Numerical investigation of the propagation characteristics of shear horizontal guided wave modes in coated pipes

Shear horizontal (SH) guided waves are commonly used for non-destructive evaluation of plates and pipes. A coating layer is often applied to the outer surface of steel pipes to prevent external corrosion. Even with a considerably lower acoustic impedance, relative to steel, the presence of this layer can significantly alter the propagation characteristics of SH guided wave mode. Due to its viscoelastic properties, guided wave modes that propagate fairly in an equivalent bare pipe may become an unsuitable choice in a coated pipe. In this paper, we investigate which SH modes are more adequate for a circumferential inspection of a coated pipe of interest. The case study consists of a 323 mm outer diameter, 6.35 mm thick steel pipe coated with a 1.5 mm polyethylene layer, in which SH waves are generated either at the low- and high-frequency-thickness regime. A semi-analytical model was used to calculate the dispersion and attenuation curves and a finite element numerical model was further exploited to investigate the propagating modes. Results show that, in the analyzed case, at the high-frequency-thickness regime, the quasi-SH0 mode at 10 mm wavelength and the quasi-SH1 mode at 20 mm wavelength presented low attenuation of about 4 dB/m and 14 dB/m. However, at the low-frequency-thickness regime, the quasi-SHO mode at 20 mm wavelength presented a high attenuation, hindering and reflected echo from a defect to be detected, since it lies at a crossing point between the dispersion curves of a guided wave mode whose profile is mostly in the metal layer with a guided wave mode whose profile is concentrated in the coating layer. Therefore, operating at the usual low-thickness-frequency regime was completed prevented, making one move the higher-frequency-regime. A careful selection of the most suitable operating mode is paramount in order to inspect a defect with ultrasonic guided waves in pipes with coating.