Impact of motion blur on stereo‐digital image correlation with the focus on a drone‐carried stereo rig

Stereo‐digital image correlation (DIC) is a wide‐spread technique in the field of experimental mechanics for measuring shape, motion, and deformation and it is frequently used for material identification by using inverse methods (e.g., virtual fields method and finite element model updating). New applications emerge due to the reached maturity level of the technique, which poses new challenges towards reaching a desired level of accuracy in operating conditions. In this work, the possibility of a drone carrying an in‐house‐made portable DIC setup is explored, and the effect of the drone‐induced vibrations on the accuracy of stereo‐DIC for shape and strain measurement is evaluated. During acquisition, the relative motion between the camera system and the measured item generates motion‐blurred images. The effect of this phenomenon on the precision of stereo‐DIC is further evaluated in this paper.     

数字图像相关测量钢绞线弹性模量的应用研究

该文利用非接触式数字图像相关方法测量了预应力钢绞线的弹性模量。测量过程中用CCD相机记录不同载荷下钢绞线表面的数字图像,再利用数字图像相关方法对所采集的序列数字图像进行分析从中精确提取不同载荷下钢绞线表面的纵向平均正应变。根据试验机的载荷信息、钢绞线的参考截面积和由数字图像相关方法测量的平均正应变绘制出钢绞线的应力-应...     

The Enhanced Digital Image Correlation Technique for Feature Tracking During Drying of Wood

Abstract: The enhanced digital image correlation (EDIC) technique is proposed as an improvement of the digital image correlation (DIC) technique in that it utilises monogenic filtering as a prefilter for extracting intrinsic local phase information from the low‐contrast images and normalized cross‐correlation (NCC) as a feature‐tracking algorithm. The monogenic filtering separates local structural information that is the local phase of an image, which is invariant with respect to the local energy of the image. Therefore, it improves the image, permitting the DIC technique to produce stable and accurate measurements of deformation for a heterogeneous and hygroscopic material like wood during drying.     

Automated Fast Initial Guess in Digital Image Correlation

A challenging task that has hampered the fully automatic processing of the digital image correlation (DIC) technique is the initial guess when large deformation and rotation are present. In this paper, a robust scheme combining the concepts of a scale‐invariant feature transform (SIFT) algorithm and an improved random sample consensus (iRANSAC) algorithm is employed to conduct an automated fast initial guess for the DIC technique. The scale‐invariant feature transform algorithm can detect a certain number of matching points from two images even though the corresponding deformation and rotation are large or the images have periodic and identical patterns. After removing the wrong matches with the improved random sample consensus algorithm, the three pairs of closest and non‐collinear matching points serve for the purpose of initial guess calculation. The validity of the technique is demonstrated by both computer simulation and real experiment.     

Formulation of Lucas–Kanade Digital Image Correlation Algorithms for Non‐contact Deformation Measurements: A Review

Digital image correlation (DIC) metrology has been increasingly used in a wide range of experimental mechanics research and applications. The DIC algorithm used so far is however limited mostly to the classic forward additive Lucas–Kanade type. In this paper, a survey is given about the formulation of other types of Lucas–Kanade DIC algorithms that have been appeared in computer vision, robotics, medical image analysis literature and so on. Concise notations consistent with the finite deformation kinematics analysis in continuum mechanics are used to describe all Lucas–Kanade DIC algorithms. An intermediate image is introduced as a frame of reference to clarify the so‐called compositional algorithms in a two‐frame DIC analysis. Explicit examples about the additive and compositional updating of deformation parameters are given for affine deformation mapping. Extensions of these algorithms to the so‐called consistent or symmetric types are also presented. The equivalency of final numerical solutions using additive, compositional and inverse compositional algorithms is shown analytically for the case of affine deformation mapping. In particular, the inverse compositional algorithm for affine image subset deformation is highlighted for its superior computational efficiency. While computationally less efficient, consistent and symmetric algorithms may be more robust and less biased and their potentials in experimental mechanics applications remain to be explored. The unified formulation of these Lucas–Kanade DIC algorithms collected all together in this paper can serve as a useful guide for researchers in experimental mechanics to further evaluate the merits as well as limitations of these non‐classic algorithms for image‐based precision displacement measurement applications.     

Application of Digital Image Correlation Method to Study Dental Composite Shrinkage

Abstract:  Although resin-based composites are widely used in dental restoration, these materials shrink during polymerisation. Polymerisation shrinkage results in distortion of the restoration and bonded tooth and also generates internal stress at the resin–tooth interface. Digital image correlation (DIC) is used to determine the in-plane displacement field by matching different zones of two characterised pictures. The objective of this study was to examine the applicability of DIC in measuring the deformation of the composite restoration and the surrounding tooth. A preliminary experiment examined the shrinkage of composites in a simulated cavity using the DIC method. The measured shrinkage pattern was consistent with a corresponding finite element model. Subsequently the deformation of composite restorations on human molars was examined using this validated DIC method. The greatest deformation was found on the free occlusal surfaces, and the least on the gingival wall. The increased deformation on the post-cured images indicated that the shrinkage continued even after termination of light activation. DIC method facilitates a full-field measurement of shrinkage profile. These experimental results did not only demonstrate the spatial and temporal relationship of displacement in a dental restoration, but also provide validation of computational models to examine the polymerisation consequence.     

Energy Balance of a Semicrystalline Polymer During Local Plastic Deformation

Abstract: This paper, in a first part, presents the characteristics of a new experimental set‐up using digital image correlation (DIC) and infrared thermography (IRT). In a second part, the kinematical data, obtained by DIC, are used to track the material surface element temperature variations deduced from thermal images. They are then combined to construct the local energy balance. To illustrate the interest of such an approach, the paper then describes the calorimetric effects accompanying the propagation of necking in a plasticised PolyAmide 11. A thermodynamic analysis of cyclic loading finally aims to show the existence of an entropic elastic effect generally associated with rubber‐like materials.     

Effect of DIC Spatial Resolution,Noise and Interpolation Error on Identification Results with the VFM

The use of experimental tests that involve full‐field measurements to characterize mechanical material properties is becoming more widespread within the engineering community. In particular digital image correlation (DIC) on white light speckles is one of the most used tools, thanks to the relatively low cost of the equipment and the availability of dedicated software. Nonetheless the impact of measurement errors on the identified parameters is still not completely understood. To this purpose, in this paper, a simulator able to numerically simulate an experimental test, which involves DIC is presented. The chosen test is the Unnotched Iosipescu test used to identify the orthotropic elastic parameters of composites. Synthetic images are generated and then analysed by DIC. Eventually the obtained strain maps are used to identify the elastic parameters with the Virtual Fields Method (VFM). The numerical errors propagating through the simulation procedure are carefully characterized. Besides, the simulator is used to compare the performances of DIC and the grid method in the identification process with the VFM. Finally, the influence of DIC settings on the identification error is studied as a function of the camera digital noise level, in order to find the best testing configuration.     

Evaluating the use of digital image correlation for strain measurement in historic tapestries using representative deformation fields

An analysis technique to assess the viability of digital image correlation (DIC) in tracking the full‐field strains across the surface of hanging historic tapestries is presented. Measurement uncertainty related to the use of the inherent tapestry image in tracking displacements is investigated through use of “synthetic” deformation fields. The latter are generated by mapping the details of a given tapestry image into finite element analyses. The combination of self‐weight loading, material non‐linearity, and image specific heterogeneity (related to slit stitching, damage, and patch‐restorations) serve to generate a bespoke deformation field complex enough to assess the reliability of DIC measurements. Accuracy is evaluated by comparing measured results with the original known deformations. The technique demonstrates that the optimum imaging settings and the choice of subset size for DIC analysis are strongly influenced by the tapestry image and the goal of the measurement, they are found using a compromise between conflicting objectives: minimising measurement error while maximising resolution.     

An Evaluation of Convergence Criteria for Digital Image Correlation Using Inverse Compositional Gauss–Newton Algorithm

A fast, robust and accurate digital image correlation (DIC) method, which uses a robust zero‐mean normalized sum of squared difference correlation criterion, a sophisticated reliability‐guided displacement tracking strategy and an efficient inverse compositional Gauss–Newton (IC‐GN) algorithm, was recently proposed for full‐field deformation measurement. As an iterative local optimization algorithm, IC‐GN algorithm iteratively solves for the incremental warp assumed on the reference subset until the preset convergence criteria are satisfied. In the literature, different convergence criteria have been set for iterative optimization algorithms. However, on the one hand, stringent convergence criteria lead to increased number of iterations and lessen the computational efficiency. On the other hand, too loose convergence conditions enhance the computational efficiency but may decrease the registration accuracy. Understanding the impact of prescribed convergence criteria on DIC measurement and how to choose proper convergence criteria are therefore fundamental problems in realizing high‐efficiency yet high‐accuracy DIC analysis. In this paper, the convergence characteristics of IC‐GN algorithm are investigated in terms of convergence speed and radius of convergence using real experimental images. The effect of various convergence criteria on the efficiency and accuracy of IC‐GN algorithm are carefully examined. Recommendations are given to select proper convergence criteria for more efficient implement of IC‐GN algorithm.     

Displacement Measurement Through Digital Image Correlation and Digital Speckle Pattern Interferometry Techniques in Cold‐Expanded Holes

Abstract: In this paper, the displacement field induced by the split‐sleeve cold expansion of holes was measured using both digital image correlation (DIC) and digital speckle pattern interferometry (DSPI) techniques. Thus, the experimental results, which were evaluated on the inlet surface of a 6082‐T6 aluminium plate, were compared with those from theoretical prediction. DIC provided accurate measurements up to the elastic–plastic boundary, whereas the DSPI technique highlighted the changes of displacement in the elastic domain. Prediction of the displacement based on the existing analytical model agreed with the experimental results achieved with both techniques. Possible explanations for the differences are discussed.     

多级FFD配准视频人脸超分辨率重建

超分辨率重建是解决视频人脸识别中人脸分辨率低的有效方法,但由于人脸畸变、表情变化等非刚性变化导致无法精确配准和重建.针对此问题,提出基于B样条的多级模型自由形式形变(FFD)弹性配准算法.先用低分辨率FFD网格全局配准,再对全局配准后的图像分块并计算对应子图块的相关性系数,对相关性系数小的子图块用高分辨率FFD网格局部细配准.在配准的寻优过程中采用基于混沌因子的自适应步长最速下降法提高寻优效率.配准后,采用POCS算法对多帧图像重建高分辨率图像来识别.在标准视频库和自建视频库上实验仿真,结果表明在人脸畸变和表情变化很大的情况下,也能够精确的配准和很好的重建,得到较高识别率.     

采用仿射变换的红外与可见光图像配准方法

图像配准是图像融合技术的基本环节和首要问题。针对红外和可见光图像的配准问题,分析了两者配准的特点。引入6参数的仿射变换模型来描述3D空间中的目标,分析了通常采用的4参数模型的不完备性。利用模型计算配准参数,并用最小二乘法达到用多个配准点拟合出最优参数解的目的。实验结果表明,此方法可以很好地消除红外和可见光图像之间的差异。     

Optimised Experimental Characterisation of Polymeric Foam Material Using DIC and the Virtual Fields Method

This article presents a methodology to optimise the design of a realistic mechanical test to characterise the material elastic stiffness parameters of an orthotropic PVC foam material in one single test. Two main experimental techniques were used in this study: Digital Image Correlation (DIC) and the Virtual Fields Method (VFM). The actual image recording process was mimicked by numerically generating a series of deformed synthetic images. Subsequent to this, the entire measurement and data processing procedure was simulated by processing the synthetic images using DIC and VFM algorithms. This procedure was used to estimate the uncertainty of the measurements (systematic and random errors) by including the most significant parameters of actual experiments, e.g. the geometric test configuration, the parameters of the DIC process and the noise. By using these parameters as design variables and by defining different error functions as object functions, an optimisation study was performed to minimise the uncertainty of the material parameter identification and to select the optimal test parameters. The confidence intervals of the identified parameters were predicted based on systematic and random errors obtained from the simulations. The simulated experimental results have shown that averaging multiple images can lead to a significant reduction of the random error. An experimental determination of the elastic coefficient of a PVC foam material was conducted using the optimised test parameters obtained from the numerical study. The identified stiffness values matched well with data from previous tests, but even more interesting was the fact that the experimental uncertainty intervals matched reasonably well with the predictions of the simulations, which is a highly original result and probably the main outcome of the present paper.     

Medical image registration‐based retrieval using distance metrics

In this article, registration and retrieval are carried out separately for medical images and then registration‐based retrieval is performed. It is aimed to provide a more thorough insight on the use of registration, retrieval, and registration‐based retrieval algorithm for medical images. The purpose of this work is to deal these techniques with anatomical imaging modalities for clinical diagnosis, treatment, intervention, and surgical planning in a more effective manner. Two steps are implemented. In the first step, the affine transformation‐based registration for medical image is processed. The second step is the retrieval of medical images processed by using seven distance metrics such as euclidean, manhattan, mahalanobis, canberra, bray‐curtis, squared chord, chi‐squared, and also by using the features like mean, standard deviation, skewness, energy, and entropy. Now images registered by affine transformation are applied for retrieval. In this work, both registration and retrieval techniques in medical domain share some common image processing steps and required to be integrated in a larger system to complement each other. Experimental results, it is evident that euclidean and manhattan produces 100% precision and 35% recall found to have higher performance in retrieval. From the four anatomical modalities considered (brain, chest, liver, and limbs) brain image has better registration. It is also found that though the registration of images changes the orientation, for better performance of images in clinical evaluation it does not widely affect the retrieval performance. In the medical domain the ultimate aim of this work is to provide diagnostic support to physicians and radiologists by displaying relevant past cases, along with proven pathologies as ground truth from experimental results. © 2013 Wiley Periodicals, Inc. Int J Imaging Syst Technol, 23, 360–371, 2013     

Study on Fracture Behaviour of a Polymer‐Bonded Explosive Simulant Subjected to Uniaxial Compression Using Digital Image Correlation Method

Abstract: Quasi‐static uniaxial compression experiments were conducted on a polymer‐bonded explosive (PBX) simulant. At macro‐scale, the deformation and fracture process of samples were recorded using a charge‐coupled‐device camera. Microscopic examination was conducted to in situ observe the deformation and fracture processes of samples using SEM equipped with a loading stage. Microscopic damage modes, including interfacial debonding and particle fracture, were observed. The digital image correlation (DIC) technique was used to calculate the recorded images, and the macro‐ and micro‐scale displacement and strain fields were determined. Crack initiation, crack propagation, fracture behaviour and failure mechanism of samples were studied. The effects of aspect ratios on fracture behaviour and failure mechanism of PBX simulant were analysed.     

Investigation of the Uncertainty of DIC Under Heterogeneous Strain States with Numerical Tests

Abstract: In this research, numerical 2D digital image correlation (DIC) tests are carried out to assess the uncertainty of DIC under heterogeneous strain states. DIC is implemented to measure the deformation of the numerically deformed images with respect to the undeformed counterparts, which are taken from the real tensile specimens. The tensile specimens are made of three materials, that is, steel DC06, steel DX54D+Z and aluminium alloy Al6016 and cut into three different geometries, namely one standard design and two complex designs. The specimens are all painted manually with random speckle patterns. The original images are deformed by imposed displacement fields, which are obtained by simulating uni‐axial tensile tests of the specimens with finite element analysis (FEA). In this way, the errors sourcing from the hardware of the image system are excluded. According to the geometries of the specimens, homogeneous and heterogeneous strain states are achieved by FEA. The optimum mesh sizes of the models are identified to minimise theirs influence on the imposed fields. The impacts of subset sizes, step sizes and strain window sizes are studied for an optimum correlation. Finally, the influence of the strain state is investigated. It is found that the DIC accuracy and precision decrease under highly heterogeneous strain states.     

High‐throughput,high‐accuracy determination of coefficient of thermal expansion of carbon fibre–epoxy composites using digital image correlation

High‐throughput, high‐accuracy determination of thermal deformation and coefficient of thermal expansion (CTE) of carbon fibre–epoxy composites using 2D‐digital image correlation (2D‐DIC) is described. With the aid of a specially designed ultra‐stable and high fidelity imaging system, which integrates a high‐quality bilateral telecentric lens with monochromatic blue light illumination, surface images of multiple samples heated by a heating furnace can be captured simultaneously. The images of these samples at different temperatures are processed by advanced DIC algorithm to extract the thermal strains and the CTE of isotropic Al alloy, anisotropic unidirectional, and bidirectional carbon fibre–epoxy composites. Pure thermal expansions of these samples obtained after removing the small rigid‐body rotations clearly indicate the isotropic and anisotropic expansions of these samples. The well‐agreed results with literature values demonstrate the effectiveness and practicality of the proposed method for high‐throughput and high‐accuracy CTE measurements.     

An approach to calculate the J‐integral by digital image correlation displacement field measurement

This paper presents a combined experimental‐numerical technique for the calculation of the J‐integral as an area integral in cracked specimens. The proposed technique is based on full‐field measurement using digital image correlation (DIC) and the finite element method. The J‐integral is probably the most generalised and widely used parameter to quantify the fracture behaviour of both elastic and elastoplastic materials. The proposed technique has the advantage that it does not require crack length measurements nor is it limited to elastic fracture mechanics, provided that only small scale yielding is present. Evaluated are three test geometries; compact tension, three‐point bend and the double torsion beam. Possible errors and their magnitude and the limitations of the method are considered.     

Research on the Application of Super Resolution Reconstruction Algorithm for Underwater Image

Underwater imaging is widely used in ocean, river and lake exploration, but it is affected by properties of water and the optics. In order to solve the lower-resolution underwater image formed by the influence of water and light, the image super-resolution reconstruction technique is applied to the underwater image processing. This paper addresses the problem of generating super-resolution underwater images by convolutional neural network framework technology. We research the degradation model of underwater images, and analyze the lower-resolution factors of underwater images in different situations, and compare different traditional super-resolution image reconstruction algorithms. We further show that the algorithm of super-resolution using deep convolution networks (SRCNN) which applied to super-resolution underwater images achieves good results.