## 4 Results
As shown in Figure 10, the results of the mechanical tensile tests also confirmed the above conclusion. The tensile strength of the unused specimen was 12.24 \(\pm\) 0.17 MPa, while the tensile strength of the used specimen was 35.65 \(\pm\) 0.73 MPa. The elongation at break of the used specimen was only about 30% of that of the unused specimen. The thermal-oxidative aging tests of fluororubber in a hot-humid environment showed that the tensile  
Figure 9: Ultrasonic echo signals and reflection coefficient curve. (**a**) The ultrasonic echo signal amplitude from IF0 of the used sealing ring was higher than that of unused sealing ring. (**b**) In the spectrum of echo signals at the upper and lower interfaces, a noticeable echo aliasing phenomenon could be seen in the ultrasonic echo signal of the used sealing ring. (**c**) Model fitting curves and measured curve of the used sealing rings at the normal incidence angle. The values of the three curves were close, and their trends were similar. (**d**) Model fitting curves and the measured curve of the unused sealing rings at the normal incidence angle. The measured curve showed both increasing and decreasing fluctuations, but the overall trend followed that of the fitting curves, exhibiting a general increasing trend. strength of the material decreased from 12.24 MPa to 7.56 MPa, while the elongation at break decreased from 400% to 280%. The main mechanism behind this degradation was the rupture of crosslinking bonds and oxidative cleavage, which was not consistent with the results obtained from the used sealing ring specimens. The sealing rings involved in this paper were in service under high temperature and high oil conditions. The decrease in elongation at break observed during the oil-thermal aging test was more consistent with the actual situation [3; 28]. This may have been due to the interaction between the oil medium molecules and the rubber, which accelerated the aging and degradation reactions of the rubber. Additionally, the diffusion and plasticizing effects of the oil medium led to an increase in the hardness of the sealing ring. Using the fitting results from the acoustic model, specific values for the sound velocity and impedance were further obtained. It was found that the sound impedance of the used sealing ring was significantly higher than that of the unused sealing ring. In addition, the sound attenuation coefficient of the unused sealing ring was noticeably higher than that of the used sealing ring, which was positively correlated with the strain variable of the specimen. Due to the limited specimen size, the fitting results from the mechanical model alone did not support this conclusion. As shown in Figure 11a,c, the specimens' cross-sections were observed using a field emission scanning electron microscope (FE-SEM), revealing that the cross-section of the unused specimen was rougher compared with the used specimen. The unused specimen showed more fibrous and coiled structures, indicating that the rubber underwent ductile fracture due to its good elasticity. On the other hand, the cross-section of the used specimen was flatter, which characterizes aging and hardening, suggesting changes in the crosslinking properties. Additionally, holes and pits, ranging from a few micrometers to tens of micrometers in diameter, were observed. These might be defects caused by rubber degradation and filler detachment, resulting in