We investigated a method to evaluate fatigue damage of steels without contact using laser speckle. In the earlier stage of fatigue in steels, slipbands appear on the surface and microscopic phase strain is stored in the slipbands. The slipbands appear more densely with progress of fatigue damage. When a laser illuminates surface of the fatigued steel, light intensity distribution of the laser speckle pattern formed by the reflected light changes with the change of surface properties caused by slipbands. It has been clarified that width of the speckle pattern broadens corresponding to spatial frequency distribution of the surface profile and thus it is presumed that speckle pattern broadness with increase of slipband density. This shows that we can detect fatigue damage by observing the laser speckle pattern broadens with increase of slipband density. This shows that we can detect fatigue damage by observing the laser speckle pattern on material surface. The method presented in this paper is based on this phenomenon. We observed change of surface property and the speckle pattern during fatigue loading under constant stress amplitude using steel specimens. Surface roughness and fractal analysis of the surface profile diagrams were obtained to evaluate surface property. Change of surface roughness and fractal dimension of the surface profile were compared to change of laser speckle pattern depending on progress of fatigue damage and relation between surface property and speckle pattern was investigated. We investigated possibility of evaluation of fatigue damage observing laser speckle pattern during fatigue.
This paper presents a new method to measure cyclic strain with no contact using metal foil gauges and laser speckle sensor. When a metal foil (such as aluminum, copper, nickel etc.) is pasted on a specimen and the specimen is loaded cyclically, slipbands are produced on the foil surface by fatigue. The occurrence of the slipbands is dominated by the strain amplitude of the specimen surface and number of the loading cycles. Thus the fatigue strain of the base metal can be estimated by observing the surface change of the foil. The method in this study is based on observation of the change of laser speckle pattern depending on the surface property change of the foil caused by fatigue. This method is intended to make a non-contact strain measurement by the application of the laser speckle technique for the detection of the surface change.
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