Advanced Orthogonal Frequency and Phase Modulated Waveform for Ultrasonic Phased Array TFM Detection in CFRP Composites

Phased array ultrasonic total focusing method (TFM) imaging has attracted much attention due to its high detection image resolution and large dynamic range. However, due to the small emission energy of its single element, the echo signal is weak and susceptible to noise interference, resulting in a relatively low signal-to-noise ratio (SNR). Especially when detecting high sound attenuation carbon fiber reinforced polymer (CFRP) composites, the signal amplitude can decrease with depth to a level lower than the electronic noise introduced by the signal acquisition system. To solve this problem, a new imaging algorithm was proposed that combines the Golay code excitation using linear frequency modulation (LFM) signal as the carrier with the TFM algorithm (LFM-Golay TFM). Meanwhile, in order to further overcome the disadvantage of the LFM-Golay code encoding excitation method requiring two excitations, an AB code encoding excitation conversion factor was introduced, two sets of echo data were obtained through processing single excitation, and TFM imaging based on LFM-Golay code single excitation (IMP-LFM-Golay TFM) is realized. Simulation and experimental studies were conducted to compare the TFM imaging performance of single-pulse excitation (Pulse TFM), Barker coding excitation (Barker TFM), LFM excitation (LFM TFM), and improved LFM-Golay coding excitation. The results show that IMP-LFM-Golay TFM has the best SNR, axial resolution, and depth resolution.

I. Introduction

Carbon fiber reinforced polymer (CFRP) composites have the excellent merits of lightweight, high strength, high specific stiffness, corrosion resistance, and adjustable anisotropy [1], [2], [3], [4], [5]. Thanks to these advantages, they are gradually replacing traditional materials in many fields, such as aerospace, automotive structure, and marine engineering [6], [7], [8], [9], [10]. However, CFRP are more sensitive to impact loads when they are fixed in their transportation, assembly, and actual use processes. The material can withstand various additional loads and alternating stresses for a long time, and different types of defects can be produced inside the material. These defects greatly reduce the mechanical properties of the material, easily leading to the failure of composite components during service, and posing a huge threat to the overall reliability and safety of the structure. Therefore, it is imperative to develop efficient and reliable nondestructive test and evaluation (NDT&E) methods for CFRP composites.