A molecular dynamics simulation is carried out to study thehyper-velocity impact fracture at a space debris' attack to a man-madesatellite. A target-projectile system is modeled as aface-center-cubic system which consists of aluminum atoms. Morse potential is adopted as the interatomic potential. Impact velocity dependence of distribution of debris clouds is studied in the different velocity conditions. As a result, the structure of debriscloud of the case of 7km/s impact differs from that of 12km/s. Thesize of fragment of the debris clouds becomes smaller with increasing impact velocity. The axial component of debris cloud velocities areshown to decrease as target thickness-projectile diameter ratio t/D is increased. These results qualitatively agree well with anexperimental result.
A molecular dynamics simulation is carried out to study the fractureunder hyper-velocity impact conditions when a space debris attack to aman-made satellite. In this paper, oblique-impact simulations areperformed since most orbital debris impacts are oblique in the realcondition. Aluminum projectile impacts aluminum target at 0−, 30−, and 45−degrees obliquity. Impact velocity is 8km/s, and target thickness-projectile diameter ratio t/D is 0.684. As the angle of obliquity isincreasing, the velocity of debris after impact is decreasing, whilethe deviation of the line of flight is increasing. These resultsqualitatively agrees well with experimental results.
P(LLA-CL) copolymer was additionally blended to PLLA/PCL polymer blend to improve the immiscibility between PLLA and PCL. Mechanical properties such as the bending modulus, strength and the maximum J-integral were then measured and the phase morphology and fracture surface were observed using FE-SEM. Crystallinity values of PLLA and PCL were also evaluated using DSC. It is seen that those mechanical properties successfully increased due to P(LLA-CL) addition. The results of FE-SEM observation and crystallinity measurement suggests that the immiscibility is improved by P(LLA-CL) blending and as a result, spherulites of PCL dispersed in the blend are reduced, resulting in effective increase of ductile deformation. It is thought that this kind of phase structural change causes the improvement of the mechanical properties.
A plate of a Ti alloy, Ti-6Al-4V, with a thickness of 9mm was laser-shock peened. The distribution of the residual stress throughout the plate thickness was nondestructively measured with the transmission and reflection conditions of the strain scanning method using hard synchrotron X-rays of 70keV energy. The distributions of the in-plane and out-of-plane strains were measured by using the 004 and 112 diffractions of α-Ti phase. In-plane strains measured in two perpendicular directions were nearly identical, confirming the equi-biaxial state of residual stresses. The out-of-plane strain measured for the 112 diffraction agreed with the value calculated from out-of-plane strains based on isotropic elasticity, while that for the 004 diffraction was higher than the prediction. The discrepancy for the case of the 004 diffraction was caused by the intergranular strain due to plastic anisotropy. The use of the 112 diffraction was recommended for the stress measurement. The residual stress on the surface was compression of about −500MPa and the compression zone extended about 1.8mm below the surface. The distribution of the half-value-breadth also shows a large increase in the subsurface compression zone.
Precise X-ray diffraction analyses are performed on epitaxial ZnO and (Zn, Mg)O (0001) films with 50 nm thick cap layers grown on Sapphire (1120) substrates by molecular beam epitaxy method. The tilting of c-axis of ZnO and (Zn, Mg)O films is as small as 0.2 degree. In-plane X-ray diffraction analysis reveals directly the twisting of a-axis of films as smaller than 0.8 degree. Orientational misalignments between films and substrates are confirmed in the order of 1 degree both from out-of-plane and from In-plane Reciprocal space mapping measurements, together with the pseudomorphic coherent growth of cap layers. It is concluded that the lattice misalignment in the out-of-plane direction is affected by the miscut of substrate surface treatment.
Fatigue crack propagation tests of bulk metallic glass, Zr55Cu30Ni5Al10, were conducted with C (T) specimens. Tests were conducted at a stress ratio of 0.1 or 0.5 under the loading frequency of 20 or 1.0Hz. The fatigue crack propagation rate was controlled by the stress intensity range. Three regions were observed in the relation between crack growth rate and stress intensity range. In the middle region, the growth rate can be expressed by Paris' law, where the power of the relation was much smaller than that of crystalline metals. It shows that fatigue cracks do not grow by the striation mechanism in the material. The appearance of fracture surfaces was much different from that of crystalline metals. Since the effect of stress ratio and loading frequency were not observed, the fatigue crack growth of the bulk metallic glass is considered to be cycle dependent, and it is controlled by cyclic component of the stress intensity factor.
The fatigue strength at N = 107 for carburized steels, which indicates the fatigue strength for specimens whose fatigue origins are not at inner defects but at as-carburized surfaces in this study, is affected by a softened structure, oxidized intergranular, which is considered to be mechanically equivalent to a defect, retained austenite and residual stress. Therefore, the influence of each factor should be separately considered for evaluating the fatigue strength at N = 107 quantitatively. From this viewpoint, this study aims to clarify the influences of hardness, which exceeds the range to which the √area parameter model is applicable, defect size and retained austenite on fatigue strength at N = 107. To clarify these influences, carburized SCM420s and quenched-tempered SUJ2s were applied, and rotating bending fatigue tests were conducted using specimens with an artificial defect. Tension-compression fatigue tests were also performed by specimens with a single edge notch in order to observe crack closure behavior. The fatigue strength at N = 107 was influenced by not only the hardness and defect size, but also the retained austenite ; the fatigue strength at N = 107 of the specimens, for which the volume of the retained austenite contained in the microstructure was about 20%, was approximately 1.5 times higher than that estimated by the √area parameter model. It is considered that the improvement of the fatigue strength is due to developed crack closure in the threshold stress intensity factor range and the development is mainly caused by the strain-induced transformation of the retained austenite generated during a fatigue test. Based on these results, a new prediction equation as a substitute for the conventional one was proposed by considering the influence of the retained austenite.
Numerical simulation method for analyzing fracture behavior of porous ceramics is developed on a basis of initiation of micro-cracks. The deformation and fracture behavior are numerically simulated under four-point bending. A mixed mean stress model is adopted as a condition of the initiation and propagation of micro-cracks. Analytical models are made of an aggregation of circular grains whose are average size is 150μm. The micro-cracks are located in a bond of each grain. In a stress-strain relationship, as a notch depth and porosity increase, the stress-strain curve becomes nonlinear. The micro-cracks initiate near large pores and a main crack propagates intricately. When the notch depth is small, the fracture occurs not from the notch root. Size of a non-damaging defect can be evaluated by the proposed method. R-curve calculated is independent of the notch depth. These results calculated agree very well with experimental results.
Recently, new porous carbon materials have been developed utilizing the natural porousstructures of some plant matters. The carbon materials are manufactured by mixing with a phenol resin, pressure forming, drying, and then carbonizing at 900°C. The authors have been studied the carbon material made from the defatted rice bran (RB carbon). However, the RB carbon includes some deliquescent components, and therefore has large hygroscopic expansion and large reduction of the mechanical strengths under wet and aquatic conditions. The low water resistance is associated with the inorganic components of P and K in the rice bran. In contrast, the rice hull does not contain such inorganic components. In this study, the authors proposed the production of the water-resistant rice-hull silica carbon (RHS carbon) from the rice hull and the correcting methods of the low water resistance for the RB carbon. The RHS carbon has low hygroscopic expansion and no reduction of the compressive strengths under aquatic conditions.
In this study, the seismic response characteristics of steel pier subjected to earthquake load are clarified by the time-history elasto-plastic response analysis method. And a seismic reliability evaluation method of steel pier considering the energy absorption performance is proposed. Through of some numerical calculations, the relationship between the energy absorption performance and damage index is examined, and then both effects of the geometrical shape and the flexural stiffness of longitudinal stiffeners on the seismic reliability index of steel pier are evaluated.
Effects of frequency and stroke on the running-in process were investigated on steels with an SRV tester by varying frequency and stroke under the same average velocity. A reciprocating bearing steel ball (SUJ2) was slid against a disc of plain carbon steel (S35C). As lubricant, paraffin base oil and DBDS (Dibenzyl Disulfide)-added oil were used. At higher frequency and smaller stroke the coefficient of friction fluctuated at the stage of initial friction but settled down to a steady friction in base oil. At lower frequency and larger stroke, the coefficient of friction continued to fluctuate, which resulted in a larger wear. The addition of DBDS significantly lowered the coefficient of friction independent of stroke. However, the wear was larger at lower frequency and larger stroke in DBDS-added oil. On the other hand, higher frequency increased the number of reciprocating contacts between specimens under a constant average velocity and it promoted the oxide film formation, which contributed to the decrease in wear.
A micrometer-scale thermopile formed on Si substrate using a photolithography technique combined with laser-beam machining is described. Initially, the surface of an 11-mm-square and 0.3-mm-thick Si substrate was oxidized to grow a SiO2 thin-film. A 300-μm-long, 100-μm-wide and 0.5-μm-thick SiO2 air bridge was then formed on the Si substrate using a photolithography technique. Next, copper and constantan were evaporated in layers on the middle of the air bridge. This evaporated thin film functioned as a type-T thermocouple. Then, in order to improve the sensitivity of the thermocouple, three of the type-T thin-film thermocouples were shaped by cutting the evaporated film into three regions using a laser-beam machining technique. Finally, the thermopile was completed by connecting the three thermocouples in series, locating their hot junctions on the center of the air bridge and cold junctions on the pad areas used as electrodes. Because the heat capacity and the heat dissipation of the hot junction areas differ from those of the cold junction area, a temperature difference occurs between the hot and cold junctions when the thermopile receives radiation. This temperature difference generates a thermo-electromotive force between the cold and hot junctions, which is a measure of the intensity of the incident radiation. Due to a small heat capacity combined with a relatively large radiation reception area, the time-constant of this type of thermopile for light-illumination was found to be less than 1 ms, which was more than ten times faster than that of conventional thermopiles presently on the market.
Zinc oxide (ZnO) whiskers were directly synthesized onto the interdigital structured electrodes with 10μm gap through an aqueous solution process. X-ray diffraction (XRD) patterns, photoluminescence of the whiskers at room temperature and field emission-scanning electron microscope (FE-SEM) images showed that the main products were wurtzite ZnO whiskers like hexagonal columns. The whiskers obtained have more than 10μm long and several μm wide. A few whiskers which were strongly bridging the electrodes could be selectively left by applying ultrasonic waves for several ten minutes. Photoconductivity of the whiskers at 390nm was clearly seen with 5V bias. The same characters of whiskers are kept over a long period of time, more than 2months.
Expansive concrete is often employed for prevention of cracks in early age. The present study focused on the tensile properties of the expansive concrete in early age. Especially, the influence of coarse aggregate on tensile strength was investigated in order to obtain an optimized mix-design for expansive concrete. The experimental results showed that expansive concrete in early age has higher tensile strength as the volume of the coarse aggregate increases. The present paper indicates such results are due to the confinement effect of the coarse aggregate in the expansive concrete. In addition, expansive strains of various kinds of concrete were investigated to confirm the confinement effect by the coarse aggregate.
Waste expanded polystyrene (waste EPS) can be recycled as an aggregate for concrete when it is melted and crushed. Despite the low bulk density of 1g/cm3, water absorption of the waste EPS aggregate is as low as 5 percentage and advantageous over the conventional lightweight aggregates. Mechanical properties of concrete specimens with a coarse aggregate of the crushed waste EPS melt were studied. It was found that the unit mass of concrete using the waste EPS coarse aggregate was approx. 1.7 to 1.8t/m3, and that compressive strength of the concrete was approx. 15 to 33N/mm2. Brittleness index of the waste EPS aggregate concrete was smaller than that of normal aggregate concrete and conventional lightweight aggregate concrete. Use of crushed waste EPS as a lightweight coarse aggregate for concrete is a promising approach for the weight reduction of normal concretes.
At very early ages, plastic shrinkage is one of the causes of cracking of concrete. It has been believed that the plastic shrinkage of cementitious material occurs when the water drying rate from the surface is larger than bleeding rate. However, in recent studies, it was reported that negative pressure may occur due to hydration, without drying, in high-strength concrete with a low water-binder ratio. This suggests that the volume change of cementitious material at very early ages is influenced not only by drying from the surface but also by self desiccation due to hydration. The purpose of this study is to clarify the effects of water-binder ratios, volume fraction of fine aggregates and curing conditions on volume change of cement paste and mortar at very early ages. Effect of each factor on volume change of cement paste and mortar was investigated through experiments on the change in pore water pressure and the shrinkage behavior at very early ages. It was found that volume change of cement paste and mortar at very early ages was markedly influenced by self desiccation as well as by drying when the water-binder ratio is low.