Synthetic Aperture Focusing Imaging In Ultrasonic Immersion Testing

Synthetic aperture focused technique(SAFT)can obtain high-resolution images using a transducer with small aperture and low working frequency,but is mainly applied in contact-coupled testing.Traditional time-domain SAFT is hard to be widely applied in practice because of a large amount of convolution calculation and complex iterative calculation,which will lead to a low imaging efficiency.Then the frequency-domain SAFT was proposed with high imaging efficiency,but in which the traditional phase shift calculation in frequency domain cannot optimize the defect image obviously and is easily affected by the irregular stratification of the specimen,the imaging resolution needs to be further improved.Focusing on the above-mentioned problems,a multi-angle spatial compound technique is introduced to suppress the speckle noise and artifacts in time-domain SAFT imaging;the chirp-z transform interpolation method is introduced to improve the imaging quality of the frequency-domain SAFT imaging.The main highlights of this paper are as follows:In order to study the propagation characteristics of acoustic beam in immersion testing and optimize the parameter setting in defect detection,the Multi-Gaussian beam model is built to simulate the ultrasonic propagation and sound field in immersion testing,and an ultrasonic measurement model is established to obtain the echo signals.Then the traditional SAFT in the time domain and frequency domain are introduced and used for imaging in the obtained echo signals.The simulation makes a foundation for studying an improving SAFT and analyzing the propagation characteristics of ultrasonic signals in actual testing.Aiming at the speckle noise and artifacts in time-domain synthetic aperture focusing imaging in immersion testing,a time-domain SAFT combined with the multi-angle spatial compound method is proposed.The acquired multi-angle echo signals are enveloped by Hilbert transform,and the deviation of the Time of Flight(TOF)and refraction path caused by the acoustic difference between water and the specimen is corrected.Then the corrected signals are superimposed to reconstruct a multi-angle spatial compound image,this method is referred as angular-space domain SAFT in immersion testing.The phased-array B-type test block is chosen for ultrasonic immersion testing,and the contrastive analysis of the imaging results in traditional time-domain SAFT,angular-space domain SAFT without deviation correction and angular-space domain SAFT are carried out.The results show that the proposed method can effectively suppress the speckle noise and artifacts in image.Aiming at the poor image accuracy in frequency-domain SAFT in immersion testing,a frequency-domain SAFT combined with the chirp-z transformation interpolation algorithm is proposed.Based on the frequency-domain SAFT with phase shift migration in multi-layer medium,after the chirp-z transform interpolation in each kx-domain,the algorithm is deduced.The imaging accuracy,Signal to Noise Ratio(SNR),Contrast to Noise Ratio(CNR)and imaging efficiency of the frequency-domain SAFT with chirp-z transform interpolation,phase shift migration,and linear interpolation are discussed through experiments.The results show that the proposed method can improve the imaging accuracy,SNR and CNR with a good imaging efficiency.In summary,according to the study of the propagation characteristics of acoustic beam in immersion testing and SAFT in time-domain and frequency-domain,the proposed time-domain SAFT combined with the multi-angle spatial compound method and frequency-domain SAFT combined with the chirp-z transformation interpolation algorithm can effectively improve the imaging quality.