基于随机分布代表性体积的颗粒增强复合材料拉伸断裂破坏模拟

为研究组分材料性能对材料断裂破坏行为的影响,基于计算细观力学,建立施加周期性边界条件的代表性体积单元,对随机分布颗粒增强复合材料的拉伸断裂破坏进行数值模拟.为选取更具代表性的体积单元,采用径向分布函数和平均近邻距离变异系数两种统计方法对生成的随机分布细观结构进行评估.并在此基础上,对复合材料的有效弹性常数进行分析,以验证所选代表性体积单元的有效性.利用选定的代表性体积单元,运用扩展有限元方法对A l2 O 3/T iB2陶瓷复合材料的拉伸断裂破坏进行了数值模拟,重点讨论了颗粒断裂韧性、界面粘结强度对裂纹萌生及扩展的影响.研究结果表明,不同的颗粒断裂韧性下,复合材料的断裂行为基本一致;界面粘结强度对裂纹的萌生及扩展路径影响较为显著,破坏载荷的变化随界面粘结强度的增加分为三个不同阶段.对于A l2 O 3/T iB2陶瓷复合材料,当界面粘结强度为0.6～60 M Pa时,可兼顾界面的韧性和复合材料的断裂韧性.     

混凝土试样在静态载荷作用下断裂过程的数值模拟研究

提出了一个模拟混凝土断裂过程的细观力学模型,并应用该模型从混凝土的细观非均匀性结构出发,对混凝土试样在单轴和双轴静态载荷作用下的断裂过程进行了数值模拟,给出双轴载荷作用下混凝土的强度包络面。数值模型结果较好地模拟了混凝土试样从裂纹萌生、扩展到宏观裂纹形成的整个断裂过程,与实验结果表现出较好的一致性。     

用扩展有限元方法模拟混凝土的复合型开裂过程

用扩展有限元法对混凝土梁复合型开裂过程进行了数值模拟。裂纹面间的力学行为采用粘聚裂纹模型来描述,通过引入切向保留刚度考虑剪力分量的影响。开裂方向的计算采用了一种简化的最大切向应力准则。对Arrea和Ingraffea的混凝土梁复合开裂实验进行了数值模拟。计算给出了裂纹萌生、扩展的过程及破坏形态,并获得了与实验结果对比良好的荷载-裂纹开口滑移曲线。结果表明,扩展有限元法通过附加特定的位移模式,使裂纹两侧不连位移场的表达独立于网格划分,是一种能够模拟准脆性材料复合开裂问题的有效方法。     

基于数字图像的混凝土破坏过程的数值模拟

把混凝土看作是由水泥砂浆为基相,粗细骨料为分散相的复合材料。在细观尺度上,应用数字图像处理技术表征混凝土材料中由骨料的形状、大小和分布对混凝土材料造成的非均匀性,在组成相材料内部细微观尺度上,采用统计方法来描述材料的非均匀性,建立了细-微观尺度耦合分析的混凝土损伤数值模型,模拟了混凝土单轴载荷作用下的破坏过程。数值模拟结果能反映出骨料分布和组成相材料的非均匀性对混凝土力学行为的影响,较好的模拟了混凝土试样从裂纹萌生扩展到宏观裂纹形成的整个破坏过程。     

冲击压缩下准脆性材料含微裂纹损伤的本构模型

基于准脆性材料中翼型拉伸裂纹的成核准则,运用细观损伤理论推导了翼型裂纹损伤对材料弹性模量的弱化作用.考虑裂纹扩展对材料动态断裂的滞后效应,建立了动态裂纹扩展准则,并给出损伤演化方程,在此基础上建立了准脆性材料单轴冲击压缩下的动态损伤本构模型.结合氧化铝陶瓷材料独特的力学响应和破坏特性,讨论了模型中微裂纹成核参数、微裂纹尺寸对动态断裂强度的影响,并用该模型计算了单轴压缩下氧化铝陶瓷的应力应变曲线,数值结果与实验结果吻合良好.     

ZrO2-Ni功能梯度材料的热冲击与热疲劳行为

通过抗热震参数分析和热循环试验研究了ZrO2-Ni功能梯度材料（FGM）的热冲击与热疲劳行为及其影响因素。结果表明,ZrO2-Ni FGM热热震参数呈梯度分布,ZrO2侧抗热冲击断裂能力强而富Ni区热疲劳抗力高。其热震破坏符合热疲劳损伤机理,裂纹的准静态扩展为其控制因素。热疲劳裂纹在梯度层内以微孔聚集、连接方式萌生和扩展,而在梯度层间无横向贯穿裂纹,克服了传统陶瓷／金属结合体的界面热应力剥离问题。     

岩体多裂纹扩展演化过程数值流形方法研究

岩体中含有大量节理、裂隙、断层等各类结构面,结构面在应力作用下的扩展与贯通是导致岩体破坏的重要原因。数值流形方法 (NMM)可以有效模拟连续和非连续问题,然而,其在多裂纹动态扩展的模拟方面仍处于探索阶段。该文以线弹性断裂力学原理为基础,提出了一种基于高阶数值流形方法的多裂纹扩展模拟算法。通过在基函数中增加关键项来考虑裂纹尖端位移场的奇异性;裂纹尖端的应力强度因子则采用了J积分来计算;Ⅰ型-Ⅱ型混合裂纹的开裂和扩展方向依据最大周向拉应力准则来判断;采用假设-修正的多裂纹扩展算法解决了多裂纹的扩展问题。根据强化后的基函数,对于不符合单纯形积分形式的被积函数,采用了泰勒级数展开式计算近似解。通过多个静态裂纹扩展的经典问题的数值模拟对计算方法的合理性和计算精度及进行了验证。     

钢筋混凝土梁抗冲击性能研究的细观数值模拟方法

为更真实地描述钢筋混凝土梁在冲击荷载作用下复杂的破坏过程,结合混凝土细观结构非均质性,假定混凝土是由骨料颗粒、砂浆基质以及界面过渡区(ITZ)组成的三相复合材料,考虑混凝土细观组分的率效应,将钢筋网嵌入至素混凝土梁,建立了钢筋混凝土梁抗冲击力学行为研究的三维细观尺度数值模型。基于该数值分析模型与方法,研究了落锤速度对钢筋混凝土梁抗冲击性能的影响规律。将细观数值模拟结果与宏观模型结果以及已有试验结果进行了对比分析,包括破坏模式、冲击力、跨中位移以及支座反力等,发现模拟结果与试验结果吻合良好,验证了细观数值分析模型在钢筋混凝土梁抗冲击性能模拟中运用的可行性。     

裂纹止裂技术及裂纹动态扩展规律研究

为了对动态荷载作用下水泥粉煤灰砂浆的裂缝动态扩展行为进行研究,提出了一种大尺寸带V型底边的半圆边裂纹(SECVB)试件,其V形底部具有止裂功能。SECVB试件的V形底部设计为180°,150°和120°三个角度。采用落锤冲击装置进行了冲击试验,并使用裂纹扩展计(CPG)用于测量裂纹扩展的相关参数。利用有限差分程序AUTODYN对裂纹扩展行为进行了数值模拟,并用有限元程序ABAQUS计算了裂纹的动态应力强度因子(DSIF);根据CPG测量的裂纹萌生时间和扩展时间来确定临界应力强度因子。试验和数值模拟结果表明,SECVB试件适合于研究动态荷载作用下水泥粉煤灰砂浆的裂纹扩展行为和止裂行为。在裂纹扩展过程中,裂纹可能在一段时间内止裂,并且裂纹在起始时刻的断裂韧度高于裂纹扩展时的断裂韧度。     

基于离散元法的脆性岩石细观蠕变失稳研究

为从细观角度探究脆性岩石的蠕变失稳过程及失稳机理,该文基于三维颗粒流程序（PFC3D）考虑岩石的时效变形损伤过程,引入岩石细观单元时效损伤的应力腐蚀模型,建立了基于离散元方法的岩石时效变形损伤破裂模型,并通过单轴压缩及单轴蠕变的室内实验和数值模拟对比验证了所建立的时效变形损伤破裂模型的合理性。数值模拟再现了岩石的初始蠕变、稳态蠕变和加速蠕变三个蠕变阶段,同时模拟结果表明,在单级加载条件下,随着应力水平提高,稳态蠕变应变率显著增大,岩石蠕变失效时间逐渐缩短,初始轴向应变、初始侧向应变和初始体应变不断增大,且细观裂纹扩展形式与单轴压缩破坏形式基本相同,都是以拉伸裂纹为主,裂纹的增长速率随着时间增加而不断增大,尤其在第三蠕变阶段裂纹增长速率迅速增大;在分级加载试验过程中,模型的轴向应变、侧向应变和体应变以及裂纹最终扩展形态与单级加载基本相同;此外将三维蠕变模拟结果与二维模拟结果进行对比,结果显示三维模型拟合程度更高。     

Thermal shock resistance of ultra-high temperature ceramics including the effects of thermal environment and external constraints

The thermal shock resistance of ceramics depends on the materials mechanical and thermal properties, also is affected by component geometry and external factors and so on. Therefore, the thermal shock resistance of ceramic materials is the comprehensive performance of their mechanical and thermal properties corresponding to the various heat conditions and external constraints. In the present work, a thermal shock resistance model of the ultra-high temperature ceramics which considered the effects of thermal environment and constraints was established. With this model, the influence of constraints on the thermal shock resistance and critical fracture temperature difference had been studied and an effective idea to improve thermal shock resistance for ceramic material and structure was found. Furthermore, the model was validated by finite element method.     

Optimizing Thermal Shock Resistance of Layered Refractories

Severe thermal shocks may cause critical thermal stresses and failure in refractories or ceramic materials. To increase the thermal shock resistance, layered material structures are suggested. In order to optimize properties of these alternative structures, thermo‐mechanical simulations are required. In this study, a finite difference method (FDM) is used for solving the partial differential equation of heat conduction with spatially varying parameters. The optimization of the strip's thermal shock resistance is exemplarily done on a 10 layered strip subjected to constant temperature jump on the top surface. Each layer can be set with different porous Al₂O₃ and MgO ceramics, whose material properties are theoretically determined. In this study, an improved optimization method is developed that consists of a combination and sequence of Monte Carlo simulations and evolution strategies to overcome certain disadvantages of both techniques.     

含h-BN复相陶瓷制备及性能研究进展

陶瓷材料密度低、抗腐蚀性及耐磨性良好,但是其硬而脆导致加工困难、抗热震性差。h-BN具有弹性模量低、硬度低的特点,其可加工性能和抗热震性能优异。将h-BN引入陶瓷基体制备含h-BN复相陶瓷,能够有效改善陶瓷材料的可加工性能和抗热震性。对含h-BN复相陶瓷的材料体系、制备工艺和性能的研究一直备受关注。本文以h-BN的引入方式为分类依据较全面地总结了含h-BN复相陶瓷的制备方法。本文对引入h-BN后所制备的含h-BN复相陶瓷的常规力学性能、抗热震性、可加工性、透波性、摩擦磨损等性能的影响进行了综述;对含h-BN复相陶瓷的制备及性能研究中存在的问题进行了概括,并对该材料体系的研究方向提出了建议。     

陶瓷材料热冲击开裂机理与裂纹间距预报

该文研究陶瓷材料热冲击开裂机理和热冲击裂纹的分布规律。1mm厚的99Al2O3陶瓷薄片的水淬实验显示:裂纹间距随热冲击温差增大而减小,在同一热冲击温差下,5个试件中的各个裂纹间距与平均间距的偏差不超过7%。理论上,结合传热学和力学方法,计算了热冲击过程中试件的瞬态温度场和应力场,阐述了陶瓷材料热冲击条件下的开裂机理。以裂纹间距和深度作为变量,利用最小能量原理,发展了热冲击裂纹间距预报的有限元方法。由于从文献获得的毕渥数数据分散度很大,并且难以直接测量,因此发展了“间接测量法”,逆向估计了实验过程中的毕渥数,并在其他温度点获得了与实验吻合很好的裂纹间距数值预报结果。该文的研究对深入理解陶瓷材料的热冲击失效机制,对陶瓷材料的改性和研制有重要的意义。     

Investigation of the Thermal Shock Behavior of Ceramic Using a Combination of Experimental Testing and FE‐Simulation Methods

Thermal shock behavior of ceramics plays a decisive role in their broad industrial applications. For enhanced understanding of damage and failure mechanism under thermal shock loading, in the present work, a combination of experimental testing and numerical simulation methods has been used. The thermal shock behavior of the alumina (99.7%) disk samples has been investigated by using a plasma test stand: the bottom of the ceramic disks were locally heated in the center by plasma beam; during the heat treatment the temperature distribution at the top of the sample was recorded with a thermographic system. To characterize the thermal shock resistance, a thermomechanical simulation was subsequently carried out. It calculates the temperature and stress distribution within the ceramic disks. The calculated critical thermal tension stresses are reported, which led to the failure of the ceramic disks under thermal shock loading. The effect of the sample thickness on the temperature and stress distribution is presented. Compared with the experimental results the simulated results show excellent agreement. As conclusion, it is possible to determine the thermal shock behavior of ceramic materials by the combination of experimental testing and numerical simulation.     

Failure mode transition in ceramics under dynamic multiaxial compression

An experimental technique based on the Kolsky pressure bar has been developed to investigate the behavior of ceramics under dynamic multiaxial compression. Experimental results for aluminum nitride (AlN), together with data available in the literature, indicate that a Mohr-Coulomb criterion and the Johnson–Holmquist model fit the experimental data for failure in a brittle manner, whereas the ceramic material exhibited pressure insensitive plastic flow at high pressures. A failure surface is constructed which represents the material failure behavior, including brittle failure, brittle/ductile transition and plastic flow, under various pressures. The effect of various material properties on the failure behavior was investigated. The Poisson's ratio is found to be a measure of brittleness for ceramic materials with low spall strength under shock wave loading conditions. Lower value of Poisson's ratio indicates that the material will fail in a brittle manner through axial splitting even under uniaxial strain loading; whereas materials with higher Poisson's ratio may be expected to deform plastically beyond the Hugoniot Elastic Limit (HEL). The applicability of the proposed failure surface to a range of ceramics is explored and the limitations of the model are outlined. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermal shock and thermal fatigue study of ceramic materials on a newly developed ascending thermal shock test equipment

A test equipment was designed to study thermal shock and thermal fatigue of ceramic materials subjected to fast heating (ascending). The equipment was designed to generate thermal stress in a test specimen by heating one surface of it by an oxy-hydrogen flame while cooling the opposite surface. The sample cracked when thermal stress exceeded its mechanical strength. The in situ crack formation was detected by an acoustic emission system coupled to the set up. The hot zone temperature was measured by an infra red pyrometer. The equipment was also designed to run thermal fatigue test cycles in automatic mode between two selected temperatures. The temperature and thermal stress distribution in the test specimen were modelled using finite element software. The effect of temperature distribution of the top and bottom surfaces on thermal stresses was studied. It was observed that the thermal stress is very sensitive to the temperature distribution on the top surface and maximum near the periphery of the top surface. This was in agreement with the experimental results in which the cracks were originated from the periphery of top surface. It was also observed that the failure temperature was higher for thicker samples.     

从微孔洞起始的热冲击裂纹的数值研究——Ⅰ: 热应力分析

陶瓷材料在热冲击荷载作用下的典型破坏模式是裂纹的起始和扩展。当高温的材料或构件突然处于低温环境时,裂纹将由材料或构件的表面开始产生和发展。因此,材料表面及其表面附近的材料微结构对裂纹的起始有着非常重要的影响。实验表明:某些具有微孔洞结构的陶瓷材料比相应的密实材料的抗热冲击性能有所提高,但相关的机制目前还没有得到定量的解释。本文作者利用商业有限元软件ABAQUS,分析了受热冲击表面上的开口微孔洞、处于表面附近的闭口微孔洞周围的热应力场,以及微孔洞的大小、微孔洞与表面的距离对热应力场的影响,其目的是为进一步分析裂纹的起始和扩展做准备。数值结果表明:一般情况下,闭口微孔洞引起的应力集中比开口微孔洞引起的应力集中更为严重,特别当闭口微孔洞与表面之间的距离很小时,和没有微孔洞情况相比,闭口微孔洞引起的最大的应力集中系数超过6。     

结构陶瓷磨削表面微裂纹的研究

结构陶瓷的磨削表面微裂纹是较常见和较危险的磨削损伤，本文应用压痕断裂力学。磨削表面热应务陶瓷微观结构的有关知识分析了结构陶瓷表面裂纹的形成机理、形状特征及奖表面的延伸，并对几种典型结构民磨削表面层的微观状态进行了大量的ＳＥＭ观察，结果表明：（１）磨削微裂纹与陶瓷的断裂韧性Ｋ１Ｃ和显微硬度Ｈ之比、陶瓷的热特性及磨削工艺参数有关；（２）磨削微裂纹的 压痕效应径向裂纹、热裂纹、（３）细晶陶瓷多为穿晶裂纹     

The effects of residual stresses and degradation on the response of viscoplastic functionally graded materials

Functionally graded materials (FGMs), having ceramic and metallic constituents, are commonly used for extreme temperature applications. Under extreme temperature changes, the mismatches in the thermo-mechanical properties of the ceramics and metallic constituents could cause pronounced thermal stresses and could lead to degradation in the properties of the constituents. High stresses in the metallic constituent lead to plastic deformations and high tensile stresses in the ceramic constituent induce cracking. An elastic–viscoplastic micromechanical model is formulated for analyzing residual stresses and strains and degradation in ceramic–metal FGMs undergoing temperature changes due to conduction of heat. A degradation parameter that depends on the temperature and strain is introduced in order to determine the level of material degradation in the ceramics and metallic constituents. The Perzyna viscoplastic model is considered for the metallic constituent while the ceramic constituent is assumed linear elastic. The material parameters in these constituents change with the degradation. The problem leads to time-dependent coupled thermal, deformation, and degradation behaviors. The micromechanical model is implemented in a displacement based finite element (FE), which is used to determine the performance of the viscoplastic functionally graded structures subject to external thermo-mechanical stimuli.