用能量法分析层状材料弹性接触问题

利用能量法分析了层状材料（薄膜／基体）弹性接触问题，得到了具有一阶精度的闭合解，给出了求解薄膜弹性模量和泊松比的表达式，并与有限元的数值解进行了比较。二者比较结果表明：在工程材料范围内，理论解与数值解相差在6％以内；同时表明单相材料中剪切模量与弹性模量之间的关系也适用层状材料中的薄膜材料。在数值解的基础上，讨论了薄膜厚度与压头半径的比值对求解精度的影响，发现此比值对精度影响不大。通过对层状材料等效泊松比与等效弹性模量的定义，给出了用压痕实验测定薄膜泊松比与弹性模量的方法。     

界面特性对短纤维金属基复合材料蠕变行为的影响

基于短纤维增强金属基复合材料(MMC)的单纤维三维模型(三相)，利用粘弹性有限元分析方法对影响金属基复合材料的蠕变行为的因素进行了较为系统的分析。研究中主要讨论了界面特性和纤维取向角对金属基复合材料的蠕变性能的影响。研究结果发现，界面特性诸如厚度、模量和应力指数都对纤维最大轴应力和稳定蠕变率产生影响：稳态蠕变率随界面模量的增大而逐渐减小，当高于基体模量时基本保持不变；纤维轴应力的变化与蠕变率正好相反。稳态蠕变率随界面厚度、应力指数的增加而增大；而轴应力则随之减小。同时不同的纤维取向也影响金属基复合材料蠕变时的轴应力分布和稳态蠕变率。     

表面裂纹蠕变分析的蠕变线弹簧模型方法

鉴于用通常的数值方法分析三维蠕变裂纹问题的困难,提出了一个三维表面裂纹蠕变断裂力学参量分析的蠕变线弹簧模型方法,并在非稳态蠕变条件下的位移、裂纹尖端J积分和C积分的工程估算公式及弹塑性线弹簧模型的基础上,建立了蠕变线弹簧模型方法的有关基本方程．具体分析计算了受均匀拉伸表面裂纹平板的J积分和C积分,并与三维有限元解进行了比较,其结果吻合良好．研究结果为进一步研究三维表面裂纹的蠕变扩展及寿命预报提供了基础．     

多壁碳纳米管-高聚物纳米复合材料的抗蠕变性能

通过双螺杆挤出机和模压成型设备制备了两种不同长径比的多壁碳纳米管（MWNT）增强的聚丙烯（PP）纳米复合材料。实验表明，通过添加1％体积含量的MWNT，聚丙烯的抗蠕变性能得到很大提高，即长时间加栽后，基体的蠕变变形量和蠕变率均显著降低。同时，在特定载荷下，纳米复合材料的蠕变寿命比纯基体提高了10倍。几种栽荷传递机理导致了材料抗蠕变性能的增强：（1）碳纳米管和基体之间较好的界面性能，（2）碳纳米管限制了基体内无定型分子链的活动性，以及（3）碳纳米管的较高的长径比。差分热扫描（DSC）的结果显示了材料蠕变前后结晶的变化和栽荷传递机理分析是一致的。这些实验结果显示，在不增加成本的基础上可以大大提高抗蠕变的聚合物纳米复合材料的工程应用。     

多孔氧化铝薄膜横观各向同性弹性性质的研究

电化学阳极氧化生成的氧化铝薄膜含有高度有序的纳米孔阵列,本文首先假设氧化铝薄膜基体(无孔部分)为各向同性,结合其周期性孔结构特点和均匀化理论,可以得到氧化铝基体和薄膜弹性性质之间的关系。然后利用单轴拉伸结合电子散斑干涉(ESPI)的方法得到薄膜面内的杨氏模量为63.4GPa,并根据均匀化方法得到的基体与薄膜弹性性质的关系进一步推出薄膜横观各向同性的其它弹性参数,如基体杨氏模量等。为证明结果的可靠性,利用推出的弹性参数建立三维有限元模型,模拟纳米压痕实验,得到的加卸载曲线与实验曲线相吻合。     

聚合物纳米复合材料蠕变性能研究进展

聚合物在室温甚至低温条件下的蠕变被认为是制约其更广泛应用的主要瓶颈之一.实验研究发现添加很低含量(1{\%}重量或体积含量)的纳米颗粒,在基本上不影响基体其他力学和物理性能的前提下,能够大幅度提高聚合物的耐蠕变性能; 另外和静态性能相比较,蠕变和松弛等特性对于聚合物微观结构的变化和分子链的相互作用更加敏感,能够在新型纳米复合材料(如多层级纳米复合材料)的力学设计中为我们提供更加丰富的微观结构及其相互作用的信息.本文综述了多种形貌纳米颗粒(包括金属氧化物、碳纳米管、层状纳米黏土等)对聚合物耐蠕变性能影响的研究现状和进展.讨论了纳米颗粒的种类、形貌和含量,以及外部应力水平和温度等因素对聚合物基体材料蠕变性能的影响规律;分析了目前一些常用的模拟和预测蠕变行为的模型, 并利用这些模型,结合纳米复合材料特点, 对蠕变实验结果进行了模拟和预测;结合多层级纳米复合材料的实验研究结果,阐述了蠕变条件下纳米复合材料分子链间相互作用的特点;进而探讨了纳米颗粒影响聚合物蠕变性能的机理,展望了该领域研究的发展态势.      

家兔肌腱的蠕变变形行为的实验研究

研究了家兔肌腱的蠕变规律，测定了稳定蠕变速率等力学参量，通过对试验结果的分析，建立了肌腱的蠕变本构关系，结果表明，蠕变应变与瞬时应力和时间有关，在一定的瞬时应力作用下，稳定蠕变速率近似为常数，稳定蠕变速率与瞬时应力近似成指数关系。     

涡轮叶片蠕变全过程的修正Graham模型的参数识别

针对定向凝固镍基合金制造的航空发动机涡轮叶片,从涡轮叶片典型部位取材并设计制造模拟试件,在980℃下不同应力水平下进行单轴蠕变试验。对试验数据进行了唯象学的数学描述,并对ANSYS有限元分析软件中Graham蠕变模型进行了修正,使修正后的模型能够比较精确地描述单轴蠕变的全过程。并应用修正Graham模型对试验过程进行数值模拟,研究表明ANSYS有限元模拟蠕变的结果和试验数据吻合较好。这说明了修正Graham模型模拟单轴蠕变过程的合理性,该模型在一定程度上为ANSYS在金属材料蠕变分析方面的二次开发提供了理论基础。     

膜-基复合材料界面剪应力的三维半解析计算

 采用影响系数法对膜-基复合材料的界面剪应力三维半解析进行 了分析研究.利用三维有限元方法对薄膜的影响系数进行了计算. 将 基体作为半无限大体，利用其平面边界作用单位力时的位移场解析 解，得到基体的影响系数. 结果表明，对膜-基复合材料界面剪应力 进行三维半解析计算，克服了完全用三维有限元对其进行计算的限 制，为该类问题的分析提供了新的途径.     

含单裂隙岩体蠕变破裂裂纹扩展演化规律研究

岩体内部存在大量不同尺寸裂隙，形成不同结构形式的裂隙岩体，裂隙的存在会降低岩体的力学性能，对于岩体工程的整体稳定性具有很大的影响。本研究基于滑动翼裂纹模型对恒定载荷下含单裂隙岩体蠕变变形进行了理论推导和分析，基于MAPLE软件直观给出恒定载荷下裂隙角度、长度对蠕变变形的影响，采用连续-非连续数值分析软件GDEM,对预制单裂隙在不同倾角、不同长度情况下岩体的蠕变破裂裂纹扩展进行数值模拟。通过理论分析和数值模拟可以看出，岩体蠕变速率随着裂隙长度的增大而增大；随着裂隙角度的增加出现先增大后减小的趋势，其中裂隙倾角为30°时蠕变速率最大。含单裂隙岩体蠕变破裂裂纹扩展规律的研究可为岩体工程蠕变破裂规律研究及蠕变治理提供一定的理论依据和参考。     

DETERMINATION OF CREEP PARAMETERS FROM INDENTATION CREEP EXPERIMENTS

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Small scale,grain size and substrate effects in nano-indentation experiment of film–substrate systems

The mechanical properties of film–substrate systems have been investigated through nano-indentation experiments in our former paper (Chen, S.H., Liu, L., Wang, T.C., 2005. Investigation of the mechanical properties of thin films by nano-indentation, considering the effects of thickness and different coating–substrate combinations. Surf. Coat. Technol., 191, 25–32), in which Al–Glass with three different film thicknesses are adopted and it is found that the relation between the hardness H and normalized indentation depth h/t, where t denotes the film thickness, exhibits three different regimes: (i) the hardness decreases obviously with increasing indentation depth; (ii) then, the hardness keeps an almost constant value in the range of 0.1–0.7 of the normalized indentation depth h/t; (iii) after that, the hardness increases with increasing indentation depth. In this paper, the indentation image is further investigated and finite element method is used to analyze the nano-indentation phenomena with both classical plasticity and strain gradient plasticity theories. Not only the case with an ideal sharp indenter tip but also that with a round one is considered in both theories. Finally, we find that the classical plasticity theory can not predict the experimental results, even considering the indenter tip curvature. However, the strain gradient plasticity theory can describe the experimental data very well not only at a shallow indentation depth but also at a deep depth. Strain gradient and substrate effects are proved to coexist in film–substrate nano-indentation experiments.     

ANALYSIS ON PSEUDO-STEADY INDENTATION CREEP

Theoretical analysis and finite element （FE） simulation have been carried out for a constant specific load rate （CSLR） indentation creep test. Analytical results indicate that both the representative stress and the indentation strain rate become constant after a transient period. Moreover, the FE simulation reveals that both the contours of equivalent stress and equivalent plastic strain rate underneath the indenter evolve with geometrical self-similarity. This suggests that pseudo-steady indentation creep occurs in the region beneath the indenter. The representative points in the region are defined as the ones with the equivalent stress equal to the representative stress. In addition, it is revealed that the proportionality between indentation strain rate and equivalent plastic strain rate holds at the representative points during the pseudo-steady indentation creep of a power law material. A control volume （CV） beneath the indenter, which governs the indenter velocity, is identified. The size of the CV at the indented surface is approximately 2.5 times the size of the impression. The stress exponent for creep can be obtained from the pseudosteady indentation creep data. These results demonstrate that the CSLR testing technique can be used to evaluate creep parameters with the same accuracy as conventional uniaxial creep tests.     

Analysis of elastic and plastic deformation associated with indentation testing of thin films on substrates

A study has been made of the elastic and plastic deformation associated with submicrometer indentation of thin films on substrates using the finite element method. The effects of the elastic and plastic properties of both the film and substrate on the hardness of the film/substrate composite are studied by determining the average pressure under the indenter as a function of the indentation depth. Calculations have been made for film/substrate combinations for which the substrate is either harder or softer than the film and for combinations for which the substrate is either stiffer or more compliant than the film. It is found, as expected, that the hardness increases with indentation depth when either the yield strength or the elastic modulus of the substrate is higher than that of the film. Correspondingly, the hardness decreases with indentation depth when the yield strength or elastic modulus of the substrate is lower than that of the film. Functional equations have been developed to predict the hardness variation with depth under these different conditions. Finite element simulation of the unloading portion of the load displacement curve permits a determination of the elastic compliance of the film/substrate composite as a function of indentation depth. The elastic properties of the film can be separated from those of the substrate using this information. The results are in good agreement with King's analytical treatment of this problem.     

Indentation responses of piezoelectric films ideally bonded to an elastic substrate

The influences of elastic substrate on the indentation force, contact radius, electric potential and electric charge responses of piezoelectric film/substrate systems are investigated by the integral transform method. The film is assumed to be ideally bonded to the substrate and the contact interaction between the indenter and the film is assumed to be frictionless, with three kinds of axisymmetric insulating and conducting indenters (i.e., punch, cone and sphere) considered. Obtained results show that when the ratio of the contact radius to the film thickness is close to zero, the influences of the elastic substrate disappear and the indentation behaviors converge to the piezoelectric half space solutions while the indentation responses approach the corresponding ones of elastic half space as the ratio gets to infinity. The transition between the piezoelectric and the elastic half space indentation solutions for the film/substrate system is quantified in terms of the film thickness and the elasticity of the substrate. Finite element analysis on an insulating sphere indentation is conducted to verify the numerical calculations and good agreement is observed. The obtained results are believed to be useful for developing experimental techniques to extract the material properties of piezoelectric film/substrate systems.     

On the determination of the Young’s modulus of thin films using indentation tests

The main difficulty with the characterization of thin coatings using depth-sensing indentation tests is related to the determination of the contributions of the substrate and the film to the measured properties. In this study, three-dimensional numerical simulations of the Vickers hardness test are used in order to examine the influence of the elastic and plastic properties of the substrate and the film on the composite’s Young’s modulus results. The hardness of the film is equal to or higher than the substrate hardness. A study of the stress distributions and the indentation geometry of composites, film/substrate, was performed, taking into account the relative mechanical properties of the film and substrate. In addition, stress evolution during indentation was studied, in order to quantify the critical indentation depth under which the substrate is not elastically deformed. The accurate evaluation of the Young’s modulus of the films using weight functions is also examined: some of these have previously been proposed and one was introduced for this study. Two different fitting procedures were used to compare the results obtained from eight fictive film/substrate combinations using six weight functions. The first procedure, commonly used, considers the substrate’s modulus as a known parameter in the fitting process. In the second, the film and the substrate’s modulus are considered as unknown variables that are calculated simultaneously during the fitting process. The validity of the conclusions obtained using the fictive materials was checked by applying the weight functions to four real composites.     

Determining the Elastic Modulus of Compliant Thin Films Supported on Substrates from Flat Punch Indentation Measurements

Instrumented indentation is a technique that can be used to measure the elastic properties of soft thin films supported on stiffer substrates, including polymer films, cellulosic sheets, and thin layers of biological materials. When measuring thin film properties using indentation, the effect of the substrate must be considered. Most existing models for determining the properties of thin films from indentation measurements were developed for metal and dielectric films bonded to semiconductor substrates and have been applied to systems with film-substrate modulus ratios between 0.1 and 10. In the present work, flat punch indentation of a thin film either bonded to or in contact with a substrate is examined using finite element modeling. A broad range of film-substrate modulus ratios from 0.0001 to 1 are investigated. As the substrate is effectively rigid compared to the film when the film-substrate modulus ratio is less than 0.0001, the results are also useful for understanding systems with lower film-substrate modulus ratios. The effects of the contact radius, film thickness, elastic properties, and friction between the film and the substrate on the measured stiffness were quantified using finite element modeling in order to understand how the elastic properties of the film can be extracted from indentation measurements. A semi-analytical model was developed to describe the finite element modeling results and facilitate the use of the results to analyze experimental measurements. The model was validated through analysis of indentation measurements of thin polyethylene sheets that were supported on substrates of various stiffness.     

基于SHPB的球形压痕实验方法

提出一种基于霍普金森压杆装置的动态球形压痕实验方法,通过将硬质合金小球置于2个试件中间,实现加载过程中2个试件的同时压入,以得到准确的压痕力与位移的关系。利用有限元软件ABAQUS/Explicit对该实验方法进行了数值模拟,从实验结果的可行性、准确性等方面,对新实验方法与传统的动态压痕实验方法进行了比较;采用新方法对铝合金材料进行了实验,并得到了压入力-位移曲线等关系。研究结果表明:采用新实验方法能得到较准确的结果,能较真实地描述压痕过程。     

Cylindrical nano-indentation on metal film/elastic substrate system with discrete dislocation plasticity analysis: A simple model for nano-indentation size effect

The cylindrical nano-indentation on metal film/elastic substrate is computationally studied using two-dimensional discrete dislocation plasticity combined with the commercial software ANSYS®, with a focus on the storage volume for geometrically necessary dislocations (GNDs) inside the films and the nano-indentation size effect (NISE). Our calculations show that almost all GNDs are stored in a rectangular area determined by the film thickness and the actual contact width. The variations of indentation contact width with indentation depth for various film thicknesses and indenter radii are fitted by an exponential relation, and then the GND density underneath the indenter is estimated. Based on the Taylor dislocation model and Tabor formula, a simple model for the dependence of the nano-indentation hardness of the film/substrate system on the indentation depth, the indenter radius and the film thickness is established, showing a good agreement with the present numerical results.     

An inverse problem in film/substrate indentation: extracting both the Young’s modulus and thickness of films

In an indentation test, the effective Young’s modulus of a film/substrate bilayer heterostructure varies with the indentation depth, a phenomenon known as the substrate effect. In previous studies investigating this, only the Young’s modulus of the film was unknown. Once the effective Young’s modulus of a film/substrate structure is determined at a given contact depth, the Young’s modulus of the film can be uniquely determined, i.e., there is a one-to-one relation between the Young’s modulus of the film and the film/substrate effective Young’s modulus. However, at times it is extremely challenging or even impossible to measure the film thickness. Furthermore, the precise definition of the layer/film thickness for a two-dimensional material can be problematic. In the current study, therefore, the thickness of the film and its Young’s modulus are treated as two unknowns that must be determined. Unlike the case with one unknown, there are infinite combinations of film thickness and Young’s modulus which can yield the same effective Young’s modulus for the film/substrate. An inverse problem is formulated and solved to extract the Young’s modulus and thickness of the film from the indentation depth-load curve. The accuracy and robustness of the inverse problem-solving method are also demonstrated.