Experimental full field investigations of resonant vibrations for piezoceramic plates by an optical interferometry method

The experimental measurement of resonant frequencies for piezoelectric material is generally performed by impedance analysis. In this paper we employ an optical interferometry method, called amplitude-fluctuation electronic speckle pattern interferometry (AF-ESPI), to investigate the vibration characteristics of piezoceramic plates. This method demonstrates its advantages of combining noise reduction, like the subtraction method, and high fringe sensitivity, like the time-averaged method. As compared with the film recording and optical reconstruction procedures used for holographic interferometry, the interferometric fringes of AF-ESPI are produced instantly by a video recording system. Based on the fact that clear fringe patterns measured by the AF-ESPI method will be shown only at resonant frequencies, both the resonant frequencies and corresponding mode shapes are obtained experimentally at the same time. Excellent quality for the interferometric fringe patterns of the mode shapes is demonstrated. We find from experimental results that the out-of-plane vibration modes (type A) with lower resonant frequencies cannot be measured by impedance analysis and only the in-plane vibration modes (type B) will be shown. However, both the out-of-plane (bending) and in-plane (extension) vibration modes of piezoceramic plates are obtained by the AF-ESPI method. Finally, numberical finite element calculations are also performed, and the results are compared with the experimental measurements. Excellent agreement for the resonant frequencies and mode shapes are obtained from both results.     

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.     

Vibration analysis of plates by real-time stroboscopic holography

The application of time-averaged holography to the analysis of vibrating surfaces is now a widespread and very powerful experimental technique. However, the realtime method of exactly superimposing the reconstructed static image on the vibrating object and observing the resulting fringe patterns under stroboscopic illumination is becoming more common. In fact, by introducing an initial family of interference fringes by the rotation of the object (or image) before vibration begins, the advantages of both methods are combined.     

电子错位散斑研究

本文从理论上分析了错位散斑实现电子干涉的可行性,推导了摄象机光靶接收的错位散斑的光强表达式,分析了电子干涉条纹的可见度,提出并实现了电子错位散斑技术的三种方法,即实时法,双曝光法和实时时间差法。获得了位移梯度等值条纹的电子干涉图。     

Additive-subtractive phase-modulated shearography with synchronized acoustic stressing

A robust electronic shearography technique is described that is immune to ambinet noise. In this technique, additive speckle-interference images containing information about the same two states of deformation of a test object undergoing acoustic stressing are acquired in each video frame. When a spatially nonuniform phase modulation is introduced in every other frame during the video sequence (using a tilting mirror) and the additive interference images are subtracted sequentially using a real-time image processor, the self-interference component of sheared-speckle interference pattern is removed. The susceptibility of this method to environmental noise caused by induced thermal noise and low frequency vibration is demonstrated to be lower than that of conventional substractive shearography methods; fringe visibility and contrast are significantly improved as well. The ability of this technique to work in a turbulent environment is demonstrated, and application to detection of structural defects in adhesively bonded structures, a problem of interest in nondestructive evaluation of structures, is shown.     

EXPERIMENTAL INVESTIGATIONS ON DYNAMIC CHARACTERISTICS OF A MULTILAYER PIEZOELECTRIC STACK ACTUATOR

Multilayer piezoelectric stack actuators are widely used in many industrial applications and the investigation on the dynamic behavior of this element is needed. In this study, two optical interferometric techniques called amplitude-fluctuation electronic speckle pattern interferometry (AF-ESPI) and laser Doppler vibrometer (LDV) are used to experimentally investigate the vibration characteristics of a single-layer piezoelectric disc and a multilayer piezoelectric stack actuator. These two techniques are full-field measurement for AF-ESPI and point-wise displacement measurement for LDV. Because the clear fringe patterns obtained by the AF-ESPI method will be shown only at resonant frequencies, both the resonant frequencies and corresponding vibration mode shapes of the piezoelectric disc and the multilayer piezoelectric stack actuator are obtained simultaneously by the AF-ESPI method. Interferometric fringe patterns for both the in-plane and out-of-plane vibration mode shapes are demonstrated. In addition to the proposed two optical techniques, numerical computations based on a commercially available finite element package are presented for comparison with the experimental results. Good agreement between the measured data by experimental methods and the numerical results predicted by FEM is found in resonant frequencies and mode shapes for the single-layer piezoelectric disc. However, some discrepancies are observed for the results obtained by AF-ESPI and impedance analysis for the multilayer piezoelectric stack actuator. A detailed discussion is made to address important issues of this problem.     

The magic of carrier fringes in moiré interferometry

Practical applications in which carrier fringes are used with moiré interferometry for strain measurements are presented. Examples illustrate how moiré carrier fringes are applied to obtain the desired data in complex laminated composite specimens. In many cases, carrier fringes permit extraction of much more detailed information, with procedures that are easier and more accurate than those using loadinduced fringes alone. The fringe vector for carrier fringes is introduced and its application to the interpretation of fringe patterns is explained. In moiré interferometry, the carrier fringes are produced easily by adjustments of optical elements that control the virtual reference grating.     

数字剪切散斑相关条纹图的形成及其分析

本文提出了用线性相关计算形成高可见度的数字剪切散斑相关条纹图的方法，为相关条纹图的自动分析奠定了良好的基础。文中还推导了附加位相与物光反射镜转角的关系，使精确地计算附加位相成为可能。同时还介绍了任意附加位相的三步相移技术。最后给出了用线性相关计算法形成的相关条纹图及计算出的位相图。     

DCM 相位梯度提取技术在光力学图像信息识别中的应用

为解决相位去包裹对光力学图像处理的影响问题,本文将一种直接从数字编码傅立叶变换或相移条纹图中逐相素提取相位梯度的方法应用于光力学图像信息识别中。该方法是一个广泛应用于光纤传感器中的微分正交乘法器(DCM)的正、余弦相位检波技术。这一技术算法简单,不需计算反正切,不受相位去包裹方法标准的限制,非常适于应用在以相位梯度作为最终所需识别的信息的光力学图像处理中。本文以载波等达因条纹图应力场计算及四幅图相移等达因法求板的接触应力为例,对该技术在光力学图像信息识别中的应用进行了论证。     

倍增云纹的显微测量及密集云纹场的应变测算

为了提高云纹法的准确度,文中提出了使倍增后的云纹宽度变细并进行显微测量的技术,以及由密集云纹场测算应变的条纹间距插值和小间距均匀法。给出了估计由此法算得的应变的误差及其修正方法。所做的曲杆实验其数据与理论值符合得很好。文中还将云纹法中闪耀光栅与振幅栅相匹配的光学空间滤波系统改进为实时系统,导出其条纹倍增率及云纹图的最佳对比度条件。     

利用光学干涉条纹图相位分析测量纳米位移

Optical methods such as Twyman-Green interferometry, moiré interferometry, holographic interferometry and speckle interferometry are useful to measure displacement and strain in the full-field of structures. Recently phase analysis method of fringe patterns obtained by these optical methods becomes popular, and it provides accurate quantitative results in the full-field. In this paper, in order to measure displacement and strain, real-time or high-speed nano-meter displacement measurement methods developed by the authors are introduced. That is, (1) out-of-plane displacement analysis by Twyman-Green interferometry using integrated phase-shifting method using Fourier transform phase-shifting method, (2) simultaneous two-dimensional in-plane displacement analysis by moiré interferometry and (3) out-of-plane displacement analysis by phase-shifting digital holographic interferometry. The theories and applications are shown.     

爆破等差与等和条纹图分析方法探讨

在一次加载同时得到爆破模型的动态等差线与等和线条纹的基础上，采用部分改变物光光程的方法求得等和条纹的级次，它对今后的爆破条纹判读具有重要意义；并以此为据证实了目前采用的爆破等差条纹判读方法的正确性；发现在两种条纹图上存在着特殊的条纹对应区域，它有益于更方便地进行几种条纹图的判读。     

加窗傅里叶滤波和相干增强扩散在条纹去噪中的比较分析

光学干涉测量具有非接触、高精度和全场测量的优点,能对形变、折射率、位移等信息进行测量。噪声滤除是光学干涉测量产生的条纹图像处理的一个关键问题。加窗傅里叶滤波(Windowed Fourier Filtering,WFF)与自适应加窗傅里叶滤波(Adaptive Windowed Fourier Filtering,AWFF)是有效的频域去噪算法。相干增强扩散(Coherence Enhancing Diffusion,CED)则是基于偏微分方程的空域去噪算法。针对条纹去噪问题,比较了WFF、AWFF和CED在不同密度和不同噪声类型的条纹图上的表现,分析了它们的适用条纹类型。     

A simple method for the digital determination of the photoelastic fringe order

A new and simple approach to the digital determination of a photoelastic fringe order using two different loads is proposed. The relationships between the intensity values of light and the isochromatic fringe orders generated from two different loads are derived. The scheme used for the automated determination of the total fringe orders of a full-field photoelastic fringe pattern is described. The usefulness of this method is demonstrated using two isochromatic fringe patterns under two different loads. Extra filters are not needed in the proposed method as in the case of the two-wavelength method.     

Cyclic-stress studies by time-averaged photoelasticity

This paper describes an experimental method whereby the amplitude of cyclic stresses may be readily determined by time-averaged photoelasticity. Using an ordinary polariscope with a monochromatic-light source, ‘time-averaged isochromatics fringes’ are formed if the photographic film in the camera is exposed with an exposure time equal to one or several periods while the photoelastic model is undergoing steady-state cyclic loading. The fringe pattern depicts amplitudes of the oscillating stresses according to the zeroth-order Bessel function. These properties permit the determination of a time-averaged cyclic stress-optic law. It is also possible to use the method to determine time-averaged isoclinics. The method has great potentiality in the study of in-plane vibrations.     

Nano-meter Displacement Measurement by Phase Analysis of Fringe Patterns Obtained by Optical Methods

0Introduction Inordertomonitorthehealthofstructures,straingagesanddisplacementtransducersare usuallyused.Thereasonisthatmanyrulesorcodesforinspectionofstructuresrequiretousestrain gagesanddisplacementtransducers,anditiseasytousethem.However,thesemethodsarebasically one pointmeasurementmethods.Theyareexpensiveandtime consumingfordistributionanalysis.Opticalmethodssuchasgrating projection,geometricmoir啨,moir啨interferometry,holographic interferometryandspeckleinterferometryareusefultomeasuredi…     

利用具有相位跳跃技术的阴影摩尔图测量电子组装元件的热翘曲

Phase-stepping technique is applied to the analysis of fringe patterns of shadow moiré of electronic packages.Sensitivity of the fringe pattern analysis is demonstrated to be significantly increased.Thermally induced warpage of electronic packages is successfully measured in real-time as the sample is driven through a simulated reflow process.The paper discusses the technique of phase stepping,noise filtering and its application to the shadow moiré method.Applications of the technology are presented.     

Inversion of speckle interferometer fringes for hole-drilling residual stress determinations

Speckle interferometric fringe patterns record stress-relief displacements induced by the drilling of blind-holes into prestressed objects. The quantitative determination of residual stress state from such stress patterns is difficult because of the ambiguity in the order of the observed fringes. The plane stress magnitudes are provided directly from selected fringe positions using a stochastic, iterative least squares minimization approach. The inversion requires prior knowledge of the experimental geometry and an appropriate uniaxial stress-relief displacement basis function derived from three-dimensional finite element calculations. Superpositioning of the rotated and scaled displacement basis functions allows the stress-relief relaxation for any biaxial state of stress to be determined. In this paper, fringe patterns were forward modeled from a large ensemble of calculated biaxial stress-relief displacement fields. Inversion of these noise-free fringe patterns reproduced the biaxial stresses with negligible error. Analysis of more realistic fringe patterns that include speckle noise gave stress magnitude errors that diminished rapidly with the number of selected points to better than 3 percent for 100 points. Sensitivity of the optical method is influenced by a number of factors, but the ensemble of model fringe patterns studied indicates that the stress magnitudes (nomalized with respect to the material's Young's modulus) from 3×10^–4 to 10^–2 can accurately be determined with visible laser radiation. The method is amenable to automation and can easily be extended to study near surface gradients in the residual stresses or applied to other optical recording techniques such as moiré and phase-shifting interferometry.     

Computer-simulated stress-optic patterns

A new method for producing theoretical fringe patterns using a small real-time computer interfaced to analog equipment is presented. The method applies equally well to isochromatic, isopachic or interferometric stress patterns of any stress field for which the analytical solution is known.     

利用具有相位跳跃技术的阴影摩尔图测量电子组装元件的热翘曲

Phase-stepping technique is applied to the analysis of fringe patterns of shadow moiré of electronic packages. Sensitivity of the fringe pattern analysis is demonstrated to be significantly increased. Thermally induced warpage of electronic packages is successfully measured in real-time as the sample is driven through a simulated reflow process. The paper discusses the technique of phase stepping, noise filtering and its application to the shadow moiré method. Applications of the technology are presented.