Moiré interferometry at VPI & SU

A relatively easy technique for producing high-frequency gratings on specimens extends moiré techniques into the high-sensitivity domain. Whole-field patterns of inplane displacement components are obtained with grating frequencies of 1200, 2400 and 4000 l/mm (30,480, 60,960 and 101,600 l/in.). Moiré interferometry is a case of two-beam interference, characterized by extensive range, excellent fringe contrast and fringe localization on the specimen surface. It is a reflection technique, compatible with opaque specimens and live observation of deformation.     

MOIRÉ INTERFEROMETRY WITH ENHANCED RESOLUTION

The implementation of this method is relatively simple. It does not require any modification to interferometric systems already in use. It can be used in the analysis of large as well as very small fields, including measurements under a microscope.
A fringe pattern with a carrier is recorded in a single exposure, just as in the standard procedure. As a consequence, any possible instability of the interferometer does not affect the accuracy of the measurements. Further, the spatial resolution of the measurements is not compromised.
Since only one exposure is needed for a given displacement component this method can be applied to dynamic as well as static measurements.
It should be emphasized that even though the number of electronically generated fringes can be arbitrary, the sensitivity is the same as in ordinary moire interferometry as long as the data are taken at the lines of minimum intensity. The improvement is in the increase of the number of data points taken at the locations of maximum resolution. In other words, all the data are taken at the maximum resolution.
This method applies not only to moire interferometry but to any experimental technique based on two-beam interference, as long as the fringe pattern has good contrast.     

Grating holographic interferometry

A holographic interferometric technique, combining an image hologram with a grating approach, is proposed for three-dimensional deformation measurements on opaque planar object surfaces. In this technique, the holographic plate is brought close to the object surface, onto which a high-frequency crossed-line diffraction grating has been replicated. The grating surface produces multiple object waves rather than the usual diffusely reflected object waves. The double-exposed single holograms can be reconstructed at multiple off-axis angles. Four independent high-contrast fringe patterns are extracted simultaneously. Displacement vectors over the entire measurement area are separated in three orthogonal directions. The resultant displacements are presented as three-dimensional meshed plots and topographic contour maps. The optical system for both recording and reconstruction of the holograms has been simplified compared to conventional holographic interferometry. Experimental errors associated with fringe readout and system geometry are reduced because of the sharp images and the well-defined spatial orientation in the reconstruction system.     

Diffraction theory of optical interference moiré and a device for production of variable virtual reference gratings: A moiré microscope

Optical interference moiré methods are analyzed using Fraunhoffer diffraction theory to relate general large surface deformations to the fringes observed. This analysis determines the Almansl strain in the current configuration from the gradients of the fringe number function. The analysis shows the advantages of an experimental scheme that allows the virtual reference grating to be varied. The ability to vary the virtual reference grating results in a larger dynamic range and the ability to maintain a fringe spacing for maximum accuracy. A moiré microscope has been constructed which has this ability. Digital image processing coupled with optical filtering and phase control is used to enhance the accuracy of the fringe measurements. The variable virtual-reference-grating capability is demonstrated by using it to highlight several aspects of the deformation field near a crack tip in a single crystal of iron-silicon.     

液层厚度对浮力-热毛细对流面型的影响

将Michelson光学干涉测量系统与图像处理技术相结合，发展形成一种实时诊断热毛细对流和浮力对流流体表面形貌的实验测量系统. 采用光学干涉测量方法研究了两端带有温差的矩形池内薄层流体的对流、表面变形、以及表面波的基本问题. 应用Fourier变换方法对实验结果进行计算和分析，得到了流体表面变形和表面波的定量的实验结果. 实验结果表明了在浮力-热毛细对流的发展过程中，首先出现流体的表面变形，之后在该变形的基础上，叠加了一个表面波的信息，该表面变形和表面波与流体的温度梯度、表面张力、以及浮力有直接的关系；表面波隐藏在表面变形内.     

On the holographic interferometry fringe gradient method for complete surface deformation measurements

A fringe gradient method for complete surface gradient deformation measurements using five holograms is presented. Feasibility of the method is studied by combining the experimental results with the computer simulations. It is shown that the goveming equations relating the fringe gradient to the displacement vector and the displacement gradient are not well conditioned. Accuracy of the experimental data required to make the method successful is analyzed.     

Application of fourier analysis to the laser based interferometric strain/displacement gage

The laser based interferometric Strain/Displacement Gage (ISDG) measures the in-plane surface deformation between two small reflecting surface markers. When illuminated with a coherent beam of light, the reflected beams from the two markers form an interference pattern, and monitoring the shift of the fringe pattern allows strain in the gage section of a specimen to be directly measured. A minimum on the fringe pattern can be isolated and tracked as the test proceeds, but this technique utilizes only a small part of the optical signal and often requires a complex programming scheme. This paper presents the application of Fourier transform and phase shifting techniques to the use of the ISDG during microsample tensile testing. The Fourier transform samples the entire fringe pattern and greatly improves the optical signal to noise ratio, and the phase shifting fringe pattern analysis has proven to be more robust and less affected by speckle or optical noise than fringe pattern minimum tracking. This results in the ability to measure larger deformations with a system resolution of ∼5 microstrain and an uncertainty of ±15.5 microstrain. An example involving the microsample tensile testing of a MEMS related LIGA Ni specimen is included to demonstrate the utility of these new techniques.     

A nonlinear finite element formulation for axisymmetric torsion of biphasic materials

In this paper, we present a finite element formulation for describing the large deformation torsional response of biphasic materials, with specific application to prediction of nonlinear coupling between torsional deformation and fluid pressurization in articular cartilage. Due to the use of a cylindrical coordinate system, a particular challenge arises in the linearization of the weak form. The torsional axisymmetric case considered gives rise to additional geometric terms, which are important for the robustness of the numerical implementation and that would not be present in a Cartesian formulation. A detailed derivation of this linearization process is given, couched in the context of a variational formulation suitable for finite element implementation. A series of numerical parametric studies are presented and compared to experimental measurements of the time dependent response of cartilage.     

Stress and deformation analysis from reduced-scale plastic-model testing

The use of scale models, which are made from plastic material, for stress and deformation analysis of missile nose-cone structures is discussed. The special strain-gage application and testing techniques, which are required because of the use of plastic materials, are detailed.The utilization of relatively inexpensive simplified models for the investigation of two specific design conditions is cited. The first case is a stress and deformation study of a thin, constant-thickness, shallow spherical shell which is supported by a circumferential line reaction and subjected to uniform external pressure. Comparisons are made with a recently published theoretical analysis of the problem.The second case is a particular design problem which is concerned with the determination of the stress and deformation in a variable-thickness, shallow spherical shell with several various-size cutouts. The shell is loaded with a varying external-pressure load which is reacted by a circumferential line load at the periphery. Influence curves for both stress and deformation are given.Some limitations of plastic-model testing are reviewed, and guides to successful use of the method are given.Paper was presented at 1959 SESA Spring Meeting held in Washington, D. C. on May 20–22.     

Analysis of behavior near a cylindrical glass inclusion by scattered-light photoelasticity

Application of scattered-light photoelastic techniques to the determination of the normal-stress differences and shearing stresses in the matrix near an Eglass rod embedded in a polyester-resin matrix under tension is discussed. The rod is semi-infinite, that is, it extends from the center of the test section through one of the grip ends. Specific methods for making the three-dimensional scattered-light observations on the composite specimen under load are described in detail. Some typical fringe patterns are presented. Results will be useful in predicting the behavior of fibrous-glass-reinforced plastic composites since both matrix and inclusion are made of prototype materials. The system is modeled in geometric scale only.     

大电流快速加热条件下LY12铝板动态变形的云纹干涉法实验研究

本文研究了中间带孔受拉铝板在大电流热冲击条件下的动态变形测试．在试件表面制作高温高频光栅，预加机械载荷后放入双光束云纹干涉光路中，用大电流加热器对试件进行快速加热，利用高速摄影机拍摄记录试件表面圆孔附近区域干涉条纹的变化情况，同时利用测温系统对试件的温度变化情况进行了测试记录．实验结果表明，用高速拍摄方法摄影热冲击条件下的云纹干涉条纹变化是可行的．     

Fracture investigation at V-notch tip using coherent gradient sensing (CGS)

Local deformation field and fracture characterization of mode I V-notch tip are studied using coherent gradient sensing (CGS). First, the governing equations that relate to the CGS measurements and the elastic solution at mode I V-notch tip are derived in terms of the stress intensity factor, material constant, notch angle and fringe order. Then, a series of CGS fringe patterns of mode I V-notch are simulated, and the effects of the notch angle on the shape and size of CGS fringe pattern are analyzed. Finally, the local deformation field and fracture characterization of mode I V-notch tip with different V-notch angles are experimentally investigated using three-point-bending specimen via CGS method. The CGS interference fringe patterns obtained from experiments and simulations show a good agreement. The stress intensity factor obtained from CGS measurements shows a good agreement with finite element results under K-dominant assumption.     

A review of MEMS-based microscale and nanoscale tensile and bending testing

Thin films at the micrometer and submicrometer scales exhibit mechanical properties that are different than those of bulk polycrystals. Industrial application of these materials requires accurate mechanical characterization. Also, a fundamental understanding of the deformation processes at smaller length scales is required to exploit the size and interface effects to develop new and technologically attractive materials. Specimen fabrication, small-scale force and displacement generation, and high resolution in the measurements are generic challenges in microscale and nanoscale mechanical testing. In this paper, we review small-scale materials testing techniques with special focus on the application of microelectromechanical systems (MEMS). Small size and high force and displacement resolution make MEMS suitable for small-scale mechanical testing. We discuss the development of tensile and bending testing techniques using MEMS, along with the experimental results on nanoscale aluminum specimens.     

A Review of Speckle Pattern Fabrication and Assessment for Digital Image Correlation

As a carrier of deformation information, the speckle pattern, or more exactly the random intensity distributions, which could be naturally occurred or artificially fabricated onto test samples’ surface, plays an indispensable role in digital image correlation (DIC). It is now well recognized that the accuracy and precision in DIC measurements not only rely on correlation algorithms, but also depend highly on the quality of the speckle pattern. Considering the huge diversity in test materials, spatial scales and experimental conditions, speckle pattern fabrication could be a challenging issue facing DIC practitioners. To obtain good speckle patterns suitable for DIC measurements, some key issues of fabrication methods and quality assessment of speckle patterns must be well addressed. To this end, this review systematically presents the speckle pattern classification and fabrication techniques for various samples and scales, as well as some typical quality assessment metrics.     

Suitability of the photoelastic implementation of polymers with material birefringence

Scattered photoelastic techniques would gain considerable momentum through better understanding of the nature and response of the available photoelastic materials. The influence of the prevailing birefringent effect in the state of load-free polymers on the radiated scattered light energy is investigated. Six different photoelastic materials are considered. The impact of material birefringences on the quality of reading the fringe orders of the scattered radiation in a stressed photoelastic medium is explored. Spectral dependence of the modulation of light vectors in acrylic materials (Plexiglas) is illustrated. The advantage of this characteristic as a convenient means of compensation in scattered photoelasticity is pointed out. Acrylic sheets, a relatively cheap transparent polymer, seem to be suitable for scattered photoelastic analysis. The moiré technique might find application in determining fringe orders in a material such as Homalite-100. Replacing the primary beam by a primary sheet saves the cost and effort consumed in the scanning process. Paper was presented at the 1988 SEM Spring Conference on Experimental Mechanics held in Portland, OR on June 5–10.     

Overview of High-temperature Deformation Measurement Using Digital Image Correlation

Background

Developments in digital image correlation (DIC) in the last decade have made it a practical and effective optical technique for displacement and strain measurement at high temperatures.

Objective

This overview aims to review the research progress, summarize the experience and provide valuable references for the high-temperature deformation measurement using DIC.

Methods

We comprehensively summarize challenges and recent advances in high-temperature DIC techniques.

Results

Fundamental principles of high-temperature DIC and various approaches to generate thermal environment or apply thermal loading are briefly introduced first. Then, the three primary challenges presented in performing high-temperature DIC measurements, i.e., 1). image saturation caused by intensified thermal radiation of heated sample and surrounding heating elements, 2) image contrast reduction due to surface oxidation of the heated sample and speckle pattern debonding, and 3) image distortion due to heat haze between the sample and the heating source, and corresponding countermeasures (i.e., the suppression of thermal radiation, fabrication of high-temperature speckle pattern and mitigation of heat haze) are discussed in detail. Next, typical applications of high-temperature DIC at various spatial scales are briefly described. Finally, remaining unsolved problems and future goals in high-temperature deformation measurements using DIC are also provided. 

Conclusions

We expect this review can guide to build a suitable DIC system for kinematic field measurements at high temperatures and solve the challenging problems that may be encountered during real tests.

     

A Method of Detecting the Onset of Localized Necking Based on Surface Geometry Measurements

To more accurately capture the onset of localized necking and obtain necking limit strains, this paper proposes a method of detecting the onset of localized necking in the Marciniak test (i.e. under in-plane deformation). The method is merely based on the measured surface geometry of the test specimen using digital image correlation (DIC) techniques. It was inspired by the observation of a sudden increase of the surface curvature obtained from 2D curvature fits along the direction across the surface of the sheet. This increase of the surface curvature is detected just before the dimples, which form the final localized neck, become obvious in the DIC measurements. The appearance of this signal is explained by a neck expansion theory defined by propagation of the instability along the direction of the neck, which is a physical behavior of materials.     

Holographic moiré in real time

The body of knowledge necessary to observe holographic-moiré patterns in real time is introduced. The basic factors influencing fringe visibility in holographic moiré are analyzed and expressions to evaluate fringe visibility for any given displacement and deformation are given. The application of the introduced theory in the case of real-time observation is discussed. It is shown that the maximum benefits of this technique are achieved by combining it with closed-circuit TV. Several examples of application are given.     

数字散斑相关方法与应用研究进展

数字散斑相关方法(digital speckle correlation method, DSCM)作为固体实验力学领域材料表面变形场测量的一种非接触式方法与其它技术相比具有一些独到的优点,其有效性已经在近20年中被众多的应用实例所证明.近年来该方法在理论上逐步完善,一些现代的数学理论和数学方法(例如:小波变换、遗传算法、神经网络等)逐渐被引入到该方法中;在实验中的各个技术环节上正在逐步的改进,其结果的精度逐步的提高,结果收敛的速度也逐渐的加快;其应用的领域正逐渐从常规材料的测试向一些新型材料测试、从宏观场逐渐向细微观尺度、从常规环境向比较恶劣的环境、从实验室测试逐步向工程现场应用、从静态准静态向动态准动态等方面发展.本文简要介绍了DSCM的基本原理和数学模型,在相关搜索的改进、相关系数的选择、灰度级的重建和图像识别方面的进展以及在材料力学性能测试,微尺度变形场测量等方面的应用.最后就这一实验测量技术的几个可能的发展趋势进行了预测.