A CONTINUUM MODEL FOR DYNAMIC DAMAGE EVOLUTION OF ANISOTROPIC BRITTLE MATERIALS

This paper is concerned with the development of a computational model for the damage evolution of brittle materials under dynamic loading. Two models for dynamic damage evolution of brittle materials with or without microflaws in general anisotropic damage state are presented; the first one is based on power function of principal tensile stress and the second one is based on damage strain energy release rate. A second-order tensor based elastic–brittle damage model is formulated which is efficient computationally and consistent in its treatment of damage evolution. Measured Weibull strength distribution may be employed to account for flaw size distribution effects on the damage accumulation rate. Methods of computing the accumulated damage of a structural component and their implementation in a finite element program together with some numerical results are presented. Finally, a comparison has been made between the two damage models.     

A MICROMECHANICAL DAMAGE THEORY FOR BRITTLE MATERIALS WITH SMALL CRACKS

For brittle solids containing numerous small cracks, a micromechanical damage theory is presented which accounts for the interactions between different small cracks and the effect of the boundary of a finite solid, and includes growth of the pre-existing small cracks. The analysis is based on a superposition scheme and series expansions of the complex potentials. The small crack evolution process is simulated through the use of fracture mechanics incorporating appropriate failure criteria. The stress–strain relations are obtained from the micromechanics analysis. Typical examples are given to illustrate the potential capability of the proposed theory. These results show that the present method provides a direct and efficient approach to deal with brittle finite solids containing multiple small cracks. The stress–strain relation curves are evaluated for a rectangular plate containing small cracks.     

A MULTIAXIAL FATIGUE DAMAGE MODEL FOR ORTHOTROPIC MATERIALS UNDER PROPORTIONAL LOADING

A new multiaxial fatigue damage model for orthotropic materials is proposed based on the strain vector. Six material constants are included in the model. These material constants represent the dependence of fatigue resistance on material orientation, and they can be obtained by conducting strain-controlled uniaxial fatigue tests along the three principal orthotropic directions of an orthotropic material. The model can also be transformed in new coordinate systems to predict the fatigue lives of new material orientations. Biaxial low-cycle fatigue tests are conducted to verify the model. The prediction of the model agrees with the experimental results reasonably well.     

MESH INDEPENDENT CELL MODELS FOR CONTINUUM DAMAGE THEORY

Abstract— The conventional use of continuum ductile damage mechanics in finite element analyses identifies the "cell" in which damage occurs with the finite elements in which the distribution of stress and strain is modelled. Since the cell size is a fixed, metallurgically-defined, property of the material being analysed, this methodology forces a minimum size for the finite element mesh. Mesh refinement is thereby disallowed. This paper presents one way of avoiding the problem by developing a mesh-independent cell model which, with a fixed cell size, allows the finite element mesh to be refined to any degree within the cells. Procedures which average some state variables within the cells are introduced to prevent the localisation of damage after a certain critical stage is reached. The method has been tested in numerical simulations of (a) the deformation of a notched tensile bar, (b) a 35 mm compact tension specimen and (c) the first of the AEA spinning cylinder tests. There is a reasonable agreement between the results of the computer simulations and those of the experiments.     

复合材料内部损伤的光纤图像显示方法

研究了复合材料损伤的光纤图像显示方法，并研制了相应的测试系统。通过数字化处理检测网络输出的图像，实现复合材料结构损伤的定量检测和直观显示。冲击损伤检测实验证明，摄像－图像处理光纤损伤测试系统与通常的光电检测系统相比具有诸多优点。     

AN IMPLICIT PROJECTION ALGORITHM FOR SIMULTANEOUS FLOW OF PLASTICITY AND DAMAGE IN STANDARD GENERALIZED MATERIALS

Motivated by mechanical analysis of cancellous bone, a 3D constitutive law describing the simultaneous flow of rate-independent plasticity and damage is developed in the framework of thermodynamics of irreversible processes with internal variables. Following the hypothesis of standard generalized materials, a free energy and a dissipation potential are postulated and the associated flow rules derived with the tools of convex analysis. On the computational side, the classical implicit projection (or catching up) algorithm used in plasticity is extended to account for the additional flow of damage. Due to the existence of a dissipation potential, linearization of the incremental algorithm provides a symmetric tangent operator. Numerical resolutions of several boundary value problems and a biomechanical application are presented to illustrate the potential of the constitutive model and demonstrate the quadratic convergence of the algorithm.     

准脆性材料损伤斑图生长的数值模拟

准脆性材料损伤斑图的生长过程研究,对于认识重大工程动力学灾变孕育、发生的机制具有特别重要的基础性意义.尤其是岩石类动力学灾害如地震、岩爆、煤矿开采中的三突,其形成机制都町归结为岩石损伤演化诱致结构灾变的模式.损伤斑图的生长过程可以再现灾变的孕育、发生过程,具有非常深刻的物理力学背景.该文根据微损伤不可逆演化原理,利用格...     

模拟混凝土滞回行为的各向异性损伤模型

该文引入相互独立的拉损伤、压损伤演化规律,分别描述受拉开裂和受压破碎导致的混凝土损伤行为,然后采用非线性卸载．线性重加载模拟滞回行为,建立了改进的各向异性损伤模型。该文还建议了模型关键参数的率定方法。应用该模型模拟混凝土受拉和拉一压往复加载试验,计算获得的应力-应变曲线与试验曲线具有良好的可比性,能够反映损伤导致的刚度...     

光纤复合材料损伤探测的光学显示技术

提出一种用于光纤复合材料损伤探测的全光学显示技术。与其他显示方法相比,应用这种技术的装置结构简单,使用方便、具有抗电磁干扰和尤其适用于飞行器实时检测等优点。文中给出了这种全光学学显示技术的基本原理、结构及实测结果。     

EXPONENTIAL MAP ALGORITHM FOR STRESS UPDATES IN ANISOTROPIC MULTIPLICATIVE ELASTOPLASTICITY FOR SINGLE CRYSTALS

This paper presents a new stress update algorithm for large-strain rate-independent single-crystal plasticity. The theoretical frame is the well-established continuum slip theory based on the multiplicative decomposition of the deformation gradient into elastic and plastic parts. A distinct feature of the present formulation is the introduction and computational exploitation of a particularly simple hyperelastic stress response function based on a further multiplicative decomposition of the elastic deformation gradient into spherical and unimodular parts, resulting in a very convenient representation of the Schmid resolved shear stresses on the crystallographic slip systems in terms of a simple inner product of Eulerian vectors. The key contribution of this paper is an algorithmic formulation of the exponential map exp: sl(3) → SL(3) for updating the special linear group SL(3) of unimodular plastic deformation maps. This update preserves exactly the plastic incompressibility condition of the anisotropic plasticity model under consideration. The resulting fully implicit stress update algorithm treats the possibly redundant constraints of single-crystal plasticity by means of an active set search. It exploits intrinsically the simple representation of the Schmid stresses by formulating the return algorithm and the associated consistent elastoplastic moduli in terms of Eulerian vectors updates. The performance of the proposed algorithm is demonstrated by means of a representative numerical example.     

弹性损伤理论的几何-拓扑描述

材料内部微观几何缺陷通常是作为物理非线性问题在本构方程中考虑。针对连续介质弹性损伤理论作几何拓扑,采用非完整标架方法把材料内部微观几何缺陷转化为材料空间的弯曲,并体现在基本几何法则中。首先由连续损伤变量定义拟塑性张量,给出这些基本张量所满足的连续性方程和基本几何法则。由此建立了弹性损伤缺陷与Riemann流形的对应关系,将物理非线性问题转化为物理线性和材料所在空间的弯曲之和。最后讨论了二维情况下,各向同性晶格材料受各向异性损伤的算例。     

A STUDY OF THE INTERNAL PARAMETERS OF DUCTILE DAMAGE THEORY

In this paper the Rousselier ductile damage model is used to simulate the softening behaviour of notched bars and crack growth in a pre-cracked specimen of an A508 pressure vessel steel. The effect of the internal parameters needed to represent the damage on the calculated results is studied. The interactions between these parameters and their effects on the calibration process are discussed. The effect of the cell size on the values of the damage parameters obtained during calibration and on the response of the specimens is briefly reviewed.     

BIMATERIAL INTERFACE CRACK TIP STRESS ANALYSIS: ANISOTROPIC THEORY

Abstract— Near crack tip stress and displacement fields are obtained for anisotropic bimaterial interface cracks. A contact zone model is used in order to get rid of the unphysical oscillatory interpenetration between the edges of the crack. Semi-infinite and the finite crack problems have been studied. Using the near crack tip results of this model crack branching angles can be predicted. These results are illustrated by numerical results for various materials.     

防洪工程新材料研究开发进展

本文分析了防洪工程建设与治理对材料的基本要求与特点。介绍了灌浆材料、纤维混凝土、聚合物混凝土、土工聚合物材料、防水涂料等几类重要材料的基本情况。提出了我国防洪工程新材料研究开发的方向。     

DETERMINATION OF THE POSSIBLE ERROR IN FATIGUE DAMAGE FOR A DISCRETE STRAIN HISTORY

Abstract— The Markov matrix, which is a statistical representation of a service load history, is generally derived using 32 or more strain levels, since this is considered an optimum level between computational effort and accuracy of fatigue life prediction. A recently developed model which predicts the distribution of closed loops and fatigue life from the Markov matrix, is limited in the number of levels which can be analysed because of computational restrictions. This investigation was undertaken to determine the change in the accuracy of fatigue life predictions as the number of strain levels used to represent the strain history is reduced. Since each discretized level is used to represent a range of strain values, a given discretized history will represent a family of histories. A technique is developed which gives the distribution of damage for the family of histories given the discretized history and the distribution of peaks and valleys within each strain interval. By comparing the predicted damage caused by the measured history to a conservative estimate of the distribution of damage for the family of histories, the range of possible error is calculated and used to determine the relationship between the number of strain levels used in the discrete history and the possible error in predicted fatigue damage.     

有机/聚合物图像传感材料的研究进展

总结了有机/聚合物图像传感器材料的主要特点,对几类典型的材料体系,如共轭聚合物/C60复合体系、共轭聚合物/无机半导体纳米微粒复合体系、酞菁类和生物材料进行了重点评述,讨论了给/受体的电子结构、光致电荷转移过程、复合材料的相分离行为对传感器光敏性的影响。指出了目前有机/聚合物图像传感材料研究中亟待解决的关键科学问题。     

日本封装材料用环氧树脂动向

论述了日本封装材料用环氧树脂需求趋势,介绍了最近开发的高耐热性、耐湿性、低粘度环氧树脂。     

A GENERALIZED FATIGUE LIMIT CRITERION AND A UNIFIED THEORY OF LOW-CYCLE FATIGUE DAMAGE

Abstract— A generalized fatigue limit criterion for multiaxial stress state conditions of isotropic materials is presented. This criterion includes four material parameters and uses two invariants of stress amplitudes and furthermore two invariants of mean stresses. It is shown that the fatigue criteria of Sines and Crossland are particular cases of the formulated criterion. Practical recommendations for the use of different fatigue limit criteria are established. Theoretical predictions are compared with experimental data. Finally a continuum damage mechanics theory for low cycle fatigue of isotropic materials is proposed. This theory describes simultaneously the influence of the stress amplitude and the mean stress on the fatigue damage suffered by materials. The proposed theory is based on four material parameters. Special damage theories with a smaller number of material parameters are obtained. Practical recommendations for the use of these fatigue damage theories are presented.     

粘弹塑性损伤时变力学在岩土工程施工分析中的应用

以损伤时变体为研究对象,将时变力学与损伤力学耦联,给出粘弹塑性损伤时变力学理论基本方程和数值计算公式。通过对某地下煤层开采过程的计算机仿真分析,探讨形状时变引起的粘弹塑性时变体损伤场和位移场的时空演化过程,并研究单煤层单向开采和对挖开采方式的路径相关性问题。研究表明,应用损伤时变理论分析,可得到力学状态连续演化过程,为施工分析与预测提供科学的有效手段;不同施工路径,力学状态中间演化过程和最终结果均有较大差别,非线性(包括损伤)材料施工产生的最终力学状态与路径相关这一现象普遍存在,这对工程分析与学术研究具有一定应用与理论价值。