两相材料界面裂纹的起裂方向及断裂参量研究

采用FRANC2D软件研究了两相材料含界面裂纹的断裂特性。通过在材料界面利用CASCA手动生成界面裂纹并在裂纹尖端附近设置1/4奇异等参元,得到了界面裂纹的复应力强度因子;数值模拟得到了界面裂纹的起裂方向,并分析了界面裂纹的破坏特征。计算结果表明:1/4奇异等参元很好地描述了裂纹尖端场的1/r(1/2)奇异性,FRANC2D软件能够模拟界面裂纹的扩展方向,可以得到界面裂纹尖端的应力场和复应力强度因子,为界面裂纹的断裂特性的进一步分析提供依据。     

界面端附近裂纹的应力强度因子

结合材料的断裂形式可分为从界面端产生裂纹（沿界面或向母材内部层折）然后断裂与稍稍离开界面端处产生裂纹然后断裂这两种情况，在金属／陶瓷类结合材料中，后者出现的概率更大，本文利用结合材料界面端的奇异应力场和叠加原理，给出了界面端附近裂纹的应力强度因子近似计算公式，并用边界元数值计算验证了其有效性。     

钨合金三点弯曲破坏的细观数值模拟

运用有限元方法模拟了钨合金材料的三点弯曲实验;采用单胞单元计算模型,分析了断口形貌与材料力学性能之间的关系;建立了宏观断裂形式与微观结构参数之间的关联.结果表明:裂纹主要产生在 W-W 界面、W-M 界面,且在基体中向前发展,钨合金材料的破坏可归结为延性材料的破坏;裂纹的尖端始终处于应力集中的状态,随着裂纹的扩展,拉应力集中不断释放,并且转移到新的裂纹尖端;裂纹尖端的单元为拉伸破坏,拉应力是导致裂纹扩展的主要因素.     

双材料梁界面力学及其应用:综述

双材料结构在工程中得到了广泛应用,如薄膜涂层、压电材料、复合材料层合板和夹层板、粘结接头、FRP加固混凝土结构等;然而,这些结构的破坏通常是从界面及其附近开始的.通常利用损伤力学的方法(材料强度的方法)来预测裂纹的萌生,利用断裂力学的方法来预测裂纹的扩展;因此,开展双材料结构的界面应力分析和具分层双材料结构的断裂以及相关分析是至关重要的.首先介绍双材料梁粘结界面应力分析的基本模型,从而为预测裂纹的萌生提供了有力工具.然后综述双材料梁界面断裂力学分析的基本方法,并详细介绍解析解求解的裂纹尖端法及其相关模型,重点强调裂纹尖端变形对分析结果的影响.最后介绍界面分层对双材料结构其它力学特性的影响,例如屈曲和振动特性.     

界面超高速自相似动态分层分析：反平面情况

对材料界面超高速自相似动态分层的反平面问题进行了解析分析。分层模拟为界面裂纹由零长度自相似扩展，扩展速度为蹭音速或超音速。首先考虑运动集中载荷作用下界面动态分层的情况，利用界面裂纹自相似扩展的运动位错模型将问题归结为奇异积分方程，并求得解析解，分析了裂纹尖端的应力奇性，获得了动应力强度因子。最后，利用叠加原理给出了ｘ＾ｎ型载荷作用下界面动态分层的解。     

基于分子动力学的结合材料界面破坏准则

结合材料的破坏通常都是从界面或其附近发生的,但界面破坏的机理及其评价准则尚未十分清楚.采用分子动力学模拟方法,可以对结合材料的界面破坏过程进行模拟,从而获得结合材料的界面应力和界面破坏之间的关系.界面破坏可以分为奇异应力场作用下的破坏,和界面应力集中引起的破坏两种.虽然在分子动力学模拟中采用了高度简化的界面模型,但对界面破坏过程的模拟,仍可以帮助人们获得结合材料界面破坏过程的规律性认识.分别模拟远场作用下界面上存在初始裂纹和界面附近存在初始裂纹两种情况下的界面破坏,根据分子动力学模拟结果,提出了一个结合材料界面破坏的准则.     

有限尺寸下双材料界面附近裂纹位置对裂纹尖端的影响

随着复合材料的应用和发展,不同材料组成的界面结构越来越受到人们的重视。界面层两侧材料的性能相异会引起材料界面端奇异性,同时界面和界面附近存在裂纹会引起裂尖处的应力奇异性。因此双材料界面附近的力学分析是比较复杂的。本文建立双材料直角界面模型,在材料界面附近预设初始裂纹,计算了有限材料尺寸对界面应力场及其附近裂纹应力强度因子的影响。运用弹性力学中的 Goursat 公式求得直角界面端在有限尺寸下的应力场以及其应力强度系数。通过叠加原理和格林函数法进一步得到在直角界面端附近的裂纹尖端应力强度因子。计算结果表明,在适当范围内改变材料内裂纹与界面之间的距离,界面附近裂纹尖端的应力强度因子随着裂纹与界面距离的增加而减少,并且逐渐趋于稳定。分析结果可以为预测双材料结构复合材料界面失效位置提供参考。     

冲击载荷下镁铝合金裂纹动态扩展过程的数值模拟

采用基于黏聚裂纹模型的扩展有限元方法,开展了镁铝合金结构冲击破坏过程的数值模拟研究。通过镁铝合金三点弯曲试样冲击实验,获得了不同子弹撞击速度下试样的冲击破坏模式。在此基础上,建立了实验结构的扩展有限元模型,并采用最大主应力准则,以及含损伤型的本构关系模拟材料的冲击断裂行为。对于裂纹尖端附近区域,采用黏聚裂纹模型模拟裂纹的断裂过程。对子弹速度分别为12.2、15.1、26.3 m/s的3种工况下镁铝合金试样的动态破坏过程进行了数值模拟研究,获得了与实验相一致的断裂模式。计算结果表明,试样以Ⅰ型断裂模式为主,裂纹沿初始预制裂纹方向扩展。当裂纹扩展到一定程度后,在试样韧带区域被撞击端附近,由于应力波及边界效应导致该区域处于复杂应力状态,试样出现复合型断裂模式,裂纹偏离原扩展路径,与本文实验结果相吻合。     

双材料界面Ⅰ型扩展裂纹尖端的弹黏塑性场

双材料界面中存在材料黏性效应, 对界面裂纹尖端场的分布和界面本身性能 的变化起着重要的影响. 考虑裂纹尖端的奇异性, 建立了双材料界面扩展裂纹尖端的弹黏塑 性控制方程. 引入界面裂纹尖端的位移势函数和边界条件, 对刚性-弹黏塑性界面I型界面 裂纹进行了数值分析, 求得了界面裂纹尖端应力应变场, 并讨论了界面裂纹尖端场随各影响 参数的变化规律. 计算结果表明, 黏性效应是研究界面扩展裂纹尖端场时的一个主要因素, 界面裂纹尖端为弹黏塑性场, 其场受材料的黏性系数、马赫数和奇异性指数控制.     

光弹法实测正交异性双材料界面裂纹断裂参数

采用光弹贴片法实测正交异性双材料界面裂纹尖端区域的应力应变场, 进而求出界面裂纹的断裂力学参量. 将正交异性双材料板加工成拉伸试件,在聚碳酸酯贴片 的单侧表面镀金属铝膜,以提高贴片的反射效率. 沿贴片后的双材料界面预制裂缝,逐渐加 大载荷,得到一系列清晰的等差线条纹图. 利用正交异性双材料界面裂纹尖端应力分量表达 式计算出应力强度因子. 实验表明,光弹贴片法可有效地分析正交异性双材料界面裂纹问题.     

Molecular-dynamics simulation-based cohesive zone representation of intergranular fracture processes in aluminum

A traction-displacement relationship that may be embedded into a cohesive zone model for microscale problems of intergranular fracture is extracted from atomistic molecular-dynamics (MD) simulations. An MD model for crack propagation under steady-state conditions is developed to analyze intergranular fracture along a flat Σ99 [1 1 0] symmetric tilt grain boundary in aluminum. Under hydrostatic tensile load, the simulation reveals asymmetric crack propagation in the two opposite directions along the grain boundary. In one direction, the crack propagates in a brittle manner by cleavage with very little or no dislocation emission, and in the other direction, the propagation is ductile through the mechanism of deformation twinning. This behavior is consistent with the Rice criterion for cleavage vs. dislocation blunting transition at the crack tip. The preference for twinning to dislocation slip is in agreement with the predictions of the Tadmor and Hai criterion. A comparison with finite element calculations shows that while the stress field around the brittle crack tip follows the expected elastic solution for the given boundary conditions of the model, the stress field around the twinning crack tip has a strong plastic contribution. Through the definition of a Cohesive-Zone-Volume-Element—an atomistic analog to a continuum cohesive zone model element—the results from the MD simulation are recast to obtain an average continuum traction-displacement relationship to represent cohesive zone interaction along a characteristic length of the grain boundary interface for the cases of ductile and brittle decohesion.     

The simulation of dynamic crack propagation using the cohesive segments method

The cohesive segments method is a finite element framework that allows for the simulation of the nucleation, growth and coalescence of multiple cracks in solids. In this framework, cracks are introduced as jumps in the displacement field by employing the partition of unity property of finite element shape functions. The magnitude of these jumps are governed by cohesive constitutive relations. In this paper, the cohesive segments method is extended for the simulation of fast crack propagation in brittle solids. The performance of the method is demonstrated in several examples involving crack growth in linear elastic solids under plane stress conditions: tensile loading of a block; shear loading of a block and crack growth along and near a bi-material interface.     

压电复合材料粘接界面断裂有限元模拟

根据数字化FRMM(Fix-Ratio Mix-Mode)断裂试验,得到了压电复合材料试件的断裂韧性和位移及应变场。本文在试验的基础上,通过非线性有限元软件ABAQUS及用户子程序UMAT进行了模拟分析,采用基于损伤力学的粘聚区模型(CZM)对压电复合材料界面的起裂和脱胶扩展进行了分析,并与VCCT方法进行了比较。计算得到的荷载位移曲线更接近于试验结果,但在裂纹扩展路径上的吻合需要对粘聚区法则进一步修正。通过进一步对CZM参数进行分析,表明界面粘结强度和界面刚度对计算结果的影响很大。研究结果表明,粘聚区模型可以很好地表征压电复合材料弱粘接界面脱胶断裂问题。     

Cohesive fracture simulation and failure modes of FRP–concrete bonded interfaces

A work-of-fracture method using three-point bend beam (3PBB) specimen, commonly employed to determine the fracture energy of concrete, is adapted to evaluate the mode-I cohesive fracture of fiber reinforced plastic (FRP) composite–concrete adhesively bonded interfaces. In this study, a bilinear damage cohesive zone model (CZM) is used to simulate cohesive fracture of FRP–concrete bonded interfaces. The interface cohesive process damage model is proposed to simulate the adhesive–concrete interface debonding; while a tensile plastic damage model is used to account for the cohesive cracking of concrete near the bond line. The influences of the important interface parameters, such as the interface cohesive strength, concrete tensile strength, critical interface energy, and concrete fracture energy, on the interface failure modes and load-carrying capacity are discussed in detail through a numerical finite element parametric study. The results of numerical simulations indicate that there is a transition of the failure modes controlling the interface fracture process. Three failure modes in the mode-I fracture of FRP–concrete interface bond are identified: (1) complete adhesive–concrete interface debonding (a weak bond), (2) complete concrete cohesive cracking near the bond line (a strong bond), and (3) a combined failure of interface debonding and concrete cohesive cracking. With the change of interface parameters, the transition of failure modes from interface debonding to concrete cohesive cracking is captured, and such a transition cannot be revealed by using a conventional fracture mechanics-based approach, in which only an energy criterion for fracture is employed. The proposed cohesive damage models for the interface and concrete combined with the numerical finite element simulation can be used to analyze the interface fracture process, predict the load-carrying capacity and ductility, and optimize the interface design, and they can further shed new light on the interface failure modes and transition mechanism which emulate the practical application.     

横观各向同性油气藏水力压裂裂纹扩展规律研究

针对横观各向同性与各向同性油气藏水力压裂裂纹扩展的差异性,基于扩展有限元法建立水力压裂力学模型,通过ABAQUS子程序开发了各向同性和横观各向同性岩体的起裂判据。在各向同性岩体数值模拟结果与解析解以及现场压裂典型曲线对比吻合的基础上,得到了包含层理构造的横观各向同性岩体水力压裂过程中裂纹扩展规律。层理类岩体水力压裂的裂缝扩展方向由地应力状态、层理方向以及岩体与层理界面抗拉强度共同决定;水力压裂过程中,注水压力在裂纹尖端产生应力集中,层理面法向分量先达到界面抗拉强度时,裂纹沿层理方向开裂,反之裂纹沿垂直最小地应力的方向扩展;裂纹扩展速度随层理抗拉强度的增加而降低;由于地层的滤失,随压裂液的注入,裂纹长宽尺度增长速率降低。     

SHEAR BEAM MODEL FOR INTERFACE FAILURE UNDER ANTIPLANE SHEAR (Ⅰ)—FUNDAMENTAL BEHAVIOR

The propagation of interlayer cracks and the resulting failure of the interface is a typical mode occurring in rock engineering and masonry structure. On the basis of the theory of elasto~plasticity and fracture mechanics, the shear beam model for the solution of interface failure was presented. The concept of `cohesive crack’ was adopted to describe the constitutive behavior of the cohesive interfacial layer. Related fundamental equations such as equilibrium equation, constitutive equations were presented. The behavior of a double shear beam bonded through cohesive layer was analytically calculated. The stable propagation of interface crack and process zone was investigated.     

A thermo-mechanical cohesive zone formulation for ductile fracture

The paper addresses the possibility to project both mechanical and thermal phenomena pertinent to the fracture process zone into a cohesive zone. A wider interpretation of the notion cohesive zone is thereby suggested to comprise not only stress degradation due to micro-cracking but also heat generation and energy transport. According to our experience, this widening of the cohesive zone concept allows for a more efficient finite element simulation of ductile fracture. The key feature of the formulation concerns the thermo-mechanical cohesive zone model, evolving within the thermo-hyperelastoplastic continuum, allowing for the concurrent modelling of both heat generation, due to the fracture process, and heat transfer across the fracture process zone. This is accomplished via thermodynamic arguments to obtain the coupled governing equation of motion, energy equation, and constitutive equations. The deformation map is thereby defined in terms of independent continuous and discontinuous portions of the displacement field. In addition, as an extension of the displacement kinematics, to represent the temperature field associated with the discontinuous heat flux across the fracture interface, a matching discontinuous temperature field involving the interface (or band) temperature is proposed. In the first numerical example, concerning dynamic quasi-brittle crack propagation in a thermo-hyperelastoplastic material, we capture the initial increase in temperature close to the crack surface due to the energy dissipating fracture process. In the second example, a novel application of ductile fracture simulation to the process of high velocity (adiabatic) cutting is considered, where some general trends are observed when varying the cutting velocity.     

An improved atomistic simulation based mixed-mode cohesive zone law considering non-planar crack growth

A novel and improved atomistic simulation based cohesive zone law characterizing interfacial debonding is developed which explicitly accounts for the non-planarity of the crack propagation. Group of atoms in the simulation constituting cohesive zones which are used to obtain local stress and crack opening displacement data are determined dynamically during the non-planar crack growth as they cannot be determined apriori. The methodology is used to study the debonding of Σ5 (2 1 0)/[0 0 1] symmetric tilt grain boundary interface in a Cu bicrystal under several mixed mode loading conditions. Simulations show that such bicrystalline specimen exhibits three types of energy dissipative mechanisms – shear coupled GB migration (SCM) away from the crack-tips, change in spacial orientation of GB structural units rendering highly disordered grain boundary near the crack tips and brittle intergranular fracture. Which combination of these three deformation mechanism will be active influencing the degree of non-planarity of the crack propagation at various stages of loading depends on the loading mode-mixity. As the ratio of shear component of the loading parallel to the GB plane and normal to the tilt axis with respect to the normal loading increases (thereby increasing the mode-mixity), overall strain-to-failure also increases and SCM tends to become the dominant deformation mechanism. Through this framework, analytical functional forms and parameters describing cohesive laws for both normal and shear traction as a function of the mode-mixity of the loading and crack opening displacement are predicted.