币形裂弦纤维桥联效应力学分析

本文基于Ｃａｓｔｉｇｌｉａｎｏ’ｓ定理和界面剪滞模型，得到了含界面相效应的复合材料币形裂纹张开位移控制方程，并按照第二类Ｆｒｅｄｈｏｌｍ积分方程的迭代解法给出其数值结果，为便于分析界面相参数增韧效果等影响，寻求了该控制方程的近似解，对近似解进行了误差估计，在此基础上得到了界面剪切模量，裂纹长度，界面厚度，纤维半径，纤维体积分数以及材料性质等参数对币形裂纹桥联效应的影响。     

晶须增韧复合材料机理的研究

本文介绍了晶须增韧复合材料的机理 ,增韧机理主要包括 :裂纹桥联、裂纹偏转、拔出效应 ;讨论了界面性质、晶须性能和基质性质对机理的影响 ;并展望了今后的研究方向。     

晶须增韧复合材料机理的研究

本文介绍了晶须增韧复合材料的机理,增韧机理主要包括：裂纹桥联、裂纹偏转、拔出效应;讨论了界面性质、晶须性能和基质性质对机理的影响;并展望了今后的研究方向。     

PVA纤维/水泥界面的力学性能及对SHCC拉伸性能的影响

通过实验分析2种PVA纤维(纤维A和纤维B)与水泥基体的界面力学行为,探讨其对应变硬化水泥基复合材料(SHCC)拉伸性能的影响。由单丝拔出实验得出,纤维A和纤维B滑移开始时的界面剪切应力分别为3.79和1.82 MPa,且界面断裂能分别为1.16和10.18J/m2。纤维A在滑移阶段的强化效应优于纤维B,纤维A配制的SHCC延性更好。预制缺口的SHCC试样在裂纹扩展阶段,纤维A桥联作用的刚度较大,控制裂纹张开的能力较强。含纤维A和纤维B的试样在裂纹张开过程中的余能分别为154.47和407.24J/m2,最大桥联应力分别为4.56和4.12 MPa,满足水泥基复合材料裂纹稳态扩展准则。     

芳纶纤维增韧碳纤维泡沫金属夹芯梁压缩性能及界面性能

为研究芳纶短纤维对复合材料夹芯材料/结构的界面及性能的影响,对具有芳纶短纤维增韧界面的碳纤维-泡沫铝夹芯梁进行了试验和细观增韧机制研究.在夹芯梁制备过程中,在碳纤维-泡沫铝界面加入低密度芳纶短纤维薄膜,通过短纤维的桥联作用,提高夹芯梁的界面黏接性能.研究了芳纶纤维增韧对夹芯梁面内压缩性能和破坏模态的影响,采用非对称双悬臂梁(ADCB)试验测量了不同增韧参数条件下,碳纤维表板与泡沫铝芯体之间的临界能量释放率.试验结果显示:在相同增韧参数条件下,Kevlar纤维增韧夹芯梁的面内压缩性能和界面临界能量释放率均较好,而混杂长度Kevlar纤维的界面增韧效果最优.通过对试件断面的SEM观测,分析了芳纶纤维增韧的细观增韧机制.     

陶瓷基体中不连续细长相的增韧机制与方位角效应

对不连续细长相在陶瓷基体中的增韧机制及其方位角效应进行了实验观察。在此基础上，建立了裂纹面桥联机制与细长相方位角之间关系的分析模型。结果表明：裂纹尖端后方裂纹面桥联作用主要受细长相架桥、细长相拔出和细长相根部基体破坏三种机制的支配；拔出机制只有当细长相轴向与裂纹面法线方向的夹角足够小时才可能发生；对于中等大小的夹角，架桥作用可以持续到细长相破断；当夹角足够大时，细长相根部基体发生破坏。     

铬粒弥散增韧氧化铝复合材料的研究

本文以金属铬粉弥散增韧氧化名陶瓷为研究对象，具体地探讨了增韧的结果与两相界面结合强度之间的关系，试验结果表明，约有２５％的铬颗粒与基体裂纹发生桥联作用，并且产生一定的颈缩塑性变形，从而吸怍能量达到增韧的目的，基体对颗粒的变形具有一定的约束作用。较高的界面结合强度使得铬颗粒在很小的塑性变形下发生脆断，而较低的界面结合强度将导致颗粒的拔出。     

晶须和相变复合增韧陶瓷的复合增韧模型

建立了晶须和相变复合增韧陶瓷的复合增韧模型,其中晶须增韧考虑了裂纹桥联和裂纹偏转两种机理;相变增韧在考虑体膨胀作用的基础上,用切应变影响因子来考虑切应变增韧效应．计算结果表明,相变增韧、桥联增韧和裂纹偏转增韧存在相干性,相变增韧降低晶须增韧效果,而晶须增韧促进相变增韧效果,SiC_w／ZrO₂（2mol％Y₂O₃）／Al₂O₃断裂韧性的计算结果与实验结果吻合．     

三维针刺C/SiC摩擦材料的拉伸性能

通过化学气相渗透法(CVI)结合反应熔体浸渗法(RMI)制备了低成本、高性能的三维针刺C/SiC摩擦材料,并对材料的组织结构、拉伸行为以及拉伸后的微结构进行了分析和研究。结果表明,材料由C、Si以及SiC等三种物相组成,密度约为2.1g/cm3,开气孔率约为4.4%;材料的拉伸强度约为114MPa～154MPa,弹性模量约为40GPa～63GPa;具有类似于金属的“塑性”,其增韧机理主要有纤维拔出、界面脱粘、裂纹偏转、纤维桥联以及裂纹分叉等。     

SiC_w/ZrO_2(6mol%Y_2O_3)陶瓷中晶须增韧的数值模型

SiCw／ZrO2（6mol％Y2O3）陶瓷的实验研究表明，晶须桥联和裂纹偏转是其主要增韧机制在两种机制协同增韧的基础上，建立了晶须增韧的数值模型，对材料的三点弯曲断裂过程的计算结果表明：载荷／位移曲线呈锯齿状，是由于晶须桥联作用使得裂纹扩展与停止这一过程反复出现而引起的；随晶须含量增加，复合材料韧性提高，晶须桥联和裂纹偏转两种增韧贡献都增加，但是占主导地位的增韧机制由裂纹偏转机制逐步过渡到裂纹桥联机制．计算结果与材料的测试结果很吻合．     

Bridge-toughening analysis of the penny crack in a fiber reinforced composite with interfacial effect

A fracture mechanics analysis of bridge effect on a fiber reinforced composite containing a penny crack is presented. The integral equation governing bridge-toughening as well as crack opening displacement (COD) for the composite with interfacial layer is derived from the Castingliano's theorem and interface shear-lag model. A numerical result of the COD equation is obtained using iteration solution of the Fredholm integral equation of the second kind. In order to investigate the effect of various parameters on the toughening, an approximate analytical solution of the equation is presented and its error analysis is performed, which demonstrated the approximation solution to be appropriate. A parametric study of the influence of the length, interfacial shear modulus, thickness of the interphase, fiber radius, fiber volume fraction and properties of materials on composite toughening is therefore carried out.     

An interface integral equation method applied to a crack impinging upon a bimaterial,frictional interface

A crack impinging upon a frictional, bimaterial interface is studied theoretically. Specifically we consider the problem of an infinitely long, cracked, two-dimensional fiber, which is embedded in an infinite plane with distinct elastic properties. The composite is subjected to tensile loading parallel to the fiber. An interface integral equation method is developed to solve this problem. This method, involving to-be-determined distributions of line forces, reduces the specific problem considered here to four coupled integral equations which are solved numerically. The bimaterial effect appears to be significant with respect to the length of the slip zone along the interface and the interfacial shear stress. However, the blunting of the crack by the frictional interface is virtually independent of the bimaterial effect.     

Effect of interphase on fibre-bridging toughness of a unidirectional FRP composite thin plate

A parametric analysis of the toughening mechanisms in a uniaxially fibre reinforced polymer (FRP) thin plate with a power-law hardening shear interphase is presented. An interfacial shear-lag model is used to analyse the relationship between the crack surface traction exerted by the intact fibres and the crack opening displacement (COD). Numerical solutions of the equations governing bridge-toughening are given. Two special kinds of interphase, i.e. linearly elastic and perfect plastic, are discussed. The results demonstrate that the toughening ratio of the composite thin plate is sensitive to several parameters, e.g. the thickness of the interphase between fibre and matrix, the hardening parameter of the interphase, the interfacial shear properties (stiffness and strength), the fibre radius and the far-field load. The results of this investigation will be beneficial to the selection of constitutive materials, the improvement of mechanical behaviour and the fabrication process of FRP composites.     

A cohesive crack model to study the fracture behaviour of fiber-reinforced brittle-matrix composites

A cohesive crack model analysis of the fracture resistance and ductility of fiber-reinforced brittle-matrix composites was performed. The bulk material is assumed to behave as a linear elastic solid. The constitutive equation for the cohesive crack is obtained through micromechanical considerations based on the shear lag model of fiber-matrix interaction and on the statistical nature of fiber failure. A parametrical study of the influence of fiber volume fraction, fiber strength and flaw distribution, interfacial shear stress and specimen geometry on the fracture resistance of these composites was carried out. The results illustrate the role of each of these factors in overall composite toughening, and suggest various strategies to improve the composites' performance.     

Feasibility study of a tungsten wire-reinforced tungsten matrix composite with ZrOx interfacial coatings

Brittleness problem imposes a severe restriction on the potential application of tungsten as high-temperature structural material. In this paper, a novel toughening method for tungsten is proposed based on reinforcement by tungsten wires. The underlying toughening mechanism is analogous to that of fiber-reinforced ceramic matrix composites. Strain energy is dissipated by debonding and frictional sliding at engineered fiber/matrix interfaces. To achieve maximum composite toughness fracture mechanical properties have to be optimized by interface coating. In this work, we evaluated six kinds of ZrO_x-based interface coatings. Interfacial parameters such as shear strength and fracture energy were determined by means of fiber push-out tests. The parameter values of the six coatings were comparable to each other and satisfied the criterion for crack deflection. Microscopic analysis showed that debonding occurred mostly between the W filament and the ZrO_x coating. Feasibility of interfacial crack deflection was also demonstrated by a three-point bending test.     

Mode II Macrocrack Initiation in Orthotropic Composite Viscoelastic Plate

Safe loads and initiation time for a straight macrocrack in viscoelastic orthotropic material that is intended to model a fiber composite plate under shear loads is investigated. The composite material is modeled by viscoelastic orthotropic medium. Determination of expression for crack shear displacement as function of time is based on the corresponding elastic solution and the method of operator continued fractions. Initiation time is obtained as a solution of integral equation for the incubation period. Numerical calculations are given for mode II macrocrack initiation.     

Modelling of inter-laminar toughening from chopped Kevlar fibers

Inter-laminar toughening from sparsely distributed chopped fibers is analyzed in this paper. A detailed micromechanical model describing the special toughening effects of those chopped Kevlar fibers lying within the delamination plane has been developed. To determine bridging forces of the Kevlar fibers lying in the matrix near a delamination-crack face, a simplified model based on coupling of matrix spalling (matrix fracture following by separation of fiber away from the matrix) and fiber pull-out is proposed. The model is then used to study toughening of carbon-fiber/epoxy composite laminates with chopped Kevlar fibers. The model shows that the energy dissipation in the pull-out process of chopped Kevlar fibers is much more than that in the matrix spalling process. The numerical results indicate that interfacial properties, interactions among Kevlar fibers and percentage of Kevlar fibers involved in crack bridging play important roles in delamination toughening. The modelling results are in good agreement with the experimental results available in the literature.     

The mechanics of matrix cracking in fiber reinforced ceramic composites containing a viscous interface

A detailed fracture mechanics analysis of matrix cracking in a fiber reinforced ceramic composite is presented for the case where the fiber—matrix interface exhibits viscous flow as can be the case when ceramic composites containing amorphous interfacial layers are subjected to loads at elevated temperatures. The analysis considers the case where matrix cracks are fully bridged by fibers, and the role of the viscous interface is to introduce a time dependence into the stress-intensity formulations. Such time-dependence arises because the bridging fibers are able to pull out of the matrix by viscous interfacial flow, with the result that the crack opening, as well as the actual (or shielded) matrix crack-tip stress-intensity factor, increase with time under the action of a constant externally applied load to the composite. The differential equation governing the mechanics of the fiber pull-out is derived. This is then applied to obtain expressions for the time-dependence of the crack opening and the effective crack-tip stress-intensity factor in terms of material and microstructural factors. These expressions predict that the matrix crack will exhibit stable crack growth, with the crack growth rate being essentially crack length (and time) independent and a function only of the applied stress and of material and microstructural factors. It is also shown that the composite lifetime is independent of the sizes of pre-existing cracks and is dependent only on a critical microstructure dependent flaw size, applied stress and microstructural factors.     

A micromechanical analysis of fiber crack propagation in composite materials due to transverse tensile loads

The fiber crack propagation in composites due to transverse tensile loads is studied using a micromechanical model and Linear Elastic Fracture Mechanics. To approach the problem, a three domain cylindrical model is introduced to simulate the fiber cracking. The model problem is then solved by the dislocation and singular integral equation techniques. The stress intensity factors of the fiber crack are calculated for various situations. It is found that fiber anisotropy has hardly any effect on the fiber crack propagation; “reverse composites” (composites in which fiber is less stiff than the matrix, such as Nicalon/SiC ceramic composite) virtually eliminate fiber crack propagation.