# A Novel Chipless Hybrid RFID Sensor for Metal Crack Detection
## 1 Introduction
There are three categories of antenna-based sensing: amplitude variation, where changes in physical quantities change the RFID transmitted and the received power; antenna resonant frequency variation, where the detuning of the antenna tag frequency concerns changes in the sensing parameter; and tag backscatter variation. [PERSON] investigated the presence of cracks with respect to the changes in the tag antenna radiation and impedance characteristics [42]. [PERSON] et al. proposed a crack width sensor that has two chipped RFID tag couplets using a PIFA antenna placed on the concrete walls to detect the crack width, and the phase shift is used as a sensing parameter to detect 1 mm cracks at 1.5 m distance [43]. [PERSON] et al. used UHF RFID tag antennas to characterize and detect surface cracks on aluminum, and phase shift in the backscattered signal was used as a sensing parameter [44]. [PERSON] et al. used a folded rectangular patch and increased the detection range to 2.1 m but the tag size was larger [45]. Recently, [PERSON] et al. presented a simulation analysis of a chipped RFID tag sensor that is sufficiently sensitive to detect 0.2 mm of crack depth, but the experimental validation is not promising due to degradation of the fabricated sensor performance in practice [37]. These sensors use application-specific chipped sensors and are not cost-effective and may not work in harsh environments. In addition, the detection capability of these chipped sensors is limited to one-parameter detection because of their operation in a narrowband spectrum. In contrast, chipless RFID provides numerous advantages over chipped RFID in terms of costs, printability, and simplicity, and it can work in harsh environments (e.g., high-temperature conditions) [46,47]. Hence, chipless RFID has received much attention in sensor applications [48,49]. Chipless RFID also provides a multi-parameter sensing capability [50]. [PERSON] et al. designed a complementary split ring resonator (CSRR) and ELC resonator with microstrip coupling to detect humidity as well as temperature. They used Rogers 5880 as the dielectric material [51]. Investigation of the detection of metallic cracks using chipless RFID sensors is quite recent. Earlier, [PERSON] et al. used time-domain reflectometry-based chipless RFID with a delay line tag structure to detect cracks [52]. [PERSON] et al. designed a circular microstrip patch antenna (CMPA) to detect metal cracks [53]. [PERSON] et al. designed and investigated an ultra-wideband, novel smart skin sensor for crack detection using multiple gap coupled split box resonators integrated with a coplanar waveguide transmission line over a Taconic dielectric material; however, it is still fed with a wired transmission line [54]. In this paper, we present the design of a small-size, low-cost ultra-high-frequency (UHF) chipless RFID tag that is capable of crack detection and characterization. We demonstrate how the position, length, and orientation of the crack can be related to the shift in the resonant frequency of the backscattered signal of the tag. The paper is organized as follows: Section 2 discusses the working principle of the RFID system and presents the simulation results. Section 3 presents the experimental results of the tag, detailing the shift in resonant frequencies in two crack orientations and discusses the challenges and limitations of the tag. Finally, conclusions are given in Section 4 with consideration of future work.