Simultaneous Measurement Of 3-D Displacement Components Caused By Crack Growth Using Circular Gratings Moire Fringes And Graphical Analysis

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Date
2012-05
Authors
Yen, Kin Sam
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Universiti Sains Malaysia
Abstract
Cracking is well-known to be a major cause of failures in concrete structures. By monitoring the growth of cracks, early remedial measure can be taken to save the structure from failure. To sense all the three crack modes namely, opening mode, sliding mode and tearing mode, a method that is able to measure the 3-D displacement components simultaneously with a single setting is needed. The moiré pattern formed by overlapping two circular gratings with slightly different pitches is proposed as a measurement tool because the moiré pattern is repeatable and unique for a relative in-plane (2-D) displacement between the two gratings. The wave properties, i.e. the amplitude and phase shift extracted from the transformed moiré fringe in the polar plane directly corresponds to the eccentricity magnitude and direction of the two gratings. Thus, graphical analysis method is developed to extract the 2-D displacement information from the circular gratings moiré pattern. In the presence of a gap between the two grating planes, different moiré patterns can be observed from the normal and off-normal viewing directions with the grating plane. The absolute difference of the 2-D displacement represented by the two moiré patterns captured from different viewing directions is related to the out-of-plane displacement between the two gratings. Based on the developed graphical analysis method, an out-of-plane displacement measurement method is established for the simultaneous 3-D displacement components determination.
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Simultaneous measurement of 3-D displacement components , caused by crack growth using circular gratings moire fringes
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