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

在这项研究中，研究了一种利用光学方法检测金属材料在疲劳加载过程中表面性质变化和估算疲劳寿命的方法。本研究的结果如下：(1)我们研制了一种光学装置来观察He-Ne激光照射物体时反射光形成的散斑图案，并建立了一套系统来输入散斑图案的图像并对图像进行分析。(2)当钢材试件以恒定的应力幅度循环加载时，散斑图案随加载循环次数的变化而变化。推导了散斑宽度的参数。(3)研究了表面粗糙度和表面轮廓的频率分布随加载周期的变化。阐明了散斑宽度、表面粗糙度和表面轮廓频率分布宽度之间的对应关系。(4)推导了不同应力幅值下散斑参数与加载循环次数之间的经验公式。(5)推导了一种利用实验公式估算疲劳寿命的方法，并与实验数据进行了比较，结果表明，在疲劳早期阶段，如疲劳寿命的四分之一，估算疲劳寿命是可能的。(6)将该方法应用于低周疲劳。结果表明，随着疲劳损伤的进行，散斑图像变宽，这与高周疲劳的情况相似，表明该方法适用于低周疲劳。

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.