Ultrasonic coded-excitation full-matrix imaging in the wavenumber domain for coated pipe welds

Welds in long-distance pipelines are susceptible to defects due to stress concentration. However, the protective coating layers outside the pipe welds hinder ultrasound penetration. As a result, current ultrasonic nondestructive testing methods require removing the protective layer before detection, which is inefficient and labor-intensive. To address this challenge, a full-matrix ultrasonic imaging method is proposed, employing coded excitation and multi-mode wavenumber-domain image reconstruction. Nonlinear frequency modulation (NLFM) is combined with the Golay pair as the coded ultrasonic excitation sequences to achieve better penetration and side lobe suppression. The multi-mode wavenumber-domain image reconstruction method solves the complex wave propagation and image migration problems for welds covered with protective layers. Experimental results demonstrate that the proposed method can detect pipe weld defects in the presence of an approximately 2.6 mm three-layer polyethylene (3LPE) coating. Various common coated pipe weld defects can be detected with an average image signal-to-noise ratio of 20.8 dB. A frame rate of 4 Hz is achieved for one imaging mode.