Abstract
Digital image correlation (DIC) is an efficient nondestructive technique for measuring surface displacement in engineering. However, standard DIC is restricted to continuous deformation, and the existing discontinuous DIC (DDIC) techniques are only able to measure small-scale cracks. In this report, a novel subset restore model and a corresponding subset size adaptation algorithm are presented to overcome this limitation for crack-state and displacement field reconstruction for large-scale cracks. The technique introduces a new subset restore method for splicing the segmented subset by tracing the motion trajectory caused by pure discontinuities. The proposed model facilitates the calculation of the rotation angle and the pivot of the subset movement. The subset size adaptation algorithm is designed based on an evaluation of the intensity gradient and correlation coefficient to allow the model to achieve high accuracy. Validation of the approach was performed using two typical crack models, by deforming a numerically synthesized Gaussian speckle image according to the deformation data from finite element analysis (FEA) results and photographing a laboratory tensile test with a high-speed CCD camera, respectively. The results validate the efficacy and high accuracy of the proposed approach compared to standard DIC in the reconstruction of the displacement fields in both continuous and discontinuous regions. The accuracy of resultant displacement reconstruction achieves approximately 0.015 pixels and 0.05 pixels in continuous region and crack vicinity, respectively.
Highlights
Cracks are among the most common defects in engineered structures. Numerous nondestructive techniques, such as eddy current testing (ECT) [1], ultrasonic flaw detection [2], infrared thermal wave method [3], electromagnetic detection [4], and crack image recognition [5,6,7], have been widely used to detect and identify cracks in structures. e accurate measurement and reconstruction of surface displacement, deformation, and crack status are the key to the acquisition of qualitative and quantitative information to better understand the mechanism of load-resistance and the failure modes of cracked structure
Rethoreet al. [14, 15] utilized the extended finite element method (XFEM) [16] and proposed an extended digital image correction (DIC) method by “enriching” the shape function of the elements to consider the presence of a crack. e image is meshed with elements similar to the finite element method, and the shape function is enriched to yield nodal displacements. is technique is fundamentally different from the standard DIC
Pan [20] suggested the use of a reliability guided (RG) DIC method to ensure that the calculation path was always along the points with the highest zero-mean normalized cross-correlation (ZNCC) coefficient and to avoid calculations for discontinuous areas
Summary
Cracks are among the most common defects in engineered structures. Numerous nondestructive techniques, such as eddy current testing (ECT) [1], ultrasonic flaw detection [2], infrared thermal wave method [3], electromagnetic detection [4], and crack image recognition [5,6,7], have been widely used to detect and identify cracks in structures. e accurate measurement and reconstruction of surface displacement, deformation, and crack status are the key to the acquisition of qualitative and quantitative information to better understand the mechanism of load-resistance and the failure modes of cracked structure. The standard DIC method is based on the hypothesis that the pending surface is continuous everywhere, which will lead to serious errors in the vicinity of cracks. Pan [20] suggested the use of a reliability guided (RG) DIC method to ensure that the calculation path was always along the points with the highest zero-mean normalized cross-correlation (ZNCC) coefficient and to avoid calculations for discontinuous areas.
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