## 3 Experiments
### Experimental Design for Detection  
The sealing ring specimens, both in service and unused, were each cut into 40 mm segments. Five segments were used for the ultrasonic measurements, five segments for the mechanical tensile testing, and two segments for electron microscope analysis.  
The steps for the ultrasonic measurement experiment of the sealing ring specimens were as follows. (1) Adjust the room temperature of the experimental space to 20 \(\pm\) 1.5 \({}^{\circ}\)C, fix the ends of the cylindrical specimen of the truncated sealing ring to ensure that the bottom is more than 2 mm away from the bottom of the container. (2) Pour purified water into the container and make sure the temperature of the purified water is 20 \(\pm\) 0.5 \({}^{\circ}\)C. (3) Observe the amplitude of the echo signal at the 0 th interface, and adjust the spatial alignment between the line-focused ultrasonic transducer and the cylindrical specimen so that their axes are parallel and the upper surface of the specimen is located at the focal point of the transducer. (4) Move the line-focused ultrasonic transducer upward by 6.5 mm, and collect echo data every 0.1 mm from top to bottom until the transducer is 6.5 mm below the focal position, obtaining the \(V(z,t)\) data. (5) Repeat the steps to obtain the \(V_{ref}(z,t)\) data of reference specimen.  
Figure 4: Fluororubber specimens for the ultrasonic measurement experiments. (**a**) The fluororubber sealing ring serviced for one year, (**b**) the brand new fluororubber sealing ring, and (**c**) fluororubber sealing gasket specimens purchased externally.  
Figure 5: (**a**) Reference specimen and (**b**) its theoretical reflection coefficient curve at different angles in 20 \({}^{\circ}\)C water. When the incident angle is within 10\({}^{\circ}\), the acoustic reflection coefficient on the surface of the specimen approaches a fixed value.  
The above steps were repeated using a line-focused ultrasonic transducer (GE Alpha ISS, Buford, GA, USA) with excitation frequencies of 2.5 MHz and 25 MHz, respectively, to analyze the reflection coefficients in the ranges of 1 MHz-4 MHz and 15 MHz-35 MHz.  
For the purchased fluororubber sealing gasket specimen, the aging test was conducted according to the relevant standards of the GB/T 3512 Test Methods for Accelerated Aging and Heat Resistance of Thermoplastic Rubber in Hot Air [27]. The test was performed at 70, 80, 90 and 100 \({}^{\circ}\)C (RH of 50%), and the testing time points were 1, 3, 6, 9 and 12 d, respectively.  
After aging, the specimens were removed from the test chamber and placed in a room temperature environment at 20 \(\pm\) 1.5 \({}^{\circ}\)C for 1 h before conducting the ultrasonic measurement experiments. Under the condition that the specimens were flatly clamped, the ultrasonic measurement steps of the fluororubber sealing gaskets specimens were not very different from those of the sealing ring specimens. The only difference was that a 25 MHz point-focused ultrasonic transducer (V324-N-SU, Olympus Corporation, Japan) was used.  
Mechanical tensile tests were carried out on a universal material testing machine (Roell Z020, Zwick, Ulm, BW, Germany) with a tensile speed of 50 mm/min. To enhance the friction and prevent the specimen from slipping during the stretching process, sandpapers were wrapped around the gripping positions at both ends of the fluororubber specimen, ensuring that the tensile limit could be accurately measured. The specimen was clamped at the lower end, and the upper fixture was adjusted to a position vertically above the lower fixture, 20 mm apart. After resetting the load to zero, the upper fixture was tightened and the tensile test began. As shown in Figure 6, the upper fixture moved upward, while the load and strain data were recorded in real-time until the specimen broke.