Abstract
High-accuracy crack growth measurement is crucial for the health assessment of concrete structures. In this work, an optical crack growth sensor using the digital sampling moiré (DSM) method is developed for two-dimensional (2D) crack growth monitoring. The DSM method generates moiré fringes from a single image through digital image processing, and it measures 2D displacements using the phase difference of moiré fringes between motion. Compared with the previous sensors using traditional photogrammetric algorithms such as the normalized cross-correlation (NCC) method, this new DSM-based sensor has several advantages: First, it is of a higher sensitivity and lower computational cost; second, it requires no prior calibration to get accurate 2D displacements which can greatly simplify the practical application for multiple crack monitoring. In addition, it is more robust to the change of imaging distance, which is determined by the height difference between two sides of a concrete crack. These advantages break the limitation of the NCC method and broaden the applicability of the crack growth sensor. These advantages have been verified with one numerical simulation and two laboratory tests.
Highlights
Concrete deterioration is usually initiated with the appearance of surface cracks
The mean absolute error (MAE) and standard deviation (SD) of the normalized cross-correlation (NCC) method range from 3 μm to 4 μm, while the corresponding values of the digital sampling moiré (DSM) method are smaller than 0.12 μm
The NCC and DSM algorithms were integrated into the optical crack growth sensor, respectively, to compare with the linear variable differential transformer (LVDT)
Summary
Concrete deterioration is usually initiated with the appearance of surface cracks. Excessive crack propagation may result in possible dysfunction or even failure of concrete structures [1]. Crack growth sensor using the normalized cross-correlation (NCC) method as the image processing algorithm was developed for 2D crack monitoring [16]. Method was developed for high-sensitivity and dynamic displacement measurements [22] In this method, a series of phase-shifted moiré fringes was generated from one image of the captured grating pattern through down-sampling and up-sampling. Results showed that the accuracy of the DSM method could approach 3.8 μm in a three-point bending test of a steel beam, which was equivalent to a 0.01 pixel in the image domain [22] Comparing this with the NCC method adopted in the wireless crack sensor, one advantage of the DSM method is that the obtained results are in the phase domain, and the corresponding physical displacements can be calculated using the predefined pitch length. The development of this sensor is presented, and its performance is tested by the numerical simulation and laboratory tests
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