Fiber reinforced polymers (FRPs) are increasingly used in thick primary load-bearing structures. Nevertheless, manufacturing and in-service defects occur with a higher chance as the FRP thickness increases and thus the potential structure defects should be detected and evaluated. To image defects in thick FRP over 10 mm thick, this study proposes a frequency-and-structure dependent time-corrected ultrasonic total focusing method (FS-TFM) based on Floquet wave theory, which differs from conventional TFM that the wave velocity correction along different propagation direction bases on not only the structural anisotropy and inhomogeneity of FRP, but also on the probing frequency. First an analytical Floquet-wave-based dynamic homogenization for a crossply FRP laminate is performed, to obtain the homogenization region and then the wave anisotropy and dispersion therein. Then numerical finite element analysis follows, to further interrogate the time domain feature of wave signals. With the understanding of wave propagation, by accurately correcting the wave velocity anisotropy related to both the inspected crossply FPR and wave frequency, the FS-TFM imaging technique is proposed to focus the defect-scattered wave energy on the defect location. In addition, the signal-to-noise ratio (SNR) analysis of side-drilled-hole (Ø2 mm) imaging via different imaging algorithms shows that the proposed FS-TFM achieves the highest SNR at the frequency approaching the upper limit of the homogenization region. Finally, the experimental validation further indicates the potential of the proposed FS-TFM for accurate defect imaging in thick FRP.

纤维增强聚合物 (FRP) 越来越多地用于厚的主要承重结构。然而，随着 FRP 厚度的增加，制造和使用中缺陷发生的可能性更高，因此应检测和评估潜在的结构缺陷。针对厚度超过10 mm的厚FRP缺陷成像，本研究提出了一种基于Floquet波理论的频率和结构相关的时间校正超声全聚焦方法（FS-TFM），该方法与传统TFM不同，波速校正沿不同的传播方向不仅取决于FRP的结构各向异性和不均匀性，还取决于探测频率。首先对正交 FRP 层压板进行基于 Floquet 波的分析动态均质化，以获得均质化区域，然后获得其中的波各向异性和色散。然后进行数值有限元分析，以进一步询问波浪信号的时域特征。基于对波传播的理解，通过精确校正与所检查的交叉层FPR和波频率相关的波速各向异性，提出了FS-TFM成像技术，将缺陷散射的波能量聚焦在缺陷位置。此外，通过不同成像算法对侧钻孔（Ø2 mm）成像的信噪比（SNR）分析表明，所提出的FS-TFM在接近均匀化上限的频率处实现了最高SNR地区。最后，实验验证进一步表明了所提出的 FS-TFM 在厚 FRP 中精确缺陷成像的潜力。