Measuring distribution of the ellipsoid of birefringence through the thickness of optical disk substrates

We have measured the birefringence of polycarbonate optical disk substrates, using ellipsometry. For a more comprehensive characterization, the probe beam was incident upon substrates in a wide range of polar angles and from different azimuths relative to track direction (?). Our measurements show that the ellipsoid of birefringence is tilted in the plane of radial (r) and normal (z) directions. The tilt angle varies through thickness, with a maximum value of approximately 10°. For beams passing through the substrate in the ?-z plane and at large incident angles, this tilt causes significant conversion (up to 100%) between p- and s-polarized components. Distributions of other parameters, such as the values of in-plane and vertical birefringence, are obtained simultaneously in the measurements.     

Optimization of periodic column growth in glancing angle deposition for photonic crystal fabrication

We investigate the growth of periodically aligned silicon microstructures for the fabrication of square spiral photonic crystals using the glancing angle deposition phi-sweep process. We report the optimization of the phi-sweep offset angle for fabrication of microstructures with more precise geometry. The effects of varying the sweep offset angle of the phi-sweep process are studied for films deposited onto a square lattice array of growth seeds. To represent one growth segment of the phi-sweep process, we fabricate 15?nm silicon thin films using several azimuthal substrate offsets from 0° to 45° at a vapor incidence angle of 85°. We also deposit silicon square spirals on square lattice arrays with the phi-sweep method, using various sweep offset angles from γ = 0° to 45°. We find that using an offset angle of γ = 26.5° optimizes the shadowing geometry, which minimizes anisotropic broadening, producing greater quality photonic crystal structures. From normal incidence reflection spectroscopy, a maximum full width at half-maximum of 273 ± 3?nm and a relative peak width (Δλ/λ) of 16.1 ± 0.1% were found for a sweep offset angle of γ = 26.5°.     

Virtual manipulation of tail postures of a gliding barn owl (Tyto alba) demonstrates drag minimization when gliding

Aerodynamic functions of the avian tail have been studied previously using observations of bird flight, physical models in wind tunnels, theoretical modelling and flow visualization. However, none of these approaches has provided rigorous, quantitative evidence concerning tail functions because (i) appropriate manipulation and controls cannot be achieved using live animals and (ii) the aerodynamic interplay between the wings and body challenges reductive theoretical or physical modelling approaches. Here, we have developed a comprehensive analytical drag model, calibrated by high-fidelity computational fluid dynamics (CFD), and used it to investigate the aerodynamic action of the tail by virtually manipulating its posture. The bird geometry used for CFD was reconstructed previously using stereo-photogrammetry of a freely gliding barn owl (Tyto alba) and we validated the CFD simulations against wake measurements. Using this CFD-calibrated drag model, we predicted the drag production for 16 gliding flights with a range of tail postures. These observed postures are set in the context of a wider parameter sweep of theoretical postures, where the tail spread and elevation angles were manipulated independently. The observed postures of our gliding bird corresponded to near minimal total drag.     

Hysteresis in Transport Critical-Current Measurements of Oxide Superconductors

We have investigated magnetic hysteresis in transport critical-current (I_c) measurements of Ag-matrix (Bi,Pb)₂Sr₂Ca₂Cu₃O_10–x (Bi-2223) and AgMg-matrix Bi₂Sr₂CaCu₂O_8+x (Bi-2212) tapes. The effect of magnetic hysteresis on the measured critical current of high temperature superconductors is a very important consideration for every measurement procedure that involves more than one sweep of magnetic field, changes in field angle, or changes in temperature at a given field. The existence of this hysteresis is well known; however, the implications for a measurement standard or interlaboratory comparisons are often ignored and the measurements are often made in the most expedient way. A key finding is that I_c at a given angle, determined by sweeping the angles in a given magnetic field, can be 17 % different from the I_c determined after the angle was fixed in zero field and the magnet then ramped to the given field. Which value is correct is addressed in the context that the proper sequence of measurement conditions reflects the application conditions. The hysteresis in angle-sweep and temperature-sweep data is related to the hysteresis observed when the field is swept up and down at constant angle and temperature. The necessity of heating a specimen to near its transition temperature to reset it to an initial state between measurements at different angles and temperatures is discussed.     

Investigation of proper specimen geometry for mode I fracture toughness testing with flattened Brazilian disc method

Investigation of geometrical parameters for flattened Brazilian disc method is important, since this is a simple and attractive method for mode I fracture toughness testing on rock cores. Evaluating numerical modeling results, a parametric equation in terms of principal stresses at the center of the disc and the loading angle of the flattened end was developed. An equation was proposed for maximum stress intensity factors at critical crack lengths around stable to unstable crack propagation. Comparing fracture toughness results of flattened Brazilian disc method to the results of the suggested cracked chevron notched Brazilian disc method, geometrical parameters for flattened Brazilian discs were investigated. Diameter, loading angle of flattened ends, and thickness of andesite rock core specimens were changed to obtain comparable results to the suggested method. The closest results to the suggested method were obtained by 54 mm diameter discs with loading angles larger than 32°, and thicknesses between 19 and 34 mm. Results were confirmed by the flattened Brazilian disc tests on a marble rock. In flattened Brazilian disc tests with smaller loading angles and larger diameters, larger fracture toughness values than the results of the suggested cracked chevron notched were obtained. However, excluding tests with large loading angles over 27°; specimen size was less effective on the results of these tests. Critical crack length parameters computed from modeling and experiments were close to each other for the flattened Brazilian disc specimens with smaller loading angles around 20° and thickness/radius ratio equal or less than 1.1.     

Mixed-mode fracture angle and fracture locus of materials subjected to compressive loading

The incipient fracture angle and fracture loci of prenotched brittle-like material subjected to compressive loading are investigated analytically and experimentally.The analysis of the problem includes parameters whose effects on fracture were pronounced via laboratory tests, namely: notch-tip curvature, subcritical microcracks emanating from the notch and crack closure process. Such considerations, jointly with the well-established fracture criteria in tensile loading (like the critical energy release rate, the critical energy density, J-integral and critical maximum stress used in this work) yielded an associated fracture locus for each criterion. Due to the mixed mode nature of the situation (K₁ and K₂) preevaluation of the fracture angle was instrumental.Data on critical (far-field) compressive load along with measured fracture angles performed on PMMA and Tungsten Carbide specimens are used to depict the most suitable fracture locus and thus to distinguish between the various fracture criteria when extended to fracture under compressive loading. An exact expression for the threshold load for complete closure of 2D elliptical cracks is used to delimit the fracture locus.     

Rupture of thin ductile tubes by oblique impact of blunt missiles: experiments

Impact tests at both normal and oblique angles of incidence were conducted on thin mild steel tubes using a moderate size of blunt missile (D_m/h=4.33) at various angles of obliquity (0°⩽φ₀⩽60°) from normal. The minimum impact speed that generated cracks through the thickness of the wall, termed the speed for rupture, was measured, and various types of rupture were identified. For a thin tube hit by a flat-nosed missile at a large angle of obliquity, the flat-nose initially cuts into the surface of the tube wall; through thickness rupture is due to tensile tearing that occurs in a region just below the crater. The speed for rupture of the tube is found to be a minimum at an angle of incidence equal to 45°; this speed is about 41% less than that required to rupture a tube of equal thickness by impact at a normal angle of obliquity. If the missile nose has a radius of curvature of the same order as the missile radius, the deformation is more diffuse in the region immediately adjacent to the contact surface; consequently the mode of failure changes from predominately shear at the edge of the missile to more uniform stretching under the contact surface. For oblique impact of missile with more rounded noses, this causes the observed speed for rupture to increase with increasing angle of obliquity.     

An improved two-point calibration method for stereo vision with rotating cameras in large FOV

At present, binocular stereo vision is gradually being applied for 3D coordinate measurements in large fields of view (FOVs). In this study, a binocular stereo vision system with fixed and non-zooming cameras in a large FOV is constructed, in which cameras can rotate horizontally and vertically. All intrinsic parameters except the focal length of the cameras are set to default values and these focal lengths are calibrated offline in advance. Only the pitch angle and yaw angle of each camera need to be obtained during rotation. Therefore, we present a novel calibration method by using two control points and transform the imaging model of the pitch and yaw angles into a quadratic equation of the tangent value of the pitch angle so that the closed-form solutions of the pitch and yaw angles can be obtained. Computer simulation and real experiments demonstrate the effectiveness of the proposed method.     

Penetration mechanics of yawed rods

The penetration of rod projectiles is a function of impact yaw. Armor steel targets were struck at 0° obliquity by long steel rods at ~2.15 km/s and various angles of yaw. Crater dimensions varied systematically with yaw angle. Trenching behavior was observed for yaw angles exceeding about 30°. Analysis indicates that the rods collapsed into the targets with no significant rotation, and that penetration chiefly depends on the parameters D and $\text{D}\text{sin θ}$ (where D is rod diameter and θ is yaw angle).     

Field-Induced CDW Phases in a Quasi-One-Dimensional Organic Conductor, HMTSF-TCNQ Under Pressure of 1 GPa in Magnetic Field of 31 T

HMTSF-TCNQ is a quasi-one-dimensional organic conductor which undergoes CDW(charge density wave) transition at 30 K at ambient pressure, where HMTSF-TCNQ is hexamethylenetetraselena fulvalene-tetracyano quino dimethane. This CDW is suppressed by the pressure of 1 GPa. At this pressure, we found field-induced successive hysteretic transitions in magnetoresistance. This reminds us of the successive field-induced SDW (spin density wave) phases in TMTSF₂X salts. However, the field range of interest is 2–3 times higher than that of TMTSF₂X salts. Therefore, we need really high field to examine these properties. It is very likely that the field induced phases are of field induced CDW (FICDW), where quantum Hall effect and many interesting phenomena are expected like in the case of FISDW. Together with the magnetoresistance study up to the field of 31 Tesla and at temperatures down to 0.4 K in various magnetic field angles respective to the crystal axes, we examined the angular dependence of magnetoresistance oscillations(AMRO). It turned out that AMRO demonstrates clearly the occurrence of field-induced phase rather than the magneto-resistance by field sweep. Since the Hall resistance, R _xy in the field-induced phases showed stepwise plateau structure against the field sweep, and its strength was in the order of magnitude of h/e ² per molecular sheet, the Hall effect is very suggestive of quantum Hall effect.     

Geometry and size effects on fracture trajectory in a limestone rock under mixed mode loading

The mixed mode I/II fracture initiation angle and the crack growth trajectory of a soft rock (Guiting limestone) were investigated experimentally and theoretically for two different shaped test specimens with various sizes. It was observed that for similar mode mixities in the two specimens, the fracture paths grew in two different trajectories. It is shown that the observed crack path and the fracture initiation angle can be predicted theoretically by using a generalized form of the maximum tangential stress criterion. The main difference in the fracture initiation angles was found to be related to the magnitude and sign of the T-stress.     

Increasing flow efficiency of high-pressure and low-pressure steam turbine stages from numerical optimization of 3D blading

3D blading of a high-pressure and low-pressure steam turbine stage is optimized using Nelder–Mead method of deformed polyhedron. Values of the minimized objective function, i.e. stage losses with the exit energy are found from 3D viscous compressible flow computations, including turbulence effects. Among the optimized parameters are stator and rotor blade numbers and stagger angles, rotor blade twist angle, stator blade sweep and lean, both straight and compound. The blade sections (profiles) are assumed not to change during the optimization. There are constraints imposed on the design parameters, including the mass flow rate and stage reaction. Optimization gives designs with new 3D blade stacking lines, and with increased efficiencies, compared with the original design.     

结构参数对复合材料V型构件固化变形影响试验及解析分析

为研究结构参数对复合材料V型构件固化变形的影响,完成了针对V型构件厚度、拐角半径、拐角角度及铺层等结构参数的变形影响研究试验。基于剪力滞后理论和弯曲理论,利用解析法建立了考虑结构参数影响的复合材料V型构件固化变形预测模型,利用模型预测了V型构件的回弹变形并分析了不同结构参数对V型构件回弹变形的影响机制。结果表明:回弹变形随着厚度的增大而减小,厚度为1~3mm之间,角度回弹变形差异最大在30%左右;回弹变形与拐角角度的补角呈约为0.014的比例;拐角半径的不同导致变形的差异不超过5%;准各向同性铺层试验件展现了最大回弹变形,0°铺层的变形减小了23.5%,90°铺层几乎不发生变形。模型分析结果表明,厚度主要通过弯曲刚度和剪切变形两方面影响回弹变形;铺层引起的力学性能和泊松效应的变化是使回弹变形有较大区别的主要原因;V型构件直边变形最大为0.20°,对回弹变形影响较大。变形预测结果与试验结果对比验证了解析法模型的准确性。     

Experimental study of the flow rectification performance of conical diffuser valves

Flow rectification performance of conical diffuser valves has been investigated experimentally. In order to quantify the valving capability of the diffuser, a diffuser performance test cell is designed. The main feature of the test cell is that the volume change of the pumping chamber is independent of the diffuser geometry and actuation frequency. Experiments have been carried out over a range of diffuser angles varying from 10° to 35° and Roshko numbers (based on diffuser throat diameter and actuation frequency) from 25 to 300. Results show that the valving performance of the diffuser is significantly affected by the tapered angle and Roshko number. The diffuser valve with diverging angle of 10° exhibits the best performance. This is consistent with our recent simulations. The maximum flow rectification efficiency that has been attained at condition of zero backpressure is around 48%. In addition, we have simultaneously measured the time-dependent pressure inside the actuation chamber. It is found that the variations of the pressure amplitude with the Roshko number are qualitatively similar to those of the valving efficiency; high rectification efficiencies are always associated with large actuation pressures. However, for a large actuation volume, the valve performance declines, even though the actuation pressure increases. A possible explanation for this phenomenon is also provided.     

大气次声源定位算法及误差分析

逐步多通道相关方法是定位次声源的主要方法。该方法会忽略阵元的海拔高度差,从而引入误差。文章分析了基于广义互相关的时间延迟估计算法,讨论了基于时延的平面波入射角定位方法。着重研究了逐步多通道相关方法的误差来源,确定了基于时延的定位方法是产生误差的主要原因,明确了大气次声源定位误差来源于忽略了阵元间的海拔高度差。基于最小二乘法推导了在不考虑阵元高度的情况下计算次声波入射角的方法。对存在高度差的4元中心三角阵型进行了误差仿真实验,在忽略阵元海拔高度差的条件下,各方向入射的次声波角度定位误差最大达到4°,特定阵型的阵元最大海拔高度差与入射角度计算误差成线性关系,并探讨了入射角度计算误差对主要参数和后续定位的影响。     

A microcontinuum analysis of the self propulsion of the spermatozoa in the cervical canal

The hydrodynamics of the self propulsion of a spermatozoa, swimming through the mucus filling the cervical channel, is investigated. The mucus is modeled as a micropolar fluid and the spermatozoa as a 2-dimensional sheet swimming at low Reynolds number between two rigid walls. The wavelengths of the propulsive waves passing down the sheet are assumed to be very large compared to the channel spacing, but the amplitude of the propulsive waves is arbitrary. Expressions for the propulsive velocity and the energy expended by the swimming sheet are obtained in terms of various parameters involved. The results are elaborated through graphs. It is found that both the propulsive velocity and the rate of working by the sheet increase as the value of the micropolar parameters $\text{N}$ increases and that of $\text{L}$ decreases.     

Substrate Doping Effect and Unusually Large Angle van Hove Singularity Evolution in Twisted Bi‐ and Multilayer Graphene

Graphene has demonstrated great potential in new‐generation electronic applications due to its unique electronic properties such as large carrier Fermi velocity, ultrahigh carrier mobility, and high material stability. Interestingly, the electronic structures can be further engineered in multilayer graphene by the introduction of a twist angle between different layers to create van Hove singularities (vHSs) at adjustable binding energy. In this work, using angle‐resolved photoemission spectroscopy with sub‐micrometer spatial resolution, the band structures and their evolution are systematically studied with twist angle in bilayer and trilayer graphene sheets. A doping effect is directly observed in graphene multilayer system as well as vHSs in bilayer graphene over a wide range of twist angles (from 5° to 31°) with wide tunable energy range over 2 eV. In addition, the formation of multiple vHSs (at different binding energies) is also observed in trilayer graphene. The large tuning range of vHS binding energy in twisted multilayer graphene provides a promising material base for optoelectrical applications with broadband wavelength selectivity from the infrared to the ultraviolet regime, as demonstrated by an example application of wavelength selective photodetector.     

锥角参数对单向聚能药柱爆破破岩效果的影响

为研究聚能穴锥角参数对爆炸应力和岩石损伤破裂范围的影响,以获得最优的聚能穴参数,从而达到最佳的破岩效果,利用有限元模拟软件LS DYNA建立了6种锥角参数下的单向聚能药柱模型。锥角的深度为15 mm,6种锥角高度分别为10、12、14、16、18 mm和20 mm。研究了岩石裂纹扩展的影响规律,测得聚能方向与非聚能方向上不同位置的有效应力,得到不同锥角参数对应的岩石单元的最大破坏距离。结果表明:聚能锥角会对爆破产生定向作用,特别是对岩石破碎和拉伸裂纹所带来的破岩效果影响明显;当锥角高度为10 mm时,距炮孔25 cm测点处聚能方向上的有效应力比非聚能方向同样距离处高110.8 MPa,同时,聚能方向上单元损伤比要比非聚能方向高21%,聚能效果最佳;随着锥角高度逐渐增大,聚能方向上岩石裂纹逐渐减少,裂纹分叉减少,单元破坏最大距离可达108.1 cm,并且呈下降趋势。     

A general weight function for inclined cracks at sharp V-notches

A general method for evaluating the Stress Intensity Factors of an inclined edge crack originated at the tip of a sharp V-notch in a semiplane is presented. An analytical Weight Function with a matrix structure was derived by extending a method developed for an inclined edge crack in an unnotched semi-plane. The effects of the principal geometrical parameters governing the problem were studied through a parametric finite element analysis, carried out for different reference loading conditions. The Weight Function can be used to produce efficient and accurate evaluations of the stress intensity factors for cracks with inclination angle in the range −72°, +72° emanating from V-notches with opening angle in the range from 18° to 144°. For a crack length up to the 10% of the characteristic notch dimension, the maximum estimated error of the Stress Intensity Factor is lower than 2% (typical errors less than 1%) in the whole ranges of the angular parameters. The capability of the proposed method to analyse cracked notches in finite-size bodies was also considered. The agreement between the results with those obtained by accurate Finite Element solutions suggests that the proposed Weight Function can be used as a general tool for evaluating the Fracture Mechanics parameters of a short crack at any V-notch tip.     

Investigation on crystallographic behaviors of nickel‐base single crystal alloy cooling holes with different spanwise injection angles

Film cooling as an important thermal protection technology is widely used in aviation and ground gas turbine blades. But film cooling holes reduce the strength of blade seriously, which have become a key region of crack nucleation. In this paper, the plastic behaviors of nickel‐base single crystal alloy turbine cooling holes in spanwise injection angles range from 0° to 40° are investigated on basis of crystallographic constitutive theory. The results show that there are both higher stress regions and lower stress regions around multi‐column cooling holes, where suffer stress interference. The maximum Mises stress occurs at the hole in the center column. The places where the maximum resolved shear stresses occurs change with load and spanwise injection angle. The maximum Mises stress around holes with injection angle of 0° is lowest. With the injection angle increases, the maximum Mises stress increases until injection angles up to 30°. In all the slip systems, the resolved shear stress of hexahedral slip system is most sensitive to the changing of spanwise injection angle and load.