Speckle interferometric damage investigation of fibre-reinforced composites

With the aid of the recently reported technique of adding up phase images modulo 2π that correspond to stepwise applied load increments, the fringe density that can be achieved in electronic speckle pattern interferometry (ESPI) has been substantially improved. This technique also allows the measurement of much larger deformations than were hitherto possible with ESPI. The analytical power of the method is demonstrated in the measurement of high local displacement gradients in carbon fibre-reinforced plastics (CFRPs). In-plane and out-of-plane displacement field measurements, performed with one single optical instrument, are compared with finite-element models. This paper reports how the technique is used to detect and quantify damage in fatigued CFRP laminates via its effect on the surface displacement field. Moreover, the measured displacement fields are used to validate a finite-element damage model. The correctness of the delamination measurement is verified with the aid of ultrasonic C-scan reference results.     

Mechanical characterization of unplasticised polyvinylchloride thick pipes by optical methods

In this work a number of techniques (electronic speckle pattern interferometry, holographic interferometry, strain gauge and finite element method) are brought to bear in order to establish consistency in the results of strain measurement. This is necessary if optical non-destructive testing methods, such as those used here, are to gain acceptance for routine industrial use. The FE model provides a useful check. Furthermore, ESPI fringe data facilitates the extension of FE models, an approach that is of growing importance in component testing.

The use of in-plane and out-of-plane sensitive electronic speckle pattern interferometry (ESPI) for non-destructive material characterization of thick unplasticised polyvinylchloride (uPVC) pipes is presented. A test rig has been designed for stressing pipes by internal pressure. ESPI gives a complete mapping of the displacement field over the area imaged by the video camera. The results for the strain of uPVC obtained from ESPI data and from strain gauges are in good agreement. The value of Young's modulus has been obtained from the fringe data and compared with results obtained using holographic interferometry and from strain gauge measurements. The FE model also produces fringe data that is consistent with the ESPI results.     

Micro- and macro deformation measurement by extension of correlation technique

In this work, we present the implementation of Electronic Speckle Pattern Interferometry (ESPI), Digital Speckle Photography (DSP) and Digital Image Correlation (DIC) as complementary techniques to measure in-plane micro and macro displacement. The main advantage of ESPI is its great sensitivity to small displacements (smaller than the size of the speckle). However, the contrast of fringes in this technique is severely affected by de-correlation effects when the in-plane displacement exceeds the size of the speckle. To eliminate the de-correlation effects, we use the DSP technique. It is possible to generate artificial speckles, usually bigger than those generated by means of illumination of the sample with laser light. By combining DSP and DIC the displacement field can be obtained when the ESPI method cannot be applied due to image de-correlation. The experimental results show that the combination of these techniques is useful to analyze deformations over a wider range.     

Improved evaluation of electronic speckle pattern interferograms by photogrammetric image analysis

Electronic speckle pattern interferometry (ESPI) is a well-established tool for non-destructive testing. It allows the quantitative determination of surface deformations and micro-movements with a sub-micrometer resolution. In the case of objects which are extended in depth, however, the evaluation and interpretation of the resulting correlation fringe patterns can be affected by perspective image distortions as well as by a varying image size. In this paper a method for combination of ESPI with a photogrammetric 3D coordinate measurement is presented. In this way, interferogram data are precisely allocated in 3D-space. Furthermore, it is possible to take into account a spatially varying sensitivity vector. The utilizability of the method is demonstrated by a deformation measurement on a stone sculpture.     

Continual deformation analysis with scanning phase method and time sequence phase method in temporal speckle pattern interferometry

If a laser beam illuminates a continual deformation object surface, it will lead to a temporal speckle pattern on the observation plane. Recording this time-dependent speckle pattern the deformation of the surface of an object can be obtained. Two methods, scanning phase method (SPM) and time sequence phase method (TSPM), have been introduced for measuring the displacement caused by the deformation in temporal speckle pattern interferometry (TSPI). Their principle is that by capturing a series of speckle interference patterns related to the object deformations, the fluctuations in the intensity of the interference patterns can be obtained. Through scanning these fluctuations and estimating both the average intensity and modulation of the temporal speckle interference patterns, the phase maps for whole-field displacements are calculated. In this way one is capable of quantitatively measuring continual displacements simply using a conventional electronic speckle pattern interferometry (ESPI) system without phase shifting or a carrier. The elaboration on the new methods is given in this paper and experiments are performed to demonstrate their performance with a conventional ESPI system.     

Holography and electronic speckle pattern interferometry in geophysics

Geophysical applications of holography and ESPI are reviewed. First, laboratory experiments of rock deformation and failure with holographic interferometry and holographic in situ stressmeters are briefly summarized. Then, holographic measurements of tunnel deformations made in Japan are described. The holographic recording system, consisting of an He---Ne gas laser and associated optical elements, was installed in a tunnel at the Amagase Crustal Movement Observatory, Japan in 1984. Tunnel deformations caused by tidal and tectonic forces have been precisely determined using the ‘real-time’ technique of holographic interferometry. Finally, some attempts to apply ESPI to geophysical measurements are introduced.     

Point load determination of static elastic moduli using laser speckle interferometry

Electronic speckle interferometry (ESPI) is used to determine the Young's modulus E and Poisson ratio ν of an isotropic material. Micron scale deformations of the surface of the block of polymethyl-methacrylate (PMMA) are induced by normal application of a known near-point force. These deformations are recorded in speckle interferometric fringe patterns. An iterative minimum error inversion technique is developed to obtain the elastic properties from the positions of fringe peaks and troughs observed in the fringe patterns. Sensitivity tests of the method on calculated fringe patterns using measured experimental uncertainties suggest the technique will provide measures of the elastic moduli to better than 5%. In an experimental test on a bloc of PMMA (acrylic) the technique gave values of E and ν that differed from corresponding measures obtained using more conventional strain-gauge methods by less than 4%.     

Measurement of vibrational energy flow in a plate with high energy flow boundary crossing using electronic speckle pattern interferometry

The measurement of vibrational energy flow is an important tool in understanding the vibrational behaviour of structures. In the past, because of transducer constraints, the measurement of vibrational energy flow was mostly restricted to single point measurements. However, recent developments in advanced laser measurement techniques, such as electronic speckle pattern interferometry (ESPI), have gained interest in applying two-dimensional, multi-point measurement techniques to the estimation of vibrational energy flow. This paper addresses the measurement of vibrational energy flow in a plate by using an ESPI based vibrational energy flow measurement technique. A radially symmetric bending wave plate vibration model is introduced and theoretical expressions for energy-based quantities are derived. To assess the accuracy of the measurement method, these theoretical quantities are compared to synthetic results derived from the ESPI energy flow measurement technique. The ESPI measurement technique is also applied to an experimental ‘infinite’ plate. Thus, a specially designed experimental apparatus was constructed so as to minimise undesired wave reflections in the plate and, thus, achieve a high energy flow boundary crossing at the edges of the plate. To reduce the effect of optical noise contamination on the ESPI measured out-of-plane plate displacement data, optimal filters were applied prior to the vibrational energy flow computation. To appraise the accuracy of the experimental method, measured vibrational power on the plate is compared with measured vibrational input power. A difference of less than 1 dB between both quantities indicates that vibrational energy flow within a rectangular plate that contains radially symmetric wave propagation can be measured to a good degree of accuracy if appropriate filtering is applied.     

High-speed ESPI and related techniques: overview and its application in the automotive industry

This paper provides an overview of recent developments and applications of high-speed electronic speckle pattern interferometry (ESPI) and related techniques ranging from high-speed pulsed laser techniques to high-speed camera methods. Different methods for extracting the phase distribution from high-speed ESPI fringe patterns are compared. Unique applications in the automotive industry using these techniques are provided. Finally, the outlook for high-speed ESPI and high-speed digital holographic interferometry are discussed.     

A comprehensive ESPI based system for combined measurement of shape and deformation of electronic components

This paper describes a very simple and reliable procedure for measuring shape and deformation of electronic components with a single experimental set-up. The procedure is based on two electronic speckle pattern interferometry (ESPI) techniques referred to as conventional ESPI and phase shifting ESPI (PS-ESPI). The present research is motivated by the fact that mismatch in thermal expansion coefficients of the different materials included in electronic packaging (EP) may cause mechanical failures since thermal stresses will change sharply through subsequent loading cycles. ESPI is particularly suitable for measurements on EP since it allows us to perform non–contact testing of non-planar heterogeneous surfaces. It is apparent that gathering detailed topographic information will certainly help us to measure accurately surface deformations of EP along with modeling correctly numerical analysis.As is known, the accuracy of results obtained with ESPI may be significantly improved by phase shifting techniques (PST). Therefore, this paper compares the relative merits of different phase-stepping strategies in order to find which strategy will perform the best for the optical set-up utilized in the experiments. Preliminary investigations on a standard specimen under three-point-bending served to choose properly the optical set-up and phase-stepping procedure which yield the best fringe visibility. Four-phases achieved the best fringe visibility and the minimum number of invalid pixels.These information have been utilized in the experimental campaign on standard and surface mounted technology (SMT) electronic components. ESPI and PS-ESPI have been used for analyzing the transient state and the steady state of devices, respectively. From the experimental results obtained here, it appears possible to measure strains induced by thermal loading cycles. The experimental set-up, based on the Lendeertz's interferometer, proved itself also able to contour specimen surface at a good level of detail. Remarkably, by using the same set-up for deformation and shape measurements we can preserve the pixel by pixel relationship between displacements and surface depth which will hold true if correlation between different exposures is not destroyed.The results obtained in this research justify using PS-ESPI in order to understand better failure mechanisms of electronic components. This fact along with the exact knowledge of object shape may be particularly useful in the different design stages (including FEM modeling and analysis) of electronics for special applications.     

Full field and microregion deformation measurement of thin films using electronic speckle pattern interferometry and array microindentation marker method

Thin films now are widely used in micro devices and structures, such as MEMS, electronic packaging, micro sensors, and so on. Their performances highly affect the reliability of the devices. Therefore, it is important to investigate the deformation and the failure mechanism of thin films. In this paper, we present two experimental methods to measure the mechanical properties of thin films. In the first method, a double-field-of-view electronic speckle pattern interferometry system (ESPI) and an integrated deformation and load measurement system are employed, which allows in situ and real-time measurements of full-field deformations of the thin films and microforces under uniaxial tensile test. In the second method, the array microindentation markers were indented on the surface of the thin film using a nanoindenter and the microregion deformations of the tested thin films were measured. In the proposed methods, the tested thin films can be made of metals, oxide ceramics, and multi-layer composites of thickness from several tens micrometers to less than a micron, and the tensile loads from 88 μN to 15 N for the first method or up to 100 N to the second one. The underlying principle of the methods and the experimental set-ups are presented. The deformations of Au and Au/Cr multi-layer films, and the pure Ni films are measured. The performance of the methods and the testing systems are also discussed.     

Noise reduction in electronic speckle pattern interferometry fringes by merging orthogonally polarised speckle fields

The noise in sawtooth fringes generated by electronic speckle pattern interferometry (ESPI) is investigated. When deformations of depolarising objects are studied, the scattered object light can be decomposed into two orthogonal linearly polarised speckle patterns which are partially decorrelated. Their correlation coefficient decreases with increasing depolarisation coefficient of the object. By suitable merging of the phase distributions of these two speckle fields on the basis of a modulation depth analysis, the rms phase error in the ESPI sawtooth fringes can be reduced significantly.     

Surface roughness investigation of semi-conductor wafers

Conventionally, surface roughness is predominantly determined through the use of stylus instruments. However, there are certain limitations involved in the method, particularly when a test specimen, such as a silicon wafer, has a smooth mirror-like surface. Hence, it is necessary to explore alternative non-contact techniques. Light scattering has recently been gaining popularity as an optical technique to provide prompt and precise inspection of surface roughness. In this paper, the total integrated scattering (TIS) model is modified to retrieve parameters on surface micro-topography through light scattering. The applicability of the proposed modified TIS model is studied and compared with an atomic force microscope. Experimental results obtained show that the proposed technique is highly accurate for measuring surface roughness in the nanometer range.     

The application of high-speed TV-holography to time-resolved vibration measurements

We describe an electronic speckle pattern interferometer (ESPI) system that has enabled non-harmonic vibrations to be measured with μs temporal resolution. The short exposure period and high framing rate of a high-speed camera at up to 40,500 frames per second allow low-power CW laser illumination and fibre-optic beam delivery to be used, rather than the high peak power pulsed lasers normally used in ESPI for transient measurement. The technique has been demonstrated in the laboratory and tested in preliminary industrial trials. The ability to measure vibration with high spatial and temporal resolution, which is not provided by techniques such as scanning laser vibrometry, has many applications in manufacturing design, and in an illustrative application described here revealed previously unmeasured “rocking” vibrations of a car door. It has been possible to make the measurement on the door as part of a complete vehicle standing on its own tyres, wheels and suspension, and where the excitation was generated by the running of the vehicle's own engine.     

Separation of vibration fringe data from rotating object fringes using pulsed ESPI

In industrial and other types of non-controlled environments, an unbalanced rotating object may present characteristic out-of-plane vibration amplitude at a specific frequency. For this type of cases and as a first step towards a complete evaluation, it is only desired to visualize the effect of the vibration on the rotating object, or vice versa, for instance to achieve object balancing. Real time optical non-intrusive measurement techniques such as pulsed electronic speckle pattern interferometry (ESPI), are well suited to study this rotating-vibrating object. The advantage offered by ESPI is that real-time fringe data is qualitatively analyzed while being observed on a TV monitor. The present paper proposes a qualitative method, based on pulsed ESPI, to separate rotation fringes from fringes solely related to vibration. The method relies on a high precision scheme that synchronizes and fixes an object point during rotation, without the use of an optomechanical object derotator.     

Deformation mapping and surface inspection of historical monuments

Electronic speckle pattern interferometry (EPSI) in various configurations (reference beam or shear) as well as double exposure speckle photography are applied for deformation monitoring in historical monuments. On-the-site measurements are made possible by a rigid construction of the optical head, common path configurations and portable image processing equipment. The delicate objects particularly call for non-intrusive measurement. Basic features of the techniques, their implementation and results from historical sites are presented. Inhomogeneities in plaster, paint or natural stones were detected and related to crack formation. The reversibility of deformations under mechanical load was checked. In frescos, debonds from the carrier wall could be discovered and the detrimental effects of crystalline efflorescences were studied. Correlation analysis in ESPI images showed the influence of moisture on a rough stone surface and exposed microbiological activity. Thus, valuable deformation data could be obtained that assist in the diagnostics of deterioration processes and provide information in the design of remedies.     

Nondestructive testing by ESPI and quasi phase shift gradient technique

A new nondestructive testing (NDT) technique, which is based on Electronic Speckle Pattern Interferometry (ESPI) and digital image processing with quasi phase shift and gradient technique, is presented. A simple phase reduction algorithm is developed, which replaced an accurate phase shifter. Compared with other phase shift techniques, this method is insensitive to environmental vibration and air disturbance, has visible procedures and results allows the object to move slowly during the inspection procedure, does not need phase unwrapping, and has a quick image processing speed. As an application, this NDT technique is used to detect defects in composite materials and the resulting deformation phase gradient image shows a better visual effect than normal ESPI.     

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.     

Estimation of masonry mechanical characteristics by ESPI fringe interpretation

Electronic speckle pattern interferometry (ESPI) can be a powerful tool for efficient non-destructive testing and evaluation of micro-deformations of masonry materials and structures. Unlike traditional transducers, ESPI requires no direct contact with the object, and the full-field visualisation it offers provides for a better understanding of the surface behaviour. This paper describes an in-plane deformation inspection system which has been built up for an automatic acquisition of interferograms at different stages of a test. The system is applied to the evaluation of some mechanical characteristics of masonry components. Qualitative and quantitative results are obtained and an overall discussion is presented.     

Construction and operation of a simple electronic speckle pattern interferometer and its use in measuring microscopic deformations

The construction and operation of a simple electronic speckle pattern interferometer (ESPI) is described. The underlying theory behind the operation of the interferometer, which is sensitive to out-of-plane displacements, is given. The interferometer uses a commercial digital still camera for image acquisition and a personal computer for image storage and analysis. The interferometer was used to measure microscopic deformations of a steel plate caused by small loads. The results were found to be in good agreement with the predictions of the elastic theory. Various possible applications of the interferometer are mentioned.