Time-averaged Electronic Speckle Pattern Research Articles

Accurate determination of the elastic moduli of optimized cantilever beams by efficient time-averaged ESPI system

Elastic moduli, including Young’s modulus, shear modulus, bulk modulus, etc, are key parameters that are used to characterize the ability of a solid material to resist various types of deformation. The moduli can be extracted from the natural frequencies of a cantilever beam. In this paper, the relationships between moduli and natural frequencies, for the first time, are quantified by the finite element method. The optimized three-dimensional proportion of the cantilever beam is selected to be implemented simple error compensation. Experimentally, to precisely obtain the natural frequencies of the cantilever beam, an efficient time-averaged electronic speckle pattern interferometry(ESPI) system has been developed. The efficiency and precision are reflected in the following aspects: firstly, according to the slender character of the cantilever beam, a large shear optical path arrangement is designed to facilitate isolation from environmental interference; secondly, a resonance search method, based on the moiré effect is employed to recognize the natural frequencies accurately and efficiently; thirdly, a novel dynamic phase-shifting method is proposed based on the arrangement of the large shear optical path for clearer visualization of the mode shape of the cantilever beam. The proposed methods are verified by three kinds of common materials. The results suggest that Young’s modulus and shear modulus derived from natural frequencies are higher than the known value, and the error compensation can significantly reduce the calculation error. Furthermore, the experiments carried out on the woven carbon fiber reinforced plastic laminates illustrate the potential of the proposed methods in the evaluation of elastic moduli of composites. Given that the exciter attached to the specimen surfaces can be replaced with some special counterparts, the proposed ESPI system has considerable potential to test the objects loaded in some extreme environments, e.g. at high temperatures or underwater, where contact detection methods are difficult to be implemented.

LDV-induced stroboscopic digital image correlation for high spatial resolution vibration measurement.

Vibration measurement, particularly mode shape measurement, is an important aspect of structural dynamic analysis since it can validate finite element or analytical vibration models. Scanning laser Doppler vibrometry (LDV) and high-speed digital image correlation have become dominant methods for experimental mode shape measurement. However, these methods have high equipment costs and several disadvantages regarding spatial or temporal performance. This paper proposes a laser Doppler vibrometer induced stroboscopic digital image correction for non-contact mode shape and operational deflection shape measurement. Our results verify that single-point LDV and normal rate cameras can be used obtain high spatial resolution mode shape and operational deflection shape. Measurement frequency range is much higher than the camera capturing rate. We also show that the proposed approach coincides well with time-averaged electronic speckle pattern interferometry.

Investigation on Vibration Response of Aluminum Foam Beams Using Speckle Interferometry

This paper firstly presents a brief review on the representations of speckle fringe pattern in vibration measurement using time-averaged electronic speckle pattern interference (ESPI) method. Based on laser phase noise, a new representation of speckle fringe intensity generated by real-time subtraction ESPI method for vibrating measurement is then proposed. In the experimental performance, the real-time subtraction ESPI method is employed to inspect the vibrating response of a closed-cell aluminum foam cantilever beam. In contract with the results of the finite elemental method simulation, the vibration mode shapes obtained by the ESPI method are well agreed with numerical prediction. The optical vibration analysis is also carried out to determine the effective Young’s modulus of aluminum foam, and the results verify the validation of the ESPI method for investigation on mechanical properties of metal foam materials.

Deformation reconstruction by means of surface optimization Part I: Time-averaged electronic speckle pattern interferometry

Electronic speckle pattern interferometry is a well-known experimental technique for full-field deformation measurements. Although speckle interferometry techniques were developed years ago and are widely used for the visualization of the operating deflection shapes of vibrating surfaces, methods for accurate reconstruction of the observed deflection shape are still an active topic of research. Determination of the relative phase of the motion of vibrating objects is especially difficult and normally phase maps are calculated by direct transformation of the experimentally obtained interferometric images. An alternative method of phase reconstruction is described that involves solving the inverse problem via surface optimization. Compared to previously developed optimization methods, this method offers a higher spatial resolution and is more suitable for analysis of complex vibration patterns.

Quantitative vibration analysis using a single fringe pattern in time-average speckle interferometry.

In this paper, a novel technique for quantitative vibration analysis using time-average electronic speckle pattern interferometry is proposed. An amplitude-varied time-average refreshing reference frame method is used to capture a fringe pattern with a better fringe contrast than the conventional reference frame technique. The recorded fringe patterns with improved contrast provide better mode shape visibility and are easier to process. A derivative-based regularized phase tracker model is used to retrieve vibration amplitudes from a single fringe pattern. The method does not require a phase shifter to obtain the mode shape or amplitude. The method provides unwrapped amplitude and amplitude derivatives maps directly, so a separate phase unwrapping process is not required. Experimental work is carried out using a circular aluminum plate test specimen and the results are compared with a finite element method modal analysis. Both experimental and numerical results show that the proposed method is robust and accurate.

Vibration mode shapes visualization in industrial environment by real-time time-averaged phase-stepped electronic speckle pattern interferometry at 10.6 μ m and shearography at 532 nm

We present our investigations on two interferometric methods suitable for industrial conditions dedicated to the visualization of vibration modes of aeronautic blades. First, we consider long-wave infrared (LWIR) electronic speckle pattern interferometry (ESPI). The use of long wavelength allows measuring larger amplitudes of vibrations compared with what can be achieved with visible light. Also longer wavelengths allow lower sensitivity to external perturbations. Second, shearography at 532 nm is used as an alternative to LWIR ESPI. Both methods are used in time-averaged mode with the use of phase-stepping. This allows transforming Bessel fringes, typical to time averaging, into phase values that provide higher contrast and improve the visualization of vibration mode shapes. Laboratory experimental results with both techniques allowed comparison of techniques, leading to selection of shearography. Finally a vibration test on electrodynamic shaker is performed in an industrial environment and mode shapes are obtained with good quality by shearography.

Investigations of the transverse vibration characteristics of piezoceramic circular plates with V-notches

This article pertains to experimental validation of the study presented in a previous paper by the same authors , in which the transverse vibration characteristics of piezoceramic circular plates with V-notches were investigated theoretically using Ritz’s method incorporating defined equivalent constants. For experimental measurement, two optical methods – amplitude-fluctuation electronic speckle pattern interferometry (AF-ESPI) and laser Doppler vibrometry (LDV) – were employed to conceptualize the analytical results. The AF-ESPI method demonstrates the advantages of combining high fringe sensitivity and noise reduction, which are achieved using the time-averaged and subtraction electronic speckle pattern interferometry methods, respectively. Both vibration mode shapes and resonant frequencies of V-notched piezoceramic plates were obtained using the AF-ESPI method; in addition, the resonant frequencies were also determined quickly using the LDV method. By varying the corner angles and notch–depth ratios, various types of piezoceramic plate (V-notched, sectorial, semicircular, segmented, and sharp-notched) were used for experimental measurements. Numerical calculations were performed using the finite element method, and the results were compared with the experimental measurements and theoretical predictions. It was shown that the resonant frequencies and mode shapes obtained using the two experimental methods agreed fairly closely with the theoretical and numerical results.

Determination of Poisson’s ratio of a beam by time-average ESPI and Euler-Bernoulli equation

In this research, a laser holographic technique is used to characterize the Poisson’s ratio of a cantilever beam. We used the principle that one can determine the Poisson’s of a material by means of its vibration characteristics which use the Euler-Bernoulli Equation to link with the specimen’s natural frequency of vibration. The Poisson’s ratio of the beam can be easily determined after the resonance frequencies of the bending and torsional vibration modes are measured by time-averaged electronic speckle pattern interferometry (TA-ESPI) with the advantages of non- destructive, non-contact and real-time measurement technique. The proposed technique is able to give high accurate Poisson’s ratio of thin materials through a simple experimental set-up and analysis.

Investigation of vibration characteristics of bonded structures by time-averaged electronic speckle pattern interferometry

In this paper, the vibration characteristics of bonded structures were experimentally investigated by incorporating amplitude fluctuation electronic speckle pattern interferometry (AF-ESPI) with modal testing technique. A special casting procedure was employed to manufacture the naturally bonded structures to avoid the influence resulted from the adhesive. Vibration characteristics of bonded structures with different geometrical combinations of constituent adherends and the influence of presence of an interfacial crack were investigated.

Time-averaged electronic speckle pattern interferometry in the presence of ambient motion Part I Theory and experiments

An electronic speckle pattern interferometer is introduced that can produce time-averaged interferograms of harmonically vibrating objects in instances where it is impractical to isolate the object from ambient vibrations. By subtracting two images of the oscillating object, rather than the more common technique of subtracting an image of the oscillating object from one of the static objects, interferograms are produced with excellent visibility even when the object is moving relative to the interferometer. This interferometer is analyzed theoretically and the theory is validated experimentally.

Speckle Noise Reduction in Vibration Measurement by Time-Average Speckle Interferometry and Digital Holography-a Unifying Approach

Time-average electronic speckle pattern interferometry and digital holography are two representative full-field coherent optical techniques used in mechanical vibration measurement. In both techniques, the quantitative data processing is affected by several difficulties. The most important are the weak contrast of Bessel-type fringes and the speckle noise. The greatest obstacle in achieving complete amplitude field estimation comes from the orthogonal components of time-averaged digital holograms or speckle interferograms, where multiplicative, high-frequency phase noise covers the deterministic, vibration-related phase. Several researchers studied these problems in relation with the double-exposure method. In the present paper, the author presents in a single, unifying approach, these double-exposure methods, common to speckle interferometry and digital holography. An important reduction of multiplicative high-frequency phase noise allows obtaining fringe-averaged patterns whose intensity noise is much lower than in classical time-average holography. The analysis allows choosing the most appropriate method leading not only to a higher fringe pattern quality, lower noise and extended measurement range, but also to a method of vibration-related phase estimation, which may include in some stages subpixel precision. The method, based on the mathematical inversion of the Bessel function on monotonic intervals, is equivalent to phase unwrapping, but it only uses time-averaged fringe patterns.

Evaluation of elastic modulus of cantilever beam by TA-ESPI

The paper proposes an evaluation technique for the elastic modulus of a cantilever beam by vibration analysis based on time average electronic speckle pattern interferometry (TA-ESPI) and Euler–Bernoulli equation. General approaches for the measurement of elastic modulus of a thin film are the Nano indentation test, Buldge test, Micro-tensile test, and so on. They each have strength and weakness in the preparation of the test specimen and the analysis of experimental results. ESPI is a type of laser speckle interferometry technique offering non-contact, high-resolution and whole-field measurement. The technique is a common measurement method for vibration mode visualization and surface displacement. Whole-field vibration mode shape (surface displacement distribution) at resonance frequency can be visualized by ESPI. And the maximum surface displacement distribution from ESPI can be used to find the resonance frequency for each vibration mode shape. And the elastic modules of a test material can be easily estimated from the measured resonance frequency and Euler–Bernoulli equation. The TA-ESPI vibration analysis technique can be used to find the elastic modulus of a material requiring simple preparation process and analysis.

Vibration measurement by the time-averaged electronic speckle pattern interferometry methods

Three different image-processing methods based on the time-averaged technique were compared by the electronic speckle pattern interferometry (ESPI) technique for vibration measurement. The three methods are the video-signal-addition method, the video-signal-subtraction method, and the amplitude-fluctuation method. Also, errors introduced by using the zero-order Bessel function directly into the analysis of the fringe pattern were investigated. The video-signal-addition method has been the most generally used ESPI technique for vibration measurement. However, without additional image and/or signal-processing procedures, the fringe pattern obtained directly by the video-signal-addition method is rather difficult to observe. The reason for poor visibility of the experimentally obtained fringe pattern with this method is explained. To increase the fringe pattern's visibility without additional image and/or signal processes, we tried two video-signal-subtraction methods. One of the two methods is the video-signal-subtraction method that has normally been used in the static applications. The other method, called the amplitude-fluctuation method, and its associated theory are reported here.

Heterodyning of fibre optic electronic speckle pattern interferometers using laser diode wavelength modulation

Modulation of the emission wavelength of a laser diode is demonstrated as a heterodyning technique for measuring the phase and amplitude of a vibrating object using continuous wave (cw) illumination in a time-averaged electronic speckle pattern interferometer. The technique is demonstrated with an interferometer constructed from single mode optical fibre.

Vibration fringes by phase stepping on an electronic speckle pattern interferometer: an analysis

A detailed analysis of the vibration fringes obtained by phase stepping on a time-averaged electronic speckle pattern interferometer is presented. It is shown that the contrast of the fringes remains relatively high for any phase step between 30 degrees and 180 degrees for low electronic noise and fringe density. Also, for the four-phase-stepped method, the vibration fringes have the same contrast as that of the pi-phase-shift method except that the high-frequency speckles are smoothed. The contrast of the fringes obtained with extra phase steps along with incoherent superimposition is shown to be higher than the single- or four-phase-step method. Both theory and experimental results are presented.

Semi-automatic measurements of small high-frequency vibrations using time averaged electronic speckle pattern interferometry

The authors describe modifications to the method of reference beam phase modulation as applied to time-averaged electronic speckle pattern interferometry. The modifications enable measurements of vibration amplitudes at frequencies in the megahertz region to be made semi-automatically across the whole surface of an object.

Vibration measurement of the human tympanic membrane--in vivo.

The vibration patterns of the human eardrum in vivo have been recorded by time average electronic speckle pattern interferometry--ESPI. The necessary stability was achieved by shortening the exposure time of each TV frame. The amplitude--and phasedistribution was mapped across the drum by phase-modulation techniques which also could show the vibrations in slow motion. By photoelectric detection of the TV image intensity, amplitudes down to 2 nm could be measured. Preliminary results are presented.

Detection and measurement of small vibrations using electronic speckle pattern interferometry

We describe a new method for real-time detection and measurement of small vibrations, based on phase modulation in time-average electronic speckle pattern interferometry. The modulation frequency is shifted relative to the vibration frequency, which makes the intensity of the reconstructed image vary at the difference frequency. The amplitude detection limits are about 20 A by visual observation and 0.1 A by photoelectric measurement using a lockin technique. No auxiliary system for fringe stabilization is required. At higher amplitude levels we can study the deformation of the object in slow motion. Measurements on different objects, including human ear preparation, are presented.

Vibration phase mapping using electronic speckle pattern interferometry

The contours of constant phases across sinusoidally vibrating objects have been mapped using reference wave phase modulation in time-average electronic speckle pattern interferometry.

Speckle Pattern Interferometry of Vibration Modes

An optical facility is described for the study of the vibrational behavior of engineering test pieces. Time-average holography and electronic speckle pattern methods of analysis are combined into one system. The speckle pattern method is used for the identification of resonances, frequency tuning, and object alignment, thereby optimizing the number of holographic exposures. An evaluation is made of the electronic speckle pattern method for the detection of vibration modes of three-dimensional objects. The method is shown to detect resonant vibration readily on object faces inclined at angles up to 60° to the viewing direction.