High-accuracy crack growth monitoring is crucial for the health assessment of concrete structures, and different kinds of crack growth sensors have been developed and applied in in-service structures. Those sensors can only measure one dimensional displacement while cracks might propagate in multiple directions. For two-dimensional (2D) crack growth monitoring, image-based sensor using traditional photogrammetric algorithms such as the normalized cross-correlation (NCC) have been developed. The image-based sensor requires prior calibration and is limited to even cracks whose two sides are of the same height. To broaden the scope of application to uneven cracks, this study developed crack growth sensors using the moiré technique, which is less sensitive to the height change of tw...[ Read more ]

High-accuracy crack growth monitoring is crucial for the health assessment of concrete structures, and different kinds of crack growth sensors have been developed and applied in in-service structures. Those sensors can only measure one dimensional displacement while cracks might propagate in multiple directions. For two-dimensional (2D) crack growth monitoring, image-based sensor using traditional photogrammetric algorithms such as the normalized cross-correlation (NCC) have been developed. The image-based sensor requires prior calibration and is limited to even cracks whose two sides are of the same height. To broaden the scope of application to uneven cracks, this study developed crack growth sensors using the moiré technique, which is less sensitive to the height change of two sides of cracks.

In the first generation of developed sensor, the digital sampling moiré (DSM) method is employed due to its simple setup and high accuracy. 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 sensor using the normalized cross-correlation (NCC) method, this DSM-based crack sensor is of higher sensitivity, requires no prior calibration, and is more robust to the change of height difference between two sides of cracks. However, this sensor is incapable to measure cracks’ in-plane rotation, which can be observed in the bending structures and the corrosion of the reinforcement concrete. To simultaneously measure the in-plane rotation and 2D translation of cracks, an enhanced-DSM (EDSM) method was proposed by integrating the 2D rigid transformation algorithm with the DSM method. This newly developed algorithm was built in the sensor and applied to do 3-DOF crack monitoring in a three-point bending test and a reinforcement concrete corrosion test.