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。     

压电双材料界面钝裂纹附近螺型位错的屏蔽效应与发射条件

研究了压电双材料界面钝裂纹附近螺型位错的屏蔽效应与发射条件.应用保角变换技术,得到了复势函数与应力场的封闭形式解,讨论了位错方位、双材料电弹常数及裂纹钝化程度对位错屏蔽效应和发射条件的影响.结果表明,Burgers矢量为正的螺型位错可以降低界面钝裂纹尖端的应力强度因子(屏蔽效应),屏蔽效应随位错方位角及位错与裂纹尖端距...     

穿透夹杂界面的半无限楔形裂纹尖端螺型位错的发射研究

摘要：研究了穿透圆形夹杂界面的半无限楔形裂纹与裂纹尖端螺型位错的干涉问题。应用复变函数解析延拓技术与奇性主部分析方法,得到了位错位于半圆形夹杂内部时,半无限基体和半圆形夹杂内复势函数的解析解。然后利用保角映射技术得到了穿透圆形夹杂界面的半无限楔形裂纹尖端螺型位错产生的应力场以及作用在位错上的位错力的解析表达式。主要讨论了螺型位错对裂纹的屏蔽效应以及从楔形裂纹尖端发射位错的临界载荷条件。研究结果表明正的螺型位错可以削弱楔形裂纹尖端的应力强度因子,屏蔽裂纹的扩展,屏蔽效应随位错方位角的增大而减小。位错发射所需的无穷远临界应力随发射角的增加而增大,最可能的位错发射角度为零度,直线裂纹尖端位错的发射比楔形裂纹尖端位错的发射更容易,硬基体抑制位错的发射。     

螺型位错偶极子与界面钝裂纹的干涉效应

研究了螺型位错偶极子与界面钝裂纹的干涉效应.应用保角变换技术,得到了复势函数与应力场的封闭解析解,讨论了位错偶极子方位、臂长及裂纹钝化程度对位错偶板子屏蔽效应和发射条件的影响.结果表明,与单个螺型位错不同,螺型位错偶极子与x轴夹角在一定范围内时才可以降低界面钝裂纹尖端的应力强度因子(屏蔽效应),屏蔽效应随偶板子臂长的增大而增强,随裂纹钝化程度的增大而增强,屏蔽区域也随裂纹钝化程度的增大而增大;位错偶极子发射所需的临界无穷远加载随偶极子臂长的增加而减小,随位错方位角及裂纹钝化程度的增加而增大;最可能的位错偶极子发射角度为0.螺型位错偶极子的发射比单个螺型位错的发射要困难.本文解答的特殊情况与相关文献给出的解答一致.     

温度对金属材料钼位错发射的影响

应用关联参照模型、随位错位置变化的柔性位移边界条件和三维分子动力学方法研究了体心立方（ＢＣＣ）金属晶体钼在不同温度下裂尖发射位错的力学行为，随着温度的提高，不但发射位错的临界应力强度因子下降而且在同一应 度因子条件下，发射位错的数量出增加，位错速度和不全位错之间的扩展距离对温度不敏感，在位错发射过程中，发现了稳定的和不稳定的两个变形状态，在稳定的有状态，位错发射后，塞积在远离裂纹尖端处；必须增加外     

增强相/夹杂内螺位错偶极子与含共焦钝裂纹椭圆夹杂的干涉效应

运用弹性力学的复势方法,研究了纵向剪切下增强相/夹杂内螺型位错偶极子与含共焦钝裂纹椭圆夹杂的干涉效应,得到了该问题复势函数的封闭形式解答,由此推导出了夹杂区域的应力场、作用在螺型位错偶极子中心的像力和像力偶矩以及裂纹尖端应力强度因子级数形式解。并分析了位错偶极子倾角 、钝裂纹尺寸和材料常数对位错像力、像力偶矩以及应力强度因子的影响。数值计算结果表明：位错像力、像力偶矩以及应力强度因子均随位错偶极子倾角做周期变化;夹杂内部的椭圆钝裂纹明显增强了硬基体对位错的排斥,减弱了软基体对位错的吸引,且对于硬夹杂,位错出现了一个不稳定平衡位置,该平衡位置随钝裂纹曲率的增大不断向界面靠近;变化 值将出现改变位错偶极子对应力强度因子作用方向的临界值。     

一维六方准晶中平行螺型位错与有限长裂纹的相互作用

本文采用解析函数理论,利用Muskhelishvili著名的复变函数方法,给出了无限大一维六方准晶中多条平行螺型位错与有限长直裂纹相互作用的解析解,并得到了裂纹尖端的场强度因子和作用在位错上的像力.数值算例讨论了镜像力、场强度因子随位错位置的变化,及位错的位置与分布对广义应力场的影响．     

位错偶极子和Ⅲ型界面裂纹的干涉效应

研究了多晶体材料中螺型位错偶极子和界面裂纹的弹性干涉作用.利用复变函数方法,得到了该问题复势函数的封闭形式解答.求出了由位错偶极子诱导的应力场和裂纹尖端应力强度应子,分析了偶极子的方向,偶臂和位置以及材料失配对应力强度因子的影响.推导了作用在螺型位错偶极子中心的像力和力偶矩,并讨论了界面裂纹几何条件和不同材料特征组合对位错偶极子平衡位置的影响规律.结果表明,裂纹尖端的螺型位错偶极子对应力强度因子会产生强烈的屏蔽或反屏蔽效应.同时,界面裂纹对螺型位错偶极子在材料中运动有很强的扰动作用.     

位错、裂纹与断裂

本文评述近些年来在位错、裂纹与断裂方面的研究进展.内容主要包括:①以屈服强度温度依赖性为例,讨论力学性能的结构敏感性,使其对位错芯结构与塑性之间的关系有一定的认识;②纯弹性断裂总是极少的,因之在加载应力作用下,裂纹顶端总是存在着或大或小的塑性区,存在着裂纹与位错的交互作用.这里以我们自己的工作为主,重点讨论裂端位错过程的研究结果.      

压电螺位错与含界面裂纹圆形涂层夹杂的干涉

研究位于基体或夹杂中任意点的压电螺型位错与含界面裂纹圆形涂层夹杂的电弹耦合干 涉问题. 运用复变函数方法,获得了基体,涂层和夹杂中复势函数的一般解答. 典型例 子给出了界面含有一条裂纹时,复势函数的精确级数形式解. 基于已获得的复势函数和广 义Peach-Koehler公式,计算了作用在位错上的像力. 讨论了裂纹几何条件,涂层厚度和材 料特性对位错平衡位置的影响规律. 结果表明,界面裂纹对涂层夹杂附近的位错运动有很大 的影响效应,含界面裂纹涂层夹杂对位错的捕获能力强于完整粘结情况;并发现界面裂纹长 度和涂层材料常数达到某一个临界值时可以改变像力的方向. 解答的特殊情形包含了以 往文献的几个结果.     

Effect of temperature on dislocation emission from crack tip for BCC metal Mo

The effect of thermally activated energy on the dislocation emission from a crack tip in BCC metal Mo is simulated in this paper. Based on the correlative reference model on which the flexible displacement boundary scheme is introduced naturally, the simulation shows that as temperature increases the critical stress intensity factor for the first dislocation emission will decrease and the total number of emitted dislocations increase for the same external load. The dislocation velocity and extensive distance among partial dislocations are not sensitive to temperature. After a dislocation emission, two different deformation states are observed, the stable and unstable deformation states. In the stable deformation state, the nucleated dislocation will emit from the crack tip and piles up at a distance far away from the crack tip, after that the new dislocation can not be nucleated unless the external loading increases. In the unstable deformation state, a number of dislocations can be emitted from the crack tip continuously under the same external load. The project is supported by the National Natural Science Foundation of China.     

Energy dissipation mechanisms in ductile fracture

The objective is to investigate energy dissipation mechanisms that operate at different length scales during fracture in ductile materials. A dimensional analysis is performed to identify the sets of dimensionless parameters which contribute to energy dissipation via dislocation-mediated plastic deformation at a crack tip. However, rather than using phenomenological variables such as yield stress and hardening modulus in the analysis, physical variables such as dislocation density, Burgers vector and Peierls stress are used. It is then shown via elementary arguments that the resulting dimensionless parameters can be interpreted in terms of competitions between various energy dissipation mechanisms at different length scales from the crack tip; the energy dissipations mechanisms are cleavage, crack tip dislocation nucleation and also dislocation nucleation from a Frank-Read source. Therefore, the material behavior is classified into three groups. The first two groups are the well-known intrinsic brittle and intrinsic ductile behavior. The third group is designated to be extrinsic ductile behavior for which Frank-Read dislocation nucleation is the initial energy dissipation mechanism. It is shown that a material is predicted to exhibit extrinsic ductility if the dimensionless parameter bρ_disl^1/2 (b is Burgers vector, ρ_disl is dislocation density) is within a certain range defined by other dimensionless parameters, irrespective of the competition between cleavage and crack tip dislocation nucleation. The predictions compare favorably to the documented behavior of a number of different classes of materials.     

QUASICONTINUUM SIMULATION OF NANOINDENTATION OF NICKEL FILM

A large-scale atom simulation of nanoindentation into a thin nickel film using thequasicontinuum method was performed. The initial stages of the plasticity deformation of nickelwere studied. Several useful results were obtained as follows: (1)The response of the load versusindentation depth—on the load versus indentation depth curve, besides the straight parts cor-responding to the elastic property of nickel, the sudden drop of the load occurred several times;(2) The phenomena of dislocation nucleation—the dislocation nucleation took place when theload descended, which makes it clear that dislocation nucleation causes the drop of the load;(3)The mechanism of the dislocation emission—the Peierls-Nabarro dislocation model and a pow-erful criterion were used to analyze the dislocation emission. And the computational value was ingood agreement with the predict value; (4) The density of geometrically necessary dislocations.A simple model was used to obtain the density of geometrically necessary dislocations beneaththe indenter. Furthermore, the influence of the boundary conditions on the simulation results wasdiscussed.     

Molecular dynamics simulation of crack-tip processes in copper

The crack tip processes in copper under mode II loading have been simulated by a molecular dynamics method. The nucleation, emission, dislocation free zone (DFZ) and pile-up of the dislocations are analyzed by using a suitable atom lattice configuration and Finnis & Sinclair potential. The simulated results show that the dislocation emitted always exhibits a dissociated fashion. The stress intensity factor for dislocation nucleation, DFZ and dissociated width of partial dislocations are strongly dependent on the loading rate. The stress distributions are in agreement with the elasticity solution before the dislocation emission, but are not in agreement after the emission. The dislocation can move at subsonic wave speed (less than the shear wave speed) or at transonic speed (greater than the shear wave speed but less than the longitudinal wave speed), but at the longitudinal wave speed the atom lattice breaks down. The project supported by the National Natural Science Foundation of China     

Peierls框架与裂纹尖端位错行为

本文在Ｐｅｉｅｒｌｓ框架下对裂纹尖端位错成核与发射问题进行了严格的数学分析，在修正Ｒｉｃｅ设相的基础上，建立了一组新的控制方程，应力场与位错密度场分别表示成第一类与第二类切比雪夫多项式的级数，相应的张开位移与滑错位移以用三角级数表示。通过离散的方法，控制方程转化为一组非线性代数方程。用Ｎｅｗｔｏｎ－Ｒａｐｈｓｏｎ方法求解这组方程，得到远场为纯剪，纯拉及两者复合情况下的解答。计算结果清楚地揭示了裂纹     

Nanoscale rotational deformation effect on dislocation emission from an elliptically blunted crack tip in nanocrystalline materials

A grain size-dependent model is theoretically established to describe the effect of a special physical micromechanism of plastic flow on the dislocation emission from an elliptical blunt crack tip in nanocrystalline solids. The micromechanism represents the fast nanoscale rotational deformation (NRD) occurring through collective events of ideal nanoscale shear near crack tips, which as a stress source is approximately equivalent to a quadrupole of wedge disclinations. By the complex variable method, the grain size-dependent criterion for the dislocation emission from an elliptical blunt crack tip is derived. The influence of the grain size and the features of NRD on the critical stress intensity factors for dislocation emission is evaluated. The results indicate that NRD releases the high stresses near the crack tip region and thereby enhances the critical stress intensity factor for dislocation emission. The NRD has great influence on the most probable angle for dislocation emission. The critical stress intensity factor will increase with the increment of the grain size, which means the emission of the dislocation becomes more difficult for larger grain size due to the effect of NRD.     

镍单晶薄膜纳米压痕的准连续介质模拟

用准连续介质方法模拟了大规模原子的镍薄膜在纳米压痕下发生初始塑性变形的行为.主要得到了:(1)载荷-位移响应.在载荷位移曲线上除了反应晶体弹性性质的直线外还有数次的载荷突然下降过程.(2)位错形核现象.与载荷-位移曲线上的载荷突然下降相对应的在受压的晶体上发现了位错形核现象,说明载荷的下降是因为位错形核引起的.(3)位错的发射机制.用Peierls-Nabarro位错模型以及能量法分析了位错的发射机制,理论值与计算值吻合较好.(4)几何必需位错密度.用一个简单的模型计算了几何必须位错密度.此外还考虑了边界条件对模拟结果的影响.     

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.