Zero-frequency wave based-SAFT imaging for high-density polyethylene with irregular surfaces

When an ultrasound tone-burst propagates in an elastic solid with quadratic nonlinearities, a pulse wave with a carrier frequency of zero (referred to as zero-frequency wave, ZFW) will be generated due to material nonlinearity. The low acoustic attenuation characteristics of ZFW is particularly well-suited for defect detection of highly attenuative materials. In our previous study, large detection depths in high-density polyethylene (HDPE) have been demonstrated using ZFW. However, in practical engineering applications, defect detection in HDPE with irregular surfaces is hindered by limitations in complex propagation path and low coupling efficiency. To address this issue, using ZFWs generated by the propagation of high-frequency ultrasonic waves, a water immersion SAFT imaging method is proposed for HDPE with irregular surfaces. The proposed method, based on the priori knowledge of geometric interface between water and the measured HDPE with irregular surface, utilizes the Snell's law to get the propagation paths of the ZFWs. The propagation time of ZFW in water and HDPE is computed using these paths, through which its time-of-flight is obtained. The time-of-flight is subsequently utilized to calculate the intensity of each pixel. Finally, the normalized imaging results are finally generated after iterative computations across all scanning points. Experimental results show that the proposed method is capable of detecting side-drilled hole defects with a diameter of 1 mm and a longitudinal depth of approximately 100 mm in HDPE with irregular surfaces including incline surface, convex surface, and concave surface. These findings validate the effectiveness of the proposed method in detecting defects in highly attenuative materials with irregular surface, demonstrating its potential as a valuable tool for non-destructive testing.