金属粉末冶金抗摩复合材料研制方面的成就

对摩擦部件材料的要求 现代机器制造业和运输业的发展是与摩擦部件的运转速度和载荷容量的提高联系在一起的。对抗摩材料的要求是由它的工作条件决定的,并在广阔的范围内变化着。但是,存在着一系列对所有抗摩材料都必须共同具备的要求,这些共同的要求就是:尽可能低的摩擦系数和高的耐磨性;最优化的整体和表面强度,这种强度使表面层的高强度与能够有效磨合相结合;足够高的韧性以尽可能排除脆性破坏;高的抗疲劳强度;在材料的工作表面上形成能防止咬粘的二次结构层的能力;相当高的导热性和最佳的热胀系数;     

谈力的功的定义

已刊登的几篇关于讨论力的功的定义的文章中,关于功的定义是准确的,严格的,完整的,并不需要作补充或修正.但如果把功的定义改为     

国际力学中心简介

在意大利风光明媚的威尼靳城北面约一百二个公里的地方,是一个约有十万居民的历史悠久的美丽的小城市乌迪内。国际力学中心就座落在它的市中心的原托尔索宫。在这座两层楼的相当富丽堂皇的建筑内,有两个小型的设备很好的报     

瑞利数对行波对流缺陷特性的影响

通过流体力学方程的数值模拟,研究了瑞利数对分离比ψ=-0.6的混合流体行波对流缺陷结构的影响。结果表明:对于给定的相对瑞利数r,缺陷的出现是间歇性的,缺陷出现的位置固定,缺陷出现的周期保持为常数;对于不同的r,缺陷形成时滚动的分裂是不对称的,可以在原滚动的上方也可以在原滚动的下方形成一个新的滚动,从而形成缺陷,缺陷出现的位置基本稳定在腔体的中部,缺陷出现周期随着r的增加而增加;在具有缺陷的行波存在的下限附近,缺陷出现周期减小得较快;在具有缺陷的行波存在的上限附近,缺陷出现周期增加得较快。有缺陷和无缺陷的行波的对流垂直流速最大值δwmax都随着时间在周期变化,但规律是不同的。具有缺陷的行波中,垂直流速最大值δwmax的周期代表缺陷出现的周期;在无缺陷的行波中,垂直流速最大值δwmax的周期代表行波的周期。     

含慢变参数的非线性振动系统的振动特性

慢变参数的振动系统一类典型的非线性系统，工程上的许多振动都于这一类，由于慢变系统的质量、刚度、干扰力等参数是随机时间缓慢的，另一般的参变系统有本质的不同，因此需根据咖以研究。本文讨论了慢变系统的分析方法。同时结合提升机罐笼与钢丝强组成的系统和慢变刚度的转子系统对自汉的和非自治的慢变系统的的夺动特性进行了研究，提出育夺区时的振幅的具体方法。     

变幅载荷下挤压强化增益的可靠性分析

1.引言航空结构设计的根本目的在于保证结构的可靠性,或者在保证可靠性的前提下使结构更优化.目前在我国的航空工业中,考虑概率因素为基础的结构设计问题已引起了广泛的兴趣.由于疲劳的统计性质,对于承受疲劳载荷的结构,失效概率的研究是不可缺少的.对于一个结构疲劳可靠性的估价取决于它的设计需要,如果一个结构需要安全地使用一段规定的期限而不检修,那么它必须在整个使用期间保持容许的低的失效概率.本文利用条件概率(即事件A 发生的条件下,事件B 发生的概率.)的概念,导出了一个变幅程序块谱作用下构件疲劳寿命的可靠性分析方法,完善了Miner 法则应用的局限性及不足,使得疲劳试验及分析的结果更具说服力.2.累积损伤计算的Miner 法则     

考虑应变硬化效应的简支梁后屈服性质

本文对于线性应变硬化刚塑性材料构成的矩形截面简支梁在中央集中载荷作用下的载椅——挠度关系进行了研究。文中不仅考虑了应变硬化效应使梁的极限弯矩增大的作用,而且还考虑了由于应变硬化而引起的塑性区的扩展所致的挠度改变对于梁的束载雄力的影响。研究表明:不仅仅材料的应变硬化特性对于梁在屈服后的承载能力有显著的影响,梁的几何特征长细比也对梁在屈服后的承载能力有显著的影响。     

两自由度的Burridge-Knopoff模型的一种近似分析方法

Burridge-Knopoff模型是速度驱动的多质量的质量弹簧系统.它是用来进行地震机理研究的一种摩擦动力系统的模型.本文考查了具有两个自由度的Burridge-Knopoff模型的摩擦激振问题.摩擦的不光滑性给系统的求解带来了很大的困难.又系统带有两个摩擦接触面,运动情况更加复杂.本文通过对摩擦力取平均的方法,得到了系统在各个运动形式下的通解,采用半解析的方法对此两自由度的系统进行了研究.通过以上方法,给出了一种周期运动的具体运动形式以及位移和速度的时程关系;并且发现在不同驱动速度的情况下,随着驱动速度的增大,系统的运动形式存在简单和复杂交替出现的现象,并给出了不同系统参数下的分岔图.     

迴轉仪的运动稳定性問題

回转仪的运动稳定性问题,具有实际的意义。例如,在飞行器的飞行自动控制中,就需要两个保持稳定运动的回转仪,它们的外环的轴,一个是垂直的,一个是水平的。 最近有许多工作用不同的方法讨论了这两种情形下回转仪运动的稳定条件。文献[2—4]研究了外环的轴为垂直寸的情形,文献[5]研究了外环的轴为水平的情形。作者     

应用于弹性问题的重心有限元法

摘要：通过几何的方法构造了在任意多边形上的具有重心型格式的平均值插值函数,并利用Galerkin法提出了应用于弹性问题的重心有限元法。重心有限元法的插值函数在多边形单元间是协调的,能够方便的施加本质边界条件。重心有限元法的插值函数对于不同边数的多边形单元具有统一的表达形式,编程实现简便易行,能够方便的应用于复杂几何区域的求解。通过重心有限元法分别进行了小片试验、悬臂梁和复合材料的有效模量的数值模拟。小片试验的计算精度达到了机器精度;悬臂梁的计算结果与解析解的吻合程度较高;复合材料的有效模量的数值模拟结果与传统有限元和解析解吻合得较好,变化趋势合理。     

On formulas for the Rayleigh wave velocity in pre-stressed compressible solids

In this paper, formulas for the velocity of Rayleigh waves in compressible isotropic solids subject to uniform initial deformations are derived using the theory of cubic equation. They are explicit, have simple algebraic forms, and hold for a general strain energy function. Unlike the previous investigations where the derived formulas for Rayleigh wave velocity are approximate and valid for only small enough values of pre-strains, this paper establishes exact formulas for Rayleigh wave velocity being valid for any range of pre-strains. When the prestresses are absent, the obtained formulas recover the Rayleigh wave velocity formula for compressible elastic solids. Since obtained formulas are explicit, exact and hold for any range of pre-strains, they are good tools for evaluating nondestructively prestresses of structures.     

Green's function for SH-waves in a cylindrically monoclinic material

Green's function for SH-waves in a cylindrically monoclinic material is considered for impulsive and time-harmonic sources. Closed form expressions for the Green's function are derived for a few limited values of anisotropic parameters. A very interesting time development of the wave front shape is illustrated and the wave front singularity is discussed for the transient SH-wave. Contours of the displacement amplitude for the time-harmonic wave are also shown.     

Hypoplasticity theory for granular materials—II: Three-dimensional axially symmetric exact solutions

In part I of this paper, we consider the governing equations of hypoplasticity theory for two-dimensional steady quasi-static plane strain compressible gravity flow and determine some exact analytical solutions applying for certain special cases. Similarly, for the three-dimensional situation considered here in part II, we undertake a similar mathematical investigation to determine some simple solutions of the governing equations for three-dimensional steady quasi-static axially symmetric compressible gravity flow for hypoplastic granular materials. We again find that for certain special cases, we are able to determine some exact solutions for the stress and velocity profiles. We comment that hypoplasticity theory generally gives rise to complicated systems of coupled non-linear differential equations, for which the determination of any analytical solutions is not a trivial matter, and that the solutions determined here might be exploited as benchmarks for full numerical schemes.     

Renormalization Group Model of Large-Scale Turbulence in Porous Media

Renormalization group methods are used to develop a macroscopic (large-scale) turbulence model for incompressible flow in porous media. The model accounts for the large-distance and large-time behavior of velocity correlations generated by the momentum equation for a randomly stirred, incompressible flow. Utilizing the renormalization procedure, the transport equations for the large-scale modes and expressions for effective transport coefficients are obtained. Expressions for renormalized turbulent viscosity, which accounts for the ultraviolet subrange of the turbulent kinetic energy spectrum, are also obtained.     

On unsteady unidirectional flows of a second grade fluid

Some properties of unsteady unidirectional flows of a fluid of second grade are considered for flows produced by the sudden application of a constant pressure gradient or by the impulsive motion of one or two boundaries. Exact analytical solutions for these flows are obtained and the results are compared with those of a Newtonian fluid. It is found that the stress at the initial time on the stationary boundary for flows generated by the impulsive motion of a boundary is infinite for a Newtonian fluid and is finite for a second grade fluid. Furthermore, it is shown that initially the stress on the stationary boundary, for flows started from rest by sudden application of a constant pressure gradient is zero for a Newtonian fluid and is not zero for a fluid of second grade. The required time to attain the asymptotic value of a second grade fluid is longer than that for a Newtonian fluid. It should be mentioned that the expressions for the flow properties, such as velocity, obtained by the Laplace transform method are exactly the same as the ones obtained for the Couette and Poiseuille flows and those which are constructed by the Fourier method. The solution of the governing equation for flows such as the flow over a plane wall and the Couette flow is in a series form which is slowly convergent for small values of time. To overcome the difficulty in the calculation of the value of the velocity for small values of time, a practical method is given. The other property of unsteady flows of a second grade fluid is that the no-slip boundary condition is sufficient for unsteady flows, but it is not sufficient for steady flows so that an additional condition is needed. In order to discuss the properties of unsteady unidirectional flows of a second grade fluid, some illustrative examples are given.     

FRP筋粘结式锚具的界面径向弹性模量分析

建立了FRP(Fibre Reinforced Polymer)筋粘结式锚具粘结界面的Rib-scale模型和Bar-scale模型,然后利用Fourier-Bessel级数推导了FRP筋、混凝土以及钢套筒等在径向应力作用下的解析解.解析解与数值解吻合较好,验证了用Fourier-Bessel级数表达的解析解的有效性....     

Boundary element simulations of spheres settling in circular,square and triangular conduits

Numerical simulations of a spherical particle sedimenting in circular, triangular and square conduits containing a viscous, inertialess, Newtonian fluid were investigated using the Boundary Element Method (BEM). Settling velocities and pressure drops for spheres falling along the centre-lines of the conduits were computed for a definitive range of sphere sizes. The numerical simulations for the settling velocities showed good agreement with existing experimental data. The most accurate analytic solution for a sphere settling along the axis of a circular conduit produced results which were almost indistinguishable from the present BEM calculations. For a sphere falling along the centre-line of a square conduit, the BEM calculations for small spheres agreed well with analytic results. No analytic results for a sphere falling along the axis of a triangular conduit were available for comparison. Extrapolation of the BEM predictions for the pressure drops, to infinitely small spheres, showed remarkable agreement with analytic results. For the circular conduit, the sphere's settling velocity and angular velocity were computed as a function of drop position for small, medium and large spheres. Excellent agreement with a reflection solution was achieved for the small sphere. In addition, end effects were investigated for centre-line drops and compared where possible with available experimental data and analytic results.Los Alamos National Laboratory, Los Alamos, New Mexico, USA.     

The re-examination of the weakly nonlinear theory of hydrodynamic stability

The weakly nonlinear theory has been widely applied in the problem of hydrodynamicstability and also in other fields.However,although its application has been successful forsome problems,yet,for other problems,the results obtained are not satisfactory,especiallyfor problems like transition or the evolution of the vortex in the free shear flow,for whichthe goal of the theoretical investigation is not seeking for a steady state,but predicting anevolutional process.In this paper,we shall examine the reason for the unsuccessfulness andsuggest ways for its amendment.     

Development of co-current air–water flow in a vertical pipe

Measurements of the cross-sectional distribution of the gas fraction and bubble size distributions were conducted in a vertical pipe with an inner diameter of 51.2 mm and a length of about 3 m for air/water bubbly and slug flow regimes. The use of a wire-mesh sensor obtained a high resolution of the gas fraction data in space as well as in time. From this data, time averaged values for the two-dimensional gas fraction profiles were decomposed into a large number of bubble size classes. This allowed the extraction of the radial gas fraction profiles for a given range of bubble sizes as well as data for local bubble size distributions. The structure of the flow can be characterized by such data. The measurements were performed for up to 10 different inlet lengths and for about 100 combinations of gas and liquid volume flow rates. The data is very useful for the development and validation of meso-scale models to account for the forces acting on a bubble in a shear liquid flow and models for bubble coalescence and break-up. Such models are necessary for the validation of CFD codes for the simulation of bubbly flows.