轮胎帘线/橡胶复合材料弹性常数的测试研究

以复合材料的微观力学理论和宏观力学理论为基础,探讨了轮有限元分析用帘线／橡胶复合材料弹性常数的测试。对哈尔平－蔡方程式、高夫－汤哥拉方程式和赤垢以－平野方程式在简化为单层横向同性体帘线／橡胶复合材料弹性常数（纵向弹性模量E1、横向弹性模量E2、面内剪切模量G12、主泊松比μ12和次泊松比μ21）计算中的应用进行了分析探讨,提出用钢丝帘线模截面积变化率来表征钢丝帘线的泊松比μc,并推荐用同固结法测试μc。可用激光散斑法、云纹法（配合微机图像处理技术）和轨道剪切法测试不同的钢丝帘线／橡胶复合材料弹性常数,对μ12大于1的现象进行了讨论。     

复合材料压力容器基体开裂损伤的研究

复合材料压力容器损伤破坏的主要形式之一是基体开裂,本文利用损伤力学方法中的细观力学对复合材料压力容器进行分析,采用等效夹杂法分析基体中任一点的应力情况,当应力达到临界值时基体开始产生裂纹,描述了基体开裂损伤的演化过程,建立裂纹密度和压力容器弹性模量之间的关系式,表现基体裂纹对压力容器刚度的影响。用压力容器的弹性模量定义损伤变量,进而建立了弹性模量和载荷循环次数之间的关系式。     

一种预测单向玄武岩纤维复合材料横向弹性模量的方法

根据单向玄武岩纤维复合材料中纤维排列方式,考虑几何对称性,并引入应变协调假设,提出了一种矩形代表性单元。根据代表性单元内纤维和基体的分布推导出单向玄武岩纤维复合材料的横向弹性模量。与实验、其他理论的结果比较表明,该代表性单元方法可以较好地预测单向玄武岩纤维复合材料的横向弹性模量。     

交变载荷作用下纤维增强密封复合材料界面脱黏研究

界面脱黏是纤维增强密封复合材料界面破坏的主要表现形式,其可视为一种特殊的裂纹扩展。本文基于剪滞模型研究了纤维增强密封复合材料中纤维和基体界面在交变载荷作用下的裂纹扩展规律。在考虑疲劳加载引起的脱黏界面损伤和损伤分布不均匀性以及材料泊松比影响的基础上,建立了等效Paris公式,得到了疲劳裂纹扩展长度、扩展速率、界面上摩擦系数和加载次数之间的关系。     

不同泊松比复合材料的裂纹尖端应力场

具有平面负泊松比的碳纤维/环氧树脂非平衡层合复合材料具有很高的断裂韧性和缺口断裂强度.本文用复变函数-变分方法计算了负泊松比材料的裂纹尖端应力场,并与常规的平衡复合材料进行了比较,重点研究了主应力方向与纤维夹角的关系.结果表明,负泊松比材料独特的裂纹尖端应力场有利于提高缺口断裂强度.     

袋式除尘器用聚苯硫醚滤料的拉伸强度及失效演化

聚苯硫醚纤维滤料被广泛用于工业炉窑烟气袋式除尘,在复杂工况服役过程中需承受各种载荷作用,拉伸载荷最为常见。利用宏观结构模型对聚苯硫醚纤维/聚四氟乙烯基布滤料的弹性常数进行测试与计算,其横向、纵向弹性模量分别为35、80 MPa,横向、纵向切变模量分别为5.7、6.0 MPa,横向、纵向泊松比分别为0.31、0.27。利用条样法对滤料的无缝线、有缝线、剪切条样等三种结构的经、纬向拉伸性能进行测试,发现不同结构的滤料具有不同的拉伸行为表现及失效过程。在滤料实际服役过程中,应充分考虑不同结构的承载能力,使滤料使用最优化。     

复合材料层合板工程常数的光纤光栅测试与分析

复合材料工程常数的精确测试是合理有效开展复合材料结构力学分析与评估的基础。采用埋入式光纤光栅测试技术对复合材料结构应变特征进行测量,可有效获取复合材料的工程常数,如弹性模量、泊松比等。本文将光纤光栅与引伸计、应变片的单向应变测试结果进行了对比,得出不同测试方法下弹性模量的结果,验证了光纤光栅测试方法的可靠性和有效性。进一步通过在复合材料试件内部分别铺设横向及纵向的光纤光栅,对复合材料试件的泊松比进行了测试,并与应变片的测试结果进行了对比。试验结果表明,光纤光栅相对于应变片测试灵敏度更高,测得的泊松比数值更为稳定。     

不同泊松比复合材料的层板断裂韧性和裂纹尖端应力场

比较了两组具有不同平面泊松比的玻璃纤维／环氧树脂复合材料层板的断裂韧性和缺口断裂强度，发现低泊松比复合材料层板具有较高的断裂韧性和缺口断裂强度；用复变函数一变分方法计算了它们的裂纹尖端应力场；研究了主应力方向与纤维夹角的关系；结果表明：低泊松比材料独特的裂纹尖端应力场有利于提高缺口断裂强度。     

复合材料的测试

本文简介了复合材料(主要是纤维增强的)所遵循的连续介质,小变形弹性理论的应力应变关系,在理论上说明了要解析一个受力构件的力与变形关系时需要些什么弹性常数。 结合到工程上的实际需要,着重介绍了常用的复合材料——正交各向异性板各个弹性常数、拉伸强度值以及应力应变曲线的试验方法;並且简单介绍了丝材、基体的上述三种数据的试验方法。 这些力学参考(包括复合材料、纤维和基体的常数与强度值)和应力应变曲线是兵器设计人员、制造技术员和使用者所必须掌握的。要不然他们研制发展新产品、维护使用现有产品以及估用或检验老产品时,则无科学的依据。     

一种单向碳化硅纤维增强钛基复合材料疲劳载荷下基体裂纹数量和位置的预测方法

正本发明公开了一种单向碳化硅纤维增强钛基复合材料疲劳载荷下基体裂纹数量和位置的预测方法,首先对单向碳化硅纤维增强钛基复合材料的基体沿长度方向划分为M个基体单元,分别确定各个基体单元在无损伤基体应力状态下、第一基体应力状态下、第二基体应力状态下各个基体单元单个循环的损伤量;然后计算单向碳化硅纤维增强钛基复合材料疲劳载荷下基体裂纹对应各     

Mechanical Properties of Two Plain-Woven Chemical Vapor Infiltrated Silicon Carbide-Matrix Composites

The elastic and inelastic properties of a chemical vapor infiltrated (CVI) SiC matrix reinforced with either plain-woven carbon fibers (C/SiC) or SiC fibers (SiC/SiC) have been investigated. It has been investigated whether the mechanics of a plain weave can be described using the theory of a cross-ply laminate, because it enables a simple mechanics approach to the nonlinear mechanical behavior. The influences of interphase, fiber anisotropy, and porosity are included. The approach results in a reduction of the composite system to a fiber/matrix system with an interface. The tensile behavior is described by five damage stages. C/SiC can be modeled using one damage stage and a constant damage parameter. The tensile behavior of SiC/SiC undergoes four damage stages. Stiffness reduction due to transverse cracks in the transverse bundles is very different from cross-ply behavior. Compressive failure is initiated by interlaminar cracks between the fiber bundles. The crack path is dictated by the bundle waviness. For SiC/SiC, the compressive behavior is mostly linear to failure. C/SiC exhibits initial nonlinear behavior because of residual crack openings. Above the point where the cracks close, the compressive behavior is linear. Global compressive failure is characterized by a major crack oriented at a certain angle to the axial loading. In shear, the matrix cracks orientate in the principal tensile stress direction (i.e., 45° to the fiber direction) with very high crack densities before failure, but only SiC/SiC shows significant degradation in shear modulus. Hysteresis is observed during unloading/reloading sequences and increasing permanent strain.     

Elastic Response and Effect of Transverse Cracking in Woven Fabric Brittle Matrix Composites

This paper examines the linear elastic tensile and fracture behavior of biaxial plain weave SiC/SiC ceramic woven fabric composites. Iso-phase mode and random-phase mode have been adopted to simulate multilayer stacking and to predict the initial linear elastic constants. It has been found that both modes predict very close results. Porosities in the composite affect the stiffness significantly, while fiber undulation shows only minimal effect. The nonlinear stress-strain relation of the composite is due to transverse cracks, which initiate mainly from interyarn pores. In the second part of this paper, two methods, classical fracture mechanics and energy balance approach, have been used to examine the crack initiation and growth. A finite element method and a modified shear-lag method have been developed to evaluate the stress distribution in the yarn with transverse cracks. The composite stiffness reduction due to transverse cracking has been obtained by both the finite element and shear-lag methods. Strain energy release rates of the growth of transverse cracks have been studied by the crack-closure procedure, using finite element methods. Effects of the yarn size and crack position on the strain energy release rate have been quantified. It is concluded that thinner yarns lead to higher critical strains for transverse cracking.     

Effects of Matrix Cracks on the Thermal Diffusivity of a Fiber-Reinforced Ceramic Composite

Effects of matrix cracks and the attendant interface debonding and sliding on both the longitudinal and the transverse thermal diffusivities of a unidirectional Nicalon/MAS composite are investigated. The diffusivity measurements are made in situ during tensile testing using a phase-sensitive photothermal technique. The contribution to the longitudinal thermal resistance from each of the cracks is determined from the longitudinal diffusivity along with measurements of crack density. By combining the transverse measurements with the predictions of an effective medium model, the thermal conductance of the interface (characterized by a Biot number) is determined and found to decrease with increasing crack opening displacement, from an initial value of ∼1 to ∼0.3. This degradation is attributed to the deleterious effects of interface sliding on the thermal conductance. Corroborating evidence of degradation in the interface conductance is obtained from the inferred crack conductances coupled with a unit cell model for a fiber composite containing a periodic array of matrix cracks. Additional notable features of the material behavior include: ( i ) reductions of ∼20% in both the longitudinal and the transverse diffusivities at stresses near the ultimate strength, ( ii ) almost complete recovery of the longitudinal diffusivity following unloading, and ( iii ) essentially no change in the transverse diffusivity following unloading. The recovery of the longitudinal diffusivity is attributed to closure of the matrix cracks. By contrast, the degradation in the interface conductance is permanent, as manifest in the lack of recovery of the transverse diffusivity.     

Interfacial shear strength studies of nicalon fibers in epoxy matrix using single fiber composite test

Interfacial properties of Nicalon (SiC) fiber in epoxy matrices of varying stiffnesses were studied using the single fiber composite test, in conjunction with stress birefringence patterns. Extensive debonding was observed with hard epoxies, but transverse matrix cracks were found in the more flexible epoxies, with the interface remaining intact. Micromechanical modeling and Monte Carlo simulation of the single fiber composite fragmentation process provided a basis to compute the interfacial shear stress from the final fragmentation length distribution. The interfacial shear stress appeared to decrease moderately with increasing matrix ductility. The large diameter Nicalon fibers create transverse cracks in the single fiber composite specimens made with flexible epoxies. Consequently, there is a high possibility of premature failure of the specimen before fiber break saturation is reached. This poses some difficulty in interpreting the results for flexible epoxies. It was also found that the interfacial shear stress values from the single fiber composite tests were always considerably higher than the ultimate shear stress values obtained from bulk epoxy (without fiber) tension tests. This effect is similar to what was seen earlier for single fiber composite tests based on graphite fibers and similar epoxy blends, though the difference between the two values was not as great.     

Temperature Dependence of Tensile Strength for a Woven Boron-Nitride-Coated Hi-Nicalon™ SiC Fiber-Reinforced Silicon-Carbide-Matrix Composite

The temperature dependence of tensile fracture behavior and tensile strength of a two-dimensional woven BN-coated Hi-Nicalon™ SiC fiber-reinforced SiC matrix composite fabricated by polymer infiltration pyrolysis (PIP) were studied. A tensile test of the composite was conducted in air at temperatures of 298 (room temperature), 1200, 1400, and 1600 K. The composite showed a nonlinear behavior for all the test temperatures; however, a large decrease in tensile strength was observed above 1200 K. Young's modulus was estimated from the initial linear regime of the tensile stress–strain curves at room and elevated temperatures, and a decrease in Young's modulus became significant above 1200 K. The multiple transverse cracking that occurred was independent of temperature, and the transverse crack density was measured from fractographic observations of the tested specimens at room and elevated temperatures. The temperature dependence of the effective interfacial shear stress was estimated from the measurements of the transverse crack density. The temperature dependence of in situ fiber strength properties was determined from fracture mirror size on the fracture surfaces of fibers. The decrease in the tensile strength of the composite up to 1400 K was attributed to the degradation in the strength properties of in situ fibers, and to the damage behavior exception of the fiber properties for 1600 K.     

Cracking and Damage in a Notched Unidirectional Fiber-Reinforced Brittle Matrix Composite

Results of four-point bend tests on notched beams of a laminated unidirectional fiber-reinforced glass matrix composite are presented. The failure sequence has been established through in situ examination. The dominant damage mode is a mixed-mode, split crack that runs parallel to the predominant fiber directions. The crack interacts with and crosses over imperfectly aligned fibers. The resulting bridging tractions are sufficient to cause the critical strain energy release rate to increase substantially as the crack extends. Several other damage modes are also observed. These include mode I (tensile) matrix cracks bridged by fibers, mode II (shear) cracks, and compressive damage at the loading points.     

Matrix cracking of 2D SiC/SiC composite characterized by in situ SEM and nano-CT

Two-dimensional plain-woven silicon carbide (SiC) fiber-reinforced SiC matrix (2D SiC/SiC) composite was prepared by polymer infiltration-pyrolysis (PIP). Matrix cracking mechanisms of the composite were investigated by in situ SEM and nano-CT to grasp tensile damage evolution. Results showed that PIP-SiC matrix possessed low-fracture energy with non-homogeneous distribution, leading to simultaneous initiation of matrix cracking outside transverse fiber bundles and in unreinforced regions. Cracks then got deflected along weak fiber/matrix interface, which accelerated crack proliferation within the composite. With an increase in the stress, cracks subsequently deflected along plain-woven layers and converged to form longitudinal macrocracks. The composite was finally delaminated via sliding.     

Elastic Properties of Laminated Calcium Aluminosilicate/Silicon Carbide Composites Determined by Resonant Ultrasound Spectroscopy

The elastic properties of unidirectional and 0°/90° crossply Nicalon-SiC-fiber-reinforced calcium aluminosilicate (CAS/SiC) ceramic-matrix composites have been measured using a resonant ultrasound spectroscopy (RUS) technique. This approach has allowed the nondestructive determination of the complete set of independent second-order elastic stiffness constants of these ceramic composites. These stiffness data have been used to obtain the orientation dependence of Young's modulus and the shear modulus. The results are in reasonably good agreement with the limited experimental data obtained from mechanical testing. The RUS measurements reveal that the unidirectional CAS/SiC composite is well modeled by transverse isotropic symmetry, indicating relatively isotropic fiber spacing in the transverse plane. The analysis indicates that the overall elastic anisotropy is also small for unidirectional and 0°/90° laminated CAS ceramic-matrix composites, a result that can be attributed to the relatively low modulus ratio of the Nicalon SiC fiber to the CAS matrix and to the moderate fiber volume fraction.     

The effect of interlayers on the transverse stresses in fiber composites

Numerical results are given of calculations of the radial, transverse, and shear stresses in the matrix surrounding a cylindrical inclusion in plane strain perpendicular to the cylinder axis, this being taken as a model of a fiber composite under transverse loading. It is shown that the presence of an interlayer on the fiber at a thickness which is a small fraction of the fiber diameter can significantly affect the stress concentrations in the matrix. The interlayer-fiber ‘composite’ can be ‘matched’ to the matrix by suitable choice of interlayer elastic moduli. In particular, if the shear modulus of the interlayer is smaller than that of the matrix and its Poisson's ratio is very small, the stress concentrations in the matrix are considerably reduced and the composite should be less subject to failure by delamination.     

Crack-Wake Debonding and Toughness in Fiber- or Whisker-Reinforced Brittle-Matrix Composites

Solutions are obtained for the mechanics of debonding in the crack wake in fiber- or whisker-reinforced composites for the case where a finite shear traction exists at the fiber/matrix interface in the debonded zone. These solutions are then applied to derive expressions for the steady-state toughness increases obtained in bonded composites wherein the toughness contribution is provided by crack-wake fiber/matrix debonding and crack bridging. The solutions for an unbonded composite containing a frictional fiber/matrix interface can be obtained from the derived equations in the limit of the fiber/matrix interface toughness equal to zero. In this limit, the debond crack length reduces to the slip length and the expressions for the crack opening and the predicted toughness increase reduce to previously derived expressions for unbonded composites. The steady-state toughness is found to depend sensitively on the interface toughness, the fiber fracture strength, and the shear tractions in the debonded zone including other material parameters, such as fiber radius and volume fraction and the moduli of the constituent phases. It is shown that in order to obtain finite toughness increases, the fiber/matrix interface toughness must be less than a critical value dependent on the fiber fracture strength, fiber radius and volume fraction, and fiber and matrix moduli. The predictions of the model are applied to published experimental results from a detailed and complete study of toughness increases in a bonded whisker-reinforced composite.