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.     

A Novel Class of Strain Measures for Digital Image Correlation

We propose a novel class strain measures for use with digital image correlation (DIC). Whereas the traditional notion of compatibility (strain as the derivative of the displacement field) is problematic when the displacement field varies substantially because of either measurement noise or material irregularity, the proposed measure remains robust, well defined and invariant under rigid body motion. Moreover, when the displacement field is smooth, the classical and proposed strain measures are approximations of each other. We demonstrate, via several numerical examples, the potential of this new strain measure for problems with steep gradients. We also show how the non‐local strain provides an intrinsic mechanism for filtering high‐frequency content from the strain profile and so has a high signal to noise ratio. This is a convenient feature considering image noise and its impact on strain calculations.     

An Evaluation of Digital Image Correlation Criteria for Strain Mapping Applications

Abstract:  The performance of four digital image correlation criteria widely used in strain mapping applications has been critically examined using three sets of digital images with various whole-field deformation characteristics. The deformed images in these image sets are digitally modified to simulate the less-than-ideal image acquisition conditions in an actual experiment, such as variable brightness, contrast, uneven local lighting and blurring. The relative robustness, computational cost and reliability of each criterion are assessed for precision strain mapping applications. Recommendations are given for selecting a proper image correlation criterion to efficiently extract reliable deformation data from a given set of digital images.     

Modal Pursuit to Detect Large Displacements and Strain Fields by Digital Image Correlation

Digital image correlation offers a useful approach to resolve displacements and strains by picture comparisons. However, many of the several methods proposed in the past suffer the occurrence of large displacements between the two compared pictures. The modal pursuit approach here introduced intends giving a possible strategy to prevent this limitation. The main advantage is that the method uses a global approach for the solution strategy. The procedure uses a displacement basis that derives from a modal analysis on a membrane structure whose shape is the same as the image region under investigation. The modes are introduced a little at a time so that the procedure is facilitated to converge, even when only two pictures (undeformed and final state) are used for the matching.     

Assessment of Analytical Models of Pure Bending of Sheet Materials Using the Digital Image Correlation Method

Bending of sheet materials is a common forming mode for shaping sheet components. Although many numerical models of bending, both analytical and numerical simulations based, are available in the literature, extensive experimental validations have been rather limited. A new bend test method and complementary three‐dimensional finite element (FE) simulation of the experiments are employed to assess the predictions from an advanced analytical and FE model of pure bending of aluminium sheet materials. The experimental set‐up developed and utilised is an open concept design that allows access to the tensile surface and through‐thickness region in the vicinity of the specimen bend line to continuously record images of the deforming specimen with two cameras. The specimen images are analysed for strains using an online strain mapping system based on digital image correction method. Tangential strain distribution results from the models in terms of material thinning in the bend region are compared with those from the experiments on AA2024 aluminium sheet material by considering the responses from the specimen edges and mid‐width regions at the bend line. Furthermore, the tangential and radial stress distributions on the through‐thickness section of the specimen from the analytical model are compared with those from the FE model. The results from experiments, FE model and analytical model are compared and discussed in the light of the experimental data and the assumptions involved in the development of the models.     

Strain Measurement of Nickel Thin Film by a Digital Image Correlation Method

Abstract:  Digital image correlation is a whole-field and non-contact strain-measuring method. It provides deformation information of a specimen by processing two digital images captured before and after the deformation. To search the deformed images, a hybrid genetic algorithm, in which a simulated annealing mutation process and adaptive mechanisms are combined with a real-parameter genetic algorithm, is adopted. This method is used to measure the strain during the microtensile testing of nickel thin film. In addition to the conventional single region, a double region in which the strain is inferred from the distance change of two regions is proposed to calculate the strain by digital image correlation. The results indicate that while the strain values obtained by single-region method are reasonable, those obtained by the double region method are more accurate. Moreover, the mechanical properties of nickel thin film could be obtained.     

Application of High Magnification Digital Image Correlation Technique to Micromechanical Strain Analysis

Abstract: This paper describes an experimental apparatus and its application for the full‐field measurement of heterogeneous strains at high magnifications. The apparatus consists of an image acquisition and analysis system, an optical microscope and a stable tensile stage. Magnified images of the specimen surface are acquired and analysed using the digital image correlation (DIC) method. The response of the heterogeneous microstructure of a nodular cast iron is investigated during a tensile test. Strains obtained by using the DIC method and averaged over the observation window correlate with strain measurements simultaneously obtained by using an extensometer. The strain maps of DIC reveal the heterogeneous development of plasticity in the nodular cast iron microstructure. The apparatus has the potential to investigate material behaviour at the microscopic scale.     

Assessing the Feasibility of Monitoring Strain in Historical Tapestries Using Digital Image Correlation

Abstract:  Digital image correlation (DIC) is used to monitor strain in a representative textile material and an historic tapestry. The validity of a 'map function' that allows 3D DIC displacement measurements to be obtained when the reference data are collected with a camera set-up different from that of the deformed data is assessed. An experiment was devised to study the effects of DIC processing parameters (interrogation cell size and overlap) on strain measurements, and to investigate if the textile contains adequate contrast for DIC to operate. The study shows that the textile's weave pattern can be used as the device for correlation. Long-term tests for monitoring creep strain using DIC both in the laboratory and in situ are presented. The results show good correspondence between strain changes in the tapestry and relative humidity.     

Evaluating J‐integral from Displacement Fields Measured by Digital Image Correlation

Path integral, domain integral and least squares methods for evaluating J‐integral from measured displacement fields for a power‐law hardening material are described in this paper. The values of the J‐integral are evaluated by applying the path and domain integral methods to the displacement fields obtained by elastoplastic finite element analysis and the displacement fields obtained through the measurement using digital image correlation. Results show that the values obtained by the domain integral method are slightly better than those by the path integral method, because the domain integral method efficiently uses the full‐field measurement data. The values of the J‐integral are also evaluated by the least squares method with the Hutchinson, Rice, and, Rosengren displacement fields. Results show that the J‐integral can be obtained by the least squares method simply and easily without any calculation of the integration. The J‐integral values obtained by the least squares method agree well with the values obtained using other methods. Because J‐integral can be evaluated easily by any method described in this paper, it is expected that these methods are applicable to various fracture problems during experimental evaluation of structural components.     

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.     

非接触数字图像相关应变测量的仿真与实验研究

数字图像相关法测量全场位移和应变是一种新的实验力学方法.该测量算法中相关函数和子区大小的选择是影响最终应变测量精度的重要因素.本文采用已知应变变形的仿真散斑图,研究了测量算法中主要相关函数和子区大小在正常光照与高斯不均匀光照条件下,对应变测量精度的影响.得出了不同测量要求下,可选择的相关函数和最佳计算窗口.通过实验验证与分析了此算法的测量精度.研究结果表明,数字图像处理技术测量应变的测试精度满足基本要求.     

Effects of Fibre Geometry and Volume Fraction on the Flexural Behaviour of Steel‐Fibre Reinforced Concrete

Abstract: This work aims in studying the mechanical behaviour of concrete, reinforced with steel fibres of different geometry and volume fraction. Experiments include compression tests and four‐point bending tests. Slump and air content tests were performed on fresh concrete. The flexural toughness, flexural strength and residual strength factors of the beam specimens were evaluated in accordance with ASTM C1609/C1609M‐05 standard. Improvement in the mechanical properties, in particular the toughness, was observed with the increase of the volume fraction of steel‐fibres in the concrete. The fibre geometry was found to be a key factor affecting the mechanical performance of the material.     

Cross‐Correlation and Differential Technique Combination to Determine Displacement Fields

Abstract: This study presents a method to measure the displacement fields on the surface of planar objects with sub‐pixel resolution, by combining image correlation with a differential technique. First, a coarse approximation of the pixel level displacement is obtained by cross‐correlation (CC). Two consecutive images, taken before and after the application of a given deformation, are recursively split in sub‐images, and the CC coefficient is used as the similarity measure. Secondly, a fine approximation is performed to assess the sub‐pixel displacements by means of an optical flow method based on a differential technique. To validate the effectiveness and robustness of the proposed method, several numerical tests were carried out on computer‐generated images. Moreover, real images from a static test were also processed for estimating the displacement resolution. The results were compared with those obtained by a commercial digital image correlation code. Both methods showed similar and reliable results according to the proposed tests.     

Digital Image Correlation Used for Experimental Investigations of Al/Mg Compounds

Al/Mg compounds produced by hydrostatic extrusion exhibit unique characteristics in terms of high strength and low weight, which are required in lightweight safety parts. The compound interface between the core and sleeve is a brittle intermetallic phase consisting of Al₂Mg₃ and Al₁₂Mg₁₇. However, a certain plastic deformability is required for subsequent processing of the semi‐finished product by forming. To obtain a basic understanding of the deformation behaviour of the interface, the Digital Image Correlation is used together with a new analysis procedure based on the approximation of the displacement field with polynomial functions. The procedure is first tested with a bending test. The specimens exhibit a remarkable deformation behaviour under bending at high temperatures, which is confirmed by the results of radial upsetting tests. Even under a multi‐axle load, the interface maintains a full material joint, though fragmentation occurs, and a new secondary interface between the fragments can be observed. In addition, an evaluation of light microscopy images and Eulerian Hencky strain values at the interface, which are based on the Digital Image Correlation results, imply a connection between the strain and the boundary layer's appearance.     

Full-Field Strain Rate Measurement by White-Light Speckle Image Correlation

Abstract:  This paper explains a numerical procedure to process sequences of digital images and to return a full-field evaluation of the strain rate. The processing procedure is based on a nonlinear least squares fitting performed globally, on the whole image, and simultaneously on several images. The use of a highly optimised code allows the analysis of long sequences in a few minutes. The results of calculations are presented as movies built by blending the colour maps of the measured strain field with the specimen pictures used in the correlation procedure. Our application is focused on studying the plastic behaviour of metals and, in particular, on highlighting any transient phenomena that might occur during yielding and strain-hardening phases on thin sheets used in the manufacture of sheet metals. A typical example for such phenomena is the Portevin-Le Châtelier effect, a repetitive yielding of alloys during plastic deformation.     

Strain Localisation in iPP/MWCNT Nanocomposites Using Digital Image Correlation

Polypropylene/multiwall carbon nanotube nanocomposites with different rates of weight incorporation (0–1%) were prepared by melt compounding and cast extrusion. The effect of maleic anhydride (5 wt%) grafted on polypropylene is studied through mechanical tests at different scales and morphological observations. In particular, the micromechanism of deformation was investigated through instrumented tensile experiments (at a macro and micro scales) using of a non‐contact method known as digital image correlation. The objective of this paper is first to characterise global behaviour (Young modulus, tensile strength, and ultimate properties) and second to go further in local analysis. In particular, optical instrumentation enables estimation of strain profile distribution onto the sample in a constricted area. Statistical parameters extracted from these local profiles are promising tools to enhance mechanical properties in link with microstructural composition. Tensile tests confirm composite reinforcement at a low level of nanocomposite incorporation, and local analysis enables quantitative measurements of adding maleic anhydride in formulations. The results reveal that addition of maleic anhydride delays strain localisation in the necked area.     

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.     

Digital Image Correlation and Noise‐filtering Approach for the Cracking Assessment of Massive Reinforced Concrete Structures

This paper describes the use of Digital Image Correlation (DIC) techniques for the cracking assessment of reinforced concrete (RC) massive beams and walls. DIC is known to provide accurate and detailed information on displacement and strain fields. Non‐contact measurements can be used to evaluate concrete cracking of destructive tests carried out on a wide range of specimen scales. When applied to large RC structures tested outdoors or in difficultly controllable conditions, DIC‐based methods may lead to erroneous results. In this study a post‐processing procedure is presented to cope with noisy full‐field measurements. The proposed cracking assessment approach is validated on a large experimental campaign. Four points bending tests are carried out on RC beams: firstly on full‐scale rectangular beams and then on mock‐ups scaled down by 1/3. In addition, fours RC walls are tested under in‐plane cyclic shear up to failure. Digital images taken throughout the tests are processed by DIC techniques to provide in‐plane displacement and strain fields. Full‐field measurements are post‐processed by the noise‐filtering technique and the cracks patterns are identified. Crack widths are measured and compared with measurements obtained from conventional point‐based sensors (linear variable differential transformer LVDT and fibre‐optic FO transducers). The proposed DIC‐based post‐processing provides accurate estimation of cracks width for most of the tests. The analyses carried out on the two groups of RC beams show a scale‐effect on the cracks width.