DAMAGE MONITORING OF METAL MATERIALS BY LASER SPECKLE ASSISTED BY IMAGE-PROCESSING TECHNIQUES - (RELATIONSHIP BETWEEN DISTRIBUTION OF LASERSPECKLE AND SURFACE-PROPERTIES)

In this study, we investigated a noncontact method of measuring plasti c strain of metals using laser speckle. This method is based on observ ation of the change in laser speckle pattern with surface profile chan ge due to plastic deformation. Analysis of the laser speckle pattern i s performed on a computer-based image processing system. The relations hips between speckle pattern and surface roughness and frequency chara cteristics of the surface profile were investigated. Surface roughness and surface profile were observed for steel specimens polished with e mery paper or plastically deformed. The surface profile diagram was an alyzed by means of the fast Fourier transform with a computer and dist ribution of the spatial frequencies was obtained. The results showed t hat distribution of light intensity of the speckle is related to frequ ency distribution of the surface profile, but it is not always related to surface roughness. It was also clarified that the laser speckle pa ttern is closely related to the magnitude of plastic strain.