Time-frequency characteristics of longitudinal modes in symmetric mode conversion for defect characterization in guided waves-based pipeline inspection

Abstract

The converted modes generated by mode conversion in guided waves-based inspection can provide plenty of defect information, and the reasonable usage of multiple wave modes can improve the results of defect inspection and evaluation. Axisymmetric longitudinal L(0,2) mode guided-waves is widely used in pipeline inspection currently, and the lower order longitudinal L(0,1) mode is easily excited simultaneously. This paper focuses the interconversion process of axisymmetric modes for incident longitudinal modes when interacting with the defects in pipeline whist ignoring the converted non-axisymmetric modes through suppressing them in reception. This process is defined as symmetric mode conversion in this research. The pattern of axisymmetric mode sequence during symmetric mode conversion is identified and the method is proposed to extract the converted axisymmetric mode components for analysis. The relationships between the defect features and the modes generated in symmetric mode conversion under the excitation of longitudinal mode waves are investigated. The conclusion obtained by numerical simulation is also experimentally verified using guided wave data from practical pipeline. The results show that longitudinal modes in symmetric mode conversion present the useful time and frequency characteristics, which provides the potential for establishing an effective defect inspection and evaluation method with longitudinal guided waves.