# A Concise State-of-the-Art Review of Crack Monitoring Enabled by RFID Technology
## 3 Crack Monitoring Based on Passive RFID Technology  
Passive RFID technology sensors have received much attention in health monitoring. Passive RFID tags generally do not contain batteries. When the passive RFID tag approaches the RFID reader, the antenna transfers the received electromagnetic wave energy as electrical energy to power the tag. Passive RFID sensors have the advantages of small size, light weight, low cost, and long service life, and the disadvantages of this technology are mainly the short transmission distance, usually within a few meters. [PERSON] et al. [20] proposed passive RFID tag antennas integrated into aircraft structural glass fiber laminates to detect internal defects (as shown in Figure 7). The results demonstrated the potential of utilizing RFID sensing in fiberglass composite laminates. [PERSON] et al. [21] developed a new crack depth detection technology based on the ThingMagic Mde platform passive UHF RFID tag (as shown in Figures 8 and 9). Wireless power transfer levels and sweep frequencies are used to match tag impedance and metal sensing effects. The distance between the label and the card reader can be adjusted to 30 cm to achieve a high quality factor. As shown in Figure 8, the existence of peaks within the period is represented by the hit state, and the absence of peaks is represented by the miss state. Thus, all crack states can be encoded as [hit miss hit], while for healthy states, only the miss state occurs, which can be employed to distinguish between healthy and cracked samples. The Power Peak Feature Extraction technique is employed to accurately detect the depth of artificial cracks on the surface of stainless steel and ferromagnetic samples, achieving a maximum error of 0.1 mm. [PERSON] et al. designed a passive RFID tag antenna sensor for crack depth detection. As the metal crack depth increases, the RFID tag resonance frequency moves to a low frequency [22]. [PERSON] et al. designed a new passive RFID patch antenna sensor for strain and crack sensing to improve reliability under temperature fluctuations. Also, in order to obtain a longer interrogation distance, another dual-mode patch antenna sensor was investigated [23]. Figure 6: (a) Layout of a 7-bit chipless RFID sensor; (b) fabricated tag [19]. Figure 7: Tensile tests of individual laminated samples with passive RFID tags: (**A**) the default placement position of the laminated sample, (**B**) the course of sample deformation, (**C**) the final state after destruction [20]. In addition, [PERSON] et al. [24; 25; 26] have conducted much research on crack monitoring based on passive RFID technology. [PERSON] et al. [24] introduced a passive RFID sensor system for crack detection (as shown in Figure 10). A novel approach utilizing 3D antenna technology was explored and validated through two case studies, i.e., the evaluation of open and closed cracks. In addition, [PERSON] et al. [25] used a circular passive RFID patch antenna sensor with an open rectangular window for crack detection (as shown in Figure 11). The sensing mechanism was comprehensively studied in tandem with mode analysis, shedding light on the fundamental principle of transforming an antenna into a crack sensor. The robustness was evaluated using varying crack positions on an aluminum sample. In order to improve the robustness of passive wireless RFID antenna sensors, [PERSON] et al. [26] reviewed the principle of magnetic resonance coupling communication for passive RFID antenna sensors with low frequency (as shown in Figure 12). The impact of communication is elucidated and effectively distinguished from perception through two steps: source frequency scanning to capture system behavior and multi-feature extraction, fusion, and selection and the normalization of selected peak-to-peak time-domain features. In this way, the robustness of RFID sensing systems can be improved. Figure 8: Power signals for stainless-steel samples with cracks with (**a**) 0.5 mm crack depth, (**b**) 1 mm crack depth, (**c**) 1.3 mm crack depth [21].