A semi-empirical model was developed to investigate the hygrothermal aging-induced degradation of mechanical properties in fiber-reinforced polymer (FRP) composites, integrating the effects of physical and chemical changes during aging. For physical changes, a shape parameter ( \(\eta\) ) was introduced to characterize the nonlinear relationship between the humidity influencing factor and moisture concentration. For chemical changes, a hydrolysis-related parameter ( \(\lambda\) ) was incorporated, as the hydrolysis of composite components follows first-order reaction kinetics. To verify the model’s applicability and accuracy, experimental data of two FRP composites (unidirectional (UD) E-glass/vinylester composites and nanofibers/epoxy composites) were used for fitting, and a comparative analysis was conducted against three classical models (Phani, Phillips, and Bulmanis). Results show the proposed model outperforms traditional ones: it has high prediction accuracy (coefficient of determination R2 averages over 0.96 for separate temperature fitting and 0.88 for multi-temperature fitting); its parameters are reasonably distributed (avoiding overfitting); it has excellent extensibility—due to the temperature-dependent term, it can predict properties under untested temperatures without refitting; and it applies to various FRP systems and mechanical properties (e.g., tensile, flexural strength).
