A Two-Layer Finite-Element Method to Predict the Water Uptake Behavior of Closed-Cell Foam in Extreme Moisture Environment

Closed-cell polymeric foams such as polymethacrylimide (PMI) and polyethylene terephthalate (PET) are widely used as lightweight cores in aerospace sandwich structures, yet their water uptake under immersion is governed by coupled diffusion and near-surface water retention thereby leading to centroid shift, mass increase, degraded structural performance, and potential flight safety risks. This work develops a two-layer finite element (FE) framework in which the global saturated water content and equivalent diffusion coefficient are first obtained from Fick fitting, and then divides the foam into inner and outer layers with distinct material properties, subsequently identified by minimizing the overall root-mean-square error (RMSE) between simulated and experimental water-gain curves. RMSE landscapes, one-dimensional profiles, and local parametric calculations show that the identified optima belong to compact low-error bands rather than isolated numerical spikes. The validated closed-cell foam model is finally transferred to a foam-cored composite rotor-blade section to demonstrate structural-scale applicability. The proposed framework offers a physically interpretable and computationally practical route for predicting immersion-induced water accumulation in closed-cell foams and foam-cored composite structures, contributing to improving structural reliability and safety.