Damage Monitoring of Metal Materials by Laser Speckle Assisted by Image Processing Techniques : Relationship between Distribution of Laser Speckle and Surface Properties

Abstract

In this study, we investigated a noncontact method of measuring plastic strain of metals using laser speckle. This method is based on observation of the change in laser speckle pattern with surface profile change due to plastic deformation. Analysis of the laser speckle pattern is performed on a computer-based image processing system. The relationships between speckle pattern and surface roughness and frequency characteristics of the surface profile were investigated. Surface roughness and surface profile were observed for steel specimens polished with emery paper or plastically deformed. The surface profile diagram was analyzed by means of the fast Fourier transform with a computer and distribution of the spatial frequencies was obtained. The results showed that distribution of light intensity of the speckle is related to frequency distribution of the surface profile, but it is not always related to surface roughness. It was also clarified that the laser speckle pattern is closely related to the magnitude of plastic strain.