层状非均匀材料边裂纹引起的J积分变化量分析

均匀材料无裂纹时沿封闭路径的J积分为零,层状非均匀材料无裂纹时沿封闭路径的J积分通常不为零且与路径相关。在位移载荷保持不变条件下引入裂纹会使J积分改变,本文分析引入裂纹所导致的远场J积分变化量,即有裂纹时与无裂纹时沿同一远场路径的J积分之差,其值等于裂尖J积分与界面J积分变化量之和。对于层状非均匀材料,虽然无裂纹时和有裂纹时的远场J积分、界面J积分都与路径相关,但当积分路径远离裂尖后,有裂纹与无裂纹时的远场J积分之差、界面J积分之差与路径无关,引入裂纹所引起的远场J积分变化量等于边界应变能密度释放量沿边界的积分。对于均匀材料半无限大平面的边裂纹,裂纹能量释放率等于无裂纹时应变能密度与8倍裂纹长度的乘积;对于层状材料的边裂纹,裂纹能量释放率等于应变能密度释放量沿边界的积分减去界面J积分变化量。     

含中心裂纹铝合金薄板裂纹稳态扩展的实验研究

一、引言对于韧性材料的含裂纹构件,通常是在大范围屈服情况下断裂的,所以必须建立弹塑性断裂理论来进行研究。用J积分判据J_(1c)或临界裂纹顶端张开位移δ_(cr)来衡量韧性材料的断裂韧性,在工程上具有实用意义。但是J_(1c)和δ_(cr)都是用来确定裂纹的初始起裂,而起裂后的裂纹稳态扩展现象很重要,特别对于硬化材料的金属薄壁构件更为明显,在裂纹缓慢稳态扩展的过程中,必须继续增加载荷,直到裂纹失稳扩展,因此要合理地确定含裂纹薄壁结构的承载能力,就需要研究裂纹稳态扩展过程。Feddersen在研究平面应力断裂问题中,用铝合金中心裂纹板试件做了大量实验,对工程设计提供了有用的分析方法,但是对于裂纹的稳态扩展过程只做了定性描述,没     

RESEARCH PROGRESS OF FRACTURE TOUGHNESS TEST METHOD FOR LOW CONSTRAINT SPECIMENS 1)

低约束试件断裂韧性测试对油气管道安全运营具有重要意义。本文回顾了低约束试件断裂韧性测试方法及发展过程,介绍了裂纹尖端张开位 移(crack tip opening displacement, CTOD)和J积分等常用断裂韧性表征参数,并对断裂韧性测试中应力强度因子、J积分塑性因子、J积分与CTOD转换因子、裂纹尺寸测量方法、数 字图像相关方法等关键问题进行对比分析,总结需要深入研究的问题,为低约束试件断裂韧性测试发展提供一定依据。     

韧性断裂的微观模型及其在约束断裂中的应用

详细分析了不同形状断裂试件及小范围屈服模型裂纹端部的损伤演化，提出了韧性断裂的宏观起裂相当于裂尖前方一特征位置处的损伤达到一临界值。利用此模型获得了与实验相一致的宏观断裂韧性及与约束无关的理论断裂韧性。     

孔洞对平面裂纹应力强度因子影响分析的广义参数Williams单元

本文采用圆形奇异区广义参数Williams单元（W单元）建立了中心裂纹与圆孔共存的平面应力模型,奇异区外围利用ABAQUS有限元软件自动网格离散技术与FORTRAN95编程前处理相结合,克服了自主编程中网格离散的局限性．算例分析了圆孔位置和几何参数对I-II混合型裂纹尖端应力强度因子(SIFs)的影响,并与扩展有限元法(XFEM)计算结果进行比较．结果表明：靠近圆孔一侧的裂尖SIFs大于远离圆孔一侧的裂尖SIFs;控制圆孔左边缘到裂纹中心的距离,则两侧裂尖SIFs随圆孔半径的增大而增大;圆孔中心与裂纹中心水平距离越远,圆孔对裂纹扩展的影响越小．同时,基于圆形奇异区的W单元直接计算得到的裂尖SIFs与扩展有限元法得到的解吻合较好,证明了W单元对奇异区离散形状不敏感,且具有高效率和高精度．     

功能梯度材料涂层平面运动裂纹分析

研究粘结于均匀材料基底上功能梯度材料涂层平面运动裂纹问题,假设功能梯度材料剪切模量和密度为坐标的指数函数,而泊松比为常数.采用Fourier变换和传递矩阵法将该混合边值问题转化为一对奇异积分方程,通过数值求解奇异积分方程组获得功能梯度材料涂层平面运动裂纹的应力强度因子.考察了结构几何尺寸、裂纹运动速度以及材料梯度参数对运动裂纹的应力强度因子的影响,发现材料梯度参数、结构几何尺寸、裂纹长度以及运动速度均对功能梯度材料动态断裂行为有显著影响.     

试样尺寸对核电厂主蒸汽管道断裂韧性的影响研究

以某核电厂SA335 材料主蒸汽管道为研究对象,首先结合SA335 的断裂阻力曲线（J-R 曲线）试验测量结果,提出了一种方法确定韧性金属材料裂纹扩展模型（G-T 模型）中的主要微观参数：初始孔隙率0 f 和初始孔隙间距D;随后,在有限元计算中引入G-T 模型模拟了SA335 紧凑拉伸试样的断裂过程,讨论了试样尺寸对于J-R 曲线的影响．结果表明：试样厚度一致时,初始裂纹长度大的试样对应偏低的断裂韧性;当试样尺寸整体缩放时,较大尺寸的试样对应偏低的断裂韧性．最后,结合实例说明了试样整体尺寸对于主蒸汽管道临界裂纹长度的影响．     

镍基双晶体晶界断裂特性研究

对单晶体以及晶界平行晶粒裂纹和晶界垂直晶粒裂纹的双晶体的断韧性进行了实验研究,设计了三种三点弯曲试件,得到了单晶体和晶界的断裂韧性值,在晶界垂直晶粒裂纹的双晶体试验中,揭示了晶界对晶粒断裂的屏蔽效应;当裂纹距晶界某一特定长度时,断裂韧性值最大,理论分析和晶体滑移有限元数值分析揭示了这种屏蔽效应的机理,双晶晶界处的变表相容性导致了理解纹尖端应国和场的重新分布,并由此产生了晶界屏蔽效应。     

功能梯度夹层多个环形界面裂纹扭转冲击

研究位于功能梯度层和外部均匀材料之间多个环形界面裂纹的扭转冲击问题,功能梯度材料 (FGM)粘结在两种不同的弹性材料之间,功能梯度层和外部材料之间环形界面裂纹的数目是任意的．引进积分变换和位错密度函数将问题化为求解Laplace域里标准的Cauchy奇异积分方程,进而化为求解代数方程;应用Laplace数值反演技术,计算时域里的动应力强度因子(DSIF)．考查了结构几何尺度和材料特性对裂尖动态断裂特性的影响．数值结果表明,DSIF存在一个主峰,到达主峰后,在其相应的静态值附近波动并最终趋于稳定;增加FGM的梯度能减小DSIF的峰值．     

基于扩展有限元法的裂尖场精度研究

扩展有限元方法基于单元分解的基本思想,通过引入位移加强函数来表征裂纹的不连续性和裂尖的奇异性。在裂尖加强单元与常规单元之间有一层混合单元,当对裂尖特定区域进行加强时,混合单元个数相应增加,混合单元个数与计算精度存在一定联系。本文提出一种正方形裂尖加强区域的选择方式,可得到较单个加强和圆形加强精度更高、更稳定的计算结果。对于不同长度的裂纹,表征裂尖场奇异性所需的裂尖加强范围存在较大差异,以正方形裂尖加强方式进行计算,得到了不同裂纹长度下最优的加强尺寸。     

纳米晶体铝在单轴拉伸下的位错堆积对微裂纹扩展的影响

摘要：针对纳米晶体材料,研究了单轴拉伸载荷作用下纳米晶体铝中的裂纹与裂纹尖端发射的位错所形成的滑移面之间的相互作用。通过分布位错法,将裂纹和滑移面等效为均匀分布的连续位错,获得了裂纹面上应力场。并引入裂纹尖端的无位错区,研究了裂纹尖端无位错区对微裂纹的萌生和主裂扩展之间的影响。结果表明,不考虑裂纹尖端无位错区时,裂纹长度较短,会先在晶界处形成微裂纹,主裂纹较长时,主裂纹会直接穿晶扩展。滑移面与裂纹尖端夹角较大时,会增加裂纹尖端发射的位错个数,从而抑制主裂纹的扩展。考虑裂纹尖端无位错区时,无位错区先于晶界处出现微裂纹,通过主裂纹与微裂纹之间位错的相互发射,导致裂纹与尖端处微裂纹汇合,有效加速了主裂纹的扩展。     

Effects of intergrain sliding on crack growth in nanocrystalline materials

Theoretical models are suggested which describe the effects of intergrain sliding on crack growth in nanocrystalline metals and ceramics. Within the models, stress concentration near cracks initiates intergrain sliding which is non-accommodated at low temperatures and effectively accommodated at intermediate temperatures. The first model is focused on the non-accommodated intergrain sliding which leads to generation of dislocations at triple junctions of grain boundaries. These dislocations cause partial stress relaxation in the vicinities of crack tips and thereby hamper crack growth. It is shown that the non-accommodated intergrain sliding increases fracture toughness by 10–30% in nanocrystalline Al, Ni and 3C–SiC. The second model deals with the case of intermediate temperatures. Within this model, intergrain sliding is effectively accommodated by diffusion-controlled climb of grain boundary dislocations. The accommodated intergrain sliding in nanocrystalline materials results in crack blunting which, in its turn, leads to an increase (by a factor ranging from 1.1 to around 3, depending on temperature) of fracture toughness.     

Brittle versus ductile transition of nanocrystalline metals

The brittle versus ductile transition for conventional metals is dictated by the competition between dislocation emission and cleavage. For nanocrystalline metals with grain size below 25 nm, however, dislocation activities are suppressed and the classic theory fails to apply. In this paper, one of the competing mechanisms that control the brittle versus ductile transition of nanocrystalline metals is found to be the grain boundary dominated creep deformation versus the grain boundary decohesion. A model is proposed to quantify the crack propagation in nanocrystalline metals. The effects of material properties, initial configuration and applied loads on the property of crack propagation are addressed. It is concluded that either the increases in the initial crack length, the applied load and the grain boundary damage, or the deterrence in creep deformation, accelerate the crack propagation, and vice versa.     

基于GMTS准则的岩石复合型断裂机理研究

使用国际岩石力学协会规定的半圆盘岩石试件,加工不同倾角的直裂纹试样,通过三点弯曲加载试验得到不同I-II复合比断裂的断裂韧性和初始断裂角．传统裂纹扩展准则忽视了常数项即T应力及更高阶项的影响,导致该扩展准则的理论预测结果存在较大缺陷,本文通过考虑常数项,建立广义最大周向应力准则(GMTS)．在此基础上,分别采用传统的裂纹扩展准则和考虑T应力的裂纹扩展准则预测不同复合比裂纹的断裂韧性和初始扩展角,然后对比理论预测结果和实验结果．分析可得：常数项即T应力对断裂的临界应力强度因子和初始断裂角的影响是不可忽略的,且II型断裂占比较大时影响更大,广义最大周向应力准则预测值与实验测试结果之间的误差最小．     

Fracture toughness from the standpoint of softening hyperelasticity

Fracture toughness of brittle materials is calibrated in experiments where a sample with a preexisting crack/notch is loaded up to a critical point of the onset of static instability. Experiments with ceramics, for example, exhibit a pronounced dependence of the toughness on the sharpness of the crack/notch: the sharper is the crack the lower is the toughness. These experimental results are not entirely compatible with the original Griffith theory of brittle fracture where the crack sharpness is of minor importance.¹To explain the experimental observations qualitatively we simulate tension of a thin plate with a small crack of a finite and varying sharpness. In simulations, we introduce the average bond energy as a limiter for the stored energy of the Hookean solid. The energy limiter induces softening, indicating material failure. Thus, elasticity with softening allows capturing the critical point of the onset of static instability of the cracked plate, which corresponds to the onset of the failure propagation at the tip of the crack. In numerical simulations we find, in agreement with experiments, that the magnitude of the fracture toughness cannot be determined uniquely because it depends on the sharpness of the crack: the sharper is the crack, the lower is the toughness.Based on the obtained results, we argue that a stable magnitude of the toughness of brittle materials can only be reached when a characteristic size of the crack tip is comparable with a characteristic length of the material microstructure, e.g. grain size, atomic distance, etc. In other words, the toughness can be calibrated only under conditions where the hypothesis of continuum fails.     

On the toughening of brittle materials by grain bridging: Promoting intergranular fracture through grain angle, strength, and toughness

The structural reliability of many brittle materials such as structural ceramics relies on the occurrence of intergranular, as opposed to transgranular, fracture in order to induce toughening by grain bridging. For a constant grain boundary strength and grain boundary toughness, the current work examines the role of grain strength, grain toughness, and grain angle in promoting intergranular fracture in order to maintain such toughening. Previous studies have illustrated that an intergranular path and the consequent grain bridging process can be partitioned into five distinct regimes, namely: propagate, kink, arrest, stall, and bridge. To determine the validity of the assumed intergranular path, the classical penetration/deflection problem of a crack impinging on an interface is re-examined within a cohesive zone framework for intergranular and transgranular fracture. Results considering both modes of propagation, i.e., a transgranular and intergranular path, reveal that crack-tip shielding is a natural outcome of the cohesive zone approach to fracture. Cohesive zone growth in one mode shields the opposing mode from the stresses required for cohesive zone initiation. Although stable propagation occurs when the required driving force is equivalent to the toughness for either transgranular or intergranular fracture, the mode of propagation depends on the normalized grain strength, normalized grain toughness, and grain angle. For each grain angle, the intersection of single path and multiple path solutions demarcates “strong” grains that increase the macroscopic toughness and “weak” grains that decrease it. The unstable transition to intergranular fracture reveals that an increasing grain toughness requires a growing region of the transgranular cohesive zone be near the cohesive strength. The inability of the body to provide the requisite stress field yields an overdriven and unstable configuration. The current results provide restrictions for the achievement of substantial toughening through intergranular fracture.     

柱状电极屈曲或裂纹扩展的临界力学参数研究

柱形结构电极是近年来使用最为广泛的锂电池电极结构之一.本文以硅材料细长柱形电极为例,研究了充电电流大小、电极长径比、初始裂纹长度以及断裂韧性对于电极的屈曲现象和裂纹扩展现象发生时间的影响.计算结果表明,屈曲与裂纹扩展现象出现的先后顺序与充电电流大小无关;具有小的长径比,大的初始裂纹长度以及较小断裂韧性的电极,裂纹扩展比屈曲现象更早发生.对于硅材料,不同长径比的电极具有不同临界断裂韧性值,当材料的断裂韧性小于该临界值,在锂化过程中裂纹扩展会先于屈曲现象发生;该临界断裂韧性值随初始裂纹长度的增加而增加.本文的结论对于电极的结构设计以及材料选取具有一定指导意义.     

Special rotational deformation as a toughening mechanism in nanocrystalline solids

A theoretical model is suggested which describes the effect of special rotational deformation on crack growth in deformed nanocrystalline ceramics and metals. Within the model, the special rotational deformation (driven by the external stress concentrated near the tip of a mode I crack) occurs in a nanograin through formation of immobile disclinations whose strengths gradually increase during the formation process conducted by grain boundary sliding and diffusion. The special rotational deformation releases, in part, local stresses near the crack tip, thus serving as a toughening mechanism in nanocrystalline materials. The effects of the special rotational deformation on the growth of pre-existent, comparatively large cracks in nanocrystalline metals and ceramics are estimated.