The stiffness degradation of fiber reinforced polymer (FRP) has significant effects on its fatigue life and reliability. In this study, the inherent correlation between stiffness degradation and strength degradation is explored, and a probability model that describing stiffness degradation of FRP is presented. Firstly, the evolution of fatigue damage in FRP is analyzed, and then it is characterized by stiffness degradation and strength degradation, respectively. Secondly, the randomness of initial stiffness and critical stiffness of FRP is considered, and a probability model of stiffness degradation is proposed based on the same damage state principle. Finally, the test data of FRP laminates are used to verify the proposed model, and the fitting accuracy is compared with several existing models. The results show that the stiffness degradation of FRP presents a fast-slow-fast trend during fatigue failure process, and the proposed model is capable of fitting stiffness degradation data of FRP with good agreement.

纤维增强聚合物（FRP）的刚度下降对其疲劳寿命和可靠性具有重要影响。在这项研究中，探讨了刚度退化和强度退化之间的内在联系，并提出了描述FRP刚度退化的概率模型。首先分析了FRP中疲劳损伤的演变，然后分别以刚度退化和强度退化为特征。其次，考虑了玻璃钢初始刚度和临界刚度的随机性，并基于相同的损伤状态原理，提出了刚度退化的概率模型。最后，使用FRP层压板的测试数据来验证所提出的模型，并将拟合精度与几种现有模型进行比较。