Nondestructive Testing of Thin Composite Structures for Subsurface Defects Detection Using Dynamic Laser Speckles

A novel nondestructive testing method for subsurface defects detection in thin composite structures using dynamic laser speckles is proposed. In this method, a laminated composite panel containing a subsurface defect is excited by a frequency scanned ultrasonic (US) wave and is illuminated by an expanded laser beam. If one of resonant frequencies of the defect coincides with the US frequency, a local area (a region of interest or ROI) of the panel optically rough surface, placed directly above the defect, begins to vibrate, and the sequences of difference speckle patterns containing the spatial response from the defect are recorded. The formation of this response is caused by both decorrelation and speckle blurring within the local speckle pattern generated by the vibrating ROI at its opposite tilts. The accumulation of difference speckle patterns increases the intensity of the spatial response. This method differs from similar ones in that defects are detected using dynamic speckle patterns of a composite rough surface, illuminated by a single expanded laser beam. The verification of the proposed method was performed using a hybrid optical-digital experimental breadboard to test composite panels containing artificial subsurface defects, as well as a real defect.本文提出了一种利用动态激光散斑（dynamic laser speckles）技术实现薄复合结构亚表面缺陷检测的新型无损检测方法。该方法中，包含亚表面缺陷的层压复合板材由扫频超声（ultrasonic, US）波激励，并经扩束激光照射。当缺陷的某一谐振频率与超声频率相匹配时，位于缺陷正上方的板材光学粗糙表面的局部区域——即感兴趣区域（Region of Interest, ROI）——将开始振动，随后可记录到包含该缺陷空间响应的差分散斑图案序列。该空间响应的形成源于振动ROI在不同倾斜姿态下所生成的局部散斑图案中，同时存在散斑去相关与散斑模糊两种效应。差分散斑图案的累积可有效提升空间响应的强度。本方法与同类检测方法的区别在于，其通过单束扩束激光照射复合粗糙表面所产生的动态散斑图案来完成缺陷检测。为验证所提方法的有效性，研究团队搭建了光-数字混合实验装置，分别对含人工亚表面缺陷的复合板材以及含真实缺陷的复合板材进行了测试。