半正弦脉冲激励下斜支承系统易损件冲击特性

以考虑易损件的斜支承弹簧系统为研究对象,建立系统几何非线性动力学方程,利用龙格—库塔法对易损件冲击响应特性进行数值分析。以易损件加速度响应峰值与脉冲激励幅值之比为易损件在半正弦波脉冲激励作用下的响应指标,无量纲脉冲激励时间作为变量,构建易损件的冲击响应谱。通过数值分析讨论脉冲激励幅值、脉冲激励时间、系统支承角以及频率比等对易损件冲击响应的影响规律。研究表明,系统支撑角、频率比等对易损件冲击响应影响显著,增加频率比可使易损件加速度响应峰值明显降低,低频率比条件下增大质量比可抑制易损件加速度响应峰值。     

缓冲包装设计引起的易损件移动现象的力学原理

目的研究产品在物流运输搬运过程中易损件会发生转移的行为。方法提出轻型小机电产品包装冲击动力学模型,建立基于2个易损件的产品冲击动力学方程,得到易损件的峰值加速度与刚度比的关系图,并且采用Runge-Kutta法进行数值求解,获取2个易损件在半正弦激励脉冲激励下的冲击时域响应、冲击谱和破损边界曲线。结果产品在不同的刚度比下,各个部件出现损坏的先后顺序不同,谁先坏本质上取决于哪个部件的峰值加速度响应首先达到其脆值。结论研究揭示了易损件随着冲击激励和包装材料特性发生移动的现象和本质原因。     

激光驱动Cs_3Sb脉冲光电阴极的研究

以脉冲激光激发光电阴极产生高亮度电子束的研究正日益受到重视。论文报告了关于Cs_3Sb光电阴极在这方面应用的实验进展。Cs_3Sb光电阴极的制备与测试均在超高真空系统中进行,采用了制备室内通电加热进Cs的制备工艺。测试光源为氙离子脉冲激光器,脉宽50～200ns。实验测得峰值光电流密度达108A/cm~2,量子产额为2%至5.6%,发射度与亮度测试采用单孔小束斑测量与数值计算比较法进行,测得亮度值为1.85×10~9A/m~2rad~2。对Cs_3Sb光电阴极进行的脉冲发射寿命测试和再激活实验表明:光致Cs脱附是发射衰减的主要原因,通过补Cs可使阴极再激活。     

矩形脉冲激励下斜支承弹簧系统冲击特性的研究

以斜支承弹簧系统为研究对象,建立了矩形脉冲激励下系统非线性动力学方程,利用龙格-库塔法对系统冲击特性进行数值分析。以系统加速度响应峰值与脉冲激励幅值之比为反映系统在冲击作用下的响应指标,脉冲激励时间、系统支撑角作为变量,构建了系统的三维冲击谱。讨论了脉冲激励幅值以及系统阻尼等对冲击谱的影响规律。研究表明,脉冲激励幅值、系统支撑角、系统阻尼等对系统冲击响应峰值影响显著,增加系统阻尼可使系统加速度响应峰值明显降低。研究结论可为斜支撑弹簧减振系统的设计提供理论依据。     

矩形脉冲激励下斜支承系统易损件的破损评价

以考虑易损件的斜支承系统为研究对象,建立矩形脉冲激励下系统无量纲非线性冲击动力学方程。以脉冲激励幅值与易损件加速度响应峰值之比作为系统无量纲临界加速度,临界加速度与无量纲脉冲时间乘积为系统无量纲临界速度,引入系统支承角或频率比或质量比,构建斜支承系统易损件破损边界曲面。利用龙格-库塔数值分析方法求解方程,分析讨论频率比、质量比、支承角等因素对易损件破损边界的影响。研究表明,增大频率比、减小系统支承角、增大质量比等可增加易损件安全区域;随无量纲脉冲激励幅值增加,易损件破损区域减小。研究结论为斜支承系统的设计提供理论依据。     

单个冲击对不锈钢管道焊接头低周疲劳寿命的影响

完成了单个冲击对１Ｃｒ１８Ｎｉ９１不锈钢管道焊接头试样低周疲劳寿命影响的试验研究。单个试验最大瞬时峰值应变率达４８０ｓ＾－１。试样未经消除焊接残余应力。采用成组法试验（每组７个试样）,对称加载模式,总应变幅为０．００２２８。结果表明,冲击影响受到焊接残余应力和冲击塑性导入机制的耦合作用。焊接残余应力与冲击应力叠加将增加材料损伤,而冲击塑性导入将减缓疲劳损伤和降低疲劳寿命分散性。前者扼制后者。考虑疲     

类金刚石薄膜的激光损伤特性测试与分析

使用脉宽12ns，波长1064nm调Q Nd：YAG激光器对真空阴极电弧沉积（VCAD）法制备的类金刚石薄膜进行抗激光损伤测试，结果表明，VCAD法镀制的薄膜抗激光损伤阈值为0．6J／cm^2。通过对热冲击效应的数值计算，得到了光斑中心的温度场和薄膜表面的应力场分布。研究表明，热应力在类金刚石薄膜的破坏过程中起主导作用，脉冲电弧沉积的DIE薄膜的激光损伤主要源于应力破坏。     

混凝土圆柱体试件在低速冲击下动力效应的研究

为研究混凝土圆柱体试件在冲击荷载作用下的动力效应,利用落锤冲击试验机,对混凝土圆柱体试件在应变率100~101/s范围内进行轴向冲击试验,并采用混凝土连续面盖帽模型（CSCM）对试验过程进行数值模拟。试验中测量不同冲击速度及冲击边界下的锤头冲击力、试件轴向应变时程曲线,获取了试件破坏形态及破坏过程的高速影像,比较分析了不同冲击速度及边界条件下,试件应力、应变峰值,应变率及动力增强系数（DIF）的变化规律。结果表明：随冲击速度的增加,试件的应力、应变峰值,应变率及动力增强系数都呈增加的趋势,冲击力作用时间则减小。混凝土平均强度与冲击速度呈抛物线关系,应变率则与冲击速度呈线性关系。模拟结果表明,CSCM混凝土本构模型在低速冲击范围内,有很好的计算精度,模拟破坏形态与试验结果吻合良好。     

基于细观结构的阴极炭块钠膨胀应力数值分析及实验验证

在细观层面上将阴极炭块视为由骨料和粘结剂组成的二相复合材料,应用蒙特卡洛方法建立二维随机骨料模型,使用ANSYS有限元软件的热分析模块求解钠膨胀应力模型,对不同形状、级配、含量的骨料模型的钠膨胀应力进行了数值模拟。结果表明:阴极炭块细观结构的非均匀性造成钠膨胀应力的不均匀分布,特别是拉应力集中导致阴极炭块破损;级配0.003~0.015 m炭骨料钠膨胀量最小,级配0.003~0.006 m炭骨料钠膨胀量最大,骨料含量80%钠膨胀量最小,骨料含量60%钠膨胀量最大。该模型的数值模拟结果与实验结果基本相符,表明该模型描述的钠膨胀和应力分布是合理和有效的,可以作为研究阴极炭块钠膨胀应力性能的一种有效辅助手段。     

不同铝电解时间下阴极炭块的损伤特性研究

对不同铝电解时间下的阴极炭块试样进行了单轴压缩实验,建立了阴极炭块的损伤本构方程。实验和理论分析结果表明:与未铝电解的炭块相比,铝电解环境下受熔盐腐蚀的炭块内部存在更多和分布更不均匀的化学腐蚀损伤,从而导致阴极炭块弹性模量、峰值应力和残余变形能力的降低,铝电解时间越长,降低程度越大。采用真实破裂过程分析软件RFPA2D对阴极炭块的单轴压缩破坏实验进行了数值模拟,通过与理论、实验结果比较发现数值模拟可有效地用于研究和再现阴极炭块变形破坏的全过程及局部的变形破坏特征。     

Velocity of sound in solid hexagonal close-packed H2 and D2

Sound velocity measurements are reported on samples of hexagonal close-packed H₂ and D₂, believed to be single crystals. In the first part of the experiments, both the longitudinal and transverse velocities (v ₁ and v _t ) of crystals grown at a given melting pressureP _M were measured at 4.2 K. For each mode, the velocities plotted vs.P _M were found to lie between approximately parallel lines that represented the maximum and minimum observable velocities under the given experimental conditions set by the acoustic cell dimensions. From these extreme velocities v _1max, v _1min, v _tmax, and v _tmin an estimation of the elastic constants was made. Knowledge of these constants in turn permitted the calculation of the velocity surface, the bulk modulus, and the Debye temperature. In spite of the uncertainties in the analysis of the present data, a meaningful comparison could be made with results from other experiments. For both and D ₂ , the elastic constants are on the average 20% below those derived from neutron scattering data. The transverse velocities, both for H ₂ and D ₂ , are consistent with those of Bezuglyi and Minyafaev for polycrystalline samples, but the longitudinal velocities are about 10% smaller than those of these authors. The bulk modulus and the Debye temperature from our experiments are compared with those from equation-of-state and calorimetric experiments. In the second part of this research, we have investigated the temperature dependence of the sound velocities at constant volume for both H ₂ and D ₂ with low concentrationsX of molecules with rotational angular momentumJ=1. From the measurements in H ₂ , we are able to calculate the temperature change in the bulk modulus, which is then compared with that calculated from calorimetric data assuming the validity of the Grüneisen equation of state. Good agreement is obtained. In solid D ₂ , were were able to deduce the temperature dependence of the adiabatic and isothermal modulusc ₃₃ and make a rough estimation for that ofc ₁₃ . In the third part we present measurements of v ₁ and v _t at 42 K and at constant volume as a function ofX in solid H ₂ . It is found that as the orthopara conversion proceeds,v ₁ increases andv _t decreases. From these measurements the bulk modulus change withX is calculated. Comparison with the bulk modulus change calculated from static pressure measurements, assuming electric quadrupole-quadrupole interaction between the (J=1) molecules, shows very good agreement. The calculated dependence of the Debye temperature onX, as obtained from a combination of sound velocity and static measurements, is smaller than that reported from recent neutron scattering results. A short section is then devoted to absorption measurements in solid H ₂ , which gave only qualitative results. Finally, in Section 4, we present measurements of the sound velocity in liquid D ₂ at saturated vapor pressure as a function of temperature and for both liquid H ₂ and D ₂ along the melting curve.Work supported by grants from the National Science Foundation and from the Army Research Office, Durham.     

Accelerated Life Testing for a Class of Linear Hazard Rate Type Distributions

Accelerated life testing for distributions with hazard rate functions of the form r(t) = Ag(t) + Bh(t) are considered. Let V ₁, …, V _k be stress levels larger than V ₀—the stress level under normal conditions [V ₀ > 0]—and let a(v) be a nondecreasing function on (0, ∞). We discuss a generalization of the common accelerated models (the power rule model and the Arrhenius model) by assuming that the hazard rate under the stress level V, is of the form (a(V _t )) ^P (Ag(t) + Bh(t)). The maximum likelihood estimators of A, B and P for complete and censored samples are studied. The estimation procedure reduces to a solution of one equation with one unknown parameter. The estimation procedure under the assumption of aging is also described. The asymptotic variance-covariance matrix is given.     

Observations and simulations of the low velocity-to-hypervelocity impact crater transition for a range of penetrator densities into thick aluminum targets

Projectile/target impact crater systems involving soda-lime glass/1100 aluminum, ferritic stainless steel/1100 aluminum, and tungsten carbide/1100 aluminum (corresponding to projectile densities of 2.2, 7.89, and 17 Mg (m³) at impact velocities ranging from 0.56 to 3.99 km/s were examined by light metallography, SEM, and TEM. Plots of crater depth/crater diameter ratio (p/D _c) versus impact velocity exhibited anomalous humps with p/D _c ranging from 0.8 to 5.5 between 1 and 2 km/s, with hypervelocity threshold or steady-state values of p/D _c (>5 km/s) ranging from 0.4 to 1.0; with the p/D _c values increasing with increasing projectile density in each case. This hump-shaped regime, with exaggerated target penetration depths, appears to occur because projectiles remain relatively intact and unfragmented. The crater geometry begins to change when the projectile fragmentation onset velocity (>2 km/s) is exceeded and fragmentation increases with increasing impact velocity. Computer simulations and validation of these simulations were developed which fairly accurately represented residual crater shapes/geometries and correlated experimentally measured microhardness maps with simulated residual yield stress contour maps. Validated computer simulations allowed representative extrapolations of impact craters well beyond the laboratory where melting and solidification occurred at the crater wall, especially for hypervelocity impact (>5 km/s).     

Physical properties of pulsed current copper electrodeposits

Square-wave unipolar pulse electroplating and bipolar pulse reverse electroplating of copper using an acid copper electrolyte of the type used for printed circuit board production has been studied. The effects of pulse-plated coatings on cathode efficiency and on the coatings properties, such as surface roughness, microhardness, stress-strain characteristics and elongation, are reported. The results show that microhardness, tensile strength and elongation are directly affected by the pulse technique used.Nomenclature A Cross-sectional area (m²) - CE Relative cathode efficiency - i Current density (A dm^–2) - i _m,i _p Mean, peak current density (A dm^–2) - M _a Actual deposit mass (g) - M _t Theoretical mass (g) - R _a Average roughness - R _tm Mean roughness - t Coating thickness (m) - t _off Off-time (ms) - t _on On-time (ms) - duty cycle (%) - density (g m^–3)     

Progress on the boundary element method to study the disturbance fields of bodies moving in an unbounded medium

For studying flow problems involved with complex physics it is now common to use numerical field methods for solving Navier-Stokes or Euler equations. However, for a large class of fluid mechanics problems, which can be dealt with linearized potential equations, the boundary element method proves to be quite useful, especially for its easy application and relatively less computational effort compared to the field methods. The boundary element method has undergone some significant advancements in the last decade with respect to the study of steady and unsteady flow problems concerning wing aerodynamics in compressible medium, flow fields of propellers and rotors and acoustical disturbance propagation from moving bodies. In this paper a few recent contributions which evolved in the DLR as research projects and as doctoral and diploma thesis of the Technical University Braunschweig are concisely described.List of symbols a Sound velocity - b Span of a wing - c _p Coefficient of static pressure - c _dp Coefficient of profile drag - c ₁, c _d, c_m Coefficient of lift, drag and moment per unit span width - c _L, c_D, c_M Total lift, drag and moment-coefficients - c _T, c_P Thrust and power-coefficient of a propeller - d Distance - D Doublet strength - e Specific heat energy - E Total energy in a moving medium element - f Frequency - F Field point - g Gravitational acceleration - h Radial distance in cylinder coordinates - I ₁, I ₂ Inducing functions - i, j, k Unit vectors in cartesian coordinates - k Wave number [/a ] - l Local wing-chord - l ₀, l _v Length of singularity element at t _oand t _v - m Notation for Fourier-component - M, M ^* Mach number based on local and critical sound speed - n Number of rotation per second - n Unit normal vector to a surface     

Hot hardness of (Fe,Cr)3C and (Fe,Cr)7C3 carbides

Hot hardness was measured on the primary carbides, (Fe, Cr)₃C and (Fe, Cr)₇C₃, in unidirectionally solidified iron-carbon-chromium hypereutectic alloys with chromium more than 4.8 wt %. The hardness-temperature relation was represented by two Ito-Shishokin formulae,H _v =A(— BT), and thus was drawn by two lines on a semilogarithmic graph. The inflection temperature where the two lines intersected was found at 730 to 860 K for (Fe, Cr)₃C carbide containing 0 to 14 wt % Cr, increasing with an increase in the chromium concentration in the carbide, and at about 910 K for (Fe, Cr)₇C₃ carbide containing 36 to 76 wt % Cr. With increasing chromium concentration in each carbide, the hardness of the carbide increased and the thermal softening coefficients decreased. The effect of chromium on the hardness, the inflection temperature and the thermal softening coefficients was more pronounced for (Fe, Cr)₃C carbide than for (Fe, Cr)₇C₃ carbide. Each of the thermal softening coefficients,B ₁(T<T _t),B ₂(T>T _t), the inflection temperature,T _t, room-temperature hardness,H _v(T _RT), and the hardness atT _t,H _v(T _t), related linearly to the chromium concentration in the carbides, and hence the hot hardness of the carbides could be expressed as functions of temperature and chromium concentration in the carbides. The relationships betweenH _v(T _RT) andH _v(T _t) and between the thermal softening coefficient,B ₂, and the activation energy for creep,Q _c(kJ mol^–1), were represented by the following equations:H _v(T _t)0.7H _v(T _RT),B ₂=1.26/Q _c.     

Upper critical field of aD-wave superconductor stabilized by antiferromagnetic fluctuations

We have derived microscopic equations for the upper critical magnetic field of aD-wave superconductor stabilized by antiferromagnetic spin fluctuations. We present numerical results for the reduced fieldh _c2(t) as a function of reduced temperaturet when the magnetic field is along thez axis and also zero-order results when it is along thex ory axis. Two differentD-wave models are considered. The angular dependence ofH _c2 forT nearT _c is given as a function of polar angleϑ in thez−x orz−y plane.     

NMR characterization of injection-moulded alumina green compacts

Injection-moulded alumina green compacts containing polypropylene, wax, and stearic acid were studied for binder distribution by proton nuclear magnetic resonance (¹H NMR) imaging. The solid imaging technique of nuclear spin-spin relaxation time,T ₂, weighted imaging at 400 MHz was used. This imaging technique utilizes a multiple pulse sequence ofD ₀-(/2)_±x --D ₁₀-D ₇-() _x -D ₇-D ₁₀-AQ-D ₀ for echo detection and phase encoding. Two-and three-dimensional images were constructed from the intensities of these nuclear echo signals. Spatially resolved two-dimensional images obtained by the application of this technique indicated that the green compacts fabricated from the same nominal binder composition did not contain the same amounts of binder. This observation agrees well with our previous conclusion drawn from nuclear spin echo studies by Hahn's pulse sequence. A 64×64×64 three-dimensional imaging revealed that the inhomogeneity of binder distribution and internal imperfection do exist at certain locations of the samples. A binder-rich folding line was also detected in one of these green compacts.     

New equations for enthalpy of vaporization from the triple point to the critical point

Two new equations are proposed for the enthalpy of vaporization from the triple point to the critical point. One of these equations containing four parameters is exceptionally good for fitting the data. The other equation containing three parameters is quite adequate for fitting the data but it is exceptionally suited for interpolation when the data do not cover the entire range. These equations have been tested using the enthalpy of vaporization of water from the triple point to the critical point and are compared with other equations.Nomenclature T _c Critical temperature, K - T _t Triple point, K - T _x Any particular temperature, K - T _r Reduced temperature - P _r Reduced pressure - R Gas constant - P Vapor pressure - X (T _c–T)/T _c - Y (T _c–T)/T - X _x (T _c–T)/(T _c–T _x) - X _t (T _c–T)/(T _c–T _t) - H _vt Enthalpy of vaporization at the triple point, kJ · mol^–1 - H _vx Enthalpy of vaporization at any temperature x, kJ · mol^–1 - Z _v Compressibilty factor of the saturated vapor - Z ₁ Compressibilty factor of the saturated liquid Relative deviation = 100[H_v(obs)–H_v(cal)]/H_v(obsd) Standard deviation = { [H _v(obs)–H _v(cal)]²/(No. points — No. parameters)}^0.5