Damage Monitoring of Metal Materials by Laser Speckle

研究了一种利用激光散斑技术非接触检测金属损伤的方法。该方法是基于观察激光散斑图案的变化，这取决于材料加载后塑性变形引起的材料表面粗糙度和纹理变化。利用图像处理系统对激光散斑图进行了自动定量分析。本研究所用的图像处理系统是基于个人计算机的系统，图像存储器的分辨率为512 × 512。将激光散斑图案的图像输入到使用CCD照相机的图像处理系统中。用C语言在微机上开发了激光散斑图分析软件。在这项研究中，重要的是找到一个合适的参数，可以准确地表达的激光散斑图案的特征。导出了一个新的参数c_<kappa>，用以统计地表示散斑图的特征。实验结果表明，该参数能较准确地反映散斑图的特征，并具有良好的再现性。以铝合金为材料，进行了静态拉伸和拉伸疲劳试验。静拉伸试验结果表明，c_与塑性应变ε_之间存在固定的关系<kappa><rho>，且可用一个简单的方程表示。因此，可以通过基于该方程的c_参数来精确地估计塑性应变的大小<kappa>。在<kappa>疲劳载荷作用下，c_参数变化较小。当应力幅较大时，c参数<kappa>随应力循环次数的增加而明显变化。通过观察激光散斑图的变化，有可能检测疲劳损伤。

This study investigated a method to detect damage of metals with no contact using the laser speckle technique. This method is based on observation of the change of laser speckle pattern depending on the surface roughness and texture change of the material caused by plastic deformation after loading of materials. The laser speckle pattern was analyzed automatically and quantitatively using an image processing system in this experiment. The Image processing system used in this study is a personal computer based one and the resolution of the image memory is 512X512. The image of the laser speckle pattern was input into the image processing system using a CCD camera. Software to analyze the laser speckle pattern was developed on the personal computer using C language. It is important in this study to find a proper parameter which can express the features of the laser speckle pattern accurately. A new parameter, c_<kappa>, was derived to statistically express the feature of the speckle pattern. It was found from the experimental result that this parameter can express the features of speckle pattern accurately and has excellent reproducibility. Experiment was carried out for both static tensile and tensile fatigue loading using an aluminium alloy as the material. The result for static tensile test shows that there is a fixed relation between c_<kappa> and plastic strain epsilon_<rho> and the relation can be expressed with a simple equation. Therefore, it is possible to accurately estimate the magnitude of plastic strain by the c_<kappa> parameter based on the equation. The change of the c_<kappa> parameter was small for fatigue loading. However, when stress amplitude was rather large, the c_<kappa> parameter changed perceptively with the increase of the stress cycles. It may be concluded that there is a possibility of detecting fatigue damage by observing the change of the laser speckle pattern.