网络爆轰驱动飞片的设计技术

提出使用炸药网络爆轰技术,设计了一种新型的低速飞片击靶加载装置,结合数值模拟和实验测 量,对驱动飞片的速度、平面性及装置结构参数进行了系统分析。实验结果表明,无氧铜飞片可以使直径 100mm、厚度3mm 的飞片在飞行距离2mm 内达到稳定的飞行状态,击靶速度在200~350m/s范围内连 续可调,飞片击靶平面范围大、速度均匀、平面性好,为实现低冲击应力的一维应变加载提供了参考。     

Interaction between collinear periodic cracks in an infinite piezoelectric body

The problem of collinear periodic cracks in an infinite piezoelectric body is studied. Effect of saturation strips at the crack-tips is taken into account. By means of the Stroh formalism and the conformal mapping technique, the general periodic solutions for collinear cracks are obtained. The stress intensity factors and the size of saturation strips are derived analytically, and their dependencies on the ratio of the periodicity on the half-length of the crack are analyzed in detail. Numerical results show the following two facts. (1) When h/l>4.0, the stress intensity factors become almost identical to those of a single crack in an infinite piezoelectric body. This indicates that the interaction between cracks can be ignored in establishing the criterion for the crack initiation in this case. (2) The speed of the saturation strip size of periodic cracks approaching that of a single crack depends on the electric load applied at infinity. In general, a large electric load at infinity is associated with a slow approaching speed.     

One-dimensional shock wave interaction with rubber and low-porosity foam

One-dimensional interaction between a planar shock wave and a rubber or low-porosity foam is investigated experimentally and numerically. The considered polyurethane foam is of high density (ρ _c=290 kg/m³) and lowporosity (ϕ=0.76), and this corresponds to an intermediate condition between rubber and high-porosity foam. Stress-strain relations for the low-porosity foam are investigated by machine tests, which show larger deformation against compressive force and higher non-linearity in stress-strain curve as compared with rubber. Also the low-porosity foam shows a hysteresis cycle. Experiments on shock wave-foam interactions are conducted by using a shock tube. Experimental time history of the surface stress of the foam at the end of the shock tube does not show shock type stress increase, but continuous excessive stress rise can be seen, and then dumping vibration approaching to gas dynamic pressure of the reflected shock wave is followed, and the highest stress amounts about 3∼4 times of the pressure after the reflected gas dynamic shock wave. Interactive motions of gas and the low-porosity foam are analyzed using the Lagrangean coordinates system. An elastic model for a low-porosity foam is assumed to be a single elastic material with the measured stress-strain relation. Results of numerical simulations are compared with the shock tube experiments, which show essentially same stress variations with experimental results.     

Likelihood and expected-time statistics of monodomain attractors in sheared discotic and rod-like nematic polymers

Employing a mesoscopic Doi tensor model, we develop transient statistical properties of sheared nematic polymer monodomains consistent with typical experimental protocols. Our goal is to convey to the experimentalist a list of expected outcomes, based not only on properties of the nematic liquid and imposed flow rate, but also on the timescale of the experiment and variability in the initial conditions. Step 1 is deterministic: we solve the model equations completely, then compile the flow-phase diagram of all monodomain attractors and phase transitions versus nematic concentration and Peclet number (shear rate normalized by molecular relaxation rate). Step 2 is to overlay on the phase diagram a statistical diagnostic of the expected time, t_A, to reach a small neighborhood of every attractor A. The statistics are taken over the arbitrary quiescent director angle on the sphere, modeling experiments that begin from rest. Step 3 is to explore parameter regimes with multiple attractors, where we statistically determine the likelihood of convergence to each attractor. These statistical properties are critical for any application of theoretical models to the interpretation of experimental data. If t_A is longer than the timescale of the experiment, attractor A is never fully resonated and the relevant stress and scattering predictions are those of the transients, not the attractor. In bi-stable and tri-stable parameter regimes, which are typical of nematic polymers, a distribution of monodomains of each type will populate the sample, so experimental data must be compared with weighted averages based on the likelihood of each attractor (see Grosso et al (2003) Phys Rev Lett 90:098304). The final step is to give statistics of shear stress and normal stress differences during the approach to each attractor type, as well as typical paths of the major director that are contrasted with the results of Van Horn et al (Rheol Acta (2003) 42(6):585–589) with Leslie-Ericksen theory.     

三维切口应力奇性指数计算

将三维切口根部的位移渐近展开式引入线弹性力学平衡方程,导得关于切口应力奇性指数的特征微分方程组。再采用插值矩阵法,一次性地计算出三维切口的各阶应力奇性指数,它们具有同阶精度,并可同时获取相应的特征角函数。算例显示该法是分析三维切口应力奇异指数的一个有效的路径。计算结果表明,若直接用平面应变理论预测三维切口应力奇性指数将导致部分重要的奇性指数丢失。     

Further consideration of the shape-factor relationship for turbulent boundary layers

The lag-entrainment predictive scheme developed by Green et al. has been modified to include the pressure-gradient parameter Π₁. In the original model suggested by Green et al. the mass-flow shape factor H₁ is related to the common shape factor H, H₁ = f(H). In the present model H₁ is related to H, Reynolds number based on the local momentum thickness θ, and Π₁; thus H₁ = f(H, R_eθ, Π₁). The modified formula for H₁, is introduced into the original lag-entrainment integral model. Calculations are made to examine the present model for the predictions of the development of boundary layers approaching separation studied experimentally by the authors. Slightly improved predictions are obtained using the model developed by El Telbany et al. However, the present model proved to give an improved representation of the development of wall shear stress in cases the two-equation turbulence model proved to be unsuccessful.     

Isotropic/orthotropic two-layered strips containing cracks terminating at and going through an interface

Crack problems for isotropic/orthotropic two-layered strips have been investigated. A system of two singular integral equations can be derived by using Fourier integral transformation and boundary conditions of crack problems. After stress singularities at crack tips or other special points are determined for internal and edge cracks, and for cracks terminating at and going through the interface, the system of singular integral equations is solved numerically by Gauss-Jacobi or Gauss-Chebyshev integration formulas for stress intensity factors at the tips and other singular points of cracks. Finally, possible crack growth behavior for cracks approaching and going through the interface is discussed.     

Calculation of motion of a spherical drop in the Bingham medium

Mathematical modeling of the experimentally observed process of approaching of two identical oil drops located in an alcohol-water solution (matrix) with an identical density is performed. It is found that the drop moves in cycles consisting of the state at rest, acceleration, and deceleration; the cycle time is about 10⁻² s. Violation of the balance of forces on the drop boundary in the state at rest is caused by the fact that the shear stresses on this boundary cannot exceed the yield stress of the matrix, and the normal stresses are determined by solving the problem of the elasticity theory, because intermolecular bonds in the quiescent matrix make it similar to a solid. The results of drop motion calculations and experimental data agree well during the entire process of drop approaching, except for the final stage, which can be attributed to the neglect of hydrodynamic interaction of the drops.     

Stress concentration due to an array or hemispherical cavities at the surface of an elastic half-space

The paper investigates the perturbation in an otherwise uniform stress field in an elastic half-space due to a doubly-periodic array of small hemispherical holes at the free surface. The solution is obtained using three potential functions of double Fourier series form in Galerkin's strain potential solution, the coefficients of which are determined using the collocation method. The unperturbed field is taken to be one of uniform plane stress parallel to the free surface. Two special cases are studied—uniform tension and uniform shear stress. Numerical results for these cases can be generalized by superposition to give solutions for a general state of biaxial plane stress. It is found that, for both tension and shear, the maximum stress concentration occurs at the bottom of the holes. The stress concentration factor increases with the ratio of hole spacing to radius, approaching the known solution for a single hemispherical hole at large ratios.     

A technique to determine total shear stress and polymer stress profiles in drag reduced boundary layer flows

Two methods of recovering the entire total shear stress profile from incomplete velocity data in turbulent boundary layers are presented and validated for both DNS simulations and experimental measurements. The first method, an exponential–polynomial curve fit, recovers the whole total shear stress profile using the data from the outer part of the boundary layer (y/δ>0.3). However, while performing well, this curve fit is sensitive to the quality of the data. The second method, a new (1−y/δ) weighted straight line fit, which is very simple and accurate, has been applied to current experiments of drag reduction in zero pressure gradient turbulent boundary layers with and without polymer injection. The total shear stress profile obtained from this fit is used to estimate the contribution of the polymer stress to the total shear stress. It shows that the polymer stress is significant only in the inner part of the boundary layer and the magnitude of the polymer stress is not always proportional to the drag reduction.     

The role of angular momentum in the laminar motion of viscous fluids

In laminar flow, viscous fluids must exert appropriate elastic shear stresses normal to the flow direction. This is a direct consequence of the balance of angular momentum. There is a limit, however, to the maximum elastic shear stress that a fluid can exert. This is the ultimate shear stress, \(\tau _\mathrm{y}\), of the fluid. If this limit is exceeded, laminar flow becomes dynamically incompatible. The ultimate shear stress of a fluid can be determined from experiments on plane Couette flow. For water at \(20\,^{\circ }\hbox {C}\), the data available in the literature indicate a value of \(\tau _\mathrm{y}\) of about \(14.4\times 10^{-3}\, \hbox {Pa}\). This study applies this value to determine the Reynolds numbers at which flowing water reaches its ultimate shear stress in the case of Taylor–Couette flow and circular pipe flow. The Reynolds numbers thus obtained turn out to be reasonably close to those corresponding to the onset of turbulence in the considered flows. This suggests a connection between the limit to laminar flow, on the one hand, and the occurrence of turbulence, on the other.     

泵头体应力集中分析

本文对石油酸化压裂车泵头体进行了三维有限元分析，找到其应力最大点。然后分析出应力集中的原因，在此基础上，通过变动相关尺寸而进行一系列的有限元计算，最后得到泵头体最大应力随关键尺寸变化的规律曲线，这个规律曲线近似为双曲线，为泵头体设计提供了可靠的理论依据。     

Unixial compression testing of M30 and JA2 gun propellants using a statistical design strategy

A statistical factorial design strategy is used to determine the uniaxial compressive mechanical response of two energetic polymers, M30 and JA2 gun propellants, as a function of strain rate,, temperature,T, specimen aspect ratio,l/d, and specimen-end lubrication,L. A model of the mechanical response,y=y(,T,l/d,L) is formed using least-squares minimization of observed behavior with a secondorder polynomial model in the independent variables. It is found that aspect ratio and end-lubrication variables can influence the overall mechanical behavior of these materials, so that their effects must be quantitatively evaluated and screened prior to the development of constitutive models. Model predictions and global root-mean-square, RMS, errors in yield stress and strain, strain-energy density at yield, compressive and failure moduli for these materials compare favorably with historical values obtained from a replicate test design strategy.     

Resistance to uniaxial extensional flow of fibre suspensions

The tensile stress due to resistance to uniaxial extensional flow of fibre suspensions in Newtonian and non-Newtonian fluids has been measured using the filament stretching technique. It has been found that addition of fibres to a Newtonian fluid increases the extensional viscosity. The steady state results agree with Bachelors theory and the stress growth behaviour is qualitatively predicted by the theory of Dinh and Armstrong. Experimental results from this work have also shown that the behaviour of a fibre suspension in viscoelastic fluid is qualitatively described by Fans equation. The added fibres increase the extensional stress growth coefficient of the viscoelastic fluid at low strain but have marginal effect on the fluid after the onset of strain-hardening.     

Stress-concentration factors at intersecting and closely approaching orthogonal coplanar holes

Using two models, each containing three groups of intersecting or closely approaching holes, 18 different cases were investigated by three-dinensional frozen-stress photoelasticity. In all cases, the coplanar holes were orthogonal to the applied uniaxial tension. Included were three cases of intersecting hoes with square corners and varying hole-diameter ratios. Five cases of intersecting holes with rounded corners were studied for two hole-diameter ratios and varying corner radius. Stresses were determined for two cases where the end of one drilled hole partially penetrated another hole leaving acute corners or feathered edges at the intersection. Three types of closely approaching holes were studied: two cases in which the end of one drilled hole nearly intersected the side of an-other hole, two cases in which the ends of two drilled holes approached each other along a common axis, and four cases (called corner-approach cases) in which the ends of two drilled holes approached each other along orthogonal axes. A stress-concentration factor of 13 was found for one of the partially penetrating drilled-hole cases. The upper limit on stress-concentration factor for a very small hole intersecting a large hole in an infinite body subjected to uniaxial stress is 8.4 for metals. This factor is reduced to 5.2 as the diameters of the intersecting holes become equal. Rounding the corners on intersecting holes reduced the stress concentration by only four to seven percent. Closely approaching drilled holes results in higher stresses than fully intersecting holes when the minimum ligament width is less than ten percent of the hole diameter. Corner approach cases do not result in high stress-concentration factors as long as the ligament width is greater than the percent of the hole diameter.     

基于界面层模型的双周期纳米夹杂复合材料反平面问题研究

利用复变函数方法并结合双准周期Riemann边值问题理论,获得了含双周期分布非均匀相（夹杂/界面层）的复合材料在远场均匀反平面应力下弹性场的全场解答．该解答可用于对纳米夹杂复合材料的应力进行分析,结合平均场理论也用于预测纳米夹杂复合材料的有效性能．计算结果表明：当夹杂尺度在纳米量级时,应力和有效反平面剪切模量具有明显的尺度依赖性,并且随着夹杂尺寸的增加,趋近于不考虑界面效应时的结果;界面层厚度和性能对应力和有效反平面剪切模量明显变化时所对应的夹杂尺度范围和趋近于无界面效应结果的快慢有显著影响;当界面厚度足够薄时,界面层模型可用于模拟零厚度界面情况．     

砂土的应力路径本构模型

将微元应力路径线性逼近,转变成与其充分接近且易于计算应变的等平均应力微元和等应力比微元,计算任意加荷应力路径所产生的塑性应变,建立了双屈服面的砂土应力路径本构模型．模型体现了岩土塑性理论分量屈服和非关联流动法则的要求,在p,q平面内根据双线性的屈服线确定了加卸载准则．结合广义非线性强度理论采用变换应力三维化方法简单、合理地使模型实现三维化．通过试验数据的验证表明,砂土应力路径本构模型可以合理地描述各种应力路径下砂土的变形和强度特性。     

偶应力问题的杂交／混合元分析

将弹性力学中Hellinger—Reissner交分原理推广到偶应力理论中，并以罚函数的形式引入其约束条件，提出了一种有效的杂交／混合单元。文中分别分析了带中心小孔平板在轴向均匀加载时的应力集中情况，以及含中问裂纹的无限平板单轴拉伸时的位移场和应力场。算例表明，该单元计算效率高，精度好，即使在材料本征长度很小时，仍然能够得到相当理想的结果。