Multi‐Stage Degradation Modeling of the Mechanical Performance of CFRP Plate in Marine Environments

ABSTRACT This study investigates the long‐term tensile performance of pultruded carbon fiber reinforced polymer (CFRP) plates exposed to seawater and proposes a three‐stage, mechanism‐informed framework to describe strength evolution. Unidirectional coupons were immersed in natural seawater at 25°C, 45°C, and 60°C for up to 12 months, followed by periodic tensile testing together with thermal, spectroscopic, and microscopic characterization to examine possible degradation processes. The results showed a non‐monotonic evolution in tensile strength, consisting of an initial decrease, an intermediate recovery, and a subsequent long‐term decline. At 60 days, the tensile strength retention reached 103.85%, 107.13%, and 101.37% at 25°C, 45°C, and 60°C, respectively. After 360 days of immersion, the corresponding retention values decreased to 87.52%, 84.26%, and 76.91%, demonstrating that elevated temperature accelerated long‐term tensile degradation. The early reduction is attributed primarily to moisture‐induced matrix plasticization, whereas the intermediate recovery may be associated with changes in the epoxy network during continued exposure. At longer times, irreversible hydrolytic degradation becomes dominant, leading to progressive strength loss. Based on these observations, a three‐stage formulation incorporating plasticization loss, transient recovery, and hydrolytic degradation was established within a unified retention function. Model parameters were calibrated using strength‐retention data at different temperatures and showed temperature dependence consistent with coupled diffusion‐ and reaction‐related processes. The proposed three‐stage model captured the main strength–time characteristics and provides a basis for durability assessment of CFRP plates in marine applications.