## 4 Results  
The reflection coefficient at the water-fluororubber interface was simulated using the viscoelastic and acoustic evaluation models presented in this study. The specific parameters used in the simulation are summarized in Table 1. \begin{table}
\begin{tabular}{c c} \hline
**Parameters** & **Value** \\ \hline \(k_{1}\) & 2.6 GPa \\ \(k_{2}\) & 0.2 GPa \\ \(\eta_{0}\) & 5 Pa-s \\ \(\eta_{1}\) & 80 Pa-s \\ Density of water \(\rho_{0}\) & 1000 kg/m\({}^{3}\) \\ Speed of sound in water \(c_{0}\) & 1480 m/s \\ \hline \end{tabular}
\end{table} 
Table 1: Simulation parameters of the reflection coefficient of the water–fluororubber interface. Figure 6: Spline stretching (**a**) and fracture (**b**). The simulation results are shown in Figure 7. It can be seen that the reflection coefficient of the water-fluororubber interface varied with both the angle and frequency. Within the small angle range, when the frequency was fixed, the reflection coefficient decreased as the angle increased, which was related to the conversion of some energy into shear waves. When the frequency changed, the reflection coefficient exhibited a minimum value. The frequency corresponding to this minimum value was related to the relaxation time \(\tau\). As shown in Figure 8a, ultrasonic echo analysis of the fluororubber gasket specimen was conducted using a 2.5 MHz line-focus ultrasonic transducer. The main frequency of the echo signal shifted from 2.65 MHz in the IF0 interface echo to 2.45 MHz in the IF1 interface echo, with a noticeable frequency shift and attenuation phenomena. The ultrasonic echo data acquisition and reflection coefficient spectrum calculation of the fluororubber sealing ring specimens were carried out using a line-focused ultrasonic transducer with a center frequency of 25 MHz and a \(-6\) dB bandwidth from 18 MHz to 32 MHz. Before the experiment, the pulse reflection energy of the ultrasonic transmitter-receiver was adjusted to 32 \(\mu\)J, and the amplification gain was set to 26 dB. As shown in Figure 9a, the echo signal waveform obtained from the IF0 interface clearly indicated that the echo amplitude of the used sealing ring specimen was significantly higher than that of the unused sealing ring specimen. The echo signal also exhibited a noticeable echo aliasing phenomenon. Figure 8: Ultrasonic echo signals and their spectra conducted using a 2.5 MHz line-focus ultrasonic transducer. **(a)** Ultrasonic echo signals including the echo signals from IF0 and IF1. **(b)** The spectrum of echo signals from IF0 and IF1. The main frequency of the echo signal shifted from 2.65 MHz (IF0 interface echo) to 2.45 MHz (IF1 interface echo). Figure 7: Simulation results of the reflection coefficient of the water–fluororubber interface. The reflection coefficient of the water–fluororubber interface varied with both the angle and frequency. Due to the higher attenuation coefficient of high-frequency ultrasound in fluororubber, the IF1 interface echo could not be effectively identified. Therefore, only the IF0 echo was used for the reflection coefficient fitting and solving. The parameters \(K_{s}\), \(H\), \(\rho\), \(k_{1}\), \(k_{2}\), \(\eta_{0}\), and \(\eta_{1}\) were obtained by fitting the reflection coefficient curve at the normal incidence angle. In the process of fitting \(k_{1}\), \(k_{2}\), \(\eta_{0}\), and \(\eta_{1}\), the density value of the fluororubber, \(\rho\), was first obtained experimentally, and then \(\rho\) was treated as a known quantity to construct the equations. The ultrasound reflection coefficient curves obtained from the measurements are shown as the star-dot curves in Figure 9c,d. The results based on the model fitting are represented by the solid red lines. The fitting results using multivariate nonlinear regression, based on the reflection coefficient spectrum, are expressed in terms of mean + variance, as shown in Table 2.  
From the mechanical model fitting results, it can be observed that the elastic modulus of the used sealing ring was significantly higher than that of the unused sealing ring.