Conventional Digital Image Correlation Method Research Articles

Dynamic Visible Monitoring of Heterogeneous Local Strain Response through an Organic Mechanoresponsive AIE Luminogen.

Local strain concentration is critically important for damage formation of structural components. Therefore, it is of particular interest in developing the structural health monitoring (SHM) method for large-scale, full-field, and on-site monitoring of local strain response in complicated structural components in service. The present work investigated a SHM method based on a pure organic mechanoresponsive luminogen (MRL), 1,1,2,2-tetrakis(4-nitrophenyl)ethane, for heterogeneous local strain concentration. Invisible heterogeneous local strain response in complicated weld joints is transformed into visible fluorescence under monotonic tension and cyclic stress loading. The local strain (<15%) calculated by fluorescence intensity has a good agreement with the results obtained by the conventional digital image correlation method, indicating good measurement accuracy of the calibrated organic MRL method. The heterogeneity of local strain in complicated weld joints increases along with elongation and number of stress cycles. Moreover, the higher mean stress and stress amplitude can induce significantly higher accumulated local strain in the relatively soft fusion zone region. Compared with conventional strain measurement methods, the present organic MRL method opens up new possibilities for large-scale, full-field, and on-site monitoring of local strain concentration and damage in complicated structural components.

DIC/Moiré hybrid method based on regular patterns for deformation measurement.

Digital Image Correlation (DIC) is a superior optical method to measure the surface deformation with a high accuracy. Currently, most researches on DIC are based on random patterns. In this paper, A DIC/Moiré hybrid method using regular patterns is proposed for deformation measurement. In this method, a Moiré fringe technique based on correlation coefficient is developed to provide accurate initial deformation estimation for DIC. Experimental results indicate a higher computational efficiency by the proposed method than the conventional DIC method. It is also found that the calculation accuracy increases using regular patterns. The advantage of obtaining accurate initial estimation by the DIC/Moiré hybrid method may enable potential application in deformation measurements.

Large In-plane Deformation Mapping and Determination of Young’s Modulus of Rubber Using Scanner-Based Digital Image Correlation

We propose a novel scanner-based digital image correlation (DIC) method to determine the full-field in-plane displacement as well as the Young’s modulus of elastomeric materials at strains up to 350%. A low-cost charged-couple device flatbed scannerwas used as the image acquisition device instead of a digital camera in the conventional DIC method. A motorized loading fixture was designed to act as the specimen clamp as well as the tensile load applicator. The rubber specimens weremade according to the ASTM:D412-06a standard andmounted into the fixture. Each specimen was scanned at increasing loads and the images were processed to obtain the full-field displacement maps using incremental cross-correlation algorithm. The Young’s modulus of the rubber at strains of 50–350% obtained using the scanner-based DIC method was compared with those obtained from a universal tensile test machine. The comparison shows a maximum deviation in Young’s modulus of 8.9 at 150% strain. The high-resolution flatbed scanner coupled with the built-in lighting was found to be an effective tool for low-cost DIC application for large deformation materials.

Improved Digital Image Correlation method

Digital Image Correlation (DIC) is a powerful technique which is used to correlate two image segments to determine the similarity between them. A correlation image is formed which gives a peak known as correlation peak. If the two image segments are identical the peak is known as auto-correlation peak otherwise it is known as cross correlation peak. The location of the peak in a correlation image gives the relative displacement between the two image segments. Use of DIC for in-plane displacement and deformation measurements in Electronic Speckle Photography (ESP) is well known. In ESP two speckle images are correlated using DIC and relative displacement is measured. We are presenting background review of ESP and disclosing a technique based on DIC for improved relative measurements which we regard as the improved DIC method. Simulation and experimental results reveal that the proposed improved-DIC method is superior to the conventional DIC method in two aspects, in resolution and in the availability of reference position in displacement measurements.

In situ strain and temperature measurement and modelling during arc welding

Experiments and numerical models were applied to investigate the thermal and mechanical behaviours of materials adjacent to the weld pool during arc welding. In the experiment, a new high temperature strain measurement technique based on digital image correlation (DIC) was developed and applied to measure the in situ strain evolution. In contrast to the conventional DIC method that is vulnerable to the high temperature and intense arc light involved in fusion welding processes, the new technique utilised a special surface preparation method to produce high temperature sustaining speckle patterns required by the DIC algorithm as well as a unique optical illumination and filtering system to suppress the influence of the intense arc light. These efforts made it possible for the first time to measure in situ the strain field 1 mm away from the fusion line. The temperature evolution in the weld and the adjacent regions was simultaneously monitored by an infrared camera. Additionally, a thermal–mechanical finite element model was applied to substantiate the experimental measurement.

Improved digital image correlation for in-plane displacement measurement

Electronic speckle photography (ESP) for in-plane displacement (IPD) and deformation measurements is well known with its more modern form, digital image correlation (DIC). Two speckle images of an optically rough surface before and after deformation, called reference and test images, are recorded and processed for IPD or deformation measurement of the test image with respect to the reference image. The reliability of ESP in measurements depends strongly on the postprocessing of the two images by DIC, which we have referred to as conventional DIC. In this paper, we are proposing a small but useful modification in the existing DIC methods by introducing some additional steps, which drastically improves the results obtained with the existing techniques. The modification to the conventional DIC method has been referred to as modified DIC. Computer-simulated and experimental results have been presented to validate the superiority of modified DIC over conventional DIC methods.

New measurement method of Poisson's ratio of PVA hydrogels using an optical flow analysis for a digital imaging system

In this paper, we propose a method to observe deformation, to measure strain and to calculate Poisson's ratio of a soft material such as a polyvinyl alcohol hydrogel by means of an optical flow analysis during a tensile test. Compared to the conventional digital image correlation method of which the basic assumption is that the local target region in an image is under linear deformation, an optical flow method can measure local deformation up to the pixel level of the image. In addition, sub-pixel measurements are possible by means of bi-cubic interpolation of each flow vector. To guarantee the accuracy of the optical flow vectors, the scale-invariant feature transform was used. Random sample consensus (RANSAC) for eliminating noisy features and then obtaining a more accurate result was applied. The mean value of Poisson's ratio is 0.4498 and the standard deviation comes to 0.0305. This optical flow method can also be applied to calculate Poisson's ratio of other soft materials.

Fast, Robust and Accurate Digital Image Correlation Calculation Without Redundant Computations

High-efficiency and high-accuracy deformation analysis using digital image correlation (DIC) has become increasingly important in recent years, considering the ongoing trend of using higher resolution digital cameras and common requirement of processing a large sequence of images recorded in a dynamic testing. In this work, to eliminate the redundant computations involved in conventional DIC method using forward additive matching strategy and classic Newton–Raphson (FA-NR) algorithm without sacrificing its sub-pixel registration accuracy, we proposed an equivalent but more efficient DIC method by combining inverse compositional matching strategy and Gauss-Newton (IC-GN) algorithm for fast, robust and accurate full-field displacement measurement. To this purpose, first, an efficient IC-GN algorithm, without the need of re-evaluating and inverting Hessian matrix in each iteration, is introduced to optimize the robust zero-mean normalized sum of squared difference (ZNSSD) criterion to determine the desired deformation parameters of each interrogated subset. Then, an improved reliability-guided displacement tracking strategy is employed to achieve further speed advantage by automatically providing accurate and complete initial guess of deformation for the IC-GN algorithm implemented on each calculation point. Finally, an easy-to-implement interpolation coefficient look-up table approach is employed to avoid the repeated calculation of bicubic interpolation at sub-pixel locations. With the above improvements, redundant calculations involved in various procedures (i.e. initial guess of deformation, sub-pixel displacement registration and sub-pixel intensity interpolation) of conventional DIC method are entirely eliminated. The registration accuracy and computational efficiency of the proposed DIC method are carefully tested using numerical experiments and real experimental images. Experimental results verify that the proposed DIC method using IC-GN algorithm and the existing DIC method using classic FA-NR algorithm generate similar results, but the former is about three to five times faster. The proposed reliability-guided IC-GN algorithm is expected to be a new standard full-field displacement tracking algorithm in DIC.

Reliability-guided digital image correlation for image deformation measurement

A universally applicable reliability-guided digital image correlation (DIC) method is proposed for reliable image deformation measurement. The zero-mean normalized cross correlation (ZNCC) coefficient is used to identify the reliability of the point computed. The correlation calculation begins with a seed point and is then guided by the ZNCC coefficient. That means the neighbors of the point with the highest ZNCC coefficient in a queue for computed points will be processed first. Thus the calculation path is always along the most reliable direction, and possible error propagation of the conventional DIC method can be avoided. The proposed novel DIC method is universally applicable to the images with shadows, discontinuous areas, and deformation discontinuity. Two image pairs were used to evaluate the performance of the proposed technique, and the successful results clearly demonstrate its robustness and effectiveness.

Displacement Identification Within Soft Tissue by Utilizing Three-Dimensional Full-Field Image Correlation Method with Parallel Solver

A new full-field digital image correlation method is proposed for identification of three-dimensional displacement field within a soft body. Two sets of three-dimensional image data, which consist of intensity values with discretely sampled positions, are obtained by X-ray CT images for undeformed and deformed states of the body. A displacement field is tentatively represented by B-spline basis functions with unknown coefficients on the overall identification domain. Following the conventional digital image correlation method, the error function of displacement field is defined by the sum of error squares of intensity values on the corresponding material points in the two sets of image data, where the relationship between deformed and undeformed positions of the material point is described by the tentative displacement field. The iterative solution search is employed for modification of the unknown coefficients to identify the actual displacement field. Use is made of the penalty function method with weak constraint of incompressibility to cope with the indefiniteness of solution caused by the intensity flatness and measurement errors of CT images. An efficient parallel algorithm for the solution search is devised by utilizing the locality of B-spline function. The decomposed coefficient matrix for each decomposed domain, derived from the linearized equation of the Levenberg-Marquardt method, is constituted parallel at computational nodes. The modified conjugate gradient method, where the sequence of a vector variable calculation is modified, efficiently solves the linear equation, using the decomposed coefficient matrices. A displacement field within a specimen under compressive load is identified for the validation of the proposed method.

1463 高次デジタル画像相関法による応力急変部のひずみ分布計測(S20-5 応力・ひずみ分布計測法(1),S20 実験力学的手法による材料・製品の評価)

A high order Digital Image Correlation Method (DCM) was developed to measure a strain distribution at notch root with a high accuracy. Only a uniform deformation in a subset was considered in the conventional DCM system. However, the strain distribution near the stress concentrated area, like a notch, was not complex. For the application was incorporated into the conventional DCM system. From the measurement result, the developed DCM system was found to be effective to measure the strain in the vicinity of notch root.