幂硬化剪切界面层效应的复合材料桥联分析

对具有幂硬化塑性剪切界面层效应的复合材料桥联进行断裂力学分析，得到了桥联增韧和裂纹张开位移的控制方程，并按照非线性Ｖｏｌｔｅｒｒａ型积分方程的迭代解给出其数值结果。并详细讨论界面相参数对桥联效应的影响。     

两端自由的叠层壳体的层间应力分布

1 前言纤维增强叠层结构的层间应力是结构强度的一个突出的问题,它常常导致结构分层破坏.近十几年以来,有关这类问题的研究方法大致可分四大类.1.近似解析解,它分别对每一层板、壳采用比较简单的平断面假设的剪切理论.2.数值解法,利用细观水平上的有限元方法求解层间应力.3.试验方法,借助于声发射及x 光拍照等手段对层间应力做定性的分析.4,三维弹性理论解法.其中文献利用最小势能原     

含层间短裂纹层合板的解析－分区变分解法

将层板横截面分为含裂纹区与不含裂纹区，在每一区内，根据夏变函数理论与特征函数展开法，得到了各自区内满足所有支配方程、裂纹表面边界条件与层间连续条件的位移与应力的特征展开式，然后利用分区广义变分原理满足裂纹表面边界以外的边界条件以及两区之间的交界条件，并由此求得奇异场控制量（广义应力强度因子）。     

复相陶瓷中颗粒桥联与相变协同增韧作用的力学分析

本文对复相陶瓷内延性金属颗粒和相变颗粒的协同增韧作用进行了力学分析，其中颗粒桥联效应还考虑了桥联颗粒与基体间工艺残余应力的影响，相变效应考虑了相变体膨胀和剪切塑性对增韧的共同贡献。数值计算结果与Ａｍａｚｉｇｏ－Ｂｕｄｉａｎｓｋｙ的结果及２Ｙ－ＴＺＰ／２０ＶＯＬ％Ｎｉ的实验结果作了比较。     

短纤维金属基复合材料应力传递的有限元分析

基于短纤维增强金属基复合材料的单纤维轴对称和三维细观力学模型,利用弹塑性有限元分析方法对该复合材料中基体与纤维间的应力传递进行研究,研究中主要讨论了基体、纤维和界面的力学性能以及纤维位向的变化对应力传递和应力分布的影响。研究表明,复合材料微结构参数的变化将显著影响基体与纤维间的应力传递和复合材料中的应力分布,复合材料设计过程中必须考虑合理的微结构特征。     

裂纹端部细短纤维的应力分析

基于裂纹端部存在与其裂纹面相垂直的二相细短纤维分析模型，采用叠加原理推导了求解纤维表面应力分布函数的积分方程，通过简化得到了该方程的解析表达显式，该积分方程的特征值方程是纤维几何参数，材料常数以及纤维相对于裂纹位置的相关函数，当材料参数不满足特征方程时，积分方程将具有唯一解；并借助数值方法，给出了纤维剪应力分布算例，和纤维对应力强度因子的影响。     

考虑界面损伤层合梁板的层间应力分析

基于精确应力分析的广义六自由度板理论,应用变分原理和损伤力学中 的应变等效原理,考虑复合材料铺设层内和层间界面处的损伤效应,建立了具两种损伤模式 的复合材料层合板的三维非线性平衡微分方程,且运用有限差分法对考虑损伤简支层合梁板 的层间应力进行了求解.     

短纤维复合材料应力传递的修正剪滞理论

对短纤维增强金属基复合材料应力传递的传统剪滞理论进行了修正和改进.修正理论中包含了一些在传统剪滞理论中没有很好考虑的因素,包括纤维端部的正应力传递和界面的结合状态.推导得到了描述短纤维复合材料应力传递机制的一些公式.通过与有限元分析得到的应力传递结果比较发现,修正理论对应力传递的描述准确性明显优于传统的剪滞理论.修正是合理而必须的.     

复合材料铺层拼接层间断裂韧性的试验研究

针对评价复合材料层合板层间断裂韧性的测量,提出了用拉伸试验法测定Ⅱ型层间断裂韧性,设计了内含铺层拼接区的分层破坏试验层合板,制备了拉伸试件.通过拉伸试验测得了拼接区开裂和分层裂纹稳态扩展过程中的载荷与变形规律:层间破坏具有Ⅱ型断裂特征,且裂纹扩展比较稳定.利用测试数据计算出断裂功,并以临界能量释放率表示层合板的Ⅱ型层间断裂韧性,结果表明用铺层拼接件拉伸法进行层间断裂韧性试验是可行的.     

复合材料桥纤维拔出的动态裂纹模型

在无限大正交各向异性体弹性平面上对复合材料桥纤维平行自由表面的内部中央裂纹提出了桥纤维拔出的动态裂纹模型。通过复变函数将其转化为Reimann-Hilbert混合边界值问题。求得了裂纹在坐标原点受载荷Px/t、Px2/t作用的解析解。利用这一解析解可通过迭加原理求得任意复杂问题的解。     

DYNAMIC CRACK MODELS ON PROBLEM OF BRIDGING FIBER PULL-OUT OF COMPOSITE MATERIALS

An elastic analysis of an internal central crack with bridging fibers parallel to the free surface in an infinite orthotropic anisotropic elastic plane was performed. A dynamic model of bridging fiber pull-out of composite materials was presented. Resultingly the fiber failure is governed by maximum tensile stress, the fiber breaks and hence the crack extension should occur in self-similar fashion. By the methods of complex functions, the problem studied can be transformed into the dynamic model to the Reimann-Hilbert mixed boundary value problem, and a straightforward and easy analytical solution is presented. Analytical study on the crack propagation subjected to a ladder load and an instantaneous pulse loading is obtained respectively for orthotropic anisotropic body. By utilizing the solution, the concrete solutions of this model are attained by ways of superposition.     

A dynamic model of bridging fiber pull-out of composite materials

An elastic analysis of an internal central crack with bridging fibers parallel to the free surface in an infinite orthotropic anisotropic elastic plane is carried out. In this paper a dynamic model of bridging fiber pull-out is presented for analyzing the distributions stress and displacement of composite materials with the internal central crack under the loading conditions of an applied non-uniform stress and the traction forces on crack faces yielded by the fiber pull-out model. Thus the fiber failure is determined by maximum tensile stress, the fiber breaks and hence the crack propagation should occur in self-similar fashion. By reducing the dynamic model to the Keldysh–Sedov mixed boundary value problem, a straightforward and easy analytical solution can be attained. When the crack extends, its fibers continue to break. Analytical study on the crack extension under the action of an inhomogeneous point force Px/t, Pt is obtained for orthotropic anisotropic body, respectively; and it can be utilized to attain the concrete solutions of the model by the ways of superposition.     

Analysis on small scale bridging in fibrous monolithic ceramics

Based on shear lag model of interface between fiber and matrix, a new formula that relates the crack opening displacement and bridging force in fibrous monolithic ceramics was constructed under the framework of small scale bridging. This formula was applied to predict the fracture resistance orR-curve response of a three-point bending specimen made of fibrous monolithic ceramics. A parametric investigation on the influences of fiber volume fraction, fiber radius, characteristics of constituents, BN's fracture toughness and specimen's geometry on the bridging forces and fracture resistance in Si₃N₄/BN composite was carried out. The upper and lower limits of theR-curve of Si₃N₄/BN in small scale bridging were derived. This research revealed the role played by the above parameters in the fracture toughness of fibrous monolithic ceramics. Supported by National Natural Science Foundation of China (59632090).     

On the toughening of brittle materials by grain bridging: Promoting intergranular fracture through grain angle, strength, and toughness

The structural reliability of many brittle materials such as structural ceramics relies on the occurrence of intergranular, as opposed to transgranular, fracture in order to induce toughening by grain bridging. For a constant grain boundary strength and grain boundary toughness, the current work examines the role of grain strength, grain toughness, and grain angle in promoting intergranular fracture in order to maintain such toughening. Previous studies have illustrated that an intergranular path and the consequent grain bridging process can be partitioned into five distinct regimes, namely: propagate, kink, arrest, stall, and bridge. To determine the validity of the assumed intergranular path, the classical penetration/deflection problem of a crack impinging on an interface is re-examined within a cohesive zone framework for intergranular and transgranular fracture. Results considering both modes of propagation, i.e., a transgranular and intergranular path, reveal that crack-tip shielding is a natural outcome of the cohesive zone approach to fracture. Cohesive zone growth in one mode shields the opposing mode from the stresses required for cohesive zone initiation. Although stable propagation occurs when the required driving force is equivalent to the toughness for either transgranular or intergranular fracture, the mode of propagation depends on the normalized grain strength, normalized grain toughness, and grain angle. For each grain angle, the intersection of single path and multiple path solutions demarcates “strong” grains that increase the macroscopic toughness and “weak” grains that decrease it. The unstable transition to intergranular fracture reveals that an increasing grain toughness requires a growing region of the transgranular cohesive zone be near the cohesive strength. The inability of the body to provide the requisite stress field yields an overdriven and unstable configuration. The current results provide restrictions for the achievement of substantial toughening through intergranular fracture.     

碳纤维/环氧树脂仿贝壳珍珠层结构强韧机制与性能优化的实验研究

材料的轻量化设计在生产实践中具有重大意义,将天然贝壳珍珠层结构应用到现有的高性能人工合成材料上,能够获得性能更加优异的轻质高强结构材料。本文采用碳纤维/环氧树脂复合材料,设计出了多种具有规则砖-泥交错叠层结构的仿贝壳珍珠层复合材料,通过力学性能测试实验、微观结构表征及力学原理分析等对不同片层单元长度及不同单元搭接形式的材料在拉伸载荷下的力学行为进行了研究,探索了其微观结构对材料强度和韧性的影响机制。结果表明,砖-泥交错叠层结构中砖块单元长度是影响材料强度和韧性的关键因素,而在此基础上通过对片层搭接形式的优化设计,可进一步改善其内部的应力分布与载荷传递机制,从而实现其强度和韧性的进一步提升与有效调控。     

The distributions of the interlaminar stress in a laminated shell with free end boundary

This paper studies the free edge effect yielded by interlaminar stress in a laminated cylindrical shell made up of fiber reinforced layer [0°], [90°] and the isotropic material layer under axisymmetric thermal load or radial pressure. Both ends of the shell are in free boundary condition. The exact solution of the problem can be obtained by using the three-dimensional theory of elasticity. For illustration, the numerical laminar stresses in a double-layer laminated shell under thermal load or radial pressure are calculated.     

On the effect of nano-particle clustering on toughening of nano-composite ceramics

In this paper, two and three-dimensional clustering models are developed to characterize the effect of nano-particle clustering on toughening of nanocomposite ceramics. It is found that crack pinning toughens the nano-composite ceramics because a higher stress intensity factor is needed for crack to propagate around or to pull-out the nano-particle. The nano-particle along the grain boundary steers the crack into the matrix grain due to the strong cohesion between the nanoparticle and the matrix. Since the fracture resistance of the grain boundary is lower than that of the grain lattice, the higher the probability of transgranular fracture induced by nano-particles, the tougher is the nano-composite. However, both crack pinning and transgranular fracture are affected by nano-particle clustering. Nanoparticle clustering, which increases with increasing volume fraction of nano-particles, leads to reduction of both the strength and toughness of the nano-composite ceramics. The larger the size of the clustered particle, and the more defects it contains, the easier it is for the crack to pass through the clustered particle, which means that the nano-particle clustering can reduce toughening induced by crack pinning and transgranular fracture. The theoretical prediction, based on the combination of the three mechanisms of nano-particles, is in agreement with the experimental data. The project supported by the National Natural Science Foundation of China (19891180) and the Research Grants Council of the HKSAR, China(HKU7081/00E)     

一种预测层合板层间剪应力的新后处理方法

层合板是航空航天领域典型的承力构件,过大的层间应力是导致其分层失效的主要原因.准确的层间应力预测往往依赖于三维平衡方程后处理方法(TPM).然而,该方法需要计算面内应力的一阶导,使得基于C0型板理论构造的线性单元无法使用TPM计算横向剪应力.本文在三维平衡方程后处理方法的基础上,提出了一种新后处理方法(NPM).新后处理方法通过虚功等效法消除了三维平衡方程后处理方法中产生的位移参数的高阶导.基于提出的新后处理方法和C0型板理论,仅需使用线性单元就可以预测层合板的横向剪应力.为了验证所提方法的有效性,本文基于修正锯齿理论(RZT)和所提方法构造了一种C0连续的三节点三角形线性板单元.数值算例表明,所提方法和三维平衡方程后处理方法具有相同的计算精度,提出的板单元能够准确高效地预测层合板的横向剪应力.此外,所提方法便于结合现有的有限元商用软件使用,基于商用软件中板壳单元获得的节点位移,使用新后处理方法极易获得准确的层间剪应力.