# Thermal-Mechanical Behaviour of Road-Embedded Wireless Charging Pads for EVs
## 11 Conclusions  
This paper presented an experimental and a finite element-based numerical methodology for analysing the structural response of IPT pads to thermally induced loading. The mechanical, thermal and mechanical properties of the materials within the pad were characterized at the system's operating temperature range. A three-phase program was used to investigate the thermal-mechanical behaviour of IPT pads. First, a partial pad was heated to uniform temperatures, followed by the complete pad being energized under two conditions: suspended in air, and then embedded in pavement. Compensation of the stress-free temperature output of resistive strain gauges and FBG sensors allowed mechanical strain to be reliably obtained.  
Under thermal loading, the pads experienced an upward bow due to the higher thermal expansion of the top half of the pads compared to the bottom half. This resulted in a bending moment on the ferrites, with tensile strains predicted and measured on the top surface, and compressive strains on the bottom surface. The embedment of the complete pad in pavement reduced ferrite strains due to the constraining boundary conditions around the system, resulting in lower strains compared to those induced through wheel loading.  
When the complete pad was suspended in air and energized, there was excellent agreement between the measured and predicted temperatures, with deviation of within 1% for the maximum temperature on the pad surface and the coil, and an 8% deviation in ferrite temperature. There was reasonable agreement between measured and predicted ferrite strains, with an average overprediction of 27% and an RMSE of 37 \(\mu\varepsilon\) at 50 \({}^{\circ}\)C ambient. Based on the results of the partial pad, a portion of this error was attributed to uncertainty in the coefficient of thermal expansion along the litz wire. Differences between the FE model and experiment increased after the pad was embedded in pavement; however, RMSE remained small relative to the failure strain of ferrite. An RMSE of 25 \(\mu\varepsilon\) was obtained at 20 \({}^{\circ}\)C ambient temperature. The increase in experiment-to-simulation deviation after embedment was mainly attributed to the high sensitivity of the 'pavement' modulus to temperature. The experimental and validated finite element modelling approaches which were developed in this study could be applied to full-scale operational IPT pads to assess and improve their mechanical robustness.  
Figure 22: Sensitivity of simulated strain in the ferrite core at 1B to the axial and transverse coefficient of thermal expansion (CTE) of coil. Strain measured at 1B is also shown (dotted line).  
**Author Contributions:** Conceptualization, K.-Y.L.; Methodology, K.-Y.L.; Validation, K.-Y.L.; Formal analysis, K.-Y.L.; Investigation, K.-Y.L.; Data curation, K.-Y.L.; Writing--original draft, K.-Y.L.; Writing--review & editing, T.A., S.B. and P.K.; Visualization, K.-Y.L.; Supervision, T.A., S.B. and P.K.; Funding acquisition, S.B. All authors have read and agreed to the published version of the manuscript.  
This work was carried out with the financial support of the Ministry of Business, Innovation and Employment, New Zealand, under grant number 3714101.  
Not applicable.  
**Data Availability Statement:** The data presented in this study is available on request from the corresponding author. The data is not publicly available due to privacy.  
The authors declare no conflict of interest.