裂纹起裂方向及扩展路径的数值模拟

采用FRANC2D断裂力学分析软件分析了含裂纹构件的断裂问题,通过在裂纹尖端附近采用1/4奇异等参元,很好的刻画了裂纹尖端场的1/√r奇异性,通过对若干典型断裂问题的计算,得到了裂纹尖端的断裂参数,模拟了有限板中心斜裂纹和边斜裂纹的扩展过程,并对裂纹的起裂方向进行了预测,与实验结果进行对比具有较好的一致性.分析结果表明,FRANC2D软件对线弹性材料的断裂问题计算结果精度很高,可以模拟裂纹的扩展路径.利用该软件的计算结果对进一步深入分析断裂问题有很大帮助,可以为工程断裂分析提供参考.     

复合型裂纹断裂的新准则

以Ⅰ-Ⅱ复合型裂纹为研究对象，对裂纹尖端的塑性区分布规律进行了理论分析。引入两组评价裂纹尖端应力场对裂纹扩展影响的参数，考虑裂纹尖端存在的局部塑性变形，并采纳如下两个假设，(1)裂纹沿最短路径穿过塑性区向弹性区扩展，(2)当在扩展方向上的弹塑性边界极半径r大于其临界极半径rC时，裂纹开始扩展。在此基础上，导出了新的复合型裂纹断裂准则，并与现有部分断裂准则及实验结果进行了对比。结果表明：新建立的复合裂纹断裂准则与实验结果吻合程度非常高。最后，阐明了以单一KIC或KIIC建立的复合裂纹扩展准则的局限性以及考虑裂纹尖端当前应力场的必要性。     

PZT-4紧凑拉伸试样的断裂分析

基于线性压电材料的复势理论，通过解析分析，导出了一种分析有限压电板裂纹问题的 解析数值方法. 首先，计算了含中心裂纹有限板的断裂参数，与Woo和Wang的解析数值 法(Int J Fract, 1993, 62: 203$\sim$218)相比较，表明该方法具有很高的精度和 很好的计算效率. 随后，采用该方法和有限元 法计算了PZT-4紧凑拉伸试样在绝缘裂纹面边界条件下断裂时的断裂参数，发现各断裂参数 的临界值分散性很大，不能作为压电材料的单参数断裂准则. 进而，针对试样真实的裂隙形 状，采用有限元法计算了裂隙尖端的应力、电位移场，比较了裂隙内介质的介电性能对裂隙 尖端场的影响，计算了带微裂纹的真实裂隙模型的断裂参数并进行了理论分析.     

空孔与运动裂纹相互作用的动焦散线实验研究

为研究预制裂纹不同偏移距离时运动裂纹与空孔的相互作用规律，采用动态焦散线实验系统，将预制裂纹的偏移距离设定为唯一变量，对含空孔的有机玻璃(PMMA)试件进行冲击三点弯实验。研究表明，存在两个临界距离:（6 mm (2 R )、9 mm (3 R )），在该偏移距离下，裂纹扩展轨迹、动态断裂特性发生显著改变:(1) 预制裂纹偏移距离不大于3 mm时，裂纹贯穿空孔，发生二次起裂，且二次起裂的速度与应力强度因子显著大于一次起裂，无偏移时裂纹轨迹的分形维数为最小值；(2) 偏移距离增大至6 mm时，裂纹不再贯穿空孔，空孔对裂纹先吸引后排斥，裂纹速度与应力强度因子先减小后增大，裂纹轨迹的分形维数达到最大值；(3) 偏移距离大于6 mm时，空孔对裂纹的吸引作用逐渐减小，大于9 mm后，空孔对裂纹的吸引不再显著，裂纹起裂后即向落锤加载方向扩展直至贯穿试件。     

分子动力学模拟含不同初始缺陷单晶镍的断裂行为与应力演化

断裂是一个跨尺度复杂的物理过程，对宏观尺度的断裂行为已有深入的研究和发展，然而对微观尺度的断裂行为及断裂过程中应力场的变化缺乏深入的理解。本文通过分子动力学模拟，研究了具有不同初始缺陷(尖锐裂纹、钝裂纹和孔洞)的单晶镍的断裂行为和应力分布特征。结果表明，不同的初始缺陷导致了不同的断裂机制、断裂强度和抗断裂性能。含初始孔洞的单晶镍样品有最高的断裂强度和最强的抗断裂性能，这与孔洞扩展过程中堆积层错的形成密切相关。其次是含初始钝裂纹的样品，在裂纹扩展过程中出现由[100]超位错发射引起的裂尖钝化；含尖锐裂纹的样品表现为脆性断裂，裂尖原子没有出现微结构的变化，其强度和抗断裂性能最低。此外，不同的初始缺陷也会导致断裂过程中应力分布的变化，对含有尖锐裂纹的脆性断裂试样，高应力(拉伸应力、平均应力和米塞斯应力)总是出现在扩展裂纹的裂尖。而对于含有钝裂纹或孔洞的韧性断裂试样，高应力不仅分布在裂尖，也分布在位错发射和堆积层错形成的区域，在裂纹/孔洞扩展之前，应力随着加载时间的增加而迅速增加，而一旦裂纹或孔洞开始扩展，应力增加非常缓慢或几乎不增加，但拉伸应力值始终大于平均应力和米塞斯应力值。这表明，在I型...     

界面裂纹萌生与扩展的分子动力学模拟

运用分子动力学模拟方法研究了裂纹在界面端处萌生与沿界面扩展的临界条件. 模拟考虑了一双相材料的3种模型，即构成90°/90°和 90°/180°夹角的两个界面端和一个界面裂纹. 模拟采用了包含原子区域与连续区域的并发型多尺度模型，即在界面端尖端和裂纹尖端附近 采用分子动力学(MD)方法，MD区域之外则按照线弹性有限元方法分析. 结果表明，在断裂启动时刻，3个模型沿界面的最大应力均达到界面理想强度；而且，其界 面能恰好足以克服界面材料的本征内聚能. 因此，界面端裂纹萌生与沿界面扩展的断裂条件可以通过界面理想强度和内聚能联系起来. 并基于模拟计算结果提出了界面断裂启动的统一准则.     

三维裂纹扩展轨迹的边界元数值模拟

提出了一种对三维裂纹扩展轨迹进行数值模拟的新方法。采用一种新的具有C^1连续性、高精度的单节点二次边界单元，使边界元(BEM)的分析效率和裂纹张开位移(COD)、应力强度因子(SIF)的精度大大提高。采用裂纹张开位移全场拟合法(GCDFP)求出裂纹面前缘的SIF，所得到的SIF达到与所用的COD资料同样的精度。使用Paris公式求出裂纹前缘各点的裂纹扩展增量，并用三次B样条函数对这些增量进行拟合，得到新的光滑裂纹前缘。根据以上思想方法，开发了具有较高的计算效率和精度的数值模拟软件。此软件可以自动跟踪裂纹扩展，得到裂纹扩展的轨迹。运用该软件对椭圆和矩形裂纹的扩展轨迹进行了数值模拟。其结果与理论上的预言完全一致，裂纹最后都趋于一个圆裂纹，具有实际指导意义。     

氢对体心立方金属裂纹尖端变形过程的影响

利用扫描电镜(SEM)原位拉伸,观察了 Fe-3％Si 单晶体裂尖的变形和断裂过程。用网格法加计算机处理系统测出了裂纹尖端的应变场及氢对应变分布的影响。发现在Ⅰ型载荷下,主裂纹尖端应变呈不对称分布,形变带内应变梯度很大。形变带内应变分布为: ε_(yy)=f(x)exp(-Ar).氢不改变裂纹尖端塑性变形特征,但能明显促进裂纹尖端局部塑性变形的非均匀增加。     

虚拟裂纹闭合法在结构断裂分析中的应用

基于三维虚拟裂纹技术(3DVCCT),利用ABAQUS用户单元子程序(UEL)编写裂纹界面单元,使3DVCCT集成于ABAQUS软件中,直接计算出裂纹的断裂参数.采用此方法对连杆杆身表面裂纹进行研究,得到了连杆裂纹的应力强度因子的分布规律.     

温度影响下砂岩的细观破坏及变形场的DSCM表征

通过扫描电镜(SEM)实时在线观察研究了温度影响下砂岩的细观破坏，观察到砂岩的脆 性断裂可同时发生在不同地方、不同矿物可能独立承载和裂纹分叉等现象. 温度低于 100°C时，主裂纹附近有许多微裂纹和支裂纹发生；而温度超过150°C之后， 表面很少出现支裂纹或二次裂纹. 随着温度的升高，砂岩的断裂韧性有先升高后降低的趋势， 150°C左右是断裂韧性变化的临界温度. 随着温度的升高，砂岩的细观断裂机制有由 脆性机制向延性机制转变的趋势，抵抗和协调变形的能力都得到增强. 细观尺度下砂岩的破 坏机制有沿颗粒断裂、穿颗粒断裂及其混合断裂，其中沿颗粒断裂机制占主导地位，这是由 于沿颗粒破坏需要消耗较少的能量，而穿颗粒断裂需消耗较多的能量. 利用数字散斑相关方 法(digital speckle correlation method, DSCM)对SEM下砂岩破坏的细观变形场 进行了测量，这表明利用DSCM表征岩石的局部变形场的连续式测量是可行的.     

Determination of mixed-mode stress intensity factors, fracture toughness, and crack turning angle for anisotropic foam material

A numerical and experimental investigation for determining mixed-mode stress intensity factors, fracture toughness, and crack turning angle for BX-265 foam insulation material, used by NASA to insulate the external tank (ET) for the space shuttle, is presented. BX-265 foam is a type of spray-on foam insulation (SOFI), similar to the material used to insulate attics in residential construction. This cellular material is a good insulator and is very lightweight. Breakup of segments of this foam insulation on the shuttle ET impacting the shuttle thermal protection tiles during liftoff is believed to have caused the space shuttle Columbia failure during re-entry. NASA engineers are interested in understanding the processes that govern the breakup/fracture of this material from the shuttle ET. The foam is anisotropic in nature and the required stress and fracture mechanics analysis must include the effects of the direction dependence on material properties. Material testing at NASA Marshall Space Flight Center (MSFC) has indicated that the foam can be modeled as a transversely isotropic material. As a first step toward understanding the fracture mechanics of this material, we present a general theoretical and numerical framework for computing stress intensity factors (SIFs), under mixed-mode loading conditions, taking into account the material anisotropy. We present SIFs for middle tension – M(T) – test specimens, using 3D finite element stress analysis (ANSYS) and FRANC3D fracture analysis software. SIF values are presented for a range of foam material orientations. Mode I fracture toughness of the material is determined based on the SIF value at failure load. We also present crack turning angles for anisotropic foam material under mixed-mode loading. The results represent a quantitative basis for evaluating the strength and fracture properties of anisotropic foam insulation material.     

Interactive finite element analysis of fracture processes: An integrated approach

A fracture analysis program has been developed which incorporates the concepts of finite element analysis, fracture mechanics, computer graphics, finite element postprocessing, automatic mesh generation, and data base design. The program FRANC (FRacture ANalysis Code) is a tool which allows a practicing engineer or a researcher to perform an incremental fracture analysis at his desk. The program draws on a substantial experience base and this paper describes the philosophy of the program and the integration of its parts. Also, a number of example problems will be presented which show some results of incremental fracture analyses.     

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。     

三维计算断裂力学

断裂力学理论从1921年Griffith研究玻璃的脆性断裂问题开始,经历了从线弹性体系到弹塑性以及蠕变理论体系、从单参数到多参数体系和从理想的二维平面理论到实际三维含裂纹结构的三维断裂理论的发展历程。针对应力强度因子K和J积分以及C(t)积分的计算方法从理想化模型的理论计算发展到实际复杂工程结构裂纹体计算的各种商业软件平台以及专业的断裂理论分析平台。尤其是随着计算机技术的发展,对三维含裂结构的静态和扩展裂纹的计算模拟已经能够融入计算机辅助设计。结合本研究组近30年来在三维疲劳断裂理论和应用研究方面的体会,简述了三维计算断裂力学从裂纹体应力应变分析和断裂参数计算到三维蠕变断裂和疲劳裂纹扩展模拟的国内外进展,并对涉及的计算方法,包括原子尺度和跨尺度的计算模拟,以及目前面临的挑战性问题作了简要介绍和分析。     

A method to discretize non‐planar fractures for 3D subsurface flow and transport simulations

A method is presented to discretize inclined non‐planar 2D fractures within a 3D finite element grid for subsurface flow and transport simulations. Each 2D fracture is represented as a triangulated surface. Each triangle is then discretized by 2D fracture elements that can be horizontal, vertical or inclined and that can be triangular or rectangular. The 3D grid representing a porous rock formation consists of hexahedra and can be irregular to allow grid refinement. An inclined fracture was discretized by (a) inclined triangles and (b) orthogonal rectangles and flow/transport simulations were run to compare the results. The comparison showed that (i) inclined fracture elements must be used to simulate 2D transient flow, (ii) results of 2D/3D steady‐state and 3D transient flow simulations are identical for both discretization methods, (iii) inclined fracture elements must be used to simulate 2D/3D transport because orthogonal fracture elements significantly underestimate concentrations, and (iv) orthogonal elements can be used to simulate 2D/3D transport if fracture permeability is corrected and multiplied by the ratio of fracture surface areas (orthogonal to inclined). Groundwater flow at a potential site for long‐term disposal of spent nuclear fuel was simulated where a complex 3D fracture network was discretized with this technique. The large‐scale simulation demonstrates that the proposed discretization procedure offers new possibilities to simulate flow and transport in complex 3D fracture networks. The new procedure has the further advantage that the same grid can be used for different realizations of a fracture network model with no need to regenerate the grid. Copyright © 2007 John Wiley & Sons, Ltd.     

用激光三角法重构混凝土断裂面三维轮廓与断裂能分析

采用基于激光三角法的测试方法对混凝土断面表面轮廓进行了三维重构,初步的断面分析结果表明,相对于传统断面分析方法,本试验方法实现了断面的无损、快速和准确的测量分析,在测量到的断面基础上,给出了本试验所得到的混凝土材料断裂能与分维的关系。     

Multi-Scale Stereo-Photogrammetry System for Fractographic Analysis Using Scanning Electron Microscopy

Current systems for photogrammetry analysis rely mainly on two-dimensional visualization methods, particularly Scanning Electron Microscopy (SEM). The absence of three-dimensional information prevents the determination of important quantitative features such as local roughness and precludes a deeper comprehension of the failure mechanisms. This paper describes a new multi-scale stereo-photogrammetry system for inspection of fracture surfaces based on SEM images. The system facilitates the reconstruction of complete 3D fracture surfaces and provides interactive visualization of the multi-scale structure, thus offering better insight into fracture surfaces at different levels of detail. In particular, a new method has been developed for geometric reconstruction of a 3D textured mesh from SEM stereo images. The mesh is represented as a 3D geometric multi-resolution structure. The sampled images are represented in the form of a multi-scale hierarchical textured structure. Thus, the global shape of the sample is represented by a 3D mesh, while its micro details are represented by textured data. This multi-scale and hierarchical structure allows interactive multi-scale navigation of the 3D textured mesh. The Regions of Interest (ROI) can actually be inspected interactively at different scales by means of optical or digital zooming. Thus, the digital model can be visualized and the behavior of the 3D material can be analyzed interactively. The contributions of this research include: (a) a new 3D multi-scale reconstruction method for SEM stereo images; (b) a new visualization module for multi-scale inspection, modeling and analysis of micro-structures for a variety of materials; and (c) 3D insight into and better understanding of fracture phenomena for material micro-structures. The feasibility of the proposed method is demonstrated on samples of different materials, and a performance analysis is applied on the resulting multi-scale model. The roughness calculation was verified against roughness calculation applied to the optical profilometer.     

受拉钢筋混凝土构件破坏过程中的等间距裂缝形成机理

采用三维真实材料破坏过程分析RFPA3D(Three-Dimension Realistic Failure Process Anal-ysis)系统,先对受拉钢筋混凝土构件在轴向拉伸作用下的破坏过程进行了数值试验研究,并与相应的实验结果进行了比较,数值试验得到的结果都与实验吻合的较好.并在此验证基础上利用RFPA系统着重分析了混凝土及钢筋在拉伸条件下的破坏过程中的应力分布情况过程及破坏机理.数值试验结果表明,受拉钢筋混凝土结构破坏过程中存在“等间距裂缝现象“,并通过破坏分析再现了裂缝形成、加密直至饱和的演化过程.本文还用三维程序,通过多个不同混凝土厚度钢筋混凝土构件的数值试验,研究了混凝土厚度对裂缝数及裂缝间距的影响,进一步发现饱和裂缝间距与混凝土厚度之间存在正比关系.随混凝土保护层厚度的增加而增大,裂缝数量随混凝土保护层厚度的增加而减少.     

The advanced simulation of fatigue crack growth in complex 3D structures

An advanced incremental crack growth algorithm for the three-dimensional (3D) simulation of fatigue crack growth in complex 3D structures with linear elastic material behavior is presented. To perform the crack growth simulation as effectively as possible an accurate stress analysis is done by the boundary-element method (BEM) in terms of the 3D dual BEM. The question concerning a reliable 3D crack growth criterion is answered based on experimental observations. All criteria under consideration are numerically realized by a predictor–corrector procedure. The agreement between numerically determined and experimentally observed crack fronts will be shown on both fracture specimens and an industrial application.     

Monte Carlo simulation of complex cohesive fracture in random heterogeneous quasi-brittle materials: A 3D study

In a recent publication (Yang et al., 2009. Monte Carlo simulation of complex cohesive fracture in random heterogeneous quasi-brittle materials. Int. J. Solids Struct. 46 (17) 3222–3234), we developed a finite element method capable of modelling complex two-dimensional (2D) crack propagation in quasi-brittle materials considering random heterogeneous fracture properties. The present study extends the method to model three-dimensional (3D) problems. First, 3D cohesive elements are inserted into the initial mesh of solid elements to model potential crack surfaces by a specially designed, flexible and efficient algorithm and corresponding computer program. The softening constitutive laws of the cohesive elements are modelled by spatially-varying 3D Weibull random fields. Monte Carlo simulations are then carried out to obtain statistical information of structural load-carrying capacity. A concrete cube under uniaxial tension was analysed as an example. It was found that as the 2D heterogeneous model, the 3D model predicted realistic, complicated fracture processes and load-carrying capacity of little mesh-dependence. Increasing heterogeneity in terms of the variance in the tensile strength random fields resulted in lower mean and higher standard deviation of peak loads. Due to constraint effects and larger areas of unsmooth, non-planar fracture surfaces, 3D modelling resulted in higher mean and lower standard deviation of peak loads than 2D modelling.