# A Concise State-of-the-Art Review of Crack Monitoring Enabled by RFID Technology
## 4 Crack Monitoring Based on Ultra-High-Frequency (UHF) RFID Technology  
The use of ultra-high-frequency (UHF) RFID tags for defect detection has broad application prospects in structural health monitoring. The transmission rate for UHF RFID technology is fast, the antenna size is small, and it can also read a large number of electronic labels in a short time. However, the penetration of this frequency band is poor, and the environment has a great impact on the use of RFID in ultra-high-frequency bands. [PERSON] and [PERSON] [27] proposed the concept of using RFID tag antenna-based sensors (TABS) for crack detection in the UHF band. The primary objective of their study was to simulate and investigate the feasibility, challenges, and underlying principles of crack monitoring using RFID technology. [PERSON] et al. [28] described a surface crack antenna reflection sensor: a chipless RFID sensor was developed to enable universal surface crack detection in a wireless manner (as illustrated in Figure 13). The sensor design was outlined, and it was demonstrated how the crack length and direction are related to the backscattered signal characteristics of the RFID sensor; a design scheme to improve the sensor sensitivity and signal fidelity was proposed. The monitoring system consists of a reader unit and a chipless antenna-based sensor, where the sensor includes a broadband antenna and a meandering transmission line terminated with an open or short circuit. By analyzing the time difference of arrival (TDOA) between two prominent RF echoes, the system can detect the presence of cracks and localize them in a structure. [PERSON] et al. [29] used a double-label system to monitor cracks in the key structural parts (as shown in Figure 14). The sensing tag principle and the coupling principle between double tags were investigated. By considering various placements of the two tags, the impact of mutual coupling on the RFID sensing performance of the tags was investigated. [PERSON] et al. introduced an electromagnetically induced transparency-inspired antenna sensor working in the ultra-high-frequency band for surface crack monitoring, with application in structural health monitoring for engineering structures [30]. The sensor utilizes a split-ring resonator for communication and embeds two back U-shaped strip resonators for sensing in high-dielectric materials, achieving simultaneous communication and sensing within the narrow  
Figure 12: Experimental setup for crack detection and characterization based on LF RFID sensing system (**a**) setup and (**b**) block diagram [26]. UHF RFID band. The sensor exhibits enhanced sensitivity, up to five times improvement, attributed to trapped-field dark modes. In addition, UHF RFID-based crack monitoring has also been successfully applied to the field of mining science. [PERSON] et al. [31] proposed a design of a passive UHF RFID tag sensor for monitoring structural cracks in coal mining conveyor belts. Furthermore, a comprehensive comparative analysis was undertaken to assess the influence of different crack orientations and widths. A model integrating RFID-based tags and conveyor belts was designed and simulated to exemplify the practical application of the proposed sensing technique. The results of the simulation demonstrate the feasibility of utilizing the proposed method for the wireless monitoring of cracks in coal mining conveyor belts. [PERSON] et al. [32] introduced the design and analysis of a new passive UHF RFID crack detection sensor for coal mine conveyor belts (as shown in Figure 15), and the proposed sensor was analyzed in detail in simulations and experiments. The experimental results of the prototype sensor finally show the change in the backscattered power of the sensor with the change in the received signal strength index (RSSI). An extensive analysis of the experimental results  
Figure 14: (**a**) Experimental setup of RFID test system and (**b**) aluminum alloy sample [29]. Figure 13: The proposed sensor, including a coplanar monopole antenna.