Digital Speckle Photography Research Articles

Digital speckle photography in the presence of displacement gradients

Digital speckle photography is a displacement field measurement method that employs laser speckles as surface markers. Since the approach requires only one reference image without a preparation of the sample and provides a fast, single-shot measurement with interferometric precision, the method is applied for in-process measurements in manufacturing engineering. Due to highly localized loads, higher-order displacement gradients occur in manufacturing processes and it is an open research question how these gradients affect the measurement errors of digital speckle photography. We simulate isotropic Gaussian surface topographies, apply a displacement field and then generate laser speckle patterns, which are evaluated with digital image correlation and subsequently the resulting random and systematic errors of the displacement field are analyzed. We found that the random error is proportional to the first-order displacement gradient and results from decorrelation of the laser speckles. The systematic error is mainly caused by the evaluation algorithm and is linearly dependent on the second-order gradient and the subset size. We evaluated in-process displacement measurements of laser hardening, grinding and single-tooth milling where we determined the relative error caused by displacement gradients to be below 2.5% based on the findings from the simulative study.

In-process measurement of mechanical loads during electrical discharge machining

In electrical discharge machining, in-process measurements of the induced mechanical stress are essential to develop process models for improving the fatigue strength of the machined part. For this purpose, high-speed digital speckle photography is introduced to measure the workpiece-side load fields with a lateral displacement resolution of 28 nm at a spatiotemporal resolution of 29 µm and 40 µs, respectively. As a result, the load effects of even individual 100 µs short discharge pulses are temporally resolved for the first time, and the occurring spatial load inhomogeneities and their dynamics are characterized to complement our understanding of the machining process.

Stages and Irregularities of Plastic Strain in Austenitic Stainless Steel within a Temperature Range of 143 < t < 420 k

This work aims at investigating the stages and irregularity of plastic strain in austenitic stainless steel 12Kh18N9T within a temperature range of 143 < T < 420 K. The localized plastic strain fronts at different stages of strain hardening have been visualized and monitored via digital speckle photography (DSP). This method consists in the high-accuracy recording of the displacement field via tracking any material surface changes and the subsequent comparison of speckle images acquired under uniaxial tension. The stress-strain diagrams were found to strongly vary with a decrease in temperature due to strain-induced-phase transition. The analysis of localized strain distributions revealed the emergence of a mobile system of equidistant strain localization foci at the strain hardening stage. The temperature dependence of plastic strain localization parameters at the linear strain hardening stage was established as well.

Исследование макроскопической локализации пластического течения в аддитивно изготовленном биметалле

The paper studies the kinetics of macroscopic localization of plastic deformation in a bimetal obtained by electron-beam surfacing of wire from low-carbon steel 09G2S on a substrate of St.3. Mechanical tests according to the scheme of uniaxial tension of flat ones were carried out at a constant rate at room temperature. The mechanical properties and stages of the plastic flow of the bimetal are compared with those of the starting materials. The kinetics of plastic deformation localization zones in different bimetal layers was studied by digital speckle photography.

Evaluation of 3D deformation field in siltstone with a pre-existing 3D surface flaw under uniaxial compression using X-ray computed tomography and digital volumetric speckle photography

The interior deformation or rupture of materials is usually derived from measuring the surface deformation of specimens. In this study, a set of uniaxial compression tests combined with in situ X-ray computed tomography (CT) was performed on siltstone specimens with a pre- existing three-dimensional (3D) surface flaw with various inclination angles (α = 30◦, 45◦, 60◦). The ratio of flaw depth to specimen thickness was 0.26. Based on the CT images of the specimens under different loads, the digital volumetric speckle photography procedure was applied to quantifying the 3D interior deformation of the specimens, and 3D displacement and major principal strain ε1 distribution in different sections were determined. The characteristics of internal strain before specimen fracturing indicated strain localization, which could describe the propagation mechanism of the single 3D surface flaw.

Simultaneous measurement of temperature and color spectrum of axisymmetric premixed flames using digital laser speckle photography and an image processing approach

The efficiency of different fuels in combustion processes has been an ongoing quest. Temperature measurements and color spectrum of premixed combustion flames are the accepted parameters to study. This work’s aim is to do so simultaneously, with a very simple setup and saving computational resources. This method does not ask to synchronize two sets of measurements, avoid the need for complicated setup alignments, and use a single digital camera. Instead, the temperature and color spectrum are measured simultaneously using digital laser speckle photography and an image processing approach. The recorded data for both measurements are acquired using only a color digital camera, where the red (r) color channel is used to record data for temperature measurements, and the blue (b) and green (g) color channels are utilized to record the flames’ color spectrum. To calculate the flames’ temperature field, the Abel inversion technique was carried out, while the image processing required crosstalk image correction, image segmentation, and image filtering. The crosstalk image correction is the main feature that establishes an unequivocal relationship between image color and color spectrum of flames. The technique was tested on flames produced with a liquefied petroleum gas fed torch using different equivalence ratios. The temperature values obtained with this method are in good agreement with those obtained with a thermocouple.

3-D strain estimation in sandstone using improved digital volumetric speckle photography algorithm

Accurately and stably estimating the strain in rocks is important to understand their failure mechanism. Digital volumetric speckle photography (DVSP) combined with X-ray computed tomography (CT) has been used to investigate the strain in rocks. This study introduced the inverse compositional Gauss–Newton algorithm into DVSP to improve the accuracy and precision of displacement and strain measurements. Granules with different particle sizes were used to simulate the mesostructure of sandstone, and a series of baseline experiments were conducted to evaluate the accuracy of the improved algorithm, which had a performance that was approximately three times better than DVSP alone. Subsequently, the algorithm was applied to red sandstone subjected to the Brazilian test and an indentation experiment. The interior deformation characteristics of the specimen were clearly depicted, which clarified the failure mechanism of the rock material.

Thermal stresses applied on helicopter blades useful to retrieve defects by means of infrared thermography and speckle patterns

This work presents a non-destructive analysis based on infrared thermography (IRT) and speckle inspection methods performed on a helicopter blade. In both cases, thermal stresses are needed in order to provoke the visualization of defects. The sample possess fabricated defects, and three techniques were selected, namely, long-pulse thermography, flash thermography, and digital speckle photography (DSP) to retrieve their positions. The first two techniques belong to the group of infrared imaging, that are inherent to the analysis of the infrared thermal patterns to detect internal anomalies in the material, whilst the last one corresponds to the optical imaging group that requires visible light to measure the material response under a thermal stimulus. The active approach, useful to produce a gradient in either, thermal and/or displacement field of the material, was used. In all cases, heat lamps were used to generate the required thermal stresses.Post-processing algorithms were applied to raw data in order to improve the defect detection. Results are finally compared to evaluate pros and cons of each method.

An optical method to determine the strain field on micro samples during electrohydraulic forming

Abstract The surface integrity of a component usually has a significant influence on its functional properties. This applies in particular to metallic micro components with sub-millimeter dimensions where little to no core material is present. Since the surface regions of these components can also exhibit changed material properties compared to macroscopic components due to size effects, the material properties cannot be adopted directly from conventional material tests such as tensile tests on macro samples of the same material. The electrohydraulic forming process opens up the possibility of directly investigating the elongation behavior of metallic micro samples in the forming process. However, an in situ strain measurement on the deformed micro sample is required in the mold, for which no measurement method is known so far. For this reason, an in situ measurement approach is presented for detecting the 2D deformation field of the micro sample surface during the electrohydraulic forming process. The approach is based on the optical methods digital speckle photography and digital image correlation. They record the movement of locally characteristic surface patterns in the 0.7 mm wide forming channel through an optical access. By calculating the gradients of the local surface motion, the strain fields on the micro samples are determined. The optical access is achieved by a 10 mm thick sapphire pane, which was tight integrated into the forming die. The 2D strain field measurement of hard sample materials (e.g. bronze) can be performed with digital speckle photography that offers a higher image contrast for optically smooth surfaces, while softer materials such as aluminum tend to get smeared at the pane, so that the captured images are evaluated with the more robust digital image correlation. An assessment of the measurement method on the basis of initial measurement results shows its principle applicability for the material characterization of micro samples.

An Improved Digital Volumetric Speckle Photography Technique with X-ray Microtomography and its Applications to Investigating Strain Localization in Red Sandstone

An Improved Digital Volumetric Speckle Photography Technique with X-ray Microtomography and its Applications to Investigating Strain Localization in Red Sandstone

3D Strain Mapping of Opaque Materials Using an Improved Digital Volumetric Speckle Photography Technique with X-Ray Microtomography

Digital volumetric speckle photography (DVSP) method has been used to strain investigation in opaque materials. In this paper, an improved DVSP algorithm is introduced, in which a multi-scale coarse–fine subset calculation process and a subvoxel shifting technique are applied to increase accuracy. We refer to the new algorithm as Multi-scale and Subvoxel shifting Digital Volumetric Speckle Photography (MS-DVSP). The displacement and strain fields of a red sandstone cylinder exposed to uniaxial compression and a woven composite beam under three-point bending are mapped in detail. The characteristics of the interior deformation of the specimens are clearly depicted, thus elucidating the failure mechanism of the materials.

Alineación Láser para Experimentación con Fotografía Digital de Speckle (DSP)

Non-contact techniques have returned to optical metrology one of the most booming branches of science, making digital speckle photography (DSP) one of the most used in the measurement of structural changes in surfaces. This technique uses a laser as a measurement system and compares the changes in the speckle pattern obtained by irradiating the surface that is subjected to deformations; however, the alignment of these systems plays an important role in the results obtained. In this work, the alignment errors of an optical system are estimated based on the analysis of the Fraunhofer diffraction pattern, as well as the statistical properties of the speckle pattern for field near and far. The results indicate that the alignment of the light beam can be determine by a fringes quantification system when passing through a pinhole and that the highly developed speckle pattern is more similar to the intensity distribution shown in the literature.

3D strain evolution in concrete using in situ X-ray computed tomography testing and digital volumetric speckle photography

The fracture process for concrete materials under compression is a complex three-dimensional phenomenon that occurs as a result of material heterogeneity and complicated mixed-mode cracking mechanisms. To understand the failure mechanism of concrete, it is necessary to have a technique that can probe the interior of the material as it is being loaded. In this paper, we use an industrial computed tomography (CT) system to obtain the volumetric image of a concrete circular cylinder under compression by taking advantage of the ability of X-rays to penetrate opaque materials. In addition, we employ a newly developed 3D strain analysis technique called digital volumetric speckle photography (DVSP) to calculate the displacement and strain distribution in two longitudinal meridian sections and a midpoint transverse section of the specimen. We are able to reveal the regions of strain localization and their evolutions, which eventually lead to the failure of the specimen.

Cross-Correlation Analysis of Digital Speckle Photography

This paper describes new algorithms for processing noisy specklograms, which permit quantitative diagnostics of the microstructure of shock-wave flows with a subpicosecond accuracy with the use of statistical analysis of numerically registered speckle fields disturbed by the refraction in the investigated flows. The software developed by us permits reconstructing up to 10,000 vector angles of probe radiation deflection in a two-dimensional region of size 20 × 30 mm2 in imaging a speckle field with optical magnification M = 1.

Digital volumetric speckle photography: a powerful experimental technique capable of quantifying interior deformation fields of composite materials

Due to the heterogeneity at different length scales, composite materials have a complex mechanical behavior. While many studies have been devoted to understanding their failure mechanism, much remain unknown. One of the main reasons is the lack of an effective experimental tool to monitor, in situ, the internal stress/strain field as the deformation progresses leading to failure. By taking advantage of the volumetric imaging capability of an X-ray CT (computed tomography), we have developed a new tool called digital volumetric speckle photography, whereby we can map quantitatively the interior deformation of almost any opaque material. In this paper, we described in detail the theory of this technique and its application to mapping the internal deformation fields of two different types of composites: fiberglass-reinforced woven composite and sandwich composite made of a polymeric foam core and two fiberglass face sheets.

New Schemes of Digital Speckle Photography

This paper presents a review and an analysis of the development of speckle photography from the first experiments performed at the National Physical Laboratory of Great Britain up to the present time. The possibility of diagnosing turbulent and three-dimensional flows by the speckle photography methods has been shown. The single-exposure scheme of speckle photography has been analyzed and the possibility of using multiaspect speckle photography in reconstructive tomography of complex three-dimensional flows has been shown. Methods based on speckle technologies such as Talbot interferometry, PIV and BOS techniques have been analyzed. Particular consideration is given to the application of new measuring technologies in diagnosing turbulent flows and reconstructive tomography of three-dimensional flows.

A New Method of Creating High-Temperature Speckle Patterns and Its Application in the Determination of the High-Temperature Mechanical Properties of Metals

Speckle techniques, such as DIC (Digital Image Correlation) and DSP (Digital Speckle Photography), are frequently used to determine the high-temperature mechanical properties of materials. Speckle techniques require the creation of a random speckle pattern on the surface of the specimen. The most commonly used approach to creating a high-contrast speckle pattern is to spray a layer of black paint particles on a white background. However, in a high-temperature environment, the paint particles tend to peel off or burn off. In this paper, we present an approach that uses a novel laser-engraving technology, where the created speckles will sustain temperatures as high as the melting temperature of the specimen. The size, density, depth and distribution of the speckles can be controlled to suit a particular situation. Since the pattern is part of the specimen, it will never disappear, until the melting temperature of the metal is reached. As an application, we used the technique to determine the elastic modulus of Ti up to 600°C and tungsten up to 1000°C.

Effect of noise on measurements of diffusivity in transparent liquid mixtures by digital speckle photography

Interfacing two liquid mixtures in a diffusion cell induces noise in the initial state of the diffusing system, which produces a gap between the diffusion boundary and the ideally boundary assumed in the theory. Measured diffusivity values systematically drift with time and they are often corrected by using a linear shift of the zero-time of the process after sufficiently long time when the system reaches the free one-dimensional diffusion regime. In data analysis methods which involve optical correlation between pairs of successive digital images of the cell, it is not easy to establish how long the transient lasts. We show that when the initial perturbation between solution and solvent relaxes slowly toward the diffusive regime no simple zero-time correction can be applied.

Digital Speckle Photography of Subpixel Displacements of Speckle Structures Based on Analysis of Their Spatial Spectra

We have investigated the capabilities of the method of digital speckle interferometry for determining subpixel displacements of a speckle structure formed by a displaceable or deformable object with a scattering surface. An analysis of spatial spectra of speckle structures makes it possible to perform measurements with a subpixel accuracy and to extend the lower boundary of the range of measurements of displacements of speckle structures to the range of subpixel values. The method is realized on the basis of digital recording of the images of undisplaced and displaced speckle structures, their spatial frequency analysis using numerically specified constant phase shifts, and correlation analysis of spatial spectra of speckle structures. Transformation into the frequency range makes it possible to obtain quantities to be measured with a subpixel accuracy from the shift of the interference-pattern minimum in the diffraction halo by introducing an additional phase shift into the complex spatial spectrum of the speckle structure or from the slope of the linear plot of the function of accumulated phase difference in the field of the complex spatial spectrum of the displaced speckle structure. The capabilities of the method have been investigated in natural experiment.

A multi-technique nondestructive approach for characterizing the state of conservation of ancient bookbindings

The aim of this work is to evaluate the potentiality of a multi-technique nondestructive approach for characterizing the state of conservation of precious bookbindings. In particular, the bookbinding of an ancient book dating back nineteenth century was inspected by infrared thermography, near-infrared reflectography and transmittography, digital speckle photography, holographic interferometry, and proton magnetic resonance relaxometry. Data were processed by different and innovative methodologies, among which, a calibration procedure of the camera for correlation analyses based on specklegrams. The results were compared, showing a promising synergy for the diagnostic purposes, on the basis of the relationship between structure, properties, and uses of the analyzed materials.