Inspection of adhesively bonded metallic plates, commonly used in aircraft structures, remains challenging for modern non-destructive testing (NDT) techniques. When a probing ultrasound (US) wave interacts with the plate boundaries, it produces multiple propagating guided waves (or Lamb modes). Analysis of these waves can be complicated due to a strong geometrical dispersion. However, recent studies showed that for specific frequencies zero-group velocity (ZGV) modes exist. Any changes in a bounded structure, like delaminations, drastically alter the conditions for zero-group velocity waves, thus making this method highly sensitive for NDT applications. Laser ultrasound (LU) provides a very broad bandwidth of the generated waves, thus it is a feasible tool for a spectroscopy-based investigation of the ZGV modes. In this paper, we demonstrate an application of high-repetition rate LU with a non-contact fiber-optic Sagnac interferometer on receive for high resolution imaging of delaminations in a structure consisting of 3 epoxy-bonded aluminum plates. The investigation is supported by numerical analysis of Lamb waves existing in the structure to determine ZGV modes sensitive to delaminations at particular bonding interfaces. Tracking the selected modes permits imaging and identification of defects. We also show that mean frequency estimation of the ZGV modes can improve the contrast-to-noise ratio compared to an amplitude of the single ZGV frequency.

对于现代无损检测（NDT）技术而言，通常用于飞机结构的粘合金属板的检查仍然具有挑战性。探测超声（US）波与板边界相互作用时，会产生多个传播的导波（或Lamb模式）。由于强烈的几何色散，对这些波的分析可能会很复杂。但是，最近的研究表明，对于特定频率，存在零组速度（ZGV）模式。有界结构中的任何变化（如分层）都会极大地改变零速波的条件，因此使该方法对NDT应用非常敏感。激光超声（LU）提供了非常宽的生成波带宽，因此它是用于基于光谱的ZGV模式研究的可行工具。在本文中，我们展示了非重复光纤Sagnac干涉仪在接收器上的高重复率LU的应用，该接收器用于由3个环氧粘合铝板组成的结构中的分层的高分辨率成像。该研究得到结构中存在的兰姆波的数值分析的支持，以确定对特定键合界面处的分层敏感的ZGV模式。跟踪所选模式可以对缺陷进行成像和识别。我们还显示，与单个ZGV频率的幅度相比，ZGV模式的平均频率估计可以提高对比度-噪声比。该研究得到结构中存在的兰姆波的数值分析的支持，以确定对特定键合界面处的分层敏感的ZGV模式。跟踪所选模式可以对缺陷进行成像和识别。我们还显示，与单个ZGV频率的幅度相比，ZGV模式的平均频率估计可以提高对比度-噪声比。该研究得到结构中存在的兰姆波的数值分析的支持，以确定对特定键合界面处的分层敏感的ZGV模式。跟踪所选模式可以对缺陷进行成像和识别。我们还显示，与单个ZGV频率的幅度相比，ZGV模式的平均频率估计可以提高对比度-噪声比。