# A Concise State-of-the-Art Review of Crack Monitoring Enabled by RFID Technology
## 2 Crack Monitoring Based on Chipless RFID Technology  
Chipless RFID technology refers to an RFID tag that does not contain a silicon chip. Chipless RFID technology has the advantage of low cost and can also store enough data. In addition, chipless RFID technology can achieve high-speed printing production. [PERSON] et al. [14] proposed chipless RFID tags for crack monitoring, as shown in Figure 1. The main part of the tag is the circular microstrip patch antenna (CMPA) resonator. In order to use the bit ID of the resonant frequency indicator tag, a slot is created in the CMPA resonator. The results show that the cracks in the metal will affect the natural frequency shift. [PERSON] and [PERSON] [15] designed a chipless RFID sensor tag array to be applied for crack detection on large metal surfaces, as shown in Figure 2. A single sensor tag containsa compass shape and a pair of L-shaped resonators. In order to identify the crack location, a series array with five rectangular resonators of different sizes is proposed to generate multiple resonant frequencies. [PERSON] et al. proposed a chip-free RFID sensor, which can be attached to metal plates for crack detection [16]. A circular microstrip patch antenna resonator was utilized as the main component of the tag, with slots created within the proposed resonator to generate resonant frequencies indicating the tag's bit ID. The performance of the chipless RFID sensor for detecting metal cracks was evaluated through simulations. The results showed that the notch in the RCS spectra of the tag sensor shifted when a crack was present on the metal plate. To improve sensor resolution, [PERSON] et al. [17] proposed a novel chipless RFID sensor that provides a high-resolution crack detection technology capable of achieving ubiquitous wireless crack sensing with high precision (as shown in Figures 3 and 4). Using time-domain reflectometry (TDR) technology, the proposed sensor facilitates a wireless, cost-effective sensing system with broad coverage for crack detection. The proposed sensor employs a super-wideband (SWB) antenna operating within the 1-20 GHz range, offering high resolution due to its exceptionally short time-domain pulse. An in-depth examination of the computational findings was conducted, and a comparative evaluation between SWB and UWB sensors was executed to underscore the superiority of the proposed sensor. Figure 1: (**a**) The layout of the proposed chipless RFID sensor tag; (**b**) the basic architecture of the metal crack detection system using the chipless RFID sensor [14]. Figure 2: Operating principle of the proposed RFID system [15]. In addition, some scholars have also proposed chipless RFID sensors with multi-sensing functions. [PERSON] et al. [18] proposed a multi-resonant chipless RFID sensor tag and a chipless RFID sensor reader for the multi-parameter estimation of metal defect characterization using principal component analysis (PCA) (as shown in Figure 5). The resonance peaks are within the 2-6 GHz band for metal defect characterization. The sensor tag is combined with a chipless RFID reader using an ultra-wideband impulse radar transceiver module and PCA for multi-parameter estimation. Research has demonstrated that the integration of a multi-resonant chipless RFID sensor tag with PCA-based feature extraction and selection techniques can effectively characterize metal cracks of varying widths and orientations. [PERSON] et al. [19] presented a compact 7-bit chipless RFID multi-parameter sensor designed with smart materials (as shown in Figure 6). The dielectric properties of this smart material vary with temperature fluctuations. Circular microstrip patch antennas were utilized to characterize the cracks. The resulting smart sensor can be affixed to any crack on a metal surface. Figure 4: Experimental setup for crack sensor interrogation [17].