对称角铺设矩形复合材料叠层板受低速冲击时的应力波传及解层破坏分析

采用碰撞力分段模型和一阶剪切理论分析了给定初始速度的铁球与四边简支的复合材料叠层板中心发生碰撞的动力学行为,包括碰撞力及接触时间的变化规律、叠层板的振动响应、应力波传、表面沉陷等。并根据忽略厚度的界面模型假设及简化的Tsai Wu张量理论对复合材料叠层板的解层破坏进行了计算和分析,并给出了破坏区大小与铁球初始速度的关系。研究表明:碰撞力与铁球的初始速度成正比;复合材料叠层板中应力波传沿固定方向的相速度在各层内相同,拉伸应力波传速度沿纤维总体占优的方向大于其垂直的方向,而剪切应力波传速度则相反。即使在较低的初始碰撞速度下, 复合材料叠层板的解层破坏也很明显,并且破坏区域随初始碰撞速度的增大而不断扩展,其形状也会发生改变。      

复合材料叠层板横向开裂的研究

在Talreja提出的研究复合材料横向开裂的损伤力学方法的基础上,文中引进了一种更合理的计算约束系数的方法,并将其用于研究复合材料叠层板的横向开裂,预估了其弹性模量的变化。经试验结果的验证,其理论的预估结果是正确的。     

基于应变模态的轨道板裂缝与脱空识别方法研究

基于应变模态对损伤敏感、轨道板应变数据易于测量的特点,提出基于应变模态的无砟轨道轨道板裂缝与脱空识别方法。研究了轨道板的应变模态理论和应变模态分析方法。用数值仿真方法分析了损伤轨道板的应变模态特征,表明一阶应变模态振型在轨道板损伤区域有明显的弯曲,可作为损伤识别参数。提出了损伤识别的应变模态曲率差指标及其计算方法,研究了其对轨道板裂缝与脱空的识别能力和识别方法。应变模态曲率差对轨道板的开裂和脱空有很强的表征能力,应变模态曲率差矩阵的F范数可表征轨道板的开裂程度。     

水下近场非接触爆炸作用下固支方板破口计算

为研究水下近场非接触爆炸载荷作用下固支方板的破口现象,根据能量守恒原理,假设冲击波能完全转化为结构的初始动能,进而转化为结构的塑性变形能。通过假设变形模式,建立塑性变形能与平板转角之间关系,进而求解平板转角。平板在中间出现破口后,裂纹向外扩展,呈现花瓣状,满足环向应变等于开裂应变条件时裂纹终止,破口达到最大。根据转角大小,可以求解破口尺寸。与通用有限元软件ABAQUS/EXPLICIT计算结果对比,两者吻合较好。工程上,可以利用本文建立的计算方法估算破口大小。     

正交叠层板横向开裂问题的研究

采用有限子层剪滞模型,并结合线弹性断裂力学,首先研究了正交叠层板在受到沿0o层方向的拉伸荷载作用下,90o层中产生横向初始开裂问题。然后采用坐标变换并结合线性叠加原理,研究了90o层发生横向基体多级开裂问题。求得了与实验结果较吻合的初始破坏应变和裂纹密度,证实了分析方法的正确性和可靠性。     

地应力对岩石爆破开裂及爆炸地震波的影响研究

深部岩体爆破开挖是高地应力和炸药爆炸产生的动应力共同作用的结果。采用SPH-FEM耦合数值模拟方法,研究了地应力场对岩石爆破开裂及开裂区外地震波能量的影响。结果表明:随着地应力水平的提高,岩石爆破破碎区的范围缩小、裂纹扩展速度降低,非静水地应力场中破碎区内裂纹主要沿最大主应力方向扩展,但地应力对爆破粉碎区的形成几乎没有影响;地应力作用下爆破开裂区形态改变影响了爆炸地震波的能量及传播特性,随着地应力的增大,更多的炸药爆炸能转化为地震波能量,产生的高频地震波随距离衰减更快,且小主应力方向上的爆炸地震波能量更大。研究成果可为深部岩体爆破优化设计提供理论参考。     

冰雹冲击复合材料层合板失效模式分析与数值模拟

针对复合材料层合板结构,建立了冰雹冲击复合材料层合板的有限元模型,在充分考虑冲击过程中冰雹的流体特性下,给出了冰雹和复合材料层合板的材料模型和损伤准则,利用显式有限元分析工具LS-DYNA研究了不同冰雹冲击速度下复合材料层合板的临界破坏速度和破坏形式。结果表明,文中给出的冰雹、复合材料层合板的材料模型和损伤准则能够合理地再现冰雹冲击复合材料板的过程;复合材料层合板(AS4/8552)在冰雹高速冲击下首先发生的是基体开裂,当冰雹速度到达125m/s时,层合板上表面纤维发生断裂,但在整个冰雹冲击过程中层合板没有发生压缩失效     

循环动荷载下泥化夹层累积变形特性研究

泥化夹层是诱发岩体工程失稳破坏的重要因素之一。为研究循环动荷载下泥化夹层的累积变形特性,深入分析主要影响因素,在黄河中游某大型水利枢纽工程勘探平硐采取试样,开展了不同工况条件下泥化夹层的动三轴试验研究。在考虑主要黏土矿物成分、黏粒含量、含水率、围压和频率等影响因素的基础上,根据试验成果,探求适合描述泥化夹层累积应变发展规律的理论模型,并基于动应力～应变关系研究了泥化夹层的动弹性模量特征。研究结果表明:(1)泥化夹层的累积应变发展规律具有破坏型特征,不符合稳定型累积应变特征,利用Monismith模型进行描述是合理的;(2)泥化夹层的动应力～应变曲线符合Hardin双曲线模型;(3)泥化夹层的累积应变随含水率、黏粒含量和频率的增大而增大,随围压的增大先减小后增大;(4)泥化夹层的动弹性模量随围压增大单调递增,随循环周次、含水率和频率的增大而单调递减;(5)主要黏土矿物成分为蒙伊混层时,泥化夹层的累积应变最大,动弹性模量随循环周次增加而衰减最快。     

冲击载荷下双预置裂纹三点弯曲梁动态断裂实验

采用数字激光焦散线实验系统,对双预置裂纹三点弯曲梁进行了动态冲击实验,分析了双预置裂纹对试件裂纹尖端扩展速率和动态应力强度因子值的影响。实验结果表明:1双预置裂纹三点弯曲梁在动态冲击实验中,B裂纹作为开裂裂纹,其起裂时间和扩展速率峰值受到冲击载荷加载点与其预置位置之间水平距离值的影响,距离越小,起裂越快,扩展速率峰值越大;2试件开裂后,裂纹的裂纹扩展速度和动态应力强度因子值随时间的变化曲线均具有快速上升然后波动下降的规律;3B裂纹起裂所需能量随着A、B裂纹间距a值减小而增大。     

斜入射下水中隐身夹芯复合材料附体结构声学设计

摘 要：将隐身夹芯复合材料代替附体结构钢质壳板,建立了斜入射条件下水中隐身夹芯复合材料结构的声学模型;从水声波动方程出发,推导了二维空间声波斜入射时的传递矩阵,及带空腔水附体结构的声反射系数和吸声系数表达式;对带空腔水附体结构模型进行了垂直入射声学试验,试验结果与传递矩阵法数值计算吻合较好;对试验模型进行了斜入射下声学性能数值计算,分析了入射角对反射系数和吸声系数的影响;考虑斜入射下夹芯层厚度、密度、损耗因子、及水层厚度等对隐身附体结构声学性能的影响,应用数值方法对水中隐身夹芯复合材料附体结构进行了声学设计,分析了各层材料参数和几何参数对隐身结构反射系数和吸声系数的影响规律。     

Effect of micro-cracks on plastic zone ahead of the macro-crack tip

From the macroscopic point of view, the plastic zone (PZ) is obtained based on the distributed dislocation technique (DDT) and von Mises yield criterion. From the microscopic point of view, PZ is determined by the DDT model. The effect of micro-cracks on PZ of the macro-crack tip is analyzed. The results show that the micro-crack has a little amplification effect on PZ of the macro-crack tip when it locates in front of PZ. As the micro-crack is close to the macro-crack tip, PZ of the macro-crack tip and the micro-crack tip will join together. When the micro-crack enters into PZ of the macro-crack tip, it has an obvious shielding effect on PZ. When the micro-crack is behind the macro-crack tip, the width of PZ decreases while the height increases. The dislocation distribution in PZ is in the form of inverse pileup. The amplification and shielding regions are divided into five strip-shaped regions, and they appear alternately. The results can provide useful information to predict plastic behaviors near crack tip. The analysis of amplification and shielding effect is important to materials design.     

Interaction of antiplane cracks with elastic waves in transversely isotropic materials

Summary The interaction of plane time-harmonic SH-waves with micro-cracks in transversely isotropic materials is investigated. Elastic wave scattering by a single micro-crack is first analyzed. The scattered displacement is expressed as a Fourier integral containing the crack opening displacement. By using this representation formula and by invoking the traction-free boundary condition on the faces of the crack, a boundary integral equation for the unknown crack opening displacement is obtained. Expanding the crack opening displacement into a series of Chebyshev polynomials and adopting a Galerkin method, the boundary integral equation is converted into an infinite system of inear algebraic equations for the expansion coefficients which is solved numerically. Numerical results are presented for the elastodynamic stress intensity factors, the scattered far-field and the scattering cross section of a single crack. Then, propagation of plane time-harmonic SH-waves in a transversely isotropicmaterial permeated by a random and dilute distribution of micro-cracks is investigated. The effects of the micro-crack density on the attenuation coefficient and the phase velocity are analyzed by appealing to a simple energy consideration and by using Kramers-Kronig relations.     

2.5D-C/SiC复合材料的拉伸损伤研究

通过2.5D-C/SiC陶瓷基复合材料的面内拉伸试验, 研究了材料在拉伸载荷作用下的力学性能和损伤演化过程, 建立了2.5D-C/SiC复合材料的应力型和应变型拉伸损伤演化模型. 结果表明, 材料沿纵向和横向的拉伸应力－应变曲线相似, 损伤过程基本相同. 对应于拉伸应力应变曲线的三个特征切线模量, 面内拉伸的损伤演化过程可以分为三个阶段: 初始损伤阶段、损伤加速阶段和损伤减缓阶段. 由应力型损伤演化模型可以推导出三个损伤阶段的两个特征应力, 其中第一特征应力可以作为工程比例极限的参考值.     

冲击荷载作用下混凝土动态本构模型的研究

基于混凝土冲击荷载作用下的实验研究,以修正Ottosen四参数破坏准则为屈服法则,引入损伤,构造了一个动态本构模型用于描述混凝土材料的冲击特性。宏观上,假设混凝土材料是一个均匀连续体;而从细观角度来看,混凝土材料内部存在大量随机分布的微裂纹损伤。假设微裂纹均匀分布,且符合理想微裂纹体系统条件,定义含裂纹材料中单位体积内微裂纹所占的比例来表征微裂纹损伤所引起的混凝土材料宏观力学性能的劣化,并给出了损伤的演化方程。通过模型计算模拟结果与实验结果比较发现,模拟曲线与实验曲线拟合良好,因而可以用该模型模拟混凝土材料在冲击荷载下的动态特性。     

Piezoelectric Behavior of an Inhomogeneous Hollow Cylinder with Thermal Gradient

An analytical solution to the axisymmetric problem of a radially polarized, radially orthotropic piezoelectric hollow cylinder with a thermal gradient and subjected to various boundary conditions is developed. The elastic coefficients, piezoelectric coefficients, stress-temperature moduli, dielectric coefficient, pyroelectric coefficients, thermal conductivity coefficient, and thermal expansion coefficients of the hollow cylinder are assumed to be graded in the radial direction according to a simple power-law distribution. The governing second-order differential equations are derived from the equilibrium equation, the charge equation of electrostatics, and steady state heat transfer equation through the radial direction of the inhomogeneous hollow cylinder. The displacement, stresses, and potential field distributions in the cylinder are examined. The influence of the inhomogeneity parameter on the numerical results is investigated.     

Micro-mechanics models for an imperfect interface under anti-plane shear load: Hypersingular integral formulations

Two micro-mechanics models are proposed for an imperfect (weak) interface between two anisotropic elastic half-spaces under anti-plane shear load. The imperfect interface in the first model contains an array of periodically distributed micro-cracks with random lengths and positions. The second model, a simplified version of the first, is a three-phase model in which a period length of the interface contains a single representative micro-crack, perfectly bonded parts and an effective region. Both models are formulated in terms of hypersingular integral equations which may be solved by boundary element procedures to calculate the effective interface stiffness and the critical failure load of the interface.     

Stress and deformation analysis of fibrous composite materials by point matching

The point matching numerical method and its generalization, the method of boundary point least squares, have been successfully applied to numerous boundary value and eigenvalue problems. The present paper demonstrates the application of these techniques to problems in the micromechanics of fibrous composite materials, i.e. determination of elastic moduli and stress concentrations for parallel-fibre materials which are loaded transversely with respect to the fibres. The solution technique utilizes exact solutions of the governing equations of plane elasticity for each component fibre and its surrounding matrix material in a typical repeating section of the composite material. The continuity conditions for stresses and displacements between fibre and matrix and the repeatability conditions at the boundary of the repeating section are satisfied approximately in a pointwise manner. Some special numerical techniques which were found to be particularly useful in applying the point matching method to these problems are delineated. The method is demonstrated for composite materials having circular, elliptical and square fibres in regular, staggered arrays. Numerical results are given which show the accuracy of the method as well as stress concentration and composite elastic moduli data.     

具有球形胞体结构的泡沫塑料弹性常数的确定

通过微分法导出了泡沫塑料剪切模量和体积模量所满足的微分方程组，并利用泡沫塑料各向同性弹性常数间满足的关系求解；得到了泡沫塑料剪切模量与体积模量的关系，确定了剪切模量与材料孔隙比的关系；并且将本文结果同其他已有模型了对比。     

Elastic behaviour and failure mechanism in epoxy syntactic foams: The effect of glass microballoon volume fractions

A representative elementary volume (REV) in epoxy syntactic foams was generated to incorporate randomly distributed glass microballoons that followed a log-normal size distribution. Finite element modelling of the REV foam was developed and experimentally validated to investigate the elastic behaviour and failure mechanism in the foams with different microballoon volume fractions (V). The localised stresses concentrate in various zones within the foam, and can cause the vertical splitting fracture of microballoons and the micro-crack formation in the matrix. Dependent on the microballoon volume fraction, micro-cracks can propagate to join adjacent micro-cracks and voids left by fractured microballoons, and finally develop into a macro-crack either in the preferred longitudinal (for low V) or diagonal (for high V) directions. This is consistent with the macroscopic observations of the fracture process in the foam specimens. It was also found that elastic characteristics of the foam vary with microballoon volume fractions.     

Level-set method for design of multi-phase elastic and thermoelastic materials

The problem of designing composite materials with desired mechanical properties is to specify the materials microstructures in terms of the topology and distribution of their constituent material phases within a unit cell of periodic microstructures. In this paper we present an approach based on a multi-phase level-set model for the geometric and material representation and for numerical solution of a least squares optimization problem. The level-set model precisely specifies the material regions and their sharp boundaries in contrast to a raster discretization of the conventional homogenization-based approaches. Combined with the classical shape derivatives, the level-set method yields a computational system of partial differential equations. In using the Eulerian computation scheme with a fixed rectilinear grid and a fixed mesh in the unit cell, the gradient descent solution of the optimization captures the interfacial boundaries naturally and performs topological changes accurately. The proposed method is illustrated with several 2D examples for the synthesis of heterogeneous microstructures of elastic and/or thermoelastic composites composed of two and three material phases.