# A Concise State-of-the-Art Review of Crack Monitoring Enabled by RFID Technology
## 7 Conclusions  
Cracking is a common hazard in engineering structures, affecting the performance and service life of structures. In order to ensure the safety and reliability of engineering structures, crack monitoring is critical. Crack monitoring based on RFID antennas has the advantages of wireless operation and low cost, which makes them highly competitive in the field of structure health monitoring. This study systematically summarizes the research progress of crack monitoring based on RFID technology in recent years. Regarding crack monitoring based on chipless RFID technology, a chipless RFID sensor using the circular patch antenna for crack detection is introduced. Also, a new chipless RFID sensor tag array for wireless crack monitoring is presented. SWB technology is employed to obtain a high-resolution chipless RFID crack sensor. In addition, some multi-resonance chipless RFID tags with multi-sensing functions for defect characterization are described. Passive RFID sensors have the advantages of small size, light weight, low cost, and long service life. This study reviewed a new crack depth detection technology based on a passive RFID tag on the ThingMagic Mae platform. A passive RFID sensor system for crack detection is introduced based on a 3D antenna and kernel principal component analysis. Also, a circular passive RFID patch antenna crack monitoring sensor with an open rectangular window and the principle of magnetic resonance coupling communication for low-frequency passive RFID antenna sensors are illustrated. For crack monitoring on the basis of UHF RFID technology, this study demonstrated how the crack length and direction are related to the backscattered signal characteristics of the RFID sensor, and a design scheme to improve the sensor sensitivity and signal fidelity is proposed. A double-label UHF RFID system is introduced to monitor cracks in the key structural parts. In addition, UHF RFID-based crack monitoring has also been successfully applied to the field of mining science. Figure 25: (**a**) Aluminum alloy tensile specimen; (**b**) load sequences; (**c**) test setup; (**d**) measurement during fatigue test [48]. For RFID-based crack monitoring applications, the two most commonly used materials are concrete materials and metal materials. This study introduces RFID-based crack monitoring applications for these two kinds of materials, including monitoring the width and depth of cracks. For concrete crack monitoring based on RFID technology, this study introduces the realization of UHPC beam crack detection based on RFID technology and a new wireless RFID sensor tag to measure the opening of the crack, as well as some UHF RFID sensors for concrete crack monitoring. For metal crack monitoring based on RFID technology, a circular patch antenna that is sensitive to both vertical and transverse cracks is presented, and it can be used to detect the location and orientation of metal cracks. In addition, this article introduced a depolarized chipless RFID sensor tag design based on double resonance features for the characterization of metal cracks, reusable RFID tags for the surface crack detection and monitoring method, and a novel sensor tag based on a spin ring resonator for crack detection on metal surfaces. In general, this study can provide technical support, a reference, and guidance for research on crack monitoring and detection based on RFID technology. Conceptualization, S.-C.R. and Q.-A.W. ; methodology, S.-C.R. and Q.-A.W. ; formal analysis, J.-F.W. and Q.-A.W. ; investigation, Z.-X.G. and Y.L. ; resources, S.-C.R. and Y.-Q.N. ; writing--original draft preparation, S.-C.R., Q.-A.W. and Z.-X.G. ; writing--review and editing, S.-C.R., Q.-A.W., J.-F.W. and Y.-Q.N. ; supervision, S.-C.R. and Q.-A.W. All authors have read and agreed to the published version of the manuscript. This research is funded by the National Natural Science Foundation of China (Grant No. 52008258), the Shenzhen Science and Technology Program (Grant Nos.