Damage Monitoring of Metal Materials by Laser Speckle

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This study investigated a method to detect damage of metals with no contact using the laser speckle technique. This method is based on observation of the change of laser speckle pattern depending on the surface roughness and texture change of the material caused by plastic deformation after loading of materials. The laser speckle pattern was analyzed automatically and quantitatively using an image processing system in this experiment. The Image processing system used in this study is a personal computer based one and the resolution of the image memory is 512X512. The image of the laser speckle pattern was input into the image processing system using a CCD camera. Software to analyze the laser speckle pattern was developed on the personal computer using C language. It is important in this study to find a proper parameter which can express the features of the laser speckle pattern accurately. A new parameter, c_<kappa>, was derived to statistically express the feature of the speckle pattern. It was found from the experimental result that this parameter can express the features of speckle pattern accurately and has excellent reproducibility. Experiment was carried out for both static tensile and tensile fatigue loading using an aluminium alloy as the material. The result for static tensile test shows that there is a fixed relation between c_<kappa> and plastic strain epsilon_<rho> and the relation can be expressed with a simple equation. Therefore, it is possible to accurately estimate the magnitude of plastic strain by the c_<kappa> parameter based on the equation. The change of the c_<kappa> parameter was small for fatigue loading. However, when stress amplitude was rather large, the c_<kappa> parameter changed perceptively with the increase of the stress cycles. It may be concluded that there is a possibility of detecting fatigue damage by observing the change of the laser speckle pattern.