颗粒缺陷相互作用下复合材料的细观损伤模型

含尖角的非椭球颗粒附近应力集中较大,诱导缺陷形成裂纹是材料损伤的重要来源.对于强界面颗粒,大刚度颗粒诱导裂纹向基体中扩展形成近似平面片状裂纹,认为诱导裂纹受颗粒应力附近应力场控制,基于有效自洽理论建立了材料细观损伤模型,得到了单向拉伸下的损伤演化,并分析了颗粒形状、尺寸、颗粒性能以及颗粒与初始缺陷相对位置等因素对材料损伤的影响.结果表明,非椭球颗粒更易诱发裂纹,同样外载应力下,损伤程度更大,含非椭球颗粒材料强度更低;含扁平型的颗粒材料裂纹损伤过程更加明显并且材料强度更大;提高颗粒刚度和含量能够增大材料强度.材料中存在尺寸过大或过小的初始裂纹时材料损伤过程不明显.     

基于球形颗粒几何排列的离散元试样高效生成方法

在球体离散元数值模拟中,颗粒的初始排列状态是影响计算效率和计算结果的重要环节。本文采用前进面几何构造算法,提出了一种基于网格搜索的球形颗粒随机排列高效算法。通过求解空间三边方程,满足了粒径设置的任意大小的颗粒依次置入前进面的外侧,并与构成前进面的三个颗粒相互接触。为获得高体积分数的颗粒簇,该算法允许颗粒改变其粒径大小。采用颗粒网格化方法可以简化前进面的搜索,并由此提高排列效率。通过计算平均配位数、体积分数和二阶结构张量的特征值,对不同粒径比下得到的立方体试样进行了分析,得到试样配位数及体积分数均随着粒径比的增大而增大,且得到的试样为各向同性。此外,空间网格的大小和初始颗粒的生成点对随机排列的效率均会产生显著的影响。最后,对非规则铁路道砟进行了精细构造及压碎模拟,发现DEM模拟得到的应力-应变曲线与试验结果基本吻合,验证了该算法得到的颗粒试样在模拟道砟裂纹起裂、扩展等过程的有效性。     

柔性基底上含过渡层纳米薄膜在双向拉伸下断裂损伤的实验研究

含过渡层的柔性基底薄膜的力学性能对现代电子元器件的广泛应用至关重要,对其力学性能的深入研究变得极为迫切。本文针对其在双向拉伸载荷下的断裂损伤进行了实验研究。对125μm聚酰亚胺上沉积的不同薄膜结构进行不同加载比下的双轴拉伸实验,通过光学显微镜观察裂纹演化过程与饱和裂纹形态。根据最小应变能密度因子理论对裂纹演化的开裂角度进行了理论分析,用有限元分析了加载比例和过渡层泊松比对结构各层双向应力比传递的影响,并在不同薄膜结构和加载比下,对裂纹演化应变进行了比较。研究结果表明,在双向拉伸载荷作用下,薄膜裂纹呈现出网状分布,裂纹演化角度与加载比、裂纹初始角度相关;一级裂纹萌生的临界应变随着加载比降低而降低,二级及以上裂纹萌生的临界应变与各级裂纹的饱和应变随加载比降低而升高;在等双轴拉伸下,不同薄膜结构的含过渡层柔性基底薄膜一级裂纹的临界应变基本一致,二级及以上裂纹的临界应变显现明显差异。     

基于细观力学分析的砂土弹塑性本构模型

利用细观力学分析中的自洽理论建立了在固结排水条件下的砂土弹塑性本构模型,在该模型中考虑了部分砂土颗粒因剪切作用在颗粒接触面滑移而引起的整体剪切模量的降低,给出了求解整体剪切模量的解析方法,并将模型预测与不同应力路径的试验结果作了比较,证明该模型是合理的.     

冲击作用下复合材料叠层板层间开裂演化模型

采用双线性特性破坏模型研究了复合材料叠层板层间开裂裂纹的演化, 通过引入弹性/剪切模量的损伤参数, 推导出损伤参数与应变之间的微分方程, 并得到裂纹耗散功率与损伤参数变化率之间的关系. 计算不同初始冲击速度下复合材料叠层板某界面上应变、应变率响应以及损伤参数的演化, 即可得到该界面发生层间开裂的情形及其对剪切模量的影响.通过检查界面各点处的损伤参数是否发生改变, 预测了冲击完成之后复合材料叠层板第1, 2层之间发生层间开裂区域的大小与位置; 该预测结果与实验数据及其他破坏准则计算结果基本相符. 计算结果表明, 在冲击过程中当界面上任意点处的剪应力超过剪切强度后, 该点附近的剪切模量开始发生衰减, 衰减大小随铁球初始冲击速度的增大而增大, 并从靠近冲击中心的位置逐渐向周围递减. 在四边简支边界条件下, 复合材料叠层板的层间开裂区域同样最先出现在界面中靠近冲击点的位置, 区域面积随初始冲击速度的增大不断扩大. 当初始冲击速度足够高时, 第1, 2层界面的两条对称轴上开始出现多个独立的开裂区域.      

水击驻波场中分散相颗粒的运动分析

根据水击在管段内形成的驻波场现象,分析了流体内分散相颗粒受到的驻波作用力;运用李雅普诺夫稳定判据研究了颗粒积聚与分离的机理;考虑到颗粒运动方程的严重刚性而很难进行数值求解,采用相空间和非对称分析方法获得了分散相颗粒的运动轨迹近似解,并进行了实验验证。结果表明:水击驻波场中分散相颗粒的受力方程中惯性项对颗粒初始运动速率的影响不可忽略;在水击驻波波节的±λ/4范围内,分散相颗粒经过一定的时间会发生积聚,其运动速度呈对称分布,最大速度出现在3λ/8位置处;随着分散相颗粒粒径和密度等物性参数以及水击驻波的频率和连续相初始速度的增大,颗粒达到平衡位置的时间呈减小趋势,且连续相的初始速度对颗粒到达波节时间的影响显著。     

磁流变弹性体若干物理量的数值分析

应用有限元方法,考虑了颗粒的磁化饱和过程与非线性磁化过程,计算得到了磁流变弹性体中的磁场分布,进而研究了在不同磁场大小、不同颗粒体积比浓度下磁流变弹性体在成链方向的相对磁导率,计算结果和实验结果取得了一致。利用Maxwell应力张量,计算了磁场引起的磁流变弹性体的附加剪切模量。分析了颗粒体积比浓度、外加磁场对磁流变弹性体的磁致剪切模量的影响。研究了颗粒为旋转椭球形状时,颗粒的放置方式与其长短轴之比对磁流变弹性体的磁致剪切模量的影响。计算结果表明,磁流变弹性体的相对磁导率随颗粒体积比浓度的增大而增大,随磁场强度的增大而减小,颗粒的形状和放置方式对磁流变效应有很大的影响。     

纳米二氧化硅水溶胶成膜特性研究

研究了纳米SiO2水溶胶的固液二相流成膜特性．结果表明，水溶胶中的Si02固体颗粒的粒径及质量分数对粘度的影响较小，对成膜能力和承载能力的影响较大；在薄膜润滑阶段，固体颗粒的粒径和质量分数对润滑状态的稳定性具有一定的影响，随着速度的增加，液体膜厚度逐渐增大，固体颗粒对液体润滑膜稳定性及性能的影响逐渐减弱；在弹流润滑阶段，固体颗粒的影响很小。     

辽西风积土动剪切模量与阻尼比特性试验研究

研究辽西风积土的动力特性离不开其动剪切模量与阻尼比这两个最基本的参数. 通过一系列动三轴试验, 分析了围压、固结比、振动频率对原状风积土的动剪切模量与阻尼比的影响情况, 并探讨了其产生的机理. 通过试验数据分析表明, 风积土的动剪切模量随围压的增大而增大, 但阻尼比随围压的增大而减小; 动剪切模量随固结比的增大而增大, 阻尼比随固结比的增大而减小; 动剪切模量和阻尼比均随振动频率的增大而增大, 但影响较小, 不如前两个因素明显.     

磁场力及膜曲率对磁敏感薄膜-基底界面 黏附性能的影响与调控

界面黏附和脱黏的可调控在攀爬装置、黏附开关、机械抓手等方面具有重要的应用需求. 针对磁敏感薄膜-基底界面, 开展了薄膜初始曲率及外加磁场对界面黏附性能影响机制的研究. 首先实验制备了具有初始曲率的磁敏感薄膜, 分别开展了具有初始曲率的磁敏感薄膜-基底界面撕脱实验及理论研究, 研究了薄膜初始曲率、弯曲刚度和外加磁场强度对界面黏附性能的影响规律. 实验和理论结果一致表明: 具有初始曲率的磁敏感薄膜-基底界面黏附力随薄膜初始曲率的增大而减小, 而外加磁场能够有效提高界面黏附力;相比于初始零曲率薄膜-基底界面稳态撕脱力与薄膜弯曲刚度无关, 薄膜弯曲刚度减弱了具有初始曲率薄膜-基底界面的稳态撕脱力. 进一步从能量角度分析了界面等效黏附性能, 揭示了薄膜弯曲能、磁场势能、界面黏附能的相互竞争机制. 最后, 基于本文的实验及理论结果, 提出了一种磁场和薄膜初始曲率协同调控的简易机械抓手, 可连续实现物体的拾取、搬运和释放功能. 本文结果不仅有助于理解多场调控的界面可逆黏附机制, 对界面黏附可控的功能器件设计亦提供了一种新方法.      

Modelling of crack growth by fracture damage zone

The strain energy density (SED) criterion is applied for analyzing the full range of mixed mode fracture from tensile to shear loading. A fracture damage zone (FDZ) local to the crack tip is defined and discussed in connection with the influence of crack geometry, loading and local material property. The size of FDZ tends to change continuously from statically to cyclically applied load conditions. It can be estimated from the uniaxial mechanical properties of the material. Both experimental and analytical results are examined for subcritical crack growth under static loading that depends on the type steel structures the fracture behavior of which could be represented by a single curve for the given specimen geometry.     

Boundary film shear elastic modulus effect in hydrodynamic contact. Part II: influence of operational parameters

The present paper is the subsequent research of the first part (Theor Comput Fluid Dyn, 2009). It investigates the boundary film shear elastic modulus effect in a hydrodynamic contact in different operating conditions. The hydrodynamic contact is one-dimensional, composed of two parallel plane surfaces, which are respectively rough rigid with rectangular micro projections in profile periodically distributed on the surface and ideally smooth rigid. The whole contact consists of cavitated area and hydrodynamic area. The hydrodynamic area consists of many micro Raleigh bearings which are discontinuously and periodically distributed in the contact. The hydrodynamic contact in a micro Raleigh bearing consists of boundary film area and fluid film area which, respectively, occur in the outlet and inlet zones. In boundary film area, the film slips at the upper contact surface due to the limited shear stress capacity of the film–contact interface, while the film does not slip at the lower contact surface due to the shear stress capacity of the film–contact interface large enough. In boundary film area, the viscosity, density, and shear elastic modulus of the film are varied across the film thickness due to the film–contact interactions, and their effective values are used in modeling which depends on the film thickness. In fluid film area, the film does not slip at either of the contact surfaces, and the shear elastic modulus of the film is neglected. It is found from the simulation results that the boundary film shear elastic modulus influences are normally negligible on the mass flow through the contact, the carried load of the contact and the overall film thickness of the contact, and the boundary film shear elastic modulus would normally influence the local film thickness in an elastic contact when the local film thickness is on the film molecule diameter scale. It is also found that the boundary film shear elastic modulus effect has the tendency of being increased with the reduction of the width of a micro contact. It is increased with the reduction of the boundary film–contact interfacial shear strength or with the increase of the critical boundary film thickness, while it is strongest at certain values of the contact surface roughness, the width ratio of fluid film area to boundary film area, and the lubricant film shear elastic modulus.

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Periodic cracking of films supported on compliant substrates

When a tensile strain is applied to a film supported on a compliant substrate, a pattern of parallel cracks can channel through both the film and substrate. A linear-elastic fracture-mechanics model for the phenomenon is presented to extend earlier analyses in which cracking was limited to the film. It is shown how failure of the substrate reduces the critical strain required to initiate fracture of the film. This effect is more pronounced for relatively tough films. However, there is a critical ratio of the film to substrate toughness above which stable cracks do not form in response to an applied load. Instead, catastrophic failure of the substrate occurs simultaneously with the propagation of a single channel crack. This critical toughness ratio increases with the modulus mismatch between the film and the substrate, so that periodic crack patterns are more likely to be observed with relatively stiff films. With relatively low values of modulus mismatch, even a film that is more brittle than the substrate can cause catastrophic failure of the substrate. Below the critical toughness ratio, there is a regime in which stable crack arrays can be formed in the film and substrate. The depth of these arrays increases, while the spacing decreases, as the strain is increased. Eventually, the crack array can become deep enough to cause substrate failure.     

Nonlocal theory solution of two collinear cracks in the functionally graded materials

In this paper, the interaction of two collinear cracks in functionally graded materials subjected to a uniform anti-plane shear loading is investigated by means of nonlocal theory. The traditional concepts of the nonlocal theory are extended to solve the fracture problem of functionally graded materials. To make the analysis tractable, it is assumed that the shear modulus varies exponentially with the coordinate vertical to the crack. By use of the Fourier transform, the problem can be solved with the help of a pair of triple integral equations, in which the unknown variable is the displacement on the crack surfaces. To solve the triple integral equations, the displacement on the crack surfaces is expanded in a series of Jacobi polynomials. Unlike the classical elasticity solutions, it is found that no stress singularity is present near the crack tips. The nonlocal elastic solutions yield a finite hoop stress at the crack tip, thus allowing us to use the maximum stress as a fracture criterion in functionally graded materials. The magnitude of the finite stress field depends on the crack length, the distance between two cracks, the parameter describing the functionally graded materials and the lattice parameter of the materials.     

Modeling crack growth during Li insertion in storage particles using a fracture phase field approach

Fracture of storage particles is considered to be one of the major reasons for capacity fade and increasing power loss in many commercial lithium ion batteries. The appearance of fracture and cracks in the particles is commonly ascribed to mechanical stress, which evolves from inhomogeneous swelling and shrinkage of the material when lithium is inserted or extracted. Here, a coupled model of lithium diffusion, mechanical stress and crack growth using a phase field method is applied to investigate how the formation of cracks depends on the size of the particle and the presence or absence of an initial crack, as well as the applied flux at the boundary. The model shows great versatility in that it is free of constraints with respect to particle geometry, dimension or crack path and allows simultaneous observation of the evolution of lithium diffusion and crack growth. In this work, we focus on the insertion process. In particular, we demonstrate the presence of intricate fracture phenomena, such as, crack branching or complete breakage of storage particles within just a single half cycle of lithium insertion, a phenomenon that was only speculated about before.     

A Zener–Stroh crack interacting with a coated inclusion with generalized Irwin plastic zone correction

The elastic–plastic fracture behavior of a Zener–Stroh crack interacting with a coated inclusion in composite materials has been investigated with crack tip plastic zone corrections. With the distributed dislocation method, the crack problem is formulated into a set of singular integral equations which are solved numerically. The plastic zone sizes at the both crack tips are determined by a generalized Irwin model where Von Mises stress yielding criterion is used. The stress intensity factor (SIF), the plastic zone size (PZS), the crack tip opening displacement (CTOD) and the effective stress intensity factor have been evaluated. In the numerical examples, the influence of the inclusion shear modulus, the coating-layer thickness and shear modulus, as well as the distance between the crack and inclusion, on the SIF, the PZS and the CTOD are discussed in detail. Numerical examples show that increasing the shear modulus or the thickness of the coating phase, the influence of the inclusion on the normalized SIF and the normalized PZS will be shielded.     

功能梯度条共线Griffith裂纹反平面剪切冲击

研究正交各向异性功能梯度条中多个共线Griffith裂纹的反平面剪切冲击问题．材料两个方向的剪切模量假定按比例同时以特定的梯度变化．采用Laplace和Fourier变换及引进位错密度函数将问题化为求解Cauchy奇异积方程，进而化为代数方程数值求解．考查材料非均匀性、正交性和功能梯度条高度对裂尖动态断裂特性的影响．动应力强度因子的数值结果显示：增加剪切模量的梯度和(或)增加垂直于裂纹面方向的剪切模量，可以抑制动应力强度因子的幅度；若功能梯度条较薄，增大条形域的高度也可抑制裂纹扩展．     

EFFECT OF VERTICAL BAFFLES ON PARTICLE MIXING AND DRYING IN FLUIDIZED BEDS OF GROUP D PARTICLES

This study reports the effect of vertical baffles on the group D powder mixing and drying characteristics in a batch fluidized bed dryer. Results obtained in this study showed that operating the fluidized bed dryer with vertical baffles gave better particle mixing. This is due to the fact that the vertical baffles acted to limit the growth of small bubbles into large bubbles and the small bubbles caused more vigorous mixing in the bed of particles before finally erupting at the bed surface. Thus, insertion of vertical baffles is a useful way to process group D particles in a fluidized bed, especially when the fluidized bed is large.     

Unstable crack motion is predictable

Yoffe's linear theory of dynamic brittle fracture suggests that crack motion will be unstable beyond ∼70% of the Rayleigh speed, a prediction that is not supported by experiment. We show by atomistic simulations that hyperelasticity, the elasticity of large strains, plays a governing role in the instability dynamics of brittle fracture. A simple, yet remarkable, scaling model based on an effective elastic modulus (the secant modulus at the stability limit) gives successful predictions for the onset speed of the crack instability.     

岩石类材料裂纹分叉非规则性几何的分形效应

本文考察了岩石裂纹(从晶粒尺寸的微裂纹到几千米的断层构造)的分叉在几何上存在的非规则性,在文献[10,11]的基础上,应用分形(fractal)几何建立了裂纹分叉的分形模型,分析了裂纹分叉的分形效应,从几何的角度研究了裂纹分叉使断裂韧性提高的物理力学现象,导出了一个比smith公式更合理的断裂韧性计算式。