Long Range Inspection of Wall Reduction of Tank Utilizing Zero-th Order Symmetric Mode Lamb Wave —Performance Demonstration of the Method Proposed—

Abstract

We developed a new long range inspection method of corrosion-induced wall reduction of storage tanks. The method utilizes the zero-th order symmetric mode (So-mode) Lamb waves excited and monitored by a specially designed rectangular compression type PZT transducer mounted on the edge of the annular plate and side wall. The system can measure both the location and damage depth from the arrival time and amplitude of the So-mode reflected by the defects, respectively. We first measured attenuation of the So-mode wave and then the depth of dish-shaped wall reduction on single carbon steel plate. Amplitude of the So-mode wave reflected by the defects was found to increase proportionally with the defect depth less than 15% of the plate thickness (10 mm). Amplitude of the reflected So-mode Lamb wave from the shallow dish-shaped defects with depth of 0.6 mm or 7.5% to the plate thickness was detected. We also detected the So-mode waves from corrosion-induced dish-shaped wall reduction with 1.45 mm depth in 10 mm plate. Artificial groove of 1 mm depth on the step-weld 8 mm plates could be detected by the proposed method.