A method for reconstruction of residual stress fields from measurements made in an incompatible region

A method is introduced by which the complete state of residual stress in an elastic body may be inferred from a limited set of experimental measurements. Two techniques for carrying out this reconstruction using finite element analysis are compared and it is shown that for exact reconstruction of the stress field via this method, the stress field must be measured over all eigenstrain-containing regions of the object. The effects of error and incompleteness in the measured part of the stress field on the subsequent analysis are investigated in a series of numerical experiments using synthetic measurement data based on the NeT TG1 round-robin weld specimen. It is hence shown that accurate residual stress field reconstruction is possible using measurement data of a quality achievable using current experimental techniques.     

Flow and fracture of rocks under general triaxial compression

Recent laboratory studies of the flow and fracture of rocks under general triaxial compression are reviewed. New developments in laboratory techniques have made it possible to measure three principal stresses and strains under general triaxial stress states, in which all three principal stresses are different.Strength and ductility of isotropic rocks are markedly affected not only by the least compression ₃, but also by the intermediate compression ₂, although these two effects are rather additional in strength, but opposite in ductility. The experimental results show that dilatancy is highly anisotropic under the general triaxial stress states.Deformational properties of anisotropic rocks have been also measured under the general triaxial compression. In this case, the effect of the intermediate compression markedly depends on the orientations of the weak planes.     

The future role of experimental mechanics in international aspects

The present and future role of measurement and experimental techniques in analyzing displacements, strain and stress in structures, structural elements as well as in technical systems is considered. There are more reasons and much wider fields to apply methods of experimental mechanics nowadays than ever before. New advanced modern techniques, new measurement equipment,automatization of the measuring and evaluating processes are available. Hybrid techniques, i.e. the combination of advanced measurement and computer techniques have been developed. Such methods allow optimization of structures and products, saving energy and raw materials, improving the quality of products and the reliability and safety of structures and technical systems, controlling the production processes as well as supervising the safety of operating systems. They can and have to contribute to minimizing risks of technical systems, and detractions of environment. To transfer knowledge on new developments in more or less advanced methods of experimental mechanics is of increasing importance to those developing countries, which are at the threshold of economic development.     

实验力学在未来国际上的地位

本文论述了测量和实验技术当前和未来在分析结构、结构单元以及技术系统的位移、应变和应力中所起的作用.今天比以往任何时候都有更多的原因在广泛的领域中采用实验力学的方法。现代先进的新技术,新的测量设备,自动的量测和评价过程已应运而生。先进的测量技术和计算机技术的结合——杂交技术已得到了发展,这些技术使我们能对结构和产品进行优化设计:节省能源和原材料,提高产品质量以及结构和技术系统的可靠性和安全度,管理生产的过程,监控运行系统的安全。这些方法能用于减少技术系统的危险性和对环境的破坏,对发展中国家传授先进的实验力学方法的知识,显得更为重要。     

纳米硬度技术的发展和应用

近二十年来,主要用于检测材料表面微米和亚微米尺度力学性质的纳米硬度技术发展迅速.首先,概述硬度的定义、分类及其适用范围.然后,系统地总结纳米硬度技术的发展,重点介绍纳米压痕硬度的测量原理及其影响因素,连续刚度测量原理,高分辨率的载荷位移测量原理,几种常用压头的几何形状,试样表面的准备和确定,相关的测试方法,仪器校准和显微观察等问题.通过压痕实验可获得硬度、弹性模量、断裂韧性、存储模量和损耗模量、蠕变应力指数等.最后,简要介绍纳米划痕硬度测量技术的发展和应用.      

二维材料实验力学综述

以石墨烯为代表的二维材料因其原子级厚度、独特物理性质,成为近年来物理、化学、材料交叉学科的研究热点,在合成制备、结构表征、应用开发等方面的研究工作表明其在微纳机电系统、光电器件与功能复合材料领域有广泛且重要的应用前景。然而,由于二维材料结构与尺度的独特性,在其基本物性的理解方面仍存在许多未解决的问题,尤其是力学性能的表征,面临着诸多挑战。本文综述了二维材料本征力学性质和界面力学行为的微纳测试与表征技术的最新进展,例如纳米压痕技术、微孔鼓泡法等,并详细探讨了影响二维材料力学性能及行为的主要因素,分析了其微观尺度下的作用机制,以期通过物理或化学手段实现力学性能的调控。     

On elastocapillarity: A review

Elastocapillary phenomena involving elastic deformation of solid structures coupled with capillary effects of liquid droplets/films can be observed in a diversity of fields,e.g.,biology and microelectromechanical systems(MEMS).Understanding the physical mechanisms underlying these phenomena is of great interest for the design of new materials and devices by utilizing the effects of surface tension at micro and nano scales.In this paper,some recent developments in the investigations on elastocapillary phenomena are briefly reviewed.Especially,we consider the deformation,adhesion,self-assembly,buckling and wrinkling of materials and devices induced by surface tensions or capillary forces.The main attention is paid to the experimental results of these phenomena and the theoretical analysis methods based on continuum mechanics.Additionally,the applications of these studies in the fields of MEMS,micro/nanometrology,and biomimetic design of advanced materials and devices are discussed.     

激光诱导应力波测量薄膜界面强度研究进展

薄膜界面强度是影响多层薄膜装置性能的重要参数。激光诱导应力波技术是在可控制和非接触条件下定量测量薄膜界面强度最有效的技术之一。在采用高强度应力波短脉冲加载实现界面层裂的同时，通过光学测量薄膜自由面瞬态位移，并利用应力波理论计算得到临界界面强度。通过精确控制试样几何形状及尺寸，包括拉伸、剪切和复合型在内的各种界面加载模式都可以实现。本文对激光诱导应力波测量薄膜界面强度研究进展进行了综述，并特别强调了不同加载模式的实现方法，高强度超薄薄膜的界面测量技术，以及如何通过辨别薄膜自由面瞬态位移光学干涉信号中的某些特殊性征来实时判断和测量界面的层裂。     

Experimental and linear viscoelastic stress distribution measurements of high density polyethylene flowing into and within a slit

We report experimental measurements fo the centreline stress build up and relaxation as molten polyethelene flows into a slit die. The time-dependent extensional stress distribution is obtained using flow birefringence techniques and these observations complement the corresponding velocity field measurements already reported. Experimental measurements of the linear viscoelastic storage and loss modulus are obtained and, from these results, the polymers are characterized in terms of a modulus spectrum. Using this modulus spectrum together with a Maxwell-type constitutive equation and the experimental centreline kinematics, we find that it is possible to simulate successfully the experimentally observed stress distributions. Our results indicate that it is essential to include the polymers' broad spectrum of relaxation times when considering time dependent flow problems.     

多层薄膜热应力模拟

针对MEMS器件和光电器件的薄膜结构在高温下产生的应力与应变会严重影响器件结构与功能的问题,本文采用Suhir异质生长薄膜热应力计算理论分析了三层薄膜结构的热应力大小分布情况,得到了不同镀膜温度、膜厚、基底厚度等条件下的热应力变化趋势,解决了困扰有限元分析的奇异点问题。通过分析模型与有限元分析结果的比对,得到该计算模型的应力分布较为符合有限元分析的结果,最大剪切应力差距约为6.1%。列举了一个通过分析关系对材料进行优化的实例。这些研究结果对恶劣工作环境下的MEMS器件以及光电子器件的薄膜设计具有一定的借鉴意义。     

地幔对流模拟实验

模拟实验是当前地幔对流研究中的一个重要手段,一个重要组成部分。本文着重介绍近年来关于地幔热对流模拟实验研究发展概况。从相似参数出发,讨论了实验装置,测试手段,并对实验结果和今后发展趋势作了评述。      

Stress Monitoring of Post-processed MEMS Silicon Microbridge Structures Using Raman Spectroscopy

Inherent residual stresses during material deposition can have profound effects on the functionality and reliability of fabricated Micro-Electro-Mechanical Systems (MEMS) devices. Residual stress often causes device failure due to curling, buckling, or fracture. Typically, the material properties of thin films used in surface micromachining are not well controlled during deposition. The residual stress; for example, tends to vary significantly for different deposition methods. Currently, few nondestructive techniques are available to measure residual stress in MEMS devices prior to the final release etch. In this research, micro-Raman spectroscopy is used to measure the residual stresses in polysilicon MEMS microbridge devices. This measurement technique was selected since it is nondestructive, fast, and provides the potential for in-situ stress monitoring. Raman spectroscopy residual stress profiles on unreleased and released MEMS microbridge beams are compared to analytical and FEM models to assess the viability of micro-Raman spectroscopy as an in-situ stress measurement technique. Raman spectroscopy was used during post-processing phosphorus ion implants on unreleased MEMS devices to investigate and monitor residual stress levels at key points during the post-processing sequences. As observed through Raman stress profiles and verified using on-chip test structures, the post-processing implants and accompanying anneals resulted in residual stress relaxation of over 90%.     

Recent advances in studies on multiphase and reacting flows in China

The recent developments and advances of studies on multiphase and reacting flows, including gas-solid, gas-liquid, liquid-solid and reacting flows, in China are reviewed. Special emphasis is laid on the fundamental studies and numerical models. Some important experimental results are also reported. But measurement techniques are not covered.     

Overview of optical techniques that measure displacements: Murray lecture

Optical techniques that measure displacements play a very important role in current experimental mechanics, material sciences and metrology. This paper presents a survey of developments in these techniques from a personal experience point of view. Three main aspects are considered. Mathematical and numerical models used in the interpretation of fringe information and the corresponding data processing techniques. Optical and electro-optical developments that have taken place to improve the sensitivity, and the efficiency of these methods to make them competitive with purely numerical methods. Applications that have arisen from the synergy between advanced computational capabilities and optics are also presented.     

涡旋运动与混沌

本文综述了利用非线性动力学方法研究涡旋运动的最新进展。通过典型例子的分析,讨论了该领域的研究方法、主要困难和存在问题,并提出今后的可能研究方向。      

The applications of unsteady, multi-dimensional studies of detonation waves to ram accelerators

Since much of the early work on the concepts on which ram accelerators are based dates back to the 1960s, although many of these are still being actively pursued, it is difficult to formulate a completely logical approach. This situation is compounded by the use of presently unacceptable treatments of unidimensional detonations in the early work and unfortunately extended to some of the more modern treatments. My approach has been to start by dealing with the early work and recent work impinging upon it, then to re-emphasise recent work on detonations, particularly that dealing with the influence of changes in confinement on quenching and re-initiation of detonations. However, some knowledge of this is inferred in suggestions made in Part 2 for possible improvements in the techniques. Latter sections cover the development of the ram accelerator, the use of various types of projectiles, developments in experimental techniques and finally on areas in space flight where the results from ram accelerators might be utilised. Received 14 January 1999 / Accepted 16 June 1999     

On the uniqueness and sensitivity of indentation testing of isotropic materials

Instrumented indentation is a popular technique to extract the material properties of small scale structures. The uniqueness and sensitivity to experimental errors determine the practical usefulness of such experiments. Here, a method to identify test techniques that minimizes sensitivity to experimental erros is in indentation experiments developed. The methods are based on considering “shape functions,” which are sets of functions that describe the force–displacement relationship obtained during the indentation test. The concept of condition number is used to investigate the relative reliability of various possible dual indentation techniques. Interestingly, it was found that many dual indentation techniques can be as unreliable as single indentation techniques. Sensitivity analyses were employed for further understanding of the uniqueness and sensitivity to experimental errors of indentation techniques. The advantage of the Monte Carlo approach over other procedures is established. Practical guidelines regarding the selection of shape functions of force–displacement relationship and geometric parameters, while carrying out indentation analysis are provided. The results suggest that indentation experiments need to be very accurate to extract reliable material properties.     

The Potential of MEMS for Advancing Experiments and Modeling in Cell Mechanics

Response to mechanical stimuli largely dictates cellular form and function. A host of extraordinary yet unexplained responses have been identified within the hierarchical cell structure. As experimental and model-based investigations in cell mechanics advance, the underlying structure-function mechanisms dictating these responses emerge. Here we explore the potential of microelectromechanical systems (MEMS) for advancing understanding of cell mechanics. To motivate the discussion, existing experimental techniques are summarized. Interrelated model-based approaches, which aim to interpret or predict observed results, are also outlined. We then focus on a representative set of MEMS-based devices designed for investigations in cell mechanics and point to the fact that, while these devices have yet to maximize their functionality through higher levels of sensor/actuator integration, they are highly complementary to existing techniques. In closing, novel MEMS sensor and actuator schemes that have yet to materialize in this field are discussed to motivate the next generation of MEMS for investigations in cell mechanics.     

Basic theory and experimental techniques of the strain-gradient method

The theories of presently used experimental methods of stress and deformation analysis which employ radiant energy as a detector are based on the assumption that light propagates rectilinearly within both undeformed and deformed bodies which are initially homogeneous and isotropic when diffraction phenomena are negligible. This assumption is not correct: light propagation within deformed bodies is nonrectilinear in a general case. Although this has already been observed and applied practically by some researchers in photoelasticity, it has not so far been generally acknowledged and accepted in experimental mechanics. On the basis of empirical data produced by the authors in the period 1948–1983, we present theories and foundations of the techniques of a new experimental method which is based on the relations between stress/strain gradients and curvatures of light beams. This method is called the strain-gradient method or, less rigorously, gradient photoelasticity.     

On the methodologies of stress analysis of composite structures: Part 2: New experimental approaches

The rapidly increasing technological importance of composite materials and composite structures is leading to the development of new, more advanced models of their actual response to mechanical and thermal loads. This in turn results in the development of new experimental and analytical methods for determination of the mechanical and thermal responses of such structures and materials to various loads. In this respect the reliability and the predictive power of various methods and techniques of stress analysis become very important since all the analytical, experimental and numerical methods used for the determination, prediction and optimization of the actual mechanical responses of composite structures and materials are based on the concepts of strain and stress. Because of the inherently three-dimensional stress and strain states in composite materials and structures and the wide use of viscoelastic polymers as the matrix and some reinforcing fiber materials, a more rigorous type of modelling than had been common in the past is needed of all the involved physical phenomena which influence the strain and stress states at the local and global levels. Also, a more rigorous analysis of practical consequences of the physical and mathematical simplifications is required to assure reliability and accuracy of various methods of stress analysis. The influence of the above-mentioned factors on the reliability and applicability of analytical and experimental procedures is illustrated by examples of actual material responses.Part 2 of this paper presents theories and techniques of three new methods of strain/stress analysis which have been developed on the basis of comprehensive physical models of involved phenomena: the isodyne, strain gradient and thermoelastic effect methods. Presented examples illustrate the efficacy of these methods.