泡沫金属压痕试验的数值模拟及其反演

在理论研究的基础上，将泡 沫金属压痕试验的有限元数值模拟结果与用无量纲分析法构造出的一系列无量纲函数相结 合，建立了泡沫金属压痕试验中载荷-压痕深度关系曲线与泡沫金属的弹塑性材料参数之 间的联系. 利用这种联系，就可以实现用压痕试验通过反演分析来确定泡沫金属的材料参数. 研究结果表明，泡沫金属材料的杨氏模量，屈服强度及塑性可压缩因子等参数均可由其压痕 试验唯一的确定，但其塑性平台区终点应变的确定还需进一步的研究.     

MEMS材料力学性能的测试技术

微电子机械系统(MEMS)技术的迅速崛起,推动了所用材料微尺度力学性能测试技术的发展.首先按作用方式将实验分成压痕/划痕、弯曲、拉伸、扭转四大类,系统介绍检测MEMS材料微尺度力学性能的微型试样、测试方法及其实验结果.测试材料主要有硅、氧化硅、氮化硅和一些金属.实验结果主要包括基本的力学性能参数如弹性模量、残余应力、屈服强度、断裂强度和疲劳强度等.最后,简要分析了未来的发展需求.      

血红细胞力学性能的纳米压痕实验和有限元模拟

采用纳米压痕技术和有限元方法研究了血红细胞的生物力学性能. 进行了血红细胞的纳米压痕实验, 得到了血红细胞的材料参数和变形形貌; 在实验基础上, 建立了血红细胞的三维有限元模型, 模拟了血红细胞的压痕载荷-位移曲线, 并考虑了参数效应. 数值模拟结果和实验数据符合很好. 通过改变压头与材料之间的摩擦系数和压头曲率半径等参数, 比较了载荷-位移曲线的变化情况. 研究表明摩擦系数对压痕载荷-位移曲线和应力分布影响很小, 而压头曲率对载荷-位移曲线的影响明显.     

原子力显微镜在二维材料力学性能测试中的应用综述

以石墨稀为代表, 二维材料有着诸多优异的性质, 在下一代电子器件等领域拥有广阔的应用前景. 目前绝大多数关于二维材料的研究都集中在其电子学和光学的性质和应用, 对于其力学性质的研究则相对欠缺, 而力学性质在二维材料的研究和应用中都有着至关重要的意义. 原子力显微镜是低维材料力学性质表征的主要手段, 例如基于原子力显微镜的纳米压痕技术. 本文首先简要介绍了二维材料的基本背景以及原子力显微镜的工作原理. 进一步展示了纳米压痕技术的工作原理和理论背景, 并回顾了利用纳米压痕技术研究二维材料面内力学性质的相关实验和理论工作, 同时探讨了原子力显微镜在表征二维材料力学性能中存在的测量误差及来源. 由于二维材料展现出强烈的各向异性, 纳米压痕技术在能够很好地测量二维材料面内力学性质的同时, 对于二维材料层间力学性质表征等方面存在明显的局限性. 第三部分介绍了一种全新的基于原子力显微镜的埃(?)压痕技术, 该技术能够将形变尺度控制在0.1 nm以内, 从而精确地表征和调控二维材料的层间范德华作用力, 即层间力学性质. 作者在第三部分介绍了通过埃压痕技术表征和调控的石墨烯、氧化石墨烯等二维材料的层间力学性质. 最后简要介绍了范德华异质结材料的基本性质, 探讨了埃压痕技术在该材料力学性质研究中的潜在应用.      

壁湍流相干结构尺度及边界层内的SL标度律

利用高分辨率、高帧率PIV系统对湍流边界层中相干结构的多种空间尺度和边界层内SL 标度律在不同尺度下的具体表达形式进行了实验研究. 实验在两个动量损失厚度雷诺数 (Re_{\theta}=628.5和Re_{\theta}=1032.9)下测量平板湍流边界层中缓冲 层、对数区和外区的二维瞬时速度场. 应用 小波分析以及传统的统计学方法，在垂直于平板和平行于平板的平面内考察平板湍流边界层 中存在的相干结构的流向和展向尺度，并与已知的相干结构尺度实验结果进行了对比分析. 利用在动量损失厚度雷诺数628.5下测得的数据，对多种脉动结构(脉动速度结构等) 的空间关系及其标度律进行了研究. 第2项工作直接利用湍流边界层空间速度分布，对多种 流场尺度结构内部的She-Leveque(简称SL)标度律及自相似律进行了验证. 结果表明，各 单一流场尺度结构内部，流向脉动速度{\pmb u}'、法向脉动速度{\pmb v}'及 脉动涡分量\d {\pmb v}'/\d {\pmb x}的统计结构量均存在明显的标度律，标度 指数的形式与自相似律和SL标度律均非常吻合，只是常数随流场尺度的不同而不同， 且呈现一定的规律性. 但对于结构量的五阶矩随距离l的研究表明，自相似律和SL 标度律成立的范围并不完全一致，同时标度律成立的范围大小与流场尺度有明显关系.     

二维材料力学行为的压痕测试

准确了解二维材料的力学性能对于推动其应用具有重要意义, 无基底压痕技术是目前最广泛采用的二维材料力学性能测试方法之一, 本文综述了二维材料压痕研究的最新进展以及所面临的问题, 并对将来的研究工作进行了展望.无基底压痕技术是将二维材料转移到带有沟槽或柱形孔的基底上, 制备二维材料"梁"和"鼓"模型, 然后利用原子力显微镜测量其在压针作用下的载荷--位移关系, 最后通过基于连续介质薄膜导出的压痕响应分析模型拟合实验结果, 估算出二维材料的弹性模量和本征强度.由于二维材料的厚度远小于连续介质薄膜, 来自于压头以及基底孔侧壁的范德华力对二维材料的压痕响应具有显著影响, 造成二维材料与传统压痕分析模型中的基本假设不符, 导致不能准确预测二维材料的弹性模量; 另外, 由于传统压痕模型无法准确描述二维材料在大变形下的非线性行为, 以及由缺陷等引起的应力集中, 导致由压痕测试表征的二维材料(特别是多晶二维材料)本征强度具有较大的偏差. 因此, 一方面需要正确了解由压痕技术获得的二维材料力学性能, 另一方面还需对目前的研究方法做进一步的改进和完善.      

颗粒增强复合材料的界面开裂与尺度效应

采用基于Huang等提出的塑性应变梯度传统理论发展的有限元方法,模拟了颗粒增强金属基复合材料的界面开裂与颗粒尺度效应.分别针对考虑颗粒与基体间界面开裂和不开裂两种情况进行分析,并将考虑界面开裂的模拟结果与实验结果进行比较,证明了模型的有效性,同时也获得应变梯度理论中所包含的材料特征尺度参量的取值.     

影响冲击载荷下脆性材料碎片尺度的因素

概括回顾近年我们针对脆性材料受冲击拉伸载荷时的断裂及破碎现象的理论分析及数值模拟成果。重点分析碎片尺度对材料参数及加载速率的依赖关系。通过量纲分析,动力学模拟以及数值实验,建立了一个普适性的无量纲关系用以评估碎片尺度。将本研究结果与其他理论及现有实验结果进行的比较表明本模型更为完善合理。     

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激光直接成形中,几何参数、材料属性和工艺参数等众多参量均会对残余应力造成影响,需 要进行系统分析. 采用量纲分析的方法,分别提取表征几何、传热和变形的3类关键无 量纲参数,并结合三维瞬态有限元分析模型来研究这些无量纲参数对热致残余应力的影响规 律. 研究表明,选用热膨胀系数、屈服应力较小的材料,残余应力会较小;工艺控制中,可 通过降低热散失、增大激光功率和提升预热温度来减小残余应力,其中预热的效果最好.     

岩体三维裂隙网络地质熵与渗透特性关系研究

裂隙网络是岩体地下水的主要流动通道,而工程岩体中裂隙网络错综复杂,裂隙网络的几何特征和连通性对其渗透性有着重要影响.为了综合量化裂隙迹长、间距、倾角、开度对裂隙网络连通性和渗透性的影响,基于信息熵原理,提出了三维裂隙网络地质熵理论和连通性指标-熵尺度,对比熵尺度与其他传统三维裂隙网络连通性指标,验证了熵尺度评价三维裂隙网络连通性和渗透性的合理性.结合锦屏一级水电站左岸边坡裂隙统计分布,建立三维裂隙网络渗流数值计算方法,分析不同裂隙迹长、倾角、间距、开度条件下三维裂隙面密度、无量纲逾渗密度、熵尺度和渗透系数的变化关系.结果表明:当体积率一定,考虑开度影响时,三维裂隙面密度和无量纲逾渗密度无法定量表征迹长和间距对裂隙网络连通性的影响;裂隙迹长与熵尺度和渗透系数呈负相关关系,裂隙间距和开度与熵尺度和渗透系数呈正相关关系,裂隙倾角变化对熵尺度和渗透系数影响较小;熵尺度与渗透系数的非线性关系近似满足二次多项式.     

纳米压痕过程的三维有限元数值试验研究

采用有限元方法模拟了纳米压痕仪的加、卸载过程，三维有限元模型考虑了纳米压痕仪的标准Berkovich压头．介绍了有限元模型的几何参数、边界条件、材料特性与加载方式，讨论了摩擦、滑动机制、试件模型的大小对计算结果的影响，进行了计算结果与标准试样实验结果的比较，证实了模拟的可靠性．在此基础上，重点研究了压头尖端曲率半径对纳米压痕实验数据的影响．对比分析了尖端曲率半径r＝0与r＝100nm两种压头的材料压痕载荷—位移曲线．结果表明，当压头尖端曲率半径r≠0时，基于经典的均匀连续介质力学本构理论、传统的实验手段与数据处理方法，压痕硬度值会随着压痕深度的减小而升高．     

CHARACTERISTIC DIMENSIONLESS NUMBERS IN MULTl-SCALE AND RATE-DEPENDENT PROCESSES

Multi-scale modeling of materials properties and chemical processes has drawn great attention from science and engineering. For these multi-scale and rate-dependent processes, how to characterize their trans-scale for-mulation is a key point. Three questions should be addressed:How do multi-sizes affect the problems?How are length scales coupled with time scales?How to identify emergence of new structure in process and its effect?For this sake, the macroscopic equations of mechanics and the kinetic equations of the microstructural transforma-tions should form a unified set that be solved simultaneously.As a case study of coupling length and time scales, the trans-scale formulation of wave-induced damage evolution due to mesoscopic nucleation and growth is discussed. In this problem, the trans-scaling could be reduced to two inde-pendent dimensionless numbers: the imposed Deborah number De=(ac)/(LV) and the intrinsic Deborah num-ber D = (nNc5)/V* ,where a. L, c, V and nN are wave speed, sample size, micr     

PERSPECTIVES IN MECHANICS OF HETEROGENEOUS SOLIDS

The Microand Nano-mechanics Working Group of the Chinese Society of Theoretical and Applied Mechanics organized a forum to discuss the perspectives,trends,and directions in mechanics of heterogeneous materials in January 2010.The international journal,Acta Mechanica Solida Sinica,is devoted to all fields of solid mechanics and relevant disciplines in science,technology,and engineering,with a balanced coverage on analytical,experimental,numerical and applied investigations.On the occasion of the 30 th anniversary of Acta Mechanica Solida Sinica,its editor-in-chief,Professor Q.S.Zheng invited some of the forum participants to review the state-of-the-art of mechanics of heterogeneous solids,with a particular emphasis on the recent research development results of Chinese scientists.Their reviews are organized into five research areas as reported in different sections of this paper.§I firstly brings in focus on microand nano-mechanics,with regards to several selective topics,including multiscale coupled models and computational methods,nanocrystal superlattices,surface effects,micromechanical damage mechanics,and microstructural evolution of metals and shape memory alloys.§II shows discussions on multifield coupled mechanical phenomena,e.g.,multi-fields actuations of liquid crystal polymer networks,mechanical behavior of materials under radiations,and micromechanics of heterogeneous materials.In §III,we mainly address the multiscale mechanics of biological nanocomposites,biological adhesive surface mechanics,wetting and dewetting phenomena on microstructured solid surfaces.The phononic crystals and manipulation of elastic waves were elaborated in §IV.Finally,we conclude with a series of perspectives on solid mechanics.This review will set a primary goal of future science research and engineering application on solid mechanics with the effort of social and economic development.     

大飞机研制中的若干复合材料力学问题

简要介绍了先进复合材料在当代大型飞机结构中的应用状况，以及相关的力学问题，着重论 述了复合材料及结构的工艺力学、跨尺度材料/结构一体化设计问题和力学性能表征、高效复 合材料结构设计和力学分析、复合材料损伤容限与耐久性设计、大型飞机的安全监测和管理 系统等. 指出，从目前国际上以及我国复合材料在大型飞机结构上的应用趋势来看，我们需 要进一步发挥广大力学工作者与相关领域科研人员和工程师的协同作用，提高我国复合材料 基础研究和应用水平，使复合材料在提升我国研制的大飞机的先进性和竞争力方面发挥重要 的作用.     

固体的统计细观力学——-连接多个耦合的时空尺度

从固体力学所面临的新的挑战------多物理、多尺度耦合及其现状的描述开始, 以层裂 过程为例, 说明了这些多尺度非平衡问题的基本困难在于, 在固体中不同尺度上有不同的微 结构层次及不同的演化物理和速率. 接下来, 概述了一些针对这一困难的独特的思路及 其成果. 第3部分强调了一些统计平均方法的范式, 以及处理包含多个时间和空间尺度的问 题的新思路, 特别是非平衡损伤演化导致宏观失效的问题. 在第4部分, 简要评述了一些连 接多个空间和时间尺度的细观力学框架, 如位错理论, 物理细观力学, Weibull理论, 随机 理论等, 并且阐述了其中蕴含的跨尺度耦合的机理. 然后, 在第5部分, 回到了描述损 伤演化过程的框架, 也就是统计细观损伤力学以及它的跨尺度封闭近似. 基于这些跨尺度框 架, 在第6部分, 对控制跨尺度耦合的可能机理进行了评述和比较. 由于对失效时灾变 的洞察与跨尺度强耦合紧密相关, 一些非平衡和强相互作用的新概念在第7部分进行了讨 论. 最后, 以一个简短的总结和一些建议结束.     

Fracture-mode map of brittle coatings: Theoretical development and experimental verification

Brittle coatings, upon sufficiently high indentation load, tend to fracture through either ring cracking or radial cracking. In this paper, we systematically study the factors determining the fracture modes of bilayer material under indentation. By analyzing the stress field developed in a coating/substrate bilayer under indentation in combination with the application of the maximum-tensile-stress fracture criterion, we show that the fracture mode of brittle coatings due to indentation is determined synergistically by two dimensionless parameters being functions of the mechanical properties of coating and substrate, coating thickness and indenter tip radius. Such dependence can be graphically depicted by a diagram called ‘fracture-mode map’, whereby the fracture modes can be directly predicated based on these two dimensionless parameters. Experimental verification of the fracture-mode map is carried out by examining the fracture modes of fused quartz/cement bilayer materials under indentation. The experimental observation exhibits good agreement with the prediction by the fracture-mode map. Our finding in this paper may not only shed light on the mechanics accounting for the fracture modes of brittle coatings in bilayer structures but also pave a new avenue to combating catastrophic damage through fracture mode control.     

Hardness-packing density scaling relations for cohesive-frictional porous materials

Recent progress in instrumented nanoindentation makes it possible today to test in situ phase properties and structures of porous materials that cannot be recapitulated ex situ in bulk form. But it requires a rigorous indentation analysis to translate indentation data into meaningful mechanical properties. This paper reports the development and implementation of a multi-scale indentation analysis based on limit analysis, for the assessment of strength properties of cohesive-frictional porous materials from hardness measurements. Based on the separation-of-scale condition, we implement an elliptical strength criterion which results from the nonlinear homogenization of the strength properties of the constituents (cohesion and friction), the porosity and the microstructure, into a computational yield design approach to indentation analysis. We identify the resulting upper bound problem as a second-order conical optimization problem, for which advanced optimization algorithms became recently available. The upper bound yield design solutions are benchmarked against solutions from comprehensive elastoplastic contact mechanics finite element solutions and lower bound solutions. Furthermore, from a detailed parameter study based on intensive computational simulations, we identify characteristic hardness-packing density scaling relations for cohesive-frictional porous materials. These scaling relations which are developed for two pore-morphologies, a matrix-pore morphology and a polycrystal (perfect disordered) morphology, are most suitable for the reverse analysis of the strength parameters of cohesive-frictional solids from indentation hardness measurements.     

Improved reverse analysis for material characterization with dual sharp indenters

It was illustrated by the author in the previous work that combinations between material properties and indentation parameters can be used as mixed parameters in dimensionless functions to capture the indentation response of materials to single and dual sharp indenters. These issues are further extended in the present study. A parametric finite element analysis was performed to investigate the conical indentation response of elasto-plastic solids. Frictional effects are studied. Conical indenters of half-included angles from 50° to 88° are considered to examine several fundamental features of instrumented sharp indentation within the frame work of limit analysis. Regarding dimensional analysis, it is found that a Taylor series expansion according to the elastic indentation work-total indentation work ratio W_e/W_t can be used to improve dimensionless functions. Within this context, a new set of dimensionless functions is explicitly constructed for hardness and indentation parameters of single and dual indenters. Based on formulated functions, a reverse analysis with dual sharp indenters, which was previously proposed by the author, is improved to extract mechanical properties of materials.     

Analysis of spherical indentation of materials with plastically graded surface layer

In the present work, a comprehensive parametric study for establishing contact mechanics of instrumented normal spherical indentation on homogeneous materials and materials with plastically graded surface layer (PGSL) was undertaken by dimensional analysis and finite element modeling. The spherical indentation response for homogeneous materials can be described only by two dimensionless parameters: strain hardening exponent and a unified parameter that can describe effects of both the normalized yield strength and the normalized indentation depth. The influences of these two parameters were investigated for a wide range of engineering materials, and the results may be used as an estimate of loading response and pile-up/sink-in behavior when the material properties are known. In the materials with PGSL, a linear gradient in yield strength, and no variation in elastic modulus and strain hardening exponent were explored. The indentation response of the materials with PGSL can be described only by three dimensionless parameters: the normalized indentation depth, the dimensionless strength gradient parameter, and the normalized PGSL thickness. The effects of these three parameters were studied systematically. The normalized pile-up/sink-in parameter is found to be an increasing function of the strength gradient parameter. The normalized pile-up/sink-in parameter increases (decreases) with increasing PGSL thickness for a fixed positive (negative) gradient case at large indentation depth. The results also indicate that the materials with positive PGSL can bear more loads and have significantly more resistance to contact crack formation.