# A Concise State-of-the-Art Review of Crack Monitoring Enabled by RFID Technology
## 6 RFID-Based Crack Monitoring Applications for Metal Materials  
In addition to the monitoring of concrete cracks, research on RFID technology for crack monitoring and detection in metal materials has also been extensive [39; 40; 41; 42; 43]. [PERSON] et al. [44] proposed a structural health monitoring sensor utilizing RFID technology, which incorporated three RFID tags for identification, as well as a circular patch antenna for crack detection. Simulation results indicate that the resonant frequency of the circular RFID patch antenna exhibits sensitivity to both vertical and transverse cracks. Therefore, the resonance frequency offset can be used to detect the location and orientation of the metal crack. Moreover, [PERSON] et al. [45] detected the width and direction of cracks in metal structures, improved the sensitivity and reliability of detection, and designed a dual-polarization chipless RFID sensor based on frequency signals. The results show that the sensor generates two groups of four resonances under the excitation of \(x\) polarization and \(y\) polarization directions. The direction of the metal crack is judged according to the direction of the resonance frequency deviation. The two polarization modes produce eight-bit resonance deviation information for judgment, which can realize the accurate identification of the cracks in various directions. What is more, [PERSON] et al. [46] introduced a depolarized chipless RFID sensor tag design based on double resonance features for the characterization of metal cracks (as shown in Figure 23). The proposed sensor integrates a modified circular patch antenna with notched edges for crack detection and diagonal bent dipoles for the RFID tag ID. Experimental results demonstrate that the dual resonance features produced by the depolarizing circular patch can effectively indicate the orientation and monitor the widths of cracks in millimeters. In addition, [PERSON] et al. [47] proposed a new application of chipless RFID based on frequency characteristics for metal crack detection and characterization at ultra-wideband frequencies (as shown in Figure 24). The design incorporates four tip-loaded dipole resonators (serving as a 4-bit ID encoder) and a circular RFID patch antenna resonator. Simulation and experimental findings demonstrate that the resonant frequency shift of the proposed sensor is a valuable indicator for determining both the orientation and width of cracks in metallic structures. Figure 22: Experimental testing: **(a)** the geometric parameters of the concrete blocks; **(b)** the casting process of the concrete with formwork; **(c)** measurements for the concrete block [38]. In addition, [PERSON] et al. [48] introduced a surface crack detection and monitoring method based on RFID tags that made the reuse of tags possible (as shown in Figure 25). Both simulated and experimental outcomes demonstrate that the proposed crack sensing tag can detect surface crack changes with millimeter-level resolution. In addition, the proposed RIFD sensor for crack detection can be reused. Furthermore, [PERSON] et al. [49] explored the dynamic monitoring capability of surface crack propagation based on an RFID tag sensor, adopted a microstrip tag antenna as the crack sensor, took its backscattered signal as the characterization parameter of crack propagation, and designed a new crack propagation test method. [PERSON] et al. [50] proposed a novel sensor tag based on a spit ring resonator (SRR) for crack detection on metal surfaces, which is excited by a waveguide to make the tag resonant. Based on the resonant frequency shift caused by the crack, the label can achieve very accurate crack detection. The strong field enhancement effect observed in the spit ring resonator resonance significantly enhances the sensitivity of the label, making it highly effective in metal crack detection. Because the label is flexible, it can be covered with a curved metal surface. On the other hand, [PERSON] et al.