Prove the ability of microwave nondestructive method combined with signal processing to determine the position of a circumferential crack in pipes

This paper presents a technique using microwaves nondestructive testing to detect circumferential crack and its location in stainless steels pipe. This approach is based on the surface current flow in the inner surface of the test pipe, when electromagnetic (EM) waves are propagating inside the pipe. Since the conductive pipe is an excellent waveguide, EM-wave can be propagated inside the pipe above the cutoff frequencies in large distances. To detect circumferential crack in pipes, a suitable EM-wave mode is generated in the inspected pipe with crack. For this purpose, a mode-converter is used to convert rectangular TE₁₀-mode to the circular TM₀₁-mode. This mode is controlled and resonated by a moving a plunger mounted in the mode-converter. To show the crack existence from experimental data, the background signal of a pipe without a crack is subtracted from that one with a crack. The location of the crack is found by knowing the time of flight (TOF) of the reflected waves and the group velocities of the waves in each part of the waveguides used in this study. The TOFs are obtained through an analysis based on the Inverse Fast Fourier Transform (IFFT) of the signals in the frequency domain and a method of signal cancellation. The obtained results show that this NDT method is suitable for determining of crack locations in large pipes without point-by-point screening since electromagnetic waves above their cutoff frequencies can propagate in pipes without significant attenuation loss. Our study is of a fundamental interest in nuclear power plants and piping systems.