Quantitative Crack Depth Monitoring Based on Traveling Wave at Millimeter-Wave Frequency

Abstract

The electromagnetic (EM) resonator-based sensor has a well-known high sensitivity for crack inspection. The nonuniform field distribution inside the resonator, however, causes an associated position-dependent sensitivity phenomenon. This article is intended to address the accurate problem of EM resonator-based sensors. Instead, a methodology based on the traveling wave is first explored. A transmission line-based sensor is implemented for accurate crack depth evaluation. The sensitivity is maintained by tightly bounded the magnetic field in the cross Section of the transmission line through loading the high dielectric material. The accuracy is naturally improved by the uniform field distributed along the propagation direction. An equivalent circuit is modeled based on the transmission line theory to validate the sensing principle. For a wide range of crack width, both simulation and measurement results demonstrate that the detection sensitivity of crack depth is up to 1.893 GHz/mm. The error bar caused by the variation of crack position can be controlled in 0.166 GHz. The performance of multiple crack detection has been verified for the proposed transmission line-based sensor at the same time. This article paves a way to extend the local structural health monitoring based on EM resonators to the large area structural health monitoring.