## 4 Results
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Specimen**} & \multicolumn{2}{c}{**Mechanical Model**} & \multicolumn{2}{c}{**Acoustic Model**} \\ \cline{2-5}  & **Parameter** & **Fitted Value** & **Parameter** & **Fitted Value** \\  & **Name** & **Time** & **Time** & **Time** \\ \hline \multirow{3}{*}{Used sealing ring} & \(k_{1}\) & 2.20 \(\pm\) 0.15 GPa & \(K_{s}\) & 2.97 \(\pm\) 0.15 GPa \\  & \(k_{2}\) & 1.42 \(\pm\) 0.23 GPa & \(H\) & (2.22 \(\pm\) 0.2) \(\times\) 10\({}^{-9}\) \\  & \(\eta_{0}\) & 6.14 \(\pm\) 0.35 Pa-s & \(\rho\) & 1380 \(\pm\) 21 kg/m\({}^{3}\) \\  & \(\eta_{1}\) & 32.55 \(\pm\) 1.26 Pa-s & \(\beta\) & (1.19 \(\pm\) 0.57) \(\times\) 10\({}^{-5}\) s/rad\(\cdot\)m \\ \hline \multirow{3}{*}{Unused sealing ring} & \(k_{1}\) & 1.99 \(\pm\) 0.08 GPa & \(K_{s}\) & 2.75 \(\pm\) 0.22 GPa \\  & \(k_{2}\) & 1.51 \(\pm\) 0.15 GPa & \(H\) & (2.15 \(\pm\) 0.18) \(\times\) 10\({}^{-9}\) \\  & \(\eta_{0}\) & 6.63 \(\pm\) 0.27 Pa-s & \(\rho\) & 1357 \(\pm\) 16 kg/m\({}^{3}\) \\  & \(\eta_{1}\) & 25.23 \(\pm\) 0.26 Pa-s & \(\beta\) & (2.37 \(\pm\) 0.63) \(\times\) 10\({}^{-5}\) s/rad\(\cdot\)m \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The fitting results of the model parameters of the fluororbuber sealing specimens. Figure 10: Strain–load curves of the fluororbuber sealing specimens. The elongation at break of the used sealing ring was 136.47%, much lower than that of the unused ring, which was 450.48%. internal voids. Therefore, the acoustic attenuation in fluororubber is primarily caused by the scattering of long-chain macromolecules and the relaxation effects inherent to the elastomer itself. As a result, the ultrasonic attenuation in the unused sealing rings was higher than in the used sealing rings. Figure 11: The SEM images of the fluororubber specimens. (**a**) Cross-section of the used sealing ring, (**b**) cross-section of the unused sealing ring, (**c**) large number of holes in the cross-section of the used sealing ring, and (**d**) microcracks on the aged surface of the used sealing ring. Purple circles in panels (**c**) and (**d**) indicated holes and microcracks, respectively. In the experiment, it was also observed that the ultrasonic reflection coefficient exhibited multiple peaks as the frequency changed, with the frequency difference between adjacent peaks approximately 3.4 MHz. This is a typical ultrasonic interference phenomenon in thin-layer media [29, 30]. At the same time, from the waveform diagrams in Figures 8a and 9a, a clear echo aliasing phenomenon was also observed. Both phenomena suggested that there were additional interfaces between the IF0 and IF1 interfaces. According to the manufacturing process of fluororubber, to enhance the sealing performance and durability of fluororubber sealing products, surface treatment processes such as coating were introduced during production. This step created a hardened layer on the surface of the fluororubber, generating a unique interface. The \"multi-peak\" phenomenon in the ultrasound reflection coefficient spectrum of the unused sealing ring was significantly stronger than that of the used sealing ring. This also indicates that after the surface hardened layer of the sealing ring eroded during service, the mechanical properties of the material degraded. As shown in Figure 11b,c, scanning electron microscopy (SEM) observations also revealed that the surface of the unused specimens was smoother than that of the used specimens. The surface of the used specimens exhibited numerous microcracks, approximately 1 to 2 microns wide and 100 microns long. The formation of these cracks may have been due to the attack of oil-based tertiary amines on the proton (H) in the --CH\({}_{2}\)CF\({}_{2}\)-- group of the molecular chain, leading to a deprotonation process, followed by a dehydrofluorination reaction. This caused damage to the main chain structure of the sealing rings, resulting in performance degradation. Under the working temperature and pressure, further cracking occurred.