Lamb wave optimized total focusing imaging for defects in thin plates

The traditional total focusing method (TFM) mainly relies on the amplitude information of the signals for delay and sum (DAS). In practical applications, signals may not always meet the prerequisite of coherent superposition However, the superposition of incoherent signals can lead to the generation of artifacts in result images. To solve this issue, an optimized total focusing imaging method (CCF-DMAS) with delay multiply and sum (DMAS) weighted by circular coherence factor (CCF) is proposed in this paper, which could realize Lamb wave total focusing imaging of defects in thin plates. Considered with the spatial coherence between the receiving array elements, the received signals is multiplied and coupled in this method. Meanwhile, the phased weighted factor is calculated based on phase information in the data to expand the differences between coherent and incoherent signals, which can narrow the main lobe, reduce sidelobes, and improve imaging resolution. In this study, an ultrasonic array data acquisition experimental system is established to excite S₀ mode Lamb wave on a zirconium alloy thin plate with through-hole defects and capture full matrix data. The new frequency components are generated by phase weighting the collected data using proposed CCF-DMAS. Then the second harmonic component is retained by bandpass filtering for total focusing imaging. The experimental results show that CCF-DMAS can effectively suppress noise and artifacts, with a signal-to-noise ratio improvement of about 39% and 22%, and an array performance index (API) improvement of about 86% and 69% compared to DAS and DMAS. It provides an effective improvement scheme for the post-processing of non-destructive testing on thin plate.