## 5 Conclusions  
An ultrasonic testing system for measuring the internal stress in GIS epoxy composites with transverse waves was established. Conclusions can be summarized as follows:  
1. The acoustoelastic effect is verified using the transverse waves in GIS epoxy composites. The velocities of ultrasonic transverse waves increase linearly with the compressive stress and decrease linearly with the tensile stress. The acoustoelastic coefficients are \(-\)1.18 \(\times\) 10\({}^{-4}\) /MPa for transverse wave polarization vertical to stress and \(-\)1.96 \(\times\) 10\({}^{-4}\) /MPa for transverse waves parallel to stress.
2. The maximum absolute values of relative errors are 35.4% and 26.3% for stress measurements with transverse wave polarization vertical and parallel to the stress direction, respectively.
3. For the tangential internal stress measurements in GIS epoxy composites, in low-stress states, applying the transverse wave with polarization parallel to the stress obtains more accurate results compared to the transverse wave with polarization vertical to the stress and longitudinal waves. The maximum absolute value of relative error is 26.3% under 10 MPa compressive stress.  
This research demonstrates that ultrasonic stress measurements for GIS composites with transverse waves are more accurate than measurements with longitudinal waves in the low-stress state. The method proposed in this article provides a basis for investigating the internal stress in GIS epoxy insulators. Studies employing Artificial Intelligence schemes to increase result accuracy and reduce measurement error caused by manual operation are carried out.  
**Supplementary Materials:** The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app132011317/s1](https://www.mdpi.com/article/10.3390/app132011317/s1)]([https://www.mdpi.com/article/10.3390/app132011317/s1](https://www.mdpi.com/article/10.3390/app132011317/s1)), Figure S1: Mesh division for tensile sample finite; Figure S2: Simulation result for epoxy sample under 30 MPa tensile stress.  
**Author Contributions:** Conceptualization: Y.R. and Z.Z.; methodology: F.T., Y.R. and Z.Z.; validation: H.Q., F.T. and W.H.; formal analysis: Y.G., F.Z. and Z.Z.; investigation: Y.R., H.M. and Z.H.; writing--original draft preparation: C.K. and Z.Z.; writing--review and editing: H.Q. and C.K.; funding acquisition: H.Q. and Z.Z. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This study was funded by scientific research projects of Shenzhen Polytechnic University, grant numbers 6022312038K and 6022312037K.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The data that support the findings of this study are available from the corresponding author upon request.  
**Conflicts of Interest:** Author [PERSON] was employed by the company Guangzhou Bureau, EHV Power Transmission Company of China Southern Power Grid. Author [PERSON] was employed by company Guangzhou Power Supply Bureau, Guangdong Power Grid. Author [PERSON] was employed by company China Southern Power Grid Research Technology Co. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.