Floquet wave theory-based time-corrected ultrasonic total focusing method for fiber-reinforced composite laminate

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.