with porosities, the metallographic observation is performed and the area fraction of defects, the area and the chord length of the largest defect are measured using Matlab's Image Processing Toolbox. The linear function is used to correlate the tensile and shear fracture strain with the characteristic size of defects. It is found that the shear ductility decreases at a faster rate than the tensile ductility with the increasing size of defects.
The present work has investigated whether the twin boundaries to be accounted or not in the Hall-Petch equation for polycrystalline metals. Pure copper, Cu-3.5at%Al, Cu-6.8at%Al and Cu-14.8at%Al alloys, which have the average grain sizes from 8.4μm to 176.0μm and different ratio of annealing twins, were produced and pulled in tension at the temperatures from 77K to 973K under a strain rate of 1.2 × 10-4s-1. The distribution of dislocations in the surface grains was also observed by using the etch pit technique. The 0.1% proof stress including twins for each specimen at room temperature well obeys to the Hall-Petch relation. The halves of frictional stress σ0 derived from the straight lines are approximately equal to the critical resolved shear stress of each single crystal, i. e. CRSS, so far reported. It is found that the proportional relation between the σ0/2 value and the square root of aluminum concentration is in good agreement for the single crystal and polycrystalline specimens. Temperature dependence on the σ0/2 values for Cu-14.8at%Al alloys is similar to the variations for CRSS of single crystal, especially at low and elevated temperature regions. The shear stress for unlocking of edge dislocations from solute atoms estimated from the Hall-Petch parameters at 293K is found to be 3.5 times larger than CRSS of single crystals, which is thought to be the unlocking stress of screw dislocations. It is considered that the role of twin boundaries is almost equivalent to the grain boundaries, because the multiplicated dislocations pile up against the twin and grain boundaries in the small deformation.
We studied the diffusion of cobalt atoms into silicon substrates by using an X-ray diffraction experiment and molecular dynamics (MD) simulations to prevent open-circuit failures induced by the agglomeration of cobalt-silicide (CoSi2) film in semiconductor devices. The experimental results revealed that the agglomeration of cobalt-silicide films was caused by the diffusion of Co-atoms from CoSi2 films with a (111) texture into Si (001) substrates. The activation energy of Co-atom diffusion measured with sheet resistance (3.6eV) agreed well with that obtained from the MD simulations (3.7eV). We developed a CoSi2 film by adding nickel (Ni), because the MD simulation results indicated that the addition of Ni effectively reduced diffusion of Co-atoms. Its effectiveness was confirmed by measuring the sheet resistance. The agglomeration rate of CoSi2 film with Ni added was one digit smaller than that of CoSi2 film without it.
In the geomechanical studies the mechanism of the shear failure for the Mode II crack extension is still an open problem. As the basic research, the purpose of this paper is, therefore, to obtain the Mode II singular fields near a crack tip. The geomaterials, in general, have a pressure sensitive yield function to yield a dilatant effect. Thus, assuming a proportional loading, we obtain the asymptotic stress and displacement fields for the Mode II crack in the linear hardening Drucker-Prager elastic-plastic material. For the Mode II crack of the Drucker-Prager material, the boundary conditions can not be given on the extension of the crack surface, since the informations of the boundary are unknown : This is due to the loss of symmetry or asymmetry of the displacements and the stresses because of the existence of a parameter of the dilatant effect. Then the boundary conditions for the Mode II crack are given on the crack surfaces ; that is, the jumps of the normal displacement, the normal stress, and the shear stress vanish. As a result, a very small normal stress appears on the crack surfaces. We finally examine the angular distribution of the displacement and stress fields which depend on the hardening parameter and the pressure sensitivity parameter.
A simplified evaluation equation that simply predicts the warp deformation behavior arising when the thermal load was given to a viscoelastic laminated beam consisting of epoxy resin and FR-4 substrate was proposed. The validity of the equation was verified by comparing its solution with the exact solution based on linear viscoelastic theory and the experimental values. The proposed simplified evaluation equation is composed of the glass transition temperature, the rubber transition temperature, the elastic modulus and the linear thermal expansion coefficient of the component material. In this study, it was clarified that the warp deformation behavior of the viscoelastic laminated beam could be simply predicted by using the proposed simplified evaluation equation where the elastic modulus is divided into three regions such as glassy state, leathery state and rubbery state.
The objective is to characterize the effects of total rubber amount on the mechanical properties of the thermoplastic polypropylene blended with two different styrene-ethylene-butadiene-styrene tri-block copolymer (SEBS) at the intermediate and high strain rates. PP and two types of SEBS were blended so that the total rubber amounts were 10 and 20 wt% against PP by the two-step blending procedure. Tensile tests are conducted at the nominal strain rates from 3 × 10-1 to 102s-1. Phase morphology is investigated to estimate the bi-modal rubber particle size distribution. In addition, the fracture surfaces were observed by scanning electron microscopy (SEM) in order to understand the difference of the toughening mechanism for PP toughened with the bimodal rubber particle size distribution in PP and SEBS blends at various total rubber contents. The large material ductility is obtained in the fracture mechanism of craze bands. The craze bands are obtained in the blend whose total SEBS content is larger than 20 wt%. In addition, the weight ratio of small SEBS particles against total SEBS particles is larger than 20% and the inter-particle distance of large SEBS particles ranging between 100 and 300nm are additional condition for crazes bands. The synergistic effect of these rubber particles gives rise to a strong increase in the ductility of these bimodal rubber-particle distributed polypropylene systems.