Development of Fatigue Life Estimation Method of Metal Materials Using Laser Speckle Sensors

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In this study, a Method is investigated to detect change in surface properties of metal materials and to estimate fatigue life during fatigue loading using optical method. Results derived from this study are shown below.(1)We developed an optical setup to observe speckle pattern formed by the reflected light when a He-Ne laser illuminates the object and also a system to input an image of the speckle pattern and analyze the image.(2)When a steel specimen is loaded cyclically with a constant stress amplitude, it was clarified that speckle pattern changes with number of loading cycles. We derived a parameter to evaluate width of the speckle pattern(3)We investigated change of surface roughness and frequency distribution of surface profiles depending on increase of loading cycles. It was clarified that there is a correspondence between width of speckle pattern, surface roughness and width of frequency distribution of surface profiles.(4)We derived an experimental equation, which expresses the relation between a parameter of the speckle pattern and number of loading cycles for different stress amplitudes.(5)We derived a method to estimate fatigue life using the experimental equation and comparison with the experimental data showed that estimation of fatigue life is possible at the early stage of fatigue, such as a quarter of fatigue life.(6)We applied this method to low cycle fatigue. The result showed that speckle pattern broadens as fatigue damage proceeds similar to the case of high cycle fatigue and it was found that this method can be applied to low cycle fatigue.