Development of Evaluation Method for Fatigue Damage in Metal Materials Using Laser Speckle

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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 investigated change of surface properties depending on increase of loading cycles under constant stress amplitude. We considered mathematical mean roughness and Fractal dimension and spatial frequency distribution of surface profiles as surface properties. Relation between these parameters and number of loading cycles were investigated. It was clarified that these surface parameters changes depending on number of loading cycles and there is a correspondence between surface properties and fatigue damage.(2)Relation between width of laser speckle pattern and number of loading cycles was investigated. Among surface properties and width of speckle pattern, spatial frequency distribution is related the most to width of speckle pattern. Width of speckle pattern has the best correlation with number of loading cycles and it was concluded that width of the speckle pattern is the most appropriate as a parameter to evaluate fatigue damage.(3)We developed an optical setup to observe speckle pattern formed by the reflected light when a He-Ne laser illuminates the object and observe speckle pattern on site during working of the testing machine.(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. We also construct the database of relations between a parameter of the speckle pattern and stress amplitude and relation between change rate of the speckle pattern and fatigue life. We devised a method to estimate fatigue life during working object based on the experimental equation and the database. Fatigue life estimation was tried using this method and the result showed that this method has a possibility for practical use.