Defect detection in near-surface blind zone using Green’s function reconstruction of diffuse field sector scanning

Aiming at the problem of defect detection in the near-surface blind zone of structure,the detection method for reconstruction of Green's function by sector scanning of diffuse field was developed.Based on the theoretical analysis of the reconstruction Green's function of diffuse field sector scanning,a 5 MHz ultrasonic phased array probe is utilized to test the near-surface defects of an aluminum block and the delay time,time window length and scanning method that affects the imaging results were analyzed.The result shows that this method can effectively detect defects from 1.5 mm to 3 mm in the near-surface blind zone of the aluminum block.The effective diffusion time of the sector scanning mode is longer than the full matrix capture mode by more than 0.5 ms.In particularly,within the effective diffusion time,as the delayed reception time and the time window length increase,the imaging quality is improved.This research work provides an effective and feasible new method for ultrasonic detection of defects in the near-surface blind zone.