Nonlinear Frequency Modulation Tfm with Second-Order Tgv and Butterworth Filter for Detection of Cfrp Composites

Abstract

In this article, a nonlinear frequency modulation (NLFM) waveform with a concave quadratic frequency function is provided, which combines the phased array ultrasonic total focusing method (TFM) to increase signal noise ratio (SNR) and detection depth of carbon fiber reinforced polymer (CFRP) composites. The second-order total generalized variation (TGV) and Butterworth high pass filters are introduced to denoise and enhance defect features in the strong background noise in CFRP composites. Experimental researches are conducted to compare the TFM imaging performance with single-pulse excitation (Pulse TFM), linear frequency modulation excitation (LFM TFM), and nonlinear frequency modulation excitation (NLFM TFM). The results show that NLFM TFM has the optimal SNR and depth resolution. With second-order TGV and Butterworth algorithm, the defect information submerged in the background noise is highlighted, and the clarity and SNR of the detected image are enhanced.

Keywords: Phased array ultrasonic, TFM, Coded excitation, CFRP, TGV, Image denoising