SAFT imaging for high-density polyethylene using quasi-static components of ultrasonic longitudinal waves.

High-density polyethylene (HDPE) is extensively utilized across various industries, including nuclear power, primarily for its exceptional properties. However, there are challenges with traditional linear ultrasound imaging systems due to the significant thicknesses and the highly attenuative of HDPE. High-frequency carrier waves can offer better imaging resolution but also suffer higher acoustic attenuation, which limits the propagation distance of primary longitudinal waves (PLW) and makes it difficult to detect defects within thick HDPEs. On the other hand, using low-frequency PLW for defect detection presents challenges in resolution despite lower attenuation and longer propagation distances. This study proposes a defect imaging method for HDPEs by using quasi-static components (QSC) generated along with high-frequency fundamental wave propagation because of the nonlinear effect. The QSC has the advantage of low attenuation because its carrier frequency is zero, which can propagate a long distance in a high acoustic attention medium like HDPE. A nonlinear ultrasonic imaging approach combining the QSC and synthetic aperture focusing technique is proposed for defect imaging in HDPEs. Experiments on HDPEs with single and multiple defects are conducted to verify the performance of the proposed method. For comparison, the imaging results using traditional linear ultrasounds with high (2.5 MHz) and low (0.5 MHz) carrier frequencies are also provided. The results show the proposed method has better imaging performance over traditional linear ultrasound imaging methods for defect defections in high acoustic attention medium.