混凝土黏聚开裂模型若干进展

黏聚模型是用来描述混凝土断裂行为的基本模型, 首先介绍了混凝土的黏聚开裂模型的基本概念,总结了确定黏聚区的本构方程的各种方法,即直接单轴拉伸测试、J积分方法、R曲线法、柔度法和逆推法.然后介绍了黏聚模型在I型和复合型裂纹问题、疲劳断裂问题中的应用以及黏聚模型与混凝土尺寸效应的关系.最后对黏聚开裂模型与桥联模型、带状裂缝模型进行了比较和总结, 指出了该模型存在的问题, 并对其以后的发展方向提出了建议.      

车辆纵振路面谱及其应用

对车辆纵振路面谱及其在地面-车辆系统中的应用进行了探讨,计算了行车纵剪力及其作用下的路面应力强度因子．与垂向路面谱相比,纵振路面谱代表的路面等级较高．行车纵剪力为均值等于0的平稳随机过程,在它的作用下,路面荷载型裂纹从上到下扩展的速度明显大于从下往上扩展的速度,扩展速度与力的 4次幂成正比．行车纵剪力对路面的裂纹损伤远远大于轮胎路面压力导致的裂纹损伤．     

On the role of the process zone in dynamic fracture

The evolution of the fracture process zone and its influence on dynamic crack propagation are investigated using a simple constitutive assumption to model the material in the fracture process zone. The mode III problem of anti-plane shear is examined using a finite difference scheme to obtain the full field solution. Unlike the classical approach of determining the steady state solutions for imposed constant speed of crack propagation, here the fracture criterion is imposed along with the formulation of the boundary-initial value problem and the coupled equations are solved numerically to determine the generation and growth of the fracture process zone as well as the crack tip. The results of the simulation indicate that the constitutive behavior of the process zone material (or equivalently the evolution of the process zone itself) plays a key role in determining the dynamics of fast fracture.     

Intergranular strain evolution near fatigue crack tips in polycrystalline metals

The deformation field near a steady fatigue crack includes a plastic zone in front of the crack tip and a plastic wake behind it, and the magnitude, distribution, and history of the residual strain along the crack path depend on the stress multiaxiality, material properties, and history of stress intensity factor and crack growth rate. An in situ, full-field, non-destructive measurement of lattice strain (which relies on the intergranular interactions of the inhomogeneous deformation fields in neighboring grains) by neutron diffraction techniques has been performed for the fatigue test of a Ni-based superalloy compact tension specimen. These microscopic grain level measurements provided unprecedented information on the fatigue growth mechanisms. A two-scale model is developed to predict the lattice strain evolution near fatigue crack tips in polycrystalline materials. An irreversible, hysteretic cohesive interface model is adopted to simulate a steady fatigue crack, which allows us to generate the stress/strain distribution and history near the fatigue crack tip. The continuum deformation history is used as inputs for the micromechanical analysis of lattice strain evolution using the slip-based crystal plasticity model, thus making a mechanistic connection between macro- and micro-strains. Predictions from perfect grain-boundary simulations exhibit the same lattice strain distributions as in neutron diffraction measurements, except for discrepancies near the crack tip within about one-tenth of the plastic zone size. By considering the intergranular damage, which leads to vanishing intergranular strains as damage proceeds, we find a significantly improved agreement between predicted and measured lattice strains inside the fatigue process zone. Consequently, the intergranular damage near fatigue crack tip is concluded to be responsible for fatigue crack growth.     

Hydraulic fracturing of a saturated porous medium-I: General theory

This investigation deals with the problem of steady state hydraulic fracture in an infinite isotropic fluid-saturated elastic porous medium induced by a uniform pressure applied to the crack surfaces. A quasi-static approach is employed in the study. A boundary value problem is formulated and then analyzed by means of the Fourier transform associated with the Wiener-Hopf technique. Stress intensity factor and potential energy release rate are found by asymptotic analysis and the superposition principle as functions of the speed of crack propagation. The material breakdown process at the crack tip is discussed based on Dugdale's model. Finally, a brief discussion of the effect of pressure drop on the hydraulic fracture process and the decrease in crack speed during crack extension is included.     

含缺口试件疲劳全寿命预测一种建议的方法

传统的研究含缺口构件的疲劳的方法是将疲劳启裂和疲劳裂纹扩展两个过程完全独立起来,用不同的方法来模拟,相互间并没有定量的关系。本文是基于最新发展的多轴疲劳损伤理论,建立了一种适用于各种载荷条件下的疲劳启裂和裂纹扩展的普适方法。根据从弹塑性分析中得到的应力应变,确定疲劳损伤模型,建立能够预测疲劳启裂、裂纹扩展速率和扩展方向的新方法。整个模拟可以分为两步:弹-塑性应力分析得到材料的应力应变分布;再运用一个通用的疲劳准则预测疲劳裂纹启裂和裂纹扩展。通过对1070号钢含缺口试件的疲劳全寿命预测,得到了与实验非常吻合的模拟结果。     

Hydraulic fracturing of a saturated porous medium-II: Special cases

This investigation deals with the problem of steady state hydraulic fracture in an infinite isotropic fluid-saturated elastic porous medium induced by a uniform pressure applied to the crack surfaces. A quasi-static approach is employed in the study. A boundary value problem is formulated and then analyzed by means of the Fourier transform associated with the Wiener-Hopf technique. Stress intensity factor and potential energy release rate are found by asymptotic analysis and the superposition principle as functions of the speed of crack propagation. The material breakdown process at the crack tip is discussed based on Dugdale's model. Finally, a brief discussion of the effect of pressure drop on the hydraulic fracture process and the decrease in crack speed during crack extension is included.     

A refined cohesive zone model that accounts for inertia of cohesive zone of a moving crack

Existing cohesive zone models assume that actual fracture zone of non-zero mass can be modeled by a line segment (cohesive zone) with no mass and inertia. In the present work, a simplified mass-spring model is presented to study inertia effect of cohesive zone on a mode-I steady-state moving crack. It is showed that fracture energy predicted by the present model increases dramatically when a finite limiting crack speed is approached. Reasonable agreement with known experiments indicates that the present model has the potential to catch the inertia effect of cohesive zone which has been ignored in existing cohesive zone models and better simulate dynamic fracture at high crack speed.     

Fatigue fracture model for thin isotropic plates with cracks in axial loading

Conclusion We have constructed a model of the growth of a fatigue crack in a thin, isotropic plate, taking the two-stage evolution of the fracture process into account. The model is based on concepts of the mechanics of a continuous defective state and on a schematic representation of the neighborhood of the tip of a fatigue crack as a plastic zone moving together with the crack. The model takes into account the influence of the cumulative defective state (damage level) along the crack propagation front on the speed of propagation.We have formulated solutions for the cases when the length of the plastic zone is constant and when it varies during the growth of fatigue cracks. We have established the fact that the plastic zone at the crack tip tends to disrupt the stability of the motion immediately at the time of inception or opening of the crack. The speed of crack propagation decreases as the plastic zone grows in size.We have shown that the problem of estimating the kinetics of fatigue cracks in thin plates can be reduced to calculating the growth rate as a function of the peak-to-peak amplitude of the stress intensity factor while preserving the structure of the governing equations of the model. We have also shown that the concept of a plastic zone of constant length induces a power-law dependence of the crack rate on K, the power exponent varying from 2 to 10–12. The Dugdale model gives a square-law dependence of the crack rate on K, which for the most part is applicable to plastic materials.S. P. Timoshenko Institute of Mechanics, National Academy of Sciences of Ukraine, Kiev. Translated from Prikladnaya Mekhanika, Vol. 30, No. 7, pp. 53–63, July, 1994.     

基于非齐次复合Poisson过程的正交异性钢桥面板细节疲劳裂纹随机扩展研究

传统的正交异性钢桥面板疲劳损伤评估常采用确定性和可靠性分析方法,忽略了疲劳裂纹扩展的随机性影响,针对这一问题,提出钢桥面板细节疲劳随机扩展分析方法。本文以南溪长江大桥为工程背景,基于长期车辆荷载监测数据,建立了车辆荷载非齐次复合Poisson过程模型。建立钢桥面板有限元模型,采用瞬态分析方法将随机车辆荷载转化成细节疲劳应力,基于线弹性断裂力学理论推导U肋-顶板焊接细节疲劳裂纹扩展时变微分方程,实现宏观关系式疲劳应力幅次数-疲劳损伤至微观表达式应力时间序列-疲劳损伤转换,讨论了车载次序及超载对疲劳裂纹扩展的影响。研究结果表明,非齐次复合泊松过程模型能够较好描述随机车流运营状态,车辆荷载的次序对疲劳裂纹扩展速率的影响不可忽略,重车排序靠前时能够促使疲劳裂纹扩展增速,南溪长江大桥细节点的车辆超载迟滞效应修正系数取值0.804。     

大体积混凝土等效裂纹断裂模型研究

根据大比尺混凝土紧凑拉伸与楔入劈拉试验成果,将裂纹在稳定扩展中断裂有程区应力分布与相应的变形（张开度ω）联系起来,通过“归一化”处理,得到了大体积混凝土断裂过程区长度的解析表达式,建立了大体积混凝土Ⅰ型断裂破坏的等效裂纹断裂模型。     

Constitutive behavior of a microcracking brittle solid in cyclic compression

A constitutive formulation is presented to determine the “driving force” for Mode I fatigue crack growth in notched plates of brittle solids stressed in uniaxial cyclic compression. For the particular case of a microcracking medium, it is demonstrated that residual tensile stresses are induced ahead of the notch during unloading from the maximum far-field compressive stress. We propose that it is this region of residual tensile stresses at the notch-tip which promotes fatigue crack growth in ceramics along the notch plane in a direction normal to the compression axis. The predictions of the analysis are compared with new experimental results on compression fatigue in brittle solids. Specifically, it is shown that the numerical estimates of the near-tip tensile zone size for microcracking ceramics compare favorably with the experimentally measured distance of stable Mode I fatigue crack growth after the first compression cycle. Experimental information on the threshold stress for microcracking, transition stress for the inducement of residual tension during unloading, and the effect of mean stress on fracture, as well as direct observations of microcracks and crack growth in compression fatigue, corroborate the assumptions and implications of the analysis.     

STOCHASTIC ELASTO-PLASTIC FRACTURE ANALYSIS OF ALUMINUM FOAMS

Model I quasi-static nonlinear fracture of aluminum foams is analyzed by considering the effect of microscopic heterogeneity. Firstly, a continuum constitutive model is adopted to account for the plastic compressibility of the metallic foams. The yield strain modeled by a two- parameter Weibull-type function is adopted in the constitutive model. Then, a modified cohesive zone model is established to characterize the fracture behavior of aluminum foams with a cohesive zone ahead of the initial crack. The tensile traction versus local crack opening displacement relation is employed to describe the softening characteristics of the material. And a Weibull statistical model for peak bridging stress within the fracture process zone is used for considering microscopic heterogeneity of aluminum foams. Lastly, the influence of stochastic parameters on the curve of stress-strain is given. Numerical examples are given to illustrate the numerical model presented in this paper and the effects of Weibull parameters and material properties on J-integral are discussed.     

基于子模型法的带有表面裂纹钢丝应力强度因子研究

钢丝裂纹应力强度因子是进行钢丝疲劳断裂寿命评估、疲劳裂纹扩展分析和钢丝断裂强度评估等工作的重要参数。本文首先介绍了裂纹扩展分析软件FRANC3D,然后基于子模型法模拟研究了拉伸荷载作用下带有表面裂纹钢丝的应力强度因子,裂纹种类包括直线形裂纹和半圆形裂纹,最后拟合得到拉伸荷载作用下带表面裂纹钢丝的应力强度因子形状修正系数表达式,分析了利用该公式进行承载力评估时产生误差的原因。研究结果表明,利用子模型模拟分析拉伸荷载作用下带有表面裂纹的钢丝应力强度因子时计算精度高,计算速度快,对计算机硬件要求低;利用该方法得到的钢丝裂纹应力强度因子,在进行索承式桥梁吊索安全性能评估时,评估结果更精确。     

自紧身管疲劳寿命的可靠性分析

基于概率断裂力学理论和Monte Carlo模拟方法，给出了自紧身管疲劳寿命的可靠性分析方法．自紧残余应力采用了符合身管材料具有强化和包辛格效应性能推导的公式计算，身管工作的应力强度因子通过权函数方法得到．实例分析结果表明，对数正态分布为身管疲劳寿命的最佳分布，同时给出了各种置信度和可靠度下的身管疲劳寿命．另外，还分析了初始裂纹尺寸和材料的断裂韧性对身管疲劳寿命的影响．该研究对保证身管的安全，避免灾难性事故的发生具有重要意义．     

Unified nano-mechanics based probabilistic theory of quasibrittle and brittle structures: II. Fatigue crack growth, lifetime and scaling

This paper extends the theoretical framework presented in the preceding Part I to the lifetime distribution of quasibrittle structures failing at the fracture of one representative volume element under constant amplitude fatigue. The probability distribution of the critical stress amplitude is derived for a given number of cycles and a given minimum-to-maximum stress ratio. The physical mechanism underlying the Paris law for fatigue crack growth is explained under certain plausible assumptions about the damage accumulation in the cyclic fracture process zone at the tip of subcritical crack. This law is then used to relate the probability distribution of critical stress amplitude to the probability distribution of fatigue lifetime. The theory naturally yields a power-law relation for the stress-life curve (S-N curve), which agrees with Basquin's law. Furthermore, the theory indicates that, for quasibrittle structures, the S-N curve must be size dependent. Finally, physical explanation is provided to the experimentally observed systematic deviations of lifetime histograms of various ceramics and bones from the Weibull distribution, and their close fits by the present theory are demonstrated.     

有限宽板中心裂纹断裂过程区的长度和位移公式

对岩石、混凝土和陶瓷等准脆性材料进行断裂分析,有必要研究裂纹前端的断裂过程区所起的作用以及各种因素对它的影响。本文提出分析有限宽中心裂纹板剪切断裂过程区的方法,此方法基于D－B模型的叠加原理,考虑了压剪断裂的摩擦阻力,并将有限板宽影响简化为载荷的修正,以非常简便的方法推导出计算断裂过程区长度和位移的公式,这些公式补充了现有文献只有“无限大”板的解的不足。利用这些公式,分析各种参数对断裂过程区长度和位移的影响变得非常方便。     

Effect of electric fields on fracture behavior of ferroelectric ceramics

The asymptotic problem of a semi-infinite crack perpendicular to the poling direction in a ferroelectric ceramic subjected to combined electric and mechanical loading is analyzed to investigate effect of electric fields on fracture behavior. Electromechanical coupling induced by the piezoelectric effect is neglected in this paper. The shape and size of the switching zone is shown to depend strongly on the relative magnitude between the applied electric field and stress field as well as on the ratio of the coercive electric field to the yield electric field. A universal relation between the crack tip stress intensity factor and the applied intensity factors of stress and electric field under small-scale conditions is obtained from the solution of the switching zone. It is found that the ratio of the coercive electric field to the yield electric field plays a significant role in determining the enhancement or reduction of the crack tip stress intensity factor. The fracture toughness variation of ferroelectrics under combined electric and mechanical loading is also discussed.