In-plane Electronic Speckle Pattern Interferometry Research Articles

In-plane ESPI with unlimited angle of view for multi-object rotation angle determination

The in-plane electronic speckle pattern interferometry (ESPI), implemented in a Michelson stellar interferometer-like configuration, offers high sensitivity and dynamic measurement. However, its limited angle of view (AOV) remains a major challenge for the rotation angle determination of multiple objects. In this Letter, we analyze the main factors that influence the AOV of the in-plane ESPI and propose an “image transmitting” approach to enlarge the AOV. With the aid of a folded dual-telescope imaging system, we develop an AOV-unlimited interferometer that can determine multi-object rotation angles in real time. The practicability of the interferometer is demonstrated by the application in real-time measuring of the rotation angles of the disks within a 2D granular system.

Spatial phase-shift dual-beam speckle interferometry.

The spatial phase-shift technique has been successfully applied to an out-of-plane speckle interferometry system. Its application to a pure in-plane sensitive system has not been reported yet. This paper presents a novel optical configuration that enables the application of the spatial phase-shift technique to pure in-plane sensitive dual-beam speckle interferometry. The new spatial phase-shift dual-beam speckle interferometry (SPS-DBSP) uses a dual-beam in-plane electronic speckle pattern interferometry configuration with individual aperture shears, avoiding the interference in the object plane by the use of a low-coherence source, and different optical paths. The measured object is illuminated by two incoherent beams that are generated by a delay line, which is larger than the coherence length of the laser. The two beams reflected from the object surface interfere with each other at the CCD plane because of different optical paths. A spatial phase shift is introduced by the angle between the two apertures when they are mapped to the same optical axis. The phase of the in-plane deformation can directly be extracted from the speckle patterns by the Fourier transform method. The capability of SPS-DBSI is demonstrated by theoretical discussion as well as experiments.

Determination of thickness uniformity of a transparent film based on in-plane ESPI and radial basis function

In-plane electronic speckle pattern interferometry (ESPI) was developed to determine the thickness uniformity of a transparent film. The method is based on the subsequent spatial carrier patterns caused by the change of the rotation angle. Full-field thickness distribution can be obtained according to the relation between the phase difference and optical path difference generated by film rotation. Moreover, radial basis function was applied to improve the image quality of interference patterns. The main principle and experimental procedure of the method were presented. The errors of measurement results were analyzed. It is shown that the thickness uniformity of the thin film can be measured rapidly and accurately. Also, the refractive index can be determined by the developed method simultaneously.

Strain rate measurements by speckle interferometry for necking investigation in stainless steel

Localization that occurs at the end of the tensile test of a ductile 316L stainless steel has been followed in detail by in-plane Electronic Speckle Pattern Interferometry (ESPI). A global description of the whole strain-rate field with an analytical function and physical descriptors such as band width, band inclination and maximum strain rate is proposed. The description with two straight bands of constant amplitude along the width of the specimens is valid from the beginning of the diffuse necking to the fracture of the specimens. It allows distinguishing between two localization scenarii which occur for specimens with a different width to thickness ratio, one with a fracture inclined along the width and the other with a fracture inclined in the thickness. For the former, the two bands keep a constant angle while for the latter, the two bands rotate progressively until they become perpendicular to the tensile direction. The bandwidth can be defined and monitored during the whole necking evolution.

Localisation Phenomenon Investigation on SMATed Stainless Steel Samples by Speckle Interferometry

Abstract: In-plane Electronic Speckle Pattern Interferometry (ESPI) has been successfully used during tensile tests on SMA-treated (Surface Mechanical Attrition Treatment) and as-received 316L stainless steel in order to measure the strain rate field. The heterogeneity in the strain rate field can be observed from a stage of deformation which doesn’t coincide with the classic Considère’s criterion (dF = 0) for the diffuse neck initiation (or plastic instability). The initiation of this heterogeneity strongly depends on treatments undergone by the material. In this paper, a comparison was performed about the heterogeneity initiation of specimens SMATed during 30 min with 2 mm shot, 3 mm shot and a non-treated sample. The formation of the two slip band system until fracture of the tensile specimen was also studied.

Measuring object shape by using in-plane electronic speckle pattern interferometry with divergent illumination

Electronic speckle pattern interferometry is a useful technique for displacement, deformation and contouring measurements. Traditionally, for contouring measurements, collimated illumination with a constant sensitivity vector is used, and the surface area analysis is limited to the illuminated area. In some industrial applications, large surfaces require to be analyzed in restricted space conditions. Considering this situation, an optical system with divergent illumination for whole-field measurements can be used. It is known that displacement fields and the optical phase are related by the sensitivity vector. Therefore, to compute the sensitivity vector, illumination position and superficial shape need to be considered, a condition that becomes an impediment for surface contouring if the superficial shape is unknown. In this work, a simple iterative algorithm based on the Gauss–Seidel technique is presented to compute contouring measurements. Contouring measurements from both ESPI and a coordinate-measuring machine (CMM) are compared. In addition, a measurement comparison considering supposed collimated and divergent illumination is presented.

Localization phenomenon investigation on Smated stainless samples by speckle interferometry

In-plane Electronic Speckle Pattern Interferometry (ESPI) has been successfull y used during tensile tests on SMA-treated (Surface Mechanical Attrition Treatment) and as received 316L stainless steel in order to measure the strain rate field. The heterogeneity in the strain rate field can be observed from a stage of deformation which doesn’t coincide with the classic Considère’s criterion (dF=0) for the diffuse neck initiation (or plastic instability). The initiation of this heterogeneity strongly depends on treatments undergone by the material. In this paper, a comparison was performed about the heterogeneity initiation of specimens SMATed during 30 minutes with 2 mm shot, 3 mm shot and a non-treated sample. The formation of the two slip band system until fracture of the tensile specimen was also studied.

Observing Deformation and Fracture of Rock with Speckle Patterns

The high resolution technique of electronic speckle pattern interferometry (ESPI) can be very useful in determining deformation of laboratory specimens and identifying initiation of failure. The in-plane ESPI theory is described and the fringe pattern of the processed ESPI image is analyzed to determine deformation and crack opening displacement. Fringes on the ESPI image represent lines of equal intensity, which relate to surface displacement. An ESPI system was constructed and calibrated for measuring uni-directional displacements. Several types of the experiments, such as uniaxial compression and fracture testing, were conducted to demonstrate the utility of ESPI.

Strain Rate Evolution in Aluminium Alloy Measured by ESPI during Tensile Test

The original and significant achievement of this work is to analyse strain rate field in aluminium alloy sheets during micro tensile test. In-plane Electronic Speckle Pattern Interferometry is used to follow the evolution of the local strain in real time. This paper is based on the detection of the localization on a relatively small area of the analysed specimen: less than 5mm × 4mm area. Moreover the speed of the tensile machine is very low, 0.2 to 0.1µm/s. The phase shifting technique is used to obtain the fringes representative of the material displacement. We can therefore get a very good accuracy in the material displacement. A heterogeneity in strain rate field can be noticed from a deformation stage which doesn’t coincide with the one calculated by the classic Considère’s criterion (dF=0) for the diffuse neck initiation (or plastic instability). We also show, the moment when one of the two slip bands systems becomes predominant, and even only one band continues to exist, this occurring widely before fracture. Finally, the fracture of the specimen occurs across this remaining band. The total strain is measured with a second camera, in white light, and is correlated together with the strain rate field to study the localization.

Strain rate measurement by Electronic Speckle Pattern Interferometry: A new look at the strain localization onset

In-plane Electronic Speckle Pattern Interferometry has been successfully used during tensile testing of semi-hard copper sheets in order to measure the strain rate. On one hand, heterogeneity in strain rate field has been found before the maximum of the tensile force ( ɛ t ≃ 19.4 and 25.4%, respectively). Thus, a localization phenomenon occurs before the classic Considère's criterion (d F = 0) for the diffuse neck initiation. On the other hand, strain rate measurement before fracture shows the moment where one of the two slip band systems becomes predominant, then strain concentrates in a small area, the shear band. Uncertainty evaluation has been carried out, which shows a very good accuracy of the total strain and the strain rate measurements.

Uncertainty analysis of displacements measured by in-plane electronic speckle-pattern interferometry with spherical wave fronts

Displacement measurements by optical interferometry depend on the induced phase difference and on the interferometer's sensitivity vector; the latter depends in turn on the illuminating sources and on the geometry of the optical arrangement. We have performed an uncertainty analysis of the in-plane displacements measured by electronic speckle-pattern interferometry with spherical incident wave fronts. We induced the displacements by applying a uniaxial tensile load on a nominally flat elastic sample. We approached the displacement uncertainty by propagating the uncertainties that we considered reasonable to assign to the measured phase difference and to the characteristic parameters of the interferometer's sensitivity vector. Special attention was paid to evaluating contributions to the displacement uncertainty. Moreover, we observed that the uncertainty decreases if the angles of incidence and the source-target distances are increased.

Stress-strain curves of flip-chip solder balls based on finite-element modeling of thermal displacements measured by electronic speckle pattern interferometry

Electronic speckle pattern interferometry (ESPI) was applied to noncontact, real-time evaluation of thermal deformation in a flip-chip solder joint. To measure the deformation of such tiny components as the solder balls in the flip-chip, the spatial resolution of ESPI was increased to submicron scale by magnifying the areas studied. Experimental-computational procedures were developed to obtain stress-strain curves for solder balls in the flip-chip based on finite-element modeling (FEM) of in-plane ESPI thermal displacement data. The stress-strain curve obtained for the flip-chip solder was compared with those for bulk solder. The microstructure was also studied to clarify the stress-strain curve results.

Extracting modal parameters of ultrasonic bar horns from ESPI FRF data

This paper demonstrates the problems associated with determining the modal parameters of curved surface structures which exhibit high modal density and a predominant longitudinal response. The inadequacies of inferring modal parameters solely from conventional normal-to-surface measurements, typically using a laser Doppler vibrometer (LDV), are highlighted. A measurement solution is proposed which relies on extracting a grid of surface in-plane vibration amplitude and phase data, which is resolved from two laser speckle fringe patterns using electronic speckle pattern interferometry (ESPI). In order to obtain frequency spectrum data by in-plane ESPI, successive surface responses are captured through a swept-sine test and fringe processing algorithms are used to calculate a vibration amplitude and phase map at chosen grid positions on the structure's surface, at selected frequency steps. Further, by measuring an electrical signal input to the driving transducer, frequency response function data is obtainable such that modes of vibration estimated by modal analysis can be correlated with finite element model data. The technique is demonstrated by modal analysis of ultrasonic bar horns such that the longitudinal modes are successfully identified.

Dual in-plane electronic speckle pattern interferometry system with electro-optical switching and phase shifting

A dual in-plane electronic speckle pattern interferometry (ESPI) system has been developed for in situ measurements. The optical setup is described here. The system uses an electro-optical switch to change between the illumination directions for x and y sensitivity. The ability of the electro-optic device to change the polarization of the laser light forms the basis of this switch. The electro-optic device is a liquid-crystal layer cemented between two optically flat glass plates. An electric field can be set up across the layer by application of a voltage to electrodes. The speckle interferometry system incorporates two additional liquid-crystal devices to facilitate phase shifting, and the overall system is controlled by advanced software, which allows switching between the two perpendicular planes in quasi real time. The fact that there are no moving parts is an advantage in any ESPI system for which mechanical stability is vital.

Measurement of the biaxial properties of nineteenth century canvas primings using electronic speckle pattern interferometry

This paper reports on the use of electronic speckle pattern interferometry (ESPI) for the measurement of the biaxial tensile properties of English 19th century canvas primings and their constituent materials. Typically, such primings are comprised of a complex structure of layers, each with different mechanical properties. ESPI has been shown to be an effective technique for investigating complex composite structures and it is especially useful for understanding the behaviour of heterogeneous materials in which non-uniform strains can occur. The flexibility of canvas primings presents a more difficult application for both biaxial tensile testing and ESPI strain measurements. A series of experiments have been carried out to measure the Poisson's ratio of the three main constituents of a 19th Century priming as composites and of an original 19th century primed canvas. The samples have been uniaxially tensioned on a biaxial tensile tester designed specifically to investigate the mechanical properties of paintings on canvas. Measurements of deformation have made using a two-dimensional in-plane ESPI configuration. The results have shown that Poisson's ratio decreases as the constituents of a painting are built up. Preliminary tests on thermally aged and original primings suggest that for a painting without cracks it is the embrittled paint which determines the mechanical response of the painting at an relative humidity of 35–40%.