The application of a speckle pattern to the surface of a structure is usually required for Digital Image Correlation (DIC)-based displacement identification. A speckle pattern generally requires the modification (e.g., painting) of the structure's surface. An improved method of laser-light speckle formation for DIC measurement of relatively large structures is proposed, where the speckle pattern is formed by laser-light interference as a consequence of the surface roughness and the laser illumination, leaving the surface of the object untouched. The interference phenomena caused by the light reflecting from the rough surface create bright spots that move with the observed structure. The size of the speckles depends on the surface roughness, the experimental setup and the optical system. The proposed method is researched using three experimental cases: two rigid-body motion measurements and a full-field operating-deflection-shape measurement. The experiments show that motion can be identified even when the amplitudes of the oscillation are as low as 0.1 µm (on a scale of 1/1000 of a pixel). While the laser speckle accuracy is promising, it was found to be less accurate than painted speckle-pattern measurements, but with further development it could become a viable alternative.
Ladisk, Faculty of Mechanical Engineering, University of Ljubljana
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