Laser Ultrasonic High Precision Imaging Method for Internal Defects of Small-Diameter Cylindrical Components

Abstract

An improved frequency-domain synthetic aperture focusing technique (F-SAFT) for laser ultrasonic testing (LUT) is proposed for internal defect detection of small-diameter cylindrical components. Firstly, a LUT automated detection platform is built, a pulsed laser is used to excite ultrasonic waves and a two-wave mixing (TWM) interferometer is used to detect ultrasonic waves. Since ultrasonic signals are affected by the thermal expansion of the pulsed laser, time-frequency analysis is used to obtain the frequency range for imaging longitudinal waves, and the influence of low-frequency clutter is eliminated through multiple filtering. Secondly, in order to balance signal acquisition efficiency and imaging quality, the peak signal-to-noise ratio (PSNR) is used to determine the optimal angular step size. Finally, the equivalent velocity of the longitudinal wave is corrected to compensate for the imaging position error caused by the separation of the ultrasonic excitation point and the detection point. The results show that the method proposed in this paper has high imaging accuracy, which could provide a new approach for in-service non-destructive testing of small-diameter cylindrical components.