# A Novel Chipless Hybrid RFID Sensor for Metal Crack Detection
## 3 Results and Discussion
FR4 material is selected as a substrate. FR4 is a widely used, low-cost, and commercially available substrate material. It is easy to process using standard PCB fabrication techniques. For proof-of-concept design, the low cost of FR4 facilitates rapid prototyping and experimentation without incurring high expenses. To investigate further, a comparative analysis of different substrates--Rogers4305B (\(\epsilon=3.66\)), Rogers4003C (\(\epsilon=3.55\)), PTFE (\(\epsilon=2.1\)), and FR4 (\(\epsilon=4.4\))--is shown in Figure 9:3 The tag design parameters are constant; only the dielectric substrate is changed. The RCS for the FR4 substrate gives better results for the tag design. Since the tag is metal mounted and metals are prone to corrosion with time, another interesting aspect was observing the behavior of the tag in corrosive and non-corrosive environments. To increase the reading distance, a 2 \(\times\) 2 array of the tag is printed and tested under the aforementioned conditions. The spacing between elements is 10 mm, as  
Figure 9:4 Simulation of RCS tag with different dielectric substrates. Figure 9:5 Simulation of different dielectric substrate thickness. shown in Figure 9:6 The printed tag array is tested on corrosive and non-corrosive samples, as shown in Figure 9:7 The samples are tested at a distance of 30 cm from the antennas. Figure 19 shows the experimental results for RCS of the tag array when placed on both a corrosive and non-corrosive metallic surface. It can be seen that the RCS has a peak at 4 GHz in a non-corrosive sample with deep frequency notches in the spectrum. For the corrosive sample, the RCS magnitudes are reduced since the effective permittivity of the metallic surface is changed. This shows the potential of the tag to detect corrosion and cracks in the metallic surface. Figure 9:8 RCS of corrosive and non-corrosive metallic material. Figure 9:9 Printed sensor tag array: 10 mm away from each other. One practical challenge is to achieve a robust sensor tag reading system. The advantage of this work is that no previous calibration of the testing setup is performed. The test is performed in a calibration-free environment. [PERSON] et al. investigated the metallic crack detection RFID CMPA tag in an anechoic chamber environment with prior calibration; however, the CMPA tag is prone to a more noisy RCS response when tested in an outside environment [53]. The proposed work shows promising results for the tag. The frequency shift in the sensor design is influenced by the effective resonator geometry and its interaction with the electromagnetic field. Cracks of varying dimensions and orientations alter the electromagnetic boundary conditions, which results in shifts toward lower frequencies due to an increase in the effective inductance or capacitance. With increase in width of the horizontal crack, there is a downward frequency shift, and with increase in the width of the vertical crack, there is an (upward shift) increase in the resonant frequency. However, detecting the frequency signature for crack depth variation is still challenging. During experiments, due to the symmetry of the tag structure, one edge of the hex tag is kept as a reference to arrange and analyze the orientations of the crack. In future research, the design and development of asymmetrical tags will be explored to achieve enhanced performance in defect characterization and classification. Also, to enable sensor deployment, a robust calibration process will be carried out, using known crack samples to establish baseline frequency responses for different crack sizes and orientations. These calibration data can be systematically stored and used for real-time comparison during practical applications, ensuring accurate and reliable performance. Advanced signal processing techniques, such as multi-frequency analysis and machine learning, can further enhance the reliability of distinguishing between different causes of frequency shifts.