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

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。研究结果如下：(1)研究了恒应力幅值下，随着加载循环次数的增加，材料表面性质的变化。我们将表面轮廓的数学平均粗糙度、分维和空间频率分布作为表面属性。研究了这些参数与加载循环次数的关系。这些表面参数随加载循环次数的变化而变化，表面性质与疲劳损伤之间存在对应关系。(2)研究了激光散斑宽度与加载循环次数之间的关系。在散斑表面特性和散斑宽度中，空间频率分布与散斑宽度的相关性最大。散斑宽度与加载循环次数相关性最好，是评价疲劳损伤最合适的参数。(3)研制了一套光学装置，用于观察He-Ne激光照射物体时反射光形成的散斑图像，并在试验机工作过程中现场观察散斑图像。(4)推导了不同应力幅值下散斑图像参数与加载循环次数之间的经验公式。建立了散斑图像参数与应力幅值之间的关系数据库以及散斑图像变化率与疲劳寿命之间的关系数据库。基于实验方程和数据库，设计了一种估算工作对象疲劳寿命的方法。用该方法进行了疲劳寿命估算试验，结果表明该方法具有实际应用的可能性。

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.