Girolamo Donato, Chang Huan-Yu, Yuan Fuh-Gwo
National Institute of Aerospace, Integrated System Health Management Laboratory, Hampton, VA 23666, USA; North Carolina State University, Department of Mechanical and Aerospace Engineering, Raleigh, NC 27695, USA.
National Institute of Aerospace, Integrated System Health Management Laboratory, Hampton, VA 23666, USA; North Carolina State University, Department of Mechanical and Aerospace Engineering, Raleigh, NC 27695, USA.
Ultrasonics. 2018 Jul;87:152-165. doi: 10.1016/j.ultras.2018.02.014. Epub 2018 Feb 16.
A fully non-contact laser-based nondestructive inspection (NDI) system is developed to detect and visualize damage in structures. The study focuses on the size quantification and characterization of a barely visible impact damage (BVID) in a honeycomb composite panel. The hardware consists of a Q-switched Nd:YAG pulse laser that probes the panel by generating broadband guided waves via thermo-elastic expansion. The laser, in combination with a set of galvano-mirrors is used to raster scan over a two-dimensional surface covering the damaged region of an impacted quasi-isotropic [60/0/-60] honeycomb composite panel. The out-of-plane velocities are measured at a fixed location normal to the surface by a laser Doppler vibrometer (LDV). An ultrasonic full wavefield assembled from the three-dimensional space-time data matrix in the interrogated area is first acquired and then processed for imaging the impacted damage area. A wavenumber filtering technique in terms of wave vectors is applied to distinguish the forward and backward wavefields in the wavenumber-frequency domain. A zero-lag cross correlation (ZLCC) imaging condition is then employed in the space-frequency domain for damage imaging. The ZLCC imaging condition consists of cross correlating the incident and reflected wavefields in the entire scanned region. The condition not only images the damage boundary between incident and reflected waves outside the damage region but also, for longer time windows, enables to capture the momentary standing waves formed within the damaged region. The ZLCC imaging condition imaged two delaminated region: a main delamination, which was a skewed elliptic with major and minor axis lengths roughly 17 mm and 10 mm respectively, and a secondary delamination region approximately 6 mm by 4 mm, however, which can only be shown at higher frequency range around 80-95 kHz. To conclude, the ZLCC results were in very good agreement with ultrasonic C-scan and X-ray computed tomographic (X-ray CT) scan results. Since the imaging condition is performed in the space-frequency domain, the imaging from ZLCC can also reveal resonance modes which are shown in the main delaminated area by windowing a narrow frequency band sequentially.
开发了一种基于激光的全非接触式无损检测（NDI）系统，用于检测结构中的损伤并进行可视化。该研究重点关注蜂窝复合板中难以察觉的冲击损伤（BVID）的尺寸量化和特征描述。硬件包括一台调Q Nd:YAG脉冲激光器，通过热弹性膨胀产生宽带导波来探测面板。该激光器与一组振镜配合使用，在二维表面上进行光栅扫描，覆盖受冲击的准各向同性[60/0/-60]蜂窝复合板的损伤区域。通过激光多普勒振动计（LDV）在垂直于表面的固定位置测量面外速度。首先获取从被询问区域的三维时空数据矩阵组装而成的超声全波场，然后对其进行处理以对受冲击损伤区域进行成像。应用基于波矢量的波数滤波技术在波数-频率域中区分向前和向后的波场。然后在空间-频率域中采用零延迟互相关（ZLCC）成像条件进行损伤成像。ZLCC成像条件包括在整个扫描区域内对入射波场和反射波场进行互相关。该条件不仅能对损伤区域外部的入射波和反射波之间的损伤边界进行成像，而且对于较长的时间窗口，还能捕捉在损伤区域内形成的瞬时驻波。ZLCC成像条件对两个分层区域进行了成像：一个主要分层区域，呈倾斜椭圆形，长轴和短轴长度分别约为17毫米和10毫米；另一个次要分层区域约为6毫米×4毫米，不过该区域仅在80 - 95千赫兹左右的较高频率范围内才能显示。总之，ZLCC结果与超声C扫描和X射线计算机断层扫描（X射线CT）结果非常吻合。由于成像条件是在空间-频率域中执行的，通过依次对窄频带进行加窗处理，ZLCC成像还可以揭示主要分层区域中出现的共振模式。
