The moire method has been extensively studied in the past and applied in various engineering applications. Several
techniques are available for generating the moire fringes in these applications, which include moire interferometry,
projection moire, shadow moire, moire deflectometry etc. Most of these methods use the superposition of linear gratings
to generate the moire patterns. The use of non-linear gratings, such as circular, radial and elongated gratings has received
less attention from the research community. The potential of
non-linear gratings in engineering measurement has been
realized in a limited number of applications, such as rotation measurement, measurement of linear displacement,
measurement of expansion coefficients of materials and measurement of strain distribution. In this work, circular
gratings of different pitch were applied to the sensing and measurement of crack displacement in concrete structures.
Gratings of pitch 0.50 mm and 0.55 mm were generated using computer software and attached to two overlapping acrylic
plates that were bonded to either side of the crack. The resulting moire patterns were captured using a standard digital
camera and compared with a set of reference patterns generated using a precision positioning stage. Using several image
pre-processing stages, such as filtering and morphological operations, and pattern matching the magnitude displacements
along two orthogonal axes can be detected with a resolution of 0.05 mm.