The Effect of Complex Corrosion Profiles on Remaining Wall Thickness Quantification Using Shear Horizontal Guided Waves

Corrosion of plate and pipe structures is a major concern to many key industries, including power, maritime, and oil and gas. Traditionally wall thickness is quantified in-service using either bulk wave ultrasonics or electromagnetic techniques. Despite their generally satisfactory accuracy, these methods involve local measurements immediately under the probe area. This limits the inspection only to areas of the structure where probe access is feasible. Unfortunately, in certain cases, direct access to the structure requiring inspection may be limited, such as when gauging under pipe supports. Moreover, spot or scan measurements can be time-consuming when a large area needs to be inspected. Periodic inspection or continuous structural health monitoring of structures with limited accessibility can be carried out using shear horizontal guided waves. By exciting a higher order mode at a wide frequency-wavelength range, it is possible to quantify the minimum remaining wall thickness using the cut-off frequency of the mode. When the selected guided wave mode interacts with the defect, frequencies below the cut-off are reflected, while higher frequencies are transmitted. Although the method is relatively simple when smooth wall thinning defects are considered, the validity of the method is under question in the case of more complex corrosion profiles, where sharp pits might occur. However, in these cases, mode SH0 can give a strong reflection from the near vertical insonified face of the pit. In this work, the interaction of modes SH1 and SH0 with complex corrosion defects is investigated. Three different cases of realistic corrosion profiles are simulated, including sharp pitting and gradual wall thinning defects. Such corrosion profiles appear commonly in structures manufactured from mild steel. A phased array-based approach is proposed, to generate and receive shear horizontal guided waves. A pair of identical phased array probes are positioned before and after the simulated corrosion profile, to monitor the reflected and transmitted waves. The targeted mode is excited selectively using a 32-element 3 mm pitch array and modes are decomposed after a 2DFFT has been performed. The cut-off frequency technique is shown to be adequate when smooth wall thinning defects are considered. When sharp pits are present, mode SH0 proved sufficient to determine the pits depth.