# A Novel Chipless Hybrid RFID Sensor for Metal Crack Detection
## 4 Conclusions  
A small RFID tag is designed using a combination of a circular patch with a hexagonal slot and a small hexagonal patch. A chipless RFID tag is used for metal crack detection. The simulation results are shown in detail to understand the behavior of the tag. Two parameters of the crack are analyzed: the crack width and the crack depth. The shift  
\begin{table}
\begin{tabular}{c c c c c c} \hline
**Reference** & **Sensor Type** & **Dimension (mm)** & **Substrate** & **Operating (GHz)** & **Results** \\ \hline
[53] & Chipless & \(35\times 15\times 1.2\) & \
\begin{tabular}{c} Taconic \\ CER \\ 10-0500 \\ \end{tabular} &
\begin{tabular}{c} CMPA \\ patch with \\ 4-bit ID, \\ simulated \\ and tested \\ in anechoic \\ chamber, \\ required \\ \end{tabular} \\ \hline
[61] & Chipless & - & - & 1–3 &
\begin{tabular}{c} CMPA \\ patch with \\ simulated \\ and tested \\ in anechoic \\ chamber, \\ real vibration \\ required \\ \end{tabular} \\ \hline
[61] & Chipless & - & - & 1–3 &
\begin{tabular}{c} CMPA \\ patch with \\ simulated \\ simulation \\ results \\ \end{tabular} \\ \hline
[62] & Chipless & \(30\times 30\times 1.6\) & FR4 & 3–5 &
\begin{tabular}{c} CMPA \\ patch with \\ simulated \\ results \\ \end{tabular} \\ \hline
[63] & Chipless & \(35\times 15\times 1.6\) & \
\begin{tabular}{c} Rogers \\ RT/Duroid® \\ 6010.2 LM \\ \end{tabular} & 2–8 &
\begin{tabular}{c} CMPA with \\ 7-bit ID, \\ simulation \\ results \\ \end{tabular} \\ \hline
[64] & Chipless & \(18\times 18\) & PCB & 2.5–6 &
[65] & Chipless & \(35\times 35\times 1.27\) & Rogers RT 6010 & 3–4.5 & \
\begin{tabular}{c} Designed \\ CMPA with \\ 4-bit ID, \\ simulation \\ results \\ \end{tabular} \\ \hline This work & Chipless & \(16\times 16\times 1.4\) (smaller \\ in size) & FR4 & 3–5 &
\begin{tabular}{c} CMPA with \\ hexagonal \\ slot, \\ simulated \\ and tested \\ in table-top \\ setup \\ \end{tabular} \\ \hline \end{tabular}
\end{table}
Table 1: Work done on chipless sensors for crack detection.  
in the resonant frequency is due to the variation in the dimensions and orientation of the metallic cracks. Horizontal cracks reduce the resonant frequency and vertical cracks increase the resonant frequency in terms of width; with depth of the vertical cracks, the resonant frequency does not change much. The proposed tag shows high sensitivity, with changes in the resonant frequency of the tag in MHz per 1 mm change in the dimensions of the crack. The table-top experimental results show the shift in resonant frequencies. The tag has the potential to detect corrosion as well as crack defects.  
The limitations and challenges of the tag are discussed briefly with a view to future improvements. Future work will involve redesigning a chipless RFID with ID capacity and testing it in a real-time environment. This ID can be a depolarizing tag that will mitigate the multipath and cross-talk interference. The reader can be a more portable transceiver module for practical applications and the use of machine learning algorithms can support better crack detection and characterization.  
Conceptualization, Y.D.K.; Methodology, Y.D.K.; Investigation, Y.D.K.; Writing--original draft, Y.D.K.; Supervision, M.H. and Y.D. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The data presented in this study are openly available in [[https://github.com/Yaminik33/RFID_Project](https://github.com/Yaminik33/RFID_Project)]([https://github.com/Yaminik33/RFID_Project](https://github.com/Yaminik33/RFID_Project)), accessed on 12 February 2025.  
The authors acknowledge and are deeply grateful to [PERSON] of the MSU ECE shop for all his assistance and support.  
The authors declare no conflicts of interest.