Sparse Array Ultrasonic Lamb Wave TDTE Imaging Method

The current Lamb wave topology imaging methods are limited to the ideal model of through-hole defects, ignoring the influence of mode coupling. However, blind hole defects, such as structural plate corrosion and cracks, are the most likely defects in practical applications, and Lamb waves are bound to appear mode-coupled at these defects. The use of the full matrix data calculations of the direct and concomitant sound fields is time-consuming and limits the industrial application of the ultrasonic Lamb wave time-domain topological energy (TDTE) imaging method. A TDTE imaging method of a sparse ultrasonic Lamb wave is proposed in this paper to solve the aforementioned two problems. The sparse array is designed based on a genetic algorithm, then the direct sound field and the accompanying sound field are calculated using the sparse array data. The mode conversion of the Lamb wave is considered when calculating the accompanying sound field, and then the accompanying sound field is reversed in time so that the direct sound field and the accompanying sound field are adaptively focused at the defect. The influence of Lamb wave mode conversion and dispersion on the imaging results is overcome. Finally, the TDTE values of the direct sound field and the accompanying sound field are used as the imaging pixel values to improve the imaging accuracy. The experimental results show that for asymmetric blind hole defects, the TDTE imaging method is more accurate than the total focus (TFM) imaging method; the sparse array designed by the genetic algorithm has good sidelobe characteristics and can effectively reduce sidelobe noise assuming the array aperture is unchanged. The TDTE imaging results based on the sparse array data show that the imaging efficiency is greatly improved compared with the full array when the receiving array is four elements under the condition of ensuring the imaging quality.