Progressive Irreversible Deterioration: Damage Accumulation and Life Prediction of CF/PEEK Composites in Cyclic Hydrothermal Environments

Carbon fiber (CF) reinforced polyether ether ketone (PEEK) (CF/PEEK) composites are gaining increasingly widespread application due to their excellent properties. However, the permanent deterioration mechanism under periodic hydrothermal aging and a reliable service life prediction framework remain unclear, severely constraining their engineering application. This study reveals that the periodic hydrothermal aging behavior of CF/PEEK composites is characterized by a fundamental transition from reversible recovery to permanent deterioration across multiple properties. The moisture absorption and desorption follow Fickian and inverse Fickian diffusion models, respectively, with reversible moisture absorption progressively converting into permanent absorption over cycles. Similarly, reversible decline in bending strength and reversible growth in surface roughness irreversibly shift to permanent deterioration and permanent growth. Notably, the first hydrothermal aging cycle induces the most severe physical and chemical degradation. It causes the most pronounced increase in surface roughness and the most significant chemical deterioration, including prominent chain scission at ether bonds, aromatic ring opening, and a marked rise in oxygen-containing functional groups. This initial damage establishes irreversible defects that accumulate in subsequent cycles, leading to permanent declines in thermomechanical properties and crystallization behavior. Based on the irreversible deterioration kinetics, a service life prediction model is established, quantifying the rapidly decreasing critical aging time with increasing cycles and highlighting the dominant role of the first aging cycle. This work provides both theoretical insight into the irreversible degradation mechanisms and a practical tool for predicting the serviceability of CF/PEEK composites in cyclic hydrothermal environments.