平面应变断裂韧度测试的有效性研究

针对平面应变断裂韧度测试厚度存在的问题,研究了几种材料测断裂韧度所需的实际厚度,拟合了平面应力到平面应变过渡状态下σz应力的分布,分析了过渡状态下的裂端应力场.通过分析不同材料特性和应力状态对断裂韧度KIC的影响,结合金属材料断裂的细观机理,认为平面应变断裂韧度KIC反映的是材料正断的性能,仅代表材料发生正断的指标.结果表明:对于某些高韧性材料,即使满足平面应变条件厚度要求,断裂以剪断为主,很难测出KIC;而对于偏脆性材料,即使不满足厚度要求,只要发生正断,也可测出KIC.结合已有试验结果,为平面应变断裂韧度KIC测定的厚度选择,提出一种合理的预测方法.借助三片模型的研究,给出平面应变断裂韧度的正断指标.     

Subcritical Growth of an Internal Circular Crack in an Aging Viscoelastic Laminated Composite

Delayed fracture of a laminated composite under tensile loads applied at infinity is studied. The composite consists of alternating elastic and aging viscoelastic layers and contains an internal penny-shaped mode I macrocrack located in parallel to the layers. A modified Leonov–Panasyuk–Dugdale crack model and the critical crack-tip opening criterion constitute a fracture model. The subcritical crack growth equations are derived using the Volterra principle and the method of operator continued fractions. The laws governing delayed fracture are studied for a specific composite material     

Delayed Fracture of a Laminated Viscoelastic Plate with a Through Crack under a Time-Dependent Load

The delayed fracture of a viscoelastic plate with a rectilinear mode I crack under a time-dependent load is studied. The plate is made of a laminated composite with an isotropic elastic reinforcing component and a viscoelastic matrix. The composite is modeled by an orthotropic homogeneous linearly viscoelastic medium with some averaged characteristics. To determine the viscoelastic characteristics of the composite, the Volterra principle and the method of continued fractions are used. The study is made within the framework of the theory of delayed fracture for viscoelastic bodies.     

平面应变下紧凑拉伸试样的动态断裂韧性的实验研究

材料的动态断裂韧性是衡量材料在动载荷作用下抵杭裂纹扩展能力的重要指标,以往的材料动态断裂韧性测试多采用三点弯曲试样,而针对紧凑拉伸试样的动态断裂韧性研究很少.本文将紧凑拉伸试样(即CT试样)简化成等效弹簧质量模型,得到了CT试样动态应力强度因子的近似表达式.对Hopkinson压杆装置进行了改进,利用改进后的实验装置进...     

纤维加强复合材料的总体粘弹性响应

在某些纤维增强复合材料(FRC)中使用金属或高分子聚合物作为基体材料。在高温等情况下,这类材料具有明显的粘弹性特性。本文采用Riemann—Liouville形式的分数阶导数模型描述基体的粘弹性特性。通过渐近均匀化方法给出了预测FRC整体三维本构关系的解析表达式。给出了应用于基体具有Makris粘弹性关系的具体形式。以圆截面纤维正方形排列的情形为例,给出了等效模量随纤维体积比的变化曲线。结果说明,这类复合材料仍具有粘弹性特性,其整体粘弹性本构关系的弹性部分综合了纤维弹性和基体弹性的贡献,粘性部分来自基体粘性的贡献,复合材料具有和基体相同的粘性系数和分数阶。为分析微结构特征对整体特性的贡献,须求解两类局部问题。可以看出,在整体的等效模量中包含了局部变形的贡献,局部变形增加了复合材料的耦合刚度。     

考虑老化的混凝土粘弹性分数导数模型

混凝土是一种具有分形结构的材料。采用分数微积分模型来研究具有分形结构材料的老化规律目前尚未见到。本文的目的是采用含分数阶导数的类标准线性体来模拟考虑老化的混凝土的蠕变和松弛规律。给出了分数导数与Abel核之间的关系。讨论了类标准线性体的蠕变柔量和松弛模量及其在考虑老化的混凝土中的应用。与传统的混凝土流变模型相比较表明，类标准线性体可以更好地同时拟合混凝土在不同龄期的蠕变和松弛曲线。而且其形式简单、统一，在计算过程中需要调整的参数很少。可以预见，类标准线性体在混凝土的结构设计和计算中将有着广泛的应用前景。     

Static Model Describing the Inhomogeneous Deformation of Unidirectional Fibrous Composites under Plane Strain

A mathematical model is proposed to describe the static deformation of a unidirectional fiber-reinforced composite under plane-strain conditions in a plane perpendicular to the fiber direction. The model is based on the stochastic static equations for an elastic homogeneous two-component medium with nonzero body forces. Applying the method of conditional moments and double Fourier transform yields a nonlocal model in the form of integro-differential equations. Expanding the integral kernels into series in expansion coefficients yields differential equations whose order depends on the number of terms in the series. The zero-order approximation leads to the theory of effective moduli, the first-order approximation to the refined theory of effective moduli, and the second-order approximation to fourth-order equilibrium equations for mean displacements (mathematical expectations) and formulas for displacements, strains, and stresses averaged over the composite and its components. All the coefficients in theses formulas are expressed in terms of the elastic constants of the components and the geometric parameters of the structure. The model is valid for heavy stress gradients. In a particular case, the static theory of two-component elastic mixtures follows from the model__________Translated from Prikladnaya Mekhanika, Vol. 41, No. 2, pp. 41–51, February 2005.     

Supercritical Deformation and Fracture of Bodies with Concentrators under Plane Stress State Conditions

The paper deals with experimental studies of inhomogeneous strain fields with regions of supercritical behavior of the material in the case of extension of plane specimens of steel 20 with concentrators of different geometry by using the method of digital image correlation. The use of a video system permits obtaining experimental data about the distribution of the fields of longitudinal, transverse, and shear components and the strain intensity. The previously considered criteria for the deformation process transition to the supercritical stage for different types of the stress–strain state were used to distinguish the regions of supercritical behavior and to analyze the evolution of the strain and temperature fields in their stable development.     

纤维排列方式对复合材料总体粘弹性常数的影响

对于金属基或高分子聚合物基复合材料，在特定情况下会表现出明显的粘弹性特性。本文采用Riemann—Liouville形式的分数阶导数模型描述基体的粘性特性，通过渐进均匀化方法给出了预测纤维加强复合材料整体本构关系的解析表达式，给出应用于基体具有Makris粘弹性关系的具体形式。最后，考察了圆截面纤维正方形排列和对角排列时的总体粘弹性弹性常数随纤维比的变化曲线。结果表明，这类复合材料仍具有粘弹性特性，其整体粘弹性本构关系的弹性部分综合了纤维弹性和基体弹性的贡献，粘性部分来自基体粘性的贡献，复合材料具有和基体相同的粘性系数和分数阶。为分析微结构特征对整体特性的贡献，须求解两类局部问题。在相同纤维体积比情况下，正方形排列的总体弹性系数大于正方形对角排列，而粘性常数相反。     

复合材料拉伸断裂过程的Monte-Carlo模拟

本文假定材料的断裂韧性Г是一个按概率密度分布的随机参数,其概率密度服从Weibull分布。应用Monte—Carlo方法模拟了层合板在拉伸条件下的断裂过程。计算结果表明：理论预测值与实验结果吻合较好。并得出结论：用Monte—Carlo模型能够较好模拟层合板的拉伸断裂过程,对层合板中基体开裂的演化进行相关的预测是可行并且有效的。     

金属材料平面应变拉伸变形局部化有限元分析

本文对于涉及韧性金属大变形中颈缩与剪切带断裂一类高度非线性变形局部化问题进行了弹塑性有限元数值模拟。采用改进的Ｊ２形变理论微分形式公式与交叉三角形四边形单元有限元网格，详细研究了应变硬化指数及初始表面不均匀特性的平面应变拉伸颈缩和剪切带形成的综合影响，给出此类问题的断裂机制图。     

Thermoelastoplastic Stress–Strain State of Laminated Shells under Axisymmetric Loading Along Arbitrary Plane Paths

The paper sets forth a method of successive approximations along loading paths. The method is used to determine the thermoelastoplastic stress–strain state of laminated shells under axisymmetric complex loading. Deformation-type constitutive equations describing loading along arbitrary plane paths are employed. A numerical example is presented     

Fracture of a Semi-Bounded Composite Material with a Near-Surface Penny-Shaped Crack under Compression

International Applied Mechanics - The nonclassical problem of fracture mechanics for a near-surface crack is solved in the case of small distances between the free surface and the crack plane. The...     

On the Delamination of a Viscoelastic Composite Circular Plate

Delamination of a symmetric circular plate made of viscoelastic composite is studied. It is assumed that the plate has a penny-shaped crack whose edges have a minor axisymmetric imperfection. The lateral surface of the plate is clamped and is compressed by uniform radial normal forces through a rigid body. The studies are made using the exact geometrically nonlinear equations of the theory of viscoelasticity. The delamination criterion is assumed unrestrained growth of the initial imperfection. The Laplace transform and FEM are employed. In particular cases, the results are compared with those for elastic composites     

A Mixed Criterion of Delayed Creep Failure Under Plane Stress

The paper discusses delayed creep failure criteria and their experimental justification. These criteria allow transition from the strength characteristics under uniaxial stress to the strength characteristics under plane stress. The criterion is chosen in the form of a mixed invariant that relates two stress components responsible for brittle and ductile failure. The limit characteristics take the effect of the principal stresses into account. The criterion was tested for isotropic metallic materials subjected to internal pressure, internal pressure with tension, pure torsion, and tension with torsion     

钢纤维增强混凝土的断裂破坏研究

在钢纤维增强混凝土的断裂力学性能的实验研究中,利用数字图像相关技术对素水泥砂浆试件和钢纤维增强水泥砂浆试件进行裂纹尖端的位移场测量,求解不同纤维分布情况下的应力强度因子从而进行增强性能的分析比较.对于单纤维桥联增强的情况,实验结果与Carpinteri的模型计算结果进行了分析比较,证明这种图像和数据处理方法对裂纹尖端的纤维位增强特性分析是有效可行的.     

Thermal Stresses in Plane Strain of Porous Elastic Solids

This paper is concerned with the linear theory of thermoelastic materials with voids. We present a method to reduce the thermoelastic problem to an isothermal one with zero body loads and with certain known boundary data. The results are used to study the thermal stresses in a tube and the thermoelastic deformation of a cylinder subjected to a uniform temperature gradient.     

集中载荷作用下各向异性平面内的椭圆孔或裂纹问题

应用复变函数Cauchy积分的方法,对含有椭圆孔或裂纹的各向异性平面,系统地导出了当其在面内受任意集中载荷作用时的复应力函数解或裂纹应力强度因子解析解,即基本解;并通过基本解的迭加,得到了在椭圆孔周或裂纹表面作用一般外载时的解,其特例证实了上述解的正确性。     

Instability of Plastic Strain and Fracture. Strain Diagram for Inhomogeneous Media

A strain diagram for a locally inhomogeneous medium (a medium with a porous structure or a system with noncut particles) is constructed on the basis of the Cosserat nonlinear–pseudocontinuum model. A modified criterion of geometrical softening that allows one to establish the dependence of the moment of stability loss on the statistical characteristics of the medium is considered.