一种基于直接计算高阶奇异积分的断裂力学双边界积分方程分析法

相对于有限元法,边界单元法在求解断裂问题上有着独特的优势,现有的边界单元法中主要有子区域法和双边界积分方程法.采用一种改进的双边界积分方程法求解二维、三维断裂问题的应力强度因子,对非裂纹边界采用传统的位移边界积分方程,只需对裂纹面中的一面采用面力边界积分方程,并以裂纹间断位移为未知量直接用于计算应力强度因子.采用一种高阶奇异积分的直接法计算面力边界积分方程中的超强奇异积分;对于裂纹尖端单元,提供了三种不同形式的间断位移插值函数,采用两点公式计算应力强度因子.给出了多个具体的算例,与现存的精确解或参考解对比,可得到高精度的计算结果.      

Evolution of viscoelastic properties of polyurethane in the course of curing

Simultaneous measurements of the components of dynamic modulus at the set of frequencies were made by Fourier-Transform Mechanical Spectroscopy (FTMS) to investigate the curing process of a segmented polyurethane.The whole process of curing is treated as consisting of three stages. In the first stage, the material can flow and this stage is complete at the gel-point. In the second stage, the curing proceeds in the network and ends with phase separation of aggregating segments (blocks) of polymeric chains. In the third stage, the curing continues in the two-phase system. The object of investigation was the transition from the second to the third stage. It was shown that the position of the maximum of loss tangent depends on frequency and does not coincide with different characteristic points marking the time dependencies of G and G. This is due to the formation of micro-particles of the newly formed phase. The FTMS method is used for separation of the modulus changes into parts related to formation of chemical and physical networks.Special means of treating calorimetric measurements identified a weak maximum heat output at the moment corresponding to the transition point.     

Viscoplasticity of agglomerated suspensions

This work is a theoretical study on the effects of agglomeration on the fluidity and plasticity of a suspension of neutrally buoyant particles in a Newtonian fluid. The dynamics of a cluster of permanently attached spherical particles in a simple shear field is analyzed. The viscous and plastic components of the drag force acting on each of the agglomerated particles is then calculated and found to depend on the size of the individual particle unit, its location being relative to the center of the cluster and the material properties of the engulfing fluid. This information in conjunction with the knowledge of the interparticle cohesive forces is used to establish criteria for the agglomerate size reduction during dispersive mixing. From the kinematics of the cluster movement and the forces acting on its particulate components the rate of energy dissipation is calculated and utilized to evaluate the viscosity and yield stress of the suspension. These rheological parameters depend on the volume fraction and architecture of the agglomerate, the number of fused particles per cluster, and the viscosity of the suspending fluid. The analysis is also extended to include the case of polydispersity of agglomerate sizes.     

南京下蜀土结构强度研究

利用常规三轴试验,研究了南京下蜀土结构强度及其影响因素。结果表明,下蜀土存在明显的结构性。当应变小于屈服应变时,土体的结构强度迅速的发挥;当应变大于屈服应变时,土体结构发生破坏,结构强度逐渐降低;最终当各个结构单元重新排列稳定时,土体结构完全被破坏。讨论了围压和含水量对结构强度的影响,结果表明南京下蜀土土体的结构强度和围压无关,和含水量有良好的幂函数关系。结构强度与非饱和土中吸附强度有一定的关系,可以用结构强度代替吸力,从而简化非饱和土抗剪强度公式。
     

Correlation between interfacial interactions and internal stresses in filled high density polyethylene

The stress relaxation and the creep behaviour of high density polyethylene (HDPE) filled with glass fibres, clay (plate-like particles) or CaCO₃ (particles with irregular shape) were measured in uniaxial extension at room temperature. It was observed that the addition of filler increased the internal stress level, as evaluated from stress relaxation data. This increase was larger than the corresponding increase in the (short-term) elastic modulus. This behaviour may be attributed to a reduced macromolecular mobility in the matrix material close to the filler surface, i.e. to formation of an interphase region in the HDPE-matrix. From the internal stress values, the thickness of this interphase region around each filler particle was estimated, assuming a uniform coverage of the particles. It was suggested that the amount of matrix material with reduced mobility (or the thickness of the interphase region) reflected the degree of adhesion between the filler and HDPE. The change in the internal stress level due to the incorporation of different fillers, which were surface treated in some cases, was also consistent with the observed creep behaviour.     

Effet de l'anisotropie élastique cristalline sur la distribution des facteurs de Schmid à la surface des polycristauxInfluence of crystalline elasticity anisotropy on Schmid factor distribution at the free surface of polycrystals

Experimental studies of the plasticity mechanisms of polycrystals are usually based on the Schmid factor distribution supposing crystalline elasticity isotropy. A numerical evaluation of the effect of crystalline elasticity anisotropy on the apparent Schmid factor distribution at the free surface of polycrystals is presented. Cubic elasticity is considered. Order II stresses (averaged on all grains with the same crystallographic orientation) as well as variations between averages computed on grains with the same crystallographic orientation but with different neighbour grains are computed. The Finite Element Method is used. Commonly studied metals presenting an increasing anisotropy degree are considered (aluminium, nickel, austenite, copper). Concerning order II stresses in strongly anisotropic metals, the apparent Schmid factor distribution is drifted towards small Schmid factor values (the maximum Schmid factor is equal to 0.43 instead of 0.5) and the slip activation order between characteristic orientations of the crystallographic standard triangle is modified. The computed square deviations of the stresses averaged on grains with the same crystallographic orientation but with different neighbour grains are a bit higher than the second order ones (inter-orientation scatter). Our numerical evaluations agree quantitatively with several observations and measures of the literature concerning stress and strain distribution in copper and austenite polycrystals submitted to low amplitude loadings. Hopefully, the given apparent Schmid factor distributions could help to better understand the observations of the plasticity mechanisms taking place at the free surface of polycrystals. To cite this article: M. Sauzay, C. R. Mecanique 334 (2006).     

Bearing capacity of strip footings with horizontal confinementLa capacité portante des fondations superficielles en présence de parois rigides

For a strip footing under axial loading, the bearing capacity is influenced by the presence of rigid walls confining the foundation soil. This problem is investigated within the framework of the theory of yield design, considering both a perfectly rough and a frictionless contact condition at the interfaces with the walls in the case of a purely cohesive soil. Upper bounds for the correction factor to be applied to the classical value of the bearing capacity are determined, as functions of the non-dimensional geometric parameter of the problem, through the kinematic approach, implementing virtual velocity fields inspired from the solution to the problem of inverted extrusion. In the perfectly rough case, it appears that the new upper bound is a significant improvement of those already available. A very simple relationship is established, which derives the upper bound for the frictionless walls from the upper bound for the rough walls. A general conclusion of the analysis is that, for the values of the geometric parameter that are likely to be encountered in practice, the increase in the bearing capacity due to the presence of the rigid walls remains very small. To cite this article: J. Salençon, C. R. Mecanique 331 (2003).     

大数据条件下结构区间可靠性与概率可靠性比较

对于结构服役期间收集到的结构相关大数据，首先对数据进行整理与分析，根据数据的特征进行分类；然后利用应力-强度干涉理论建立结构大数据条件下区间可靠性计算模型与随机可靠性计算模型，按大数据所分类型，分别计算每一类型大数据中结构区间非概率可靠度指标与随机概率可靠度，在区间非概率可靠度指标与随机概率可靠度计算结果中，最小区间可靠性指标是结构最终的区间可靠性指标，最小随机可靠度是结构最终的随机可靠度．最后，利用两个算例说明了大数据条件下结构区间可靠性计算方法与概率可靠性计算方法的优点与缺点．     

Numerical solution for the flow of viscoelastic fluids around an inclined circular cylinder.

Finite difference solutions have been obtained by the perturbation method to investigate the influence of shear thinning and elasticity on the flow around an inclined circular cylinder of finite length in a uniform flow. In this numerical analysis a generalized upper-convected Maxwell model, in which the viscosity changes according to the Cross model, has been used.The local flow over the cylinder is only slightly deflected. However, in the wake flow behind the cylinder the particle path is remarkably influenced by the axial flow and rapidly flows up parallel to the cylinder's axis. Then it gradually rejoins direction of the incoming flow. It is found that viscoelastic fluids are prone to flow axially in the vicinity of the cylinder. The numerical predictions generally agree with the flow visualization results.The numerical solutions also demonstrate that elasticity has a strong effect on the velocity profile especially around both ends of the cylinder; elasticity increases the asymmetric profiles of both circumferential velocity and axial velocity with respect to equal to 90° and decreases a difference in the circumferential velocity between the windward end and the leeward end.For non-Newtonian fluids, the length of the wake flow is influenced by not only the Reynolds number but also the cylinder diameter and it is larger for the cylinder with the smaller diameter at the same Reynolds number.Partly presented at the 9th Australasian Fluid Mechanics Conference, University of Auckland, New Zealand, 8–12 December, 1986     

Interaction of a spherical shockwave and an elastic circular cylindrical shell

The paper studies the interaction of a spherical shock wave with an elastic circular cylindrical shell immersed in an infinite acoustic medium. The shell is assumed infinitely long. The wave source is quite close to the shell, causing deformation of just a small portion of the shell, which makes it possible to represent the solution by a double Fourier series. The method allows the exact determination of the hydrodynamic forces acting on the shell and analysis of its stress state. Some characteristic features of the stress state are described for different distances to the wave source. Formulas are proposed for establishing the safety conditions of the shell.Translated from Prikladnaya Mekhanika, Vol. 40, No. 9, pp. 94–104, September 2004.     

Nonlinear behavior of matrix-inclusion composites under high confining pressure: application to concrete and mortar

This paper is devoted to a micromechanics-based simulation of the response of concrete to hydrostatic and oedometric compressions. Concrete is described as a composite made up of a cement matrix in which rigid inclusions are embedded. The focus is put on the role of the interface between matrix and inclusion which represent the interfacial transition zone (ITZ). A plastic behavior is considered for both the matrix and the interfaces. The effective response of the composite is derived from the modified secant method adapted to the situation of imperfect interfaces. To cite this article: T.H. Le et al., C. R. Mecanique 336 (2008).     

汶川地震诱发文家沟巨型滑坡 碎屑流基本特征及成因机制初步分析


2008年5月12日,汶川M80地震在四川省绵竹市清平乡文家沟内诱发一巨型滑坡。通过现场调查得知,滑坡前后缘高差455m,厚度20～30m,滑面为基岩层面,初始方量2750×107m3。滑体在运动中转化为碎屑流。滑坡-碎屑流总的水平运动距离为4022m,垂直运动距离为1443m,遗留的堆积物体积达5×107m3。滑坡距映秀—北川断裂仅36km,位于其下盘,地震烈度达XI度。滑坡导致文家沟中48人遇害,并形成一条完整的地震次生地质灾害链。初步分析表明滑坡启动速度快,滑坡向碎屑流转化过程明显、地点明确。碎屑流运动过程复杂,伴有强烈的“气垫效应”和“前缘气浪冲击效应”。作者认为,文家沟滑坡的高启动速度是长持时强烈地震动作用的结果,与山体的猛烈碰撞是导致滑体解体并转化为碎屑流的原因。     

延安宝塔山景区滑坡地质灾害风险评估

机匣与叶片的相对转动是影响涡轮叶顶间隙流动的重要因素之一. 对LISA 1.5级轴流涡轮间隙内部流动的数值计算结果表明：叶片转动对涡轮间隙流动有阻塞作用. 叶片静止时,由于阻塞作用消失,导致间隙入口速度增大,间隙流量增加,并且通过间隙的 流体全部卷起形成间隙涡. 同时在叶片顶部吸力面侧前缘、中部各形成一个间隙涡,使得间 隙流动损失增加. 而且转速下降会加剧动叶出口截面气流过偏/偏转不足现象. 同时叶片静止 时,间隙前部各个弦长截面内静压自间隙入口开始一直呈增加趋势,直到叶片尾缘附近截面, 间隙截面内静压才逐渐稳定.     

Transient removal of a contaminated fluid from a cavity

A numerical and experimental study of the time-dependent hydrodynamic removal of a contaminated fluid from a cavity on the floor of a duct is presented. The duct flow has a parabolic inlet velocity profile and laminar flows are considered in a Reynolds number range between 50 and 1600 based on the duct height. The properties of the contaminated cavity fluid are assumed to be the same as for the fluid flowing in the duct. Attention is focussed on the convective transport of contaminated fluid out from the cavity and the effect of duct flow acceleration on the cleaning process. Passive markers which are convected with the flow are used in the numerical simulation for the purpose of identifying the contaminated cavity fluid. It is shown that the cleansing of the cavity is more pronounced during the unsteady start-up of the duct flow and the rate of cleaning decreases as the flow reaches a steady state. The cleaning process is enhanced as the cavity aspect ratio is increased and as the duct Reynolds number increases. A ‘volumetric’ approach based on the spread of markers is shown to be useful in determining the fraction of the cavity that remains contaminated after steady conditions have been reached. The distribution of the contaminant in a cavity during the unsteady stage and after steady conditions are reached are identified using passive markers.     

PIV MEASUREMENT OF CLOSE IMPINGING JET ON FLAT PLATE 1)

运用时间分辨粒子成像测速系统(time-resolved particle image velocimetry, TR-PIV)对近距离下射流冲击平板时的流场进行了直接测量, 通过对两个正交的平面流场开展测量, 揭示了冲击距离和雷诺数对射流间隙内三维流动特征及涡系结构演化规律的影响. 结果表明: 射流间隙存在三种典型的涡系结构, 分别为双涡环模式、单涡环模式和卷吸模式, 但在大流量湍流状态下, 射流可能会冲破涡环, 形成随机的高速出流, 各流动模式的出现主要与射流流态及壁面约束作用有关. 运用涡量分析对三种典型涡系结构的能量传递和损失特性进行了比较. 结果表明: 近距离冲击时, 射流的能量通过涡环模式向外传递. 在双涡环模式下, 两个涡环的旋向相反, 端面的约束作用使得两个涡环都被严格约束在射流棒端面之内, 且一次涡环强度显著大于二次涡环强度. 最后, 运用本征正交分解方法对射流间隙内的流动模态及其能量分布进行了分析. 单涡和双涡模式前十阶模态分析结果表明: 能量脉动在较低阶时即以配对的模式出现, 这表明一次涡环与二次涡环均具有良好的对称性, 同时在双涡模式中, 一次涡环是占主导作用的大尺度流动结构. 卷吸模式的前三阶模态分析表明: 射流的能量集中在射流上游, 能量随紊动扩散急剧衰减.     

Flow birefringence and rheological measurements on shear induced micellar structures

Aqueous solutions of cationic surfactant systems with strongly binding counterions show the striking phenomenon of shear induced phase transitions. At low shear rates or angular frequencies, the solutions exhibit Newtonian flow. At high rates of shear, however, the rheological properties change dramatically. Above a well defined threshold value of the velocity gradient, a supermolecular structure can be formed from micellar aggregates. This shear induced structure (SIS) behaves like a gel and exhibits strong flow birefringence. The formation of the shear induced structure is very complicated and depends on the specific conditions of the surfactant system. In this paper we discuss new results which have been obtained from rheological measurements and from flow birefringence data. We examine the stability of the shear induced state as a function of temperature, surfactant concentration and salt concentration and we analyse the effect of solubilisation of alcohols and hydrocarbons. The results are interpreted in terms of a kinetic model which accounts for the observed behavior.Dedicated to the 60. birthday of Prof. H. Harnisch, Hoechst AGPartly presented at the 2nd Conference of European Rheologists, Prague, June 17–20, 1986     

An anomalous phenomenon occurring in the flow of viscoelastic fluids out of ducts

When viscoelastic fluids flow out from horizontal ducts many cracks and protruding ridges are formed on the jet surface. As far as the authors know, this phenomenon has not yet been reported. The occurrence of the anomalous phenomenon is not affected by inlet flow condition or duct shape.The phenomenon may be divided into three regimes, namely, a stable state, a breakage state, and a multiplication state. In the breakage state one ridge divides into two parts after its growth, and in the multiplication state a new ridge is suddenly formed at a crack point. With increasing shear rate, the flow pattern of the jet changes from the stable state to the breakage state, and then to the multiplication state.Furthermore, a recoil effect is observed. In this effect many air bubbles rush into the duct from the exit when the flow is quickly stopped.     

An improved model of thin cylindrical piezoelectric layers between isotropic elastic media

Radially polarized piezoelectric fibers are now considered to be used in smart composites. The piezoelectric layers in these fibers are generally anisotropic in the transverse direction and therefore difficult to analyze when dynamic loads are involved. The present paper presents a theoretical study of the dynamic behavior of radially polarized cylindrical piezoelectric layers between isotropic elastic media. A new interphase model is developed to provide simple yet accurate evaluation of the dynamic response of such anisotropic elastic layers. Unlike the traditional interface-spring model, the current interphase model satisfies the equations of motion of the layers and can provide reliable prediction of the stress and displacement. A comparison between the developed model, the interface-spring model and the finite element analysis is conducted. The results clearly show the advantages of the current model over the traditional interface-spring model in simulating anisotropic layers. Numerical examples based on this interphase model for different interphase thicknesses, loading frequencies and material combinations are presented to evaluate the dynamic behavior of multilayered elastic media.     

巫山县望霞乡桐心村危岩体变形破坏机制分析

本文通过详细调查危岩体所处斜坡变形现象发现,斜坡下部有约1m厚煤层被开采,斜坡上有塌陷和岩体开裂两种变形现象,两种变形均在继续发展;且这两者均与煤层开采有较大关联。工程地质类比法认为采煤引起的沉陷-掀斜变形机制对地表变形起主导作用。采用UDEC数值模拟表明,煤层开采后,位移变形由下向上传递。地表沉陷量向临空面增加,水平位移量也向临空面增加。上部坡体变形受垂直裂隙控制,下部坡体变形受水平层面和煤层控制;整体形成沉陷-掀斜。计算变形曲线显示变形有两个阶段,突破式跳跃变形阶段和缓慢变形阶段;如果没有新的开采活动,随着时间的推移山体最终会自行稳定下来。临空面的孤立危岩体在斜坡系统平衡后仍有较大速率,其稳定性较差,建议对其进行清除。