Digital Image Correlation Algorithm Research Articles

Optical-numerical method based on a convolutional neural network for full-field subpixel displacement measurements.

The subpixel displacement estimation is an important step to calculation of the displacement between two digital images in optics and image processing. Digital image correlation (DIC) is an effective method for measuring displacement due to its high accuracy. Various DIC algorithms to compare images and to obtain displacement have been implemented. However, there are some drawbacks to DIC. It can be computationally expensive when processing a sequence of continuously deformed images. To simplify the subpixel displacement estimation and to explore a different measurement scheme, a convolutional neural network with a transfer learning based subpixel displacement measurement method (CNN-SDM) is proposed in this paper. The basic idea of the method is to compare images of an object decorated with speckle patterns before and after deformation by CNN, and thereby to achieve a coarse-to-fine subpixel displacement estimation. The proposed CNN is a classification model consisting of two convolutional neural networks in series. The results of simulated and real experiments are shown that the proposed CNN-SDM method is feasibly effective for subpixel displacement measurement due its high efficiency, robustness, simple structure and few parameters.

Significance of Parallel Computing on the Performance of Digital Image Correlation Algorithms in MATLAB

Digital Image Correlation (DIC) is a powerful tool used to evaluate displacements and deformations in a non-intrusive manner. By comparing two images, one from the undeformed reference states of the sample and the other from the deformed target state, the relative displacement between the two states is determined. DIC is well-known and often used for post-processing analysis of in-plane displacements and deformation of the specimen. Increasing the analysis speed to enable real-time DIC analysis will be beneficial and expand the scope of this method. Here we tested several combinations of the most common DIC methods in combination with different parallelization approaches in MATLAB and evaluated their performance to determine whether the real-time analysis is possible with these methods. The effects of computing with different hardware settings were also analyzed and discussed. We found that implementation problems can reduce the efficiency of a theoretically superior algorithm, such that it becomes practically slower than a sub-optimal algorithm. The Newton–Raphson algorithm in combination with a modified particle swarm algorithm in parallel image computation was found to be most effective. This is contrary to theory, suggesting that the inverse-compositional Gauss–Newton algorithm is superior. As expected, the brute force search algorithm is the least efficient method. We also found that the correct choice of parallelization tasks is critical in attaining improvements in computing speed. A poorly chosen parallelization approach with high parallel overhead leads to inferior performance. Finally, irrespective of the computing mode, the correct choice of combinations of integer-pixel and sub-pixel search algorithms is critical for efficient analysis. The real-time analysis using DIC will be difficult on computers with standard computing capabilities, even if parallelization is implemented, so the suggested solution would be to use graphics processing unit (GPU) acceleration.

Experimental evaluation of the viscoelasticity of porcine vitreous.

This study aims to estimate the material properties of the porcine vitreous while testing it in close to its natural physiological conditions. Eighteen porcine eyes were tested within 48 h post-mortem. A custom-built computer-controlled test rig was designed to support, load and monitor the behaviour of eye globes while being subjected to dynamic rotation cycles mimicking saccade eye movement. Specimens were glued to the base of a container, surrounded by gelatin, frozen and cut in half to expose the vitreous. After thawing, the container was subjected to concentric dynamic rotations of up to 5°, 10° or 15°, while taking 50 MP photos of the specimen every 2 ms. The images were analysed by a digital image correlation algorithm to trace the movement of marked points on the vitreous surface with different radii from the centre of the posterior chamber. The initial camera image was used in building a finite-element model of the test set-up, which was used in an inverse analysis exercise to estimate the material properties of the vitreous. Angular displacements of the monitored points were up to 3.3°, 4.1° and 3.9° in response to eye rotations of 5°, 10° and 15°, respectively. With the experimental relationships between eye rotation and angular displacements used as target behaviour, the inverse analysis exercise estimated the initial shear modulus, the long-term shear modulus and the viscoelastic decay constant of the porcine vitreous as 2.10 ± 0.15 Pa, 0.50 ± 0.04 Pa and 1.20 ± 0.09 s-1, respectively. Consideration of the viscoelasticity of the vitreous was essential to represent its experimental behaviour. Testing the vitreous in close to its normal physiological conditions produced estimations of the initial shear modulus and long-term shear modulus that were, respectively, smaller and larger than reported values (Zimberlin et al. 2010 Soft Matter 6, 3632-3635. (doi:10.1039/b925407b), Liu et al. 2013 J. Biomech. 46, 1321-7. (doi:10.1016/j.jbiomech.2013.02.006), Rossi et al. 2011 Invest. Ophthalmol. Vis. Sci. 52, 3994-4002. (doi:10.1167/iovs.10-6477)).

Cost‐Effective and Ultraportable Smartphone‐Based Vision System for Structural Deflection Monitoring

This work demonstrates the viability of using a smartphone‐based vision system to monitor the deflection of engineering structures subjected to external loadings. The video images of a test structure recorded by a smartphone camera are processed using the state‐of‐the‐art subset‐based digital image correlation (DIC) algorithm to extract the vertical image displacement of discrete calculation points defined on the test object. The measured vertical image displacement can be converted to deflection (vertical displacement) by easy‐to‐implement scaling factor determination approaches. For accuracy validation, laboratory experiments using a cantilever beam subjected to external loadings were performed. The deflection and inherent frequency of the test cantilever beam measured by the proposed smartphone‐based vision system were compared with those measured by conventional dial gauges and a dynamic strain gauge. The relative errors were estimated as 1% and 0.15% for deflection and inherent frequency, respectively. Outdoor real bridge deflection monitoring tests were also carried out on an overpass with subway passing by, and the measured deflection‐time curves agree well with actual situations. The cost‐effective, ultraportable, and easy‐to‐use smartphone‐based vision system not only greatly decreases the hardware investment and complexity in deflection measurement system construction, but also increases the convenience and efficiency of deflection monitoring of engineering structures.

Dynamic Deformation Behaviour of Chiral Auxetic Lattices at Low and High Strain-Rates

The mechanical behaviour of three different auxetic cellular structures, hexa-chiral 2D, tetra-chiral 2D and tetra-chiral 3D, was experimentally investigated in this study. The structures were produced with the powder bed fusion method (PBF) from an austenitic stainless steel alloy. The fundamental material mechanical properties of the sample structures were determined with classic quasi-static compressive tests, where the deformation process was captured by a high-resolution digital camera. The Split Hopkinson Pressure Bar (SHPB) apparatus was used for dynamic impact testing at two impact velocities to study the strain-rate dependency of the structures. Two synchronised high-speed cameras were used to observe the impact tests. The captured images from both quasi-static and dynamic experiments were processed using a custom digital image correlation (DIC) algorithm to evaluate the displacement/strain fields and the Poisson’s ratio. Predominant auxetic behaviour was observed in all three structures throughout most of the deformation process both under quasi-static and impact loading regimes. The tetra-chiral 2D structure showed the most significant auxetic behaviour. Significant stress enhancement in all tested structures was observed in dynamic testing. The Poisson’s ratio strain-rate dependency was confirmed for all three auxetic structures.

Coupling tomographic and thermographic measurements for in-situ thermo-mechanical tests

In order to determine the thermo-mechanical properties of a complex 3D woven ceramic composite material, an experiment at high and inhomogeneous temperature and its dedicated full-field measurement procedure is developed. 3D tomographic images of the tested sample are captured at different stages of loading in a synchrotron beamline, and an infrared camera captures a side view of the sample as it rotates in the x-ray beam. A pin-hole projective model of the thermographic camera allows one to map the thermal field measured under numerous orientations onto a 3D mesh of the sample built from an initial tomographic image or a model. The projective model has to be calibrated, and an original procedure is proposed thanks to an integrated digital image correlation algorithm based on the ‘silhouette’ of the sample (as only the protruding edges outlining the sample shape can be seen clearly). This procedure is illustrated with an experimental case study.

A pre-processing method for digital image correlation on rotating structures

Inter-frame structural motions are required to be small for classic Digital Image Correlation (DIC) algorithms, which severely limits the application of the technique in large rotation cases. To solve the problem, multiple methods to obtain initial guesses of deformation parameters have been introduced over recent years. However, the success rate is still limited by rotation angles and the efficiency is low due to extra calculations with these methods. To further develop the application of DIC in large rotation cases, Phase Mapping Method (PMM) is introduced in this paper. The principle of the proposed method is to rearrange the obtained images to be sorted by the structural rotating phases, which are previously mapped into the range of 0 to 2π. As a result, for an image set of n frames, the average inter-frame rotating angle is reduced to 2πn-1, and the original image set is transferred into an equivalent semi-static one by PMM. Additionally, a fast phase-detection approach is introduced to automate PMM in cases that the structural rotating speeds are unknown. By performing multiple tests, it is proved that PMM, as a pre-processing method, can significantly improve the success rate and the efficiency of DIC, whether used alone or concurrently with other algorithms. The first natural frequencies and mode shapes of a rotating beam under different rotating speeds are extracted through vibration test, from which the hardening effect is observed.

Residual Intensity as a Morphological Identifier of Twinning Fields in Microscopic Image Correlation

In the microscopic observation of deforming metals, it is well known that crystallite defects that accommodate strain can occasionally become visible, namely, they introduce image contrast to their locality. For microscopic digital image correlation (DIC) applications, this is typically known as a disturbance. Here, we explore a potential upside of these image-intensity offsets, to present a new mode of differential imaging that exclusively displays the underlying plasticity agents. For this, the intensity-offset signal is isolated with residual intensity, essentially the differential between reference- and deformed-configuration intensity of each material point. The premise is showcased over an autocatalytic twin band in Magnesium AZ31, with an advanced DIC instrument that utilizes bright-field optical microscopy. With robust area-scanning that utilizes in-situ corrective measures, a material field of around 5000 grains (13 μm average size) is sampled with a maximal intragranular resolution (~250 data points per grain) for this technique. For added robustness against the intensity alterations, a DIC algorithm (Augmented Lagrangian DIC) that enforces global kinematic compatibility constraints is utilized. The calculated residual intensity map yields a detailed image of the twin networks that show a strong positional alignment with the strain localizations. At the band boundary, the twins (and their accompanying strain localization) protrude into the dormant material in a comb-like pattern. With a combination of high-resolution optics and defects that alter the surface topography, residual intensity presents a new in-situ microscopy mode that is tied to the DIC analysis. This principle also offers potential micro-deformation imaging capabilities for various other material-microscopy combinations.

High-throughput measurement of coefficient of thermal expansion using a high-resolution digital single-lens reflex camera and digital image correlation.

High-throughput measurement of thermal deformation and determination of coefficient of thermal expansion (CTE) using a high-resolution digital single lens reflex (DSLR) camera and digital image correlation (DIC) is described. To mitigate the mosaic effect caused by the Bayer filter of DSLR cameras, a color image pre-processing method, which adjusts the brightness and equalizes the color channels of the raw image, is carried out. In addition, a Gaussian pre-filtering step is adopted for denoising the images captured with DSLR cameras to enhance the subpixel registration accuracy. Then, by processing the recorded images using the state-of-the-art DIC algorithm, full-field displacements and strains can be determined. Compared with conventional industrial cameras, a DSLR camera offers not only portability, compactness, and economy but also much higher resolution of recorded images, allowing CTE characterization with higher throughput. Real experiments, including a verification experiment of the color image pre-processing technique, a benchmark CTE determination of Al alloy, and a high-throughput CTE determination of 15 samples of three different metals, validated the feasibility and accuracy of the proposed technique. The proposed method is cost-effective and time-saving, showing great potential in the high-throughput CTE measurement and other high-throughput strain measurement scenarios.

A 117 Line 2D Digital Image Correlation Code Written in MATLAB

Digital Image Correlation (DIC) has become a popular tool in many fields to determine the displacements and deformations experienced by an object from images captured of the object. Although there are several publications which explain DIC in its entirety while still catering to newcomers to the concept, these publications neglect to discuss how the theory presented is implemented in practice. This gap in literature, which this paper aims to address, makes it difficult to gain a working knowledge of DIC, which is necessary in order to contribute towards its development. The paper attempts to address this by presenting the theory of a 2D, subset-based DIC framework that is predominantly consistent with state-of-the-art techniques, and discussing its implementation as a modular MATLAB code. The correlation aspect of this code is validated, showing that it performs on par with well-established DIC algorithms and thus is sufficiently reliable for practical use. This paper, therefore, serves as an educational resource to bridge the gap between the theory of DIC and its practical implementation. Furthermore, although the code is designed as an educational resource, its validation combined with its modularity makes it attractive as a starting point to develop the capabilities of DIC.

Methodology for DIC-based evaluation of the fracture behaviour of solder materials under monotonic and creep loadings

New lead-free solder alloys for electronic packaging contain doping elements conferring upon them improved properties in terms of ease of processibility and strength under high temperature operation conditions. Studies on their resistance to quasi-static rupture remain however relatively limited. In this work, the SAC-based solder alloy, known as InnoLot, is considered and its rupture under monotonic and creep loadings is investigated experimentally and numerically. The Digital Image Correlation (DIC) technique is employed to evaluate the stress intensity factor, as well as the J- and C(t)-integrals, from full-field measurements on notched specimens. The numerical formulations of the theoretical concepts for computation of the physical fracture quantities are provided in the framework of the DIC algorithm. Results derived from the DIC experiments are compared with predictions given by finite element models, and good agreement is found in general. The observed discrepancies can be explained by lack of resolution of the optical setup measurements in the infinitesimal strain regime or by uncertainty associated with the identified material parameters used in the evaluation of stresses from the acquired strains.

A Study on Tensile Strain Distribution and Fracture Coordinate of Nanofiber Mat by Digital Image Correlation System

Strain gauges are commonly used for tension tests to obtain the strain of a metal test specimen. They make contact, however, so the gauges are not applicable to every type of test specimen. That is the reason why a non-contact type measurement system is required. Nanofibrous mats, manufactured by electrospinning, have different structures and thicknesses. Displacement and strain distributions for all ranges of the specimen have never been demonstrated for nanofiber mats so far. Wrinkled nanofibrous mats of polyurethane were made and then tension-tested. The Digital Image Correlation (DIC) method was employed to measure displacement, then to calculate strain for all areas of the specimen. The DIC system consisted of a CMOS camera, control PC and operating software with a DIC algorithm: then, the Center of Gravity (COG) algorithm was used for this system. A cross-head speed of 3 mm/min was set for the tension test. The image record speed was one frame a second. In total, 400 image frames were obtained from the start, and then displacement and strain distributions were acquired for a 400 second tension test. The strain distribution from DIC system showed good agreement with the test result by a universal testing machine.

High-precision speckle-tracking X-ray imaging with adaptive subset size choices

Speckle-tracking imaging has the advantages of simple setup and high-sensitivity to slowly varying phase gradients. Subset size choice is regarded as a trade-off problem for speckle-tracking X-ray imaging where one needs to balance the spatial resolution and accuracy, where the subset was defined as the region of interest of windowing choice for digital image correlation algorithm. An adaptive subset size choice method based on a Fourier transform for effectively detecting sample phase information without foreknowledge of the sample structure is presented in this study. The speckle-tracking phase-contrast and the form of dark-field imaging based on this method have the advantages of (i) high resolution and time saving compared to large subset choice and (ii) partially improvement the influence from experimental noises, background fluctuations, and false signals compared to small subset choice at the same time. This method has proven to be particularly robust in the experimental condition of poor signal-to-noise ratio. The proposed method may be expanded to all speckle-based imaging methods and other imaging techniques based on the subset or window matching.

Modal Parameter Estimation of a Compliant Panel Using Phase-based Motion Magnification and Stereoscopic Digital Image Correlation

This paper demonstrates the use of broad-band phase based motion magnification (PMM) to improve the modal parameter estimation from high-speed stereoscopic digital image correlation (DIC). PMM is used as a diagnostic technique to investigate the free vibration response of a panel. The compliant panel, consisting of a thin polycarbonate sheet, forms a test section wall in a supersonic blow-down wind tunnel, where it is used to investigate supersonic fluid-structure interaction in the presence of shock wave boundary layer interaction. The panel is excited by an impact hammer and the transient deformation is captured using high-speed cameras. The original and motion-magnified images are input to a digital image correlation algorithm to calculate the out-of-plane deformation of the panel. The measured deformation is used to extract the modal parameters of the compliant panel. By using PMM as a preprocessing tool in a broad frequency band containing multiple structural modes, the signal to noise ratio of the measured deformation is improved. The use of PMM improves the estimated mode shapes, increasing the MAC value of the first mode compared to FEM predictions from 0.29 to 0.99. Motion magnification also improves the coherence between measured input force and panel deformation by up to 13% if suitable parameters are chosen.

A Comprehensive Survey on Crack detection of Bone using various techniques

Digital image processing plays a key role in manipulation of image and extracting the maximum amount of data from image with help of various algorithm. Digital image correlation algorithm determines the displacement and deformation of pattern across several images. Creating innovation are developing every day in various fields, particularly in restoration condition. Notwithstanding, still some old strategies are very famous. X-ray or CT images are one among the system for identification of bone cracks. during this article, we offer a comprehensive overview of various algorithm and techniques of displacement measurement generally and crack detection especially using digital image processing. we've been successful in highlighting each and each key feature and aspect of crack detection in bone which can take the add this domain further

A Method for Markerless Tracking of the Strain Distribution of Actively Contracting Cardiac Muscle Preparations

Current techniques for tracking the displacement and strain patterns within contracting cardiac specimens often require ‘invasive’ markers to be fixed on the sample, offer only modest spatial resolution, and are computationally inefficient. We present a new technique for determining, without requiring the addition of markers, the dynamic displacement and strain distribution within isolated cardiac muscle samples. We use an accurate and robust new digital image correlation algorithm to measure deformations from a stitched-together time-series of images of a contracting trabecula, and a video recording of a contracting myocyte, captured by an optical transmission microscope. This technique yielded a 2D map of local deformation and strain throughout each specimen, over the time-course of a twitch, at a spatial resolution far exceeding that of previous techniques. In the trabecula we examined, our method enabled us to quantify its non-uniform strain and sarcomere length dynamics even when it was contracting with its length fixed. We noted the non-uniform relaxation of the specimen, where part of the trabecula appeared to be in contraction while the rest was in extension. The images of the myocyte revealed a strain distribution that ranged from −7% to −18%. By applying this method, it is now possible to accurately quantify internal deformation in isolated muscle samples to subpixel resolution, which is essential for interpreting any motion-dependent measurements such as sarcomere length and change in cross-sectional area, all of which are preparation-specific.

High-precision video extensometer based on a simple dual field-of-view telecentric imaging system

We present a novel dual field-of-field (dual-FOV) video extensometer for high-precision tensile/compressive strain measurement in material testing. The present dual-FOV video extensometer, based on the combined use of a bilateral/object-space telecentric imaging system and a cube prism, can orthogonally capture two separated FOVs on the sample surface. For any two gauge points selected in the two separated FOVs, the gauge length can be theoretically computed from the systematic parameters of the imaging system. By tracking the movement of the gauge points using advanced subset-based digital image correlation (DIC) algorithm, temporal changes of the gauge length, and consequently the average strains of the test sample can be obtained. Due to the enlarged gauge length, logitudinal strains can be measured with higher sensitivity and precision compared with previously reported video extensometers that use only a telecentric lens. The precision, accuracy, and practicability of the proposed video extensometer are validated by real tensile tests.

Acoustical and Optical Determination of Mechanical Properties of Inorganically-Bound Foundry Core Materials.

Inorganically-bound sand cores are used in many light-metal foundries to form cavities in the cast part, which cannot be realised by the mould itself. To enable FEM simulations with core materials, their mechanical properties have to be measured. In this article, we adapt methods to determine the Young’s and shear modulus, the Poisson ratio and the fracture strain of sand cores. This allows us to fully parametrise an ideal brittle FEM model. We found that the Young’s and shear modulus can be obtained acoustically via the impulse excitation technique. The fracture strain was measured with a high-speed camera and a digital image correlation algorithm.

Airship skin strain measurement based on adaptive digital image correlation

Aiming at the non-uniform strain in the welding of airship skin, this paper studies the traditional digital image correlation algorithm, optimizes the shape and size of the subset, and presents an adaptive subset algorithm. Under the premise of ensuring a substantially constant subset area, the modified algorithm uses the initial u and v values to optimize the shape of the subset, find the optimal aspect ratio. Besides, the modified digital image correlation dynamically adjusts the shape of the subset according to the threshold value, making the algorithm better applicable to non-uniform deformation and large deformation. As the results show, in the simulation experiment, under the different strains such as 1000 μɛ, 5000 μɛ, and 9000 μɛ, the standard error measured by the modified method is reduced to more than 50% compared with the traditional method. Meanwhile, during the real speckle experiment, the error in the modified digital image correlation algorithm was reduced from 9.3e−04 to 7.2e−04. Furthermore, the modified digital image correlation only costs 3% more time, has a little effect on computational efficiency. Therefore, the modified method can smooth the measured strain, and make the robustness and accuracy better during the airship skin measurement.

Microscopic and Macroscopic Measurements of Poisson’s Ratio of ASTM B557M Using Digital Image Correlation and Local Search Algorithm

Microscopic and Macroscopic Measurements of Poisson’s Ratio of ASTM B557M Using Digital Image Correlation and Local Search Algorithm