Application of synthetic aperture focusing technique to defect quantification for immersion ultrasonic testing

Among the common defect quantification methods for immersion ultrasonic testing, some are high cost while others have relatively large quantitative error. Given the problem, a new 6 dB drop defect quantification method based on frequency domain synthetic aperture focusing technique (FSAFT-DQM) was studied. The FSAFT imaging method for spherically focused transducer used by immersion ultrasonic testing was established. Experiments conducted on specimens containing either circular cylindrical cavities or flat bottom hole show that defect depth has little effect on FSAFT-DQM's accuracy; compared with 6 dB drop method, flat bottom hole's quantitative error of FSAFT-DQM drops from 71.9% to within 10.0% with step length below half-wavelength of transducer's center frequency and the error decreases from 17.2% to 5.6% by image interpolation algorithm with step length between half-wavelength and wavelength. The research results show that FSAFT-DQM improves the precision of defect quantification effectively.