Deformation and long-term damage of particulate composites under thermal loads

A theory of long-term damage of particulate composite materials under thermal load is proposed. The damage of the composite components is modeled by randomly dispersed micropores. The failure criterion for a single microvolume is determined by its stress-rupture strength, which, in turn, is determined by the dependence of the time to brittle failure on the difference between the equivalent stress and its limit, which characterizes the ultimate strength according to the Schleicher–Nadai criterion. The damage (porosity) balance equation is derived for an arbitrary time, taking the thermal effect into account. Algorithms for calculating microdamage and macrostresses as functions of time are developed     

Deformation and long-term damage of physically nonlinear particulate composites

The theory of long-term damage is generalized to particulate composite materials with physically nonlinear components. The damage of the components is modeled by randomly dispersed micropores. The damage criterion for a microvolume is characterized by its stress-rupture strength. It is determined by the dependence of the time to brittle failure on the difference between the equivalent stress and its limit, which is the ultimate strength, according to the Huber–Mises criterion, and assumed to be a random function of coordinates. An equation of damage (porosity) balance in the components at an arbitrary time is formulated. Algorithms of calculating the time dependence of are developed. The effect on the nonlinearity of the matrix on the damage and macrodeformation curves is examined     

含孔复合材料层合板静拉伸三维逐渐损伤分析

针对面内静拉伸纤维增强复合材料含中孔层合板，发展了参数化三维逐渐损伤模型. 该模型 可以模拟含中孔层合板损伤起始、发展及最终结构破坏整个过程，并能较好地预测含中孔层 合板的破坏模式和破坏强度. 采用所发展的模型和有限元三维逐渐损伤分析技术即应力分 析、失效判定准则及损伤过程中材料性能退化等，对其他文献所提供的9种不同类型含中孔层合板进行了损伤扩展分析及强度预测，同时对层合板的损伤基本机理、类型及其相互关联作用进行了探讨，计算结果与文献实验结果非常吻合.     

Microstructure damage at ensemble of points for grain composites

Fracture concentration zones are considered in microstructure elements of grain composites. Mathematical model of micro-heterogeneous medium with random properties of elements is used for calculations. The distribution laws for the modules of elasticity and ultimate strengths in the elements as well as the tensor of macroscopic deformations for the composite serve as the initial data. Different types of stresses are evaluated. Correlation functions for micro stresses are obtained by the Green’s tensor method.Random microstructure strength condition is a difference between the stress and the ultimate strength at any point of an ensemble with a particular configuration. The probability of simultaneously exceeding the ultimate strength in this set of elements determines the likelihood of failure of this ensemble of points and the relative damage at the micro level.The damage is calculated using multivariate normal distribution. Structure of correlation matrix of distribution depends on the type of fracture concentration zones. Correlation functions of microstructure strength condition depend on the distance between the points of the ensemble. Calculations of multipoint damage are provided for several configurations of points, in particular, for the three points on a straight line segment, and for the five points in the vertices and the center of a tetrahedron. For two-dimensional distribution density, the smoothing surface formulas are derived, taking into account the moments of stresses up to and including the fourth order.The influence of microstructure properties and the type of ensemble of points on composite damage is demonstrated. Study of microstructure damage enables the prediction of early stages of construction material failure.     

Deformation and long-term damage of homogeneous and composite materials of stochastic structure (Review)

The studies of mathematical models for the coupled processes of deformation and long-time damage of stochastic composite materials are systematized. Damage is modeled by stochastically arranged micropores. The damage of a single microvolume is characterized by its stress-rupture strength determined by the dependence of the time to brittle fracture on the difference between the equivalent stress and its limit, which is the ultimate strength, according to the Huber–Mises or Schleicher–Nadai criteria, and assumed to be a random function of coordinates. The equation of damage balance at an arbitrary time and the equations relating macrostresses and macrostrains constitute a closed system. Algorithms of calculating the time dependence of microdamage and macrostresses are developed. The effect of temperature and nonlinearity on the curves is studied     

Microstructure damage related to stress–strain curve for grain composites

Mathematical model of micro-heterogeneous medium with random deformation and strength properties of microstructure is developed assuming that the tensor of macroscopic deformations is known for the structure. Green–Somigliana tensor is used to obtain the formulas for random stress distribution in microstructure elements. The probability of the stress exceeding the ultimate strength in an element determines the probability of fracture in this element and the relative micro-damage. The correlation functions of stochastic microstructure ultimate strength condition are calculated for various types of stress. Normal distribution is used to calculate the damage. The distribution density can be adjusted through the stress moments to the fourth order.Micro-fractions change the composite’s macro modules of elasticity. Therefore, changes the relationship between stress and strain. Setting an increment step on the macro-strain axis, the stress–strain curve is plotted taking into account changes in composite properties. Stress–strain curves are obtained for different types of load.The increase of the factor of safety corresponds with the reduction of microstructure damage permitted in the design. Critical microstructure damage also depends on the dispersion of the microstructure properties. It is shown that the microstructure properties of composite significantly influence the behavior of materials under load and the shape of stress–strain curve. Findings are compared with experiment data.     

基于纤维单元的钢筋混凝土桥墩地震损伤评价

基于纤维模型的粱柱单元特别是柔度法单元有着较高的计算精度,已被较多地用于评价钢筋混凝土桥墩的位移（廷性）能力。随着基于性能／位移抗震设计理论的发展,相继提出了残余位移、极限曲率及曲率延性系数、纵筋和混凝土的最大应变、纵筋低周疲劳损伤等桥墩地震损伤量化指标。选用刚度法和柔度法纤维单元,考虑材料非线性、几何非线性和结点锚固...     

基于修正Park-Ang模型的圆钢管构件地震损伤评估

为合理评估薄壁圆钢管构件的地震损伤程度,基于Park-Ang地震损伤模型,通过修正相关参数,建立了适用于圆钢管构件的地震损伤模型。利用圆钢管构件在低周反复荷载作用下的骨架曲线,给出了损伤模型中各待定参数的计算方法,在此基础上通过反演构件破坏点损伤因子为1.0时的模型组合系数,并利用曲线拟合法得到模型组合系数与构件轴压比、长细比和径厚比等参数间的关系式,从而建立构件的地震损伤评估模型。结果表明,修正的Park-Ang损伤模型可较好地评估圆钢管构件的损伤程度,构件达到极限破坏时的损伤因子值收敛于1.0且离散性较小,构件的破坏程度可划分为基本完好、轻微与中等破坏、严重破坏和失效破坏,对应的损伤因子界限值分别为0.02,0.41和1.00。     

NDE of metal damage: ultrasonics with a damage mechanics model

This research describes a nondestructive method for the quantitative estimation of property variations due to damage in metal materials. The method employs a damage mechanics model, which accounts for stiffness degradation and damage evolution of a metal medium with a measurement of ultrasonic velocity. In order to describe the progressive deterioration of materials prior to the initiation of macrocracks, we have developed a new damage mechanics model. Thereafter, a finite element model valid for numerically describing such damage process has been developed by ABAQUS/Standard code, and correlations between damage state, elastic stiffness and plastic strain could be found by the results of the finite element simulation. The property variations due to damage evolution are calculated based on the Mori–Tanaka theory, and then the ultrasonic velocity can be predicted by Christoffel’s equation. When the measured velocity is coupled with the theoretically predicted velocity, the unknown damage variable is solved, from which other residual properties are determined by the predictions of damage model. The proposed technique is performed on type 304 stainless steel bars. The numerical results obtained by the simulation were compared with experimental ones in order to verify the validity of the proposed finite element model and good agreement was found. It is shown that the damaged properties of metals can be estimated accurately by the proposed method.     

Deformation and long-term damage of orthotropic composites with limited stress-rupture microstrength

The theory of long-term microdamage of homogeneous materials based on the mechanics of stochastically inhomogeneous materials is generalized to a composite with orthotropic inclusions. The damage of the composite components is modeled by randomly dispersed micropores. The damage criterion for a microvolume is characterized by its stress-rupture strength. It is determined by the dependence of the time to brittle failure on the difference between the equivalent stress and its limit, which is the tensile strength, according to the Huber–Mises criterion, and assumed to be a random function of coordinates. Given macrostresses or macrostrains, an equation of damage (porosity) balance in the composite components at an arbitrary time is derived. The time dependence of microdamage and macrostresses or macrostrains in a discrete-fiber-reinforced composite with limited stress-rupture microstrength described by a fractional-power function is plotted     

角钢约束混凝土中长柱轴压力学性能试验及承载力计算

为了研究角钢约束混凝土中长柱在轴压荷载作用下的力学性能,以长细比、缀板间距、混凝土强度等级为变化参数,完成了8个试件的静力加载试验。通过试验观察了试件的破坏形态,获取了其极限承载力、刚度、位移延性和耗能系数等力学性能指标;分析了各变化参数对力学性能指标的影响,利用统一强度理论、极限理论和叠加理论对试件的承载力进行计算。研究结果表明:长细比越小越容易发生柱端破坏,长细比加大后易发生柱中破坏;减小缀板间距,试件的极限承载力和变形能力均得到提高。随着混凝土强度等级的提高,试件的极限承载能力和初始弹性刚度得以提高,但位移延性和耗能能力有所降低。采用统一强度理论的计算值略大于试验值,采用极限分析理论和叠加理论的计算值均小于试验值。     

钢－混凝土组合梁非线性有限元分析

提出了部分剪力连接钢-混凝土组合梁非线性有限元分析的简化模型，该模型同时考虑了钢、混凝土材料的非线性本构关系和剪力连接件的滑移非线性对组合梁刚度和强度的影响，推导了考虑滑移的剪力连接件单元刚度矩阵。利用该模型计算了连续组合梁的极限荷载、挠度、应力以及滑移，与已有的理论计算结果和实验结果吻合，证明本方法分析钢-混凝土组合梁非线性的可靠性和实用性。     

Influence of heating on the deformation and long-term damage of unreinforced materials

A theory of long-term damage of homogeneous materials under thermal load is proposed. The damage of the material is modeled by randomly dispersed micropores. The failure criterion for a single microvolume is determined by its stress-rupture strength, which, in turn, is determined by the dependence of the time to brittle failure on the difference between the equivalent stress and its limit, which characterizes the ultimate strength according to the Schleicher–Nadai criterion. The damage (porosity) balance equation is derived for an arbitrary time, taking the thermal effect into account. Algorithms for calculating microdamage and macrostresses as functions of time are developed, and respective curves are plotted. The effect of temperature on the macrodeformation and damage curves is studied     

Modelling unilateral damage effect in strongly anisotropic materials by the introduction of the loading mode in damage mechanics

Damage weakens the mechanical characteristics of materials. But this weakening can disappear if the cracks close again: this is called the unilateral effect of damage. We propose a model of this phenomenon using damage mechanics in the case of a diffuse network of identical microcracks. The microcracks state is defined with two internal state variables. These two variables are control parameters of the geometry of the microcracks. They also define the loading mode in damage mechanics as in fracture mechanics. In order to limit the anisotropy induced by the microcracks, hence by the loading, we suppose that the geometry of damage spreads into preferential directions. Therefore, this model is essentially applied to materials with a strong anisotropy where the defects follow the constituents of the material. An application is given for composite laminates.     

Micromechanics of long-term damage of particulate composites with unlimited microdurability

The theory of long-term damage of homogeneous materials is generalized to particulate composite materials. The damage of the composite components is modeled by randomly dispersed micropores. The damage criterion for a microvolume is characterized by its stress-rupture strength. It is determined by the dependence of the time to brittle failure on the difference between the equivalent stress and its limit, which is the tensile strength, according to the Huber–Mises criterion, and assumed to be a random function of coordinates. An equation of damage (porosity) balance in the composite components at an arbitrary time is formulated. Algorithms of calculating the time dependence of microdamage and macrostresses or macrostrains are developed and relevant curves are plotted in the case of unlimited microdurability Translated from Prikladnaya Mekhanika, Vol. 44, No. 11, pp. 7–17, November 2008.     

冲击回波法对混凝土内部损伤检测的试验研究

通过预加荷载给混凝土试件内部引入一定程度的损伤,分别测量了损伤前后试件内部弹性波的传播速度以及抗压强度,探讨了弹性波波速的降低程度与抗压强度劣化之间的关系。研究表明:在MEM卓越周期图中,主要峰值数量增加和底部边界面反射信号周期加长可作为判别损伤产生的依据;弹性波波速与混凝土内部的损伤程度具有很好的相关性,混凝土损伤程度越大,波速降低值越大;弹性波波速降低程度与抗压强度劣化之间具有线性相关性。根据试验数据拟合出了线性方程。说明采用冲击回波法对混凝土内部损伤程度检测是可行的,在混凝土结构的可靠性与安全性分析方面具有广阔的应用前景。     

基于响应面法的复合材料连接结构可靠性分析

复合材料的力学性能参数比金属材料呈现出更大的分散性。本文提出采用加权线性响应面法对复合材料单钉连接结构进行可靠性分析。采用三维有限元模型进行复合材料单钉连接的确定性渐进失效分析。单向铺层的力学性能参数和强度参数均考虑为输入随机变量,用于可靠性分析。详细研究了不同失效准则和不同刚度退化方法的影响。结果表明,Olmedo-Santiuste失效准则与Camanho-Matthews刚度退化方法的组合能够给出与试验均值最接近的强度分析结果。另外,还仔细研究了响应面参数f的影响。结果表明,取f=1的线性加权响应面能够为复合材料单钉连接可靠性分析提供较好的统计特性逼近值。     

基于三维DIC方法的高强钢拉伸力学性能测定

高强度钢在建筑等工程领域发挥着极为重要的作用,因此准确测定其力学性能具有至关重要的意义．鉴于传统机械引伸计在小尺寸试样变形测试中的不便性,利用三维数字图像相关(3D-DIC)方法,对8.8级螺栓和Q690钢这两类试样在单轴拉伸试验全过程中的变形进行了测试,分别得到了应力-应变曲线、弹性模量、屈服强度、强度极限、断后延伸率和断面收缩率,由于试样在屈服阶段应变增加而应力基本不变,因此同时研究了该阶段中试样从弹性变形演化到塑性变形的发展规律．实验结果表明三维DIC在小尺寸试样力学性能测试方面具有很强的优越性,可用来灵活地测量变形并研究变形的演化规律．     

钢纤维增强砂浆的轴拉损伤力学模型

基于Li Fa Ming的平行棒模型,对短钢纤维增强砂浆的平板试件,在直接拉伸条件下的破坏行为进行了分析,假设试件由N根相互平行的复合棒组成,每根复合棒又由一根纤维棒和S根砂浆棒组成,考虑纤维在基体中分布的方向因子和长度因子.砂浆的损伤可按连续损伤力学进行处理,将Loland模型和Mazars模型加以改进来描述.依据多根纤维的拉拨模型,假定纤维与基体间界面的损伤由纤维脱粘长度与纤维插入长度的比值来描述,复合材料的损伤包括基体的损伤和纤维的损伤,借助已有的试验数据和文献资料来确定本构模型中的各种参数,成功建立了短钢纤维增强砂浆直接拉伸应力一应变全曲线模型.所建模型与试件在直接拉伸试验下的应力-应变全曲线进行了对比,结果较为吻合.     

基于双剪统一强度理论的复式钢管混凝土轴压承载力计算

考虑内、外钢管对混凝土的双重约束作用, 分析复式钢管混凝土的轴压应力状态, 采用双剪统一强度理论分析混凝土和钢管各自的极限轴压强度, 得出复式钢管混凝土轴压承载力公式; 通过推导钢管在极限状态的强度折减系数及钢管与混凝土之间的紧箍力值, 计算出的轴压承载力与试验数据进行对比, 吻合较好, 验证了双剪统一强度理论在复式钢管混凝土轴压承载力计算中的适用性; 并给出了内圆钢管的径厚比和直径与轴压承载力提高系数的关系, 为复式钢管混凝土的优化设计提供理论依据.