Ultrasonic Evaluation Method for Mechanical Performance Degradation of Fluororubber Used in Nuclear Power Facility  
[PERSON]  
1 School of Mechanical Engineering, Zhejiang University, Hangzhou 310027, China; [EMAIL_ADDRESS] (L.W. ); [EMAIL_ADDRESS] (C.Z. ); [EMAIL_ADDRESS] (A.S.); [EMAIL_ADDRESS] (B.J) 1  
[PERSON]  
1 School of Mechanical Engineering, Zhejiang University, Hangzhou 310027, China; [EMAIL_ADDRESS] (L.W. ); [EMAIL_ADDRESS] (C.Z. ); [EMAIL_ADDRESS] (A.S.); [EMAIL_ADDRESS] (B.J) 1  
[PERSON]  
2 Computer Application Institute of Nuclear Industry, China National Nuclear Corporation, Beijing 100822, China; wutong@  
of nuclear-grade fluororubber and predicting the sealing performance and service life of in-service or stored sealing components play a crucial role in preventing accidents caused by leakage. Currently, the main methods for predicting rubber performance degradation and service life include traditional kinetic methods, computer simulation prediction methods, and constitutive model methods [3]. The kinetic method based on the Arrhenius model is used to fit empirical kinetic equations by using experimentally obtained aging data with aging times. It also uses the Arrhenius equation to establish the relationship between the aging reaction rate constants at different temperatures. This life prediction method has certain limitations in practical applications. [PERSON] et al. [1] standardized the chemical, mechanical, and thermal stability under the testing conditions of the Arrhenius method. [PERSON] employed finite element analysis based on fundamental material properties such as viscoelasticity, hyperelasticity, and chemical degradation, combined with compression stress relaxation performance, to predict the sealing performance of elastomer components under real-world conditions. The constitutive model method involves establishing the \"stress-strain\" relationship of rubber to simulate its performance changes during the aging process, thereby predicting the service life under different stress levels [4]. For example, [PERSON] proposed an empirical constitutive hyperelasticity model that takes into account the effects of aging temperature and time on its parameters, which can effectively reduce the need for mechanical property testing experiments. There are various nondestructive testing techniques for rubber materials including X-ray radiographic inspection, thermography, laser speckle interference, nuclear magnetic resonance, terahertz imaging, ultrasonic testing, and so on [5; 6; 7]. Among these, ultrasonic nondestructive testing uses ultrasonic waves as the carrier, which are mechanical waves. The acoustic properties of these waves, such as time-domain signals, sound velocity, attenuation, impedance, and scattering, have a direct correlation with the material's mechanical properties (e.g., elastic modulus, shear modulus, and viscoelasticity) as well as its microstructure [8; 9]. This makes ultrasonic testing one of the most promising nondestructive methods for evaluating the mechanical properties of rubber. Ultrasonic testing has proven to be an effective nondestructive evaluation technique for detecting defects and assessing material degradation due to its sensitivity to changes in mechanical properties [10]. Traditional linear ultrasonic methods, which rely on sound velocity and attenuation coefficients, are effective for determining the elastic properties of materials and identifying macroscopic defects [11; 12; 13]. However, these methods are limited in their ability to detect damage before the onset of crack formation [14]. In contrast, nonlinear ultrasonic testing is particularly adept at detecting subtle defects by analyzing waveform distortions during sound wave propagation through materials without causing damage. When a defect is present, the stress-strain relationship at the defect site deviates from [PERSON]'s law, transitioning from linear to nonlinear behavior.