The generation and propagation of cracks are critical factors that affect the performance and life of large structures. Therefore, in order to minimize maintenance costs and ensure personal safety, it is necessary to monitor key structures. The sensor based on ultra-high frequency radio frequency identification (UHF RFID) antenna has the advantages of passive wireless, low cost, and great potential in the field of metallic structure health monitoring. In this paper, aimed at the key parts of a metallic structure, a dual-tag system is used for crack monitoring. In conjunction with mode analysis, the principles of the sensing tag and the coupling principles of the dual-tag are analyzed. Considering that the dual-tag is placed in different methods, the effect of mutual coupling on the sensing performance of the tag is studied. The results show that the frequency of the sensing tag can be tuned by adding the interference tag, and the dual-tag sensor system has reasonable sensitivity. The results also provide potential guidance for the optimal placement of multiple tags in the near-field region.

Wireless monitoring of cracks is a critical task in evaluating the state of metal structure. Radio-frequency identification (RFID) antenna sensors have been intensively investigated for crack detection. Nevertheless, the development of RFID antenna sensors satisfying the requirements of high sensitivity and coverage remains a challenge. In this paper, an ultrahigh frequency (UHF) RFID antenna sensor with coupled ring resonators has been proposed, where the outer ring is designed for radiation and sensing, and the inner ring is utilized for impedance matching. The sensor is directly placed on metallic surface, forming an electromagnetically induced transparency-like (EIT-like) effect to enhance sensing capabilities. Due to a relatively uniform electromagnetic field constrained under the outer ring, surface cracks can be detected in different positions and directions. The sensor is sensitive to both depth and width of crack, with a sensitivity of -5.46 MHz/mm in crack depth and -10 MHz/mm in crack width detection. According to the results of this investigation, a potential sensing technique for practical applications is demonstrated.

Cracks in metal structures are common defects that require monitoring over large areas, often necessitating numerous sensors. This wiring and layout increase design and deployment complexity. Traditional electromagnetic resonant sensors, while simple and sensitive, are limited in coverage. In response, this paper proposes a wireless passive sensor based on electrically reconfigurable resonant rings. These rings, integrated directly onto a microstrip line, confine electromagnetic fields within miniaturized resonant units, significantly enhancing sensitivity. Through electrically control the working sequence of the resonant rings, the sensor can eliminate the impact of inter-unit mutual coupling, enabling the detection of crack depth and width in sub-millimeter level. The proposed sensor has been configured to two states: status 1 (resonant frequency is 955 MHz) and status 2 (resonant frequency is 903 MHz). The measured sensitivities for crack width characterization are 163.9 MHz/mm² and 153.5 MHz/mm², respectively and the measured sensitivities for crack depth characterization are 125.75 MHz/mm² and 128.5 MHz/mm², respectively. This paper demonstrates its suitability for large-scale coverage in metal components.

Developing an indicator for metal materials to characterize cracks is urgent. However, traditional sensor-based technology has drawbacks such as high costs for installation and maintenance when using wired connections. In this paper, we studied the Radio Frequency Identification sensors created through three-dimensional printing technology to characterize surface cracks in metals. This approach simplifies the manufacturing process, silver nanoparticles are printed layer by layer on substrate to form the sensor pattern. The functionality of the sensor is verified through simulations and experiments involving samples with various crack sizes. Our findings demonstrate that when cracks pass over the sensor, there is a distinct response in terms of a shift in resonant frequency, moreover, the sensor offers a reading range greater than 0.7 m at resonance frequency without requiring power supply or wired connection for data transmission purposes. This research showcases the design of a smart structure that is compact, easy-to-fabricate, and potential for applications related to structural health monitoring and crack sensing.

In this work, a wireless passive high-sensitivity displacement sensor operated in the ultra-high frequency (UHF) radio frequency identification (RFID) band is proposed for displacement monitoring. The sensor is designed based on the principle of electromagnetically induced transparency (EIT)-like effect, the strip dipole (SD) rendering a bright mode, and a pair of U-shaped ring resonators (USRR) producing a dark mode. The coupling between these two modes is extremely sensitive to micro displacement, enabling high-sensitivity displacement detection. The bright mode can directly capture the electromagnetic wave energy emitted by the RFID reader antenna and activate the RFID tag chip, enabling passive sensing. The resonant characteristics within the threshold power to activate the tag are utilized for displacement characterization, ensuring reliable wireless measurement. The proposed wireless passive displacement sensor has undergone simulation and experimental validation. The results indicate that the prototype sensor is capable of detecting displacement variations as small as 0.01 mm, consistently achieving deep sub-millimeter resolution, and demonstrating a detection sensitivity of 110 MHz/mm.