Cracks are common defects in metallic components, the presence of which can significantly affect service life and operational stability. Sensors based on electromagnetic resonators have relatively high sensitivity; however, they are limited in size, which restricts their coverage and makes large-area monitoring unattainable. The uneven internal field distribution within the resonator is a critical factor contributing to sensitivity variation at different locations. In this study, a vertical U-shaped ring structure is excited using a microstrip line. This allows the sensor to achieve large-area monitoring while maintaining sensitivity. The shift in resonance frequency is investigated and extracted as a characteristic feature for crack identification. The sensitivity of the measurement is 0.95 GHz/mm² for depth and 0.685 GHz/mm² for width. The proposed sensor can be used to detect potential cracks in metal structures.

Fatigue cracks and other forms of damage can have a significant impact on the normal operation of metal facilities, necessitating the deployment of multiple sensors for monitoring within large structures. The arrangement of these sensors must take into account factors such as the shape, size, and complexity of the monitoring area, as well as the optimal positioning and spacing of sensor nodes. This requirement for comprehensive coverage while minimizing costs presents considerable challenges for structural health monitoring (SHM) techniques. In this article, the feasibility of crack detection with a simple microstrip line (ML) is studied in the millimeter-wave band. The detection sensitivity is 0.283/mm2, the precision is 13.61%, and the minimum crack depth that can be identified is 0.2 mm (when crack width $\ge 1.0$ mm). An equivalent circuit model for this type of traveling-wave sensor is established in conjunction with field analysis, and the accuracy of the model is verified by comparing full-wave simulation and the circuit model. The proposed sensor can act as a distributed sensor for the SHM applications.