Modeling nonlinear, hysteretic, and irreversible moisture-induced deformation of CFRP based on two-phase diffusion theory

Abstract

Water or moisture absorption causes swelling and plasticization of polymers and polymer matrices in composites. Predicting the long-term deformation of composite structures is crucial for the feasibility study of advanced space observation satellites. This study developed a model to explain the nonlinear, hysteretic, and irreversible moisture-induced deformation of epoxy-based CFRP structures during moisture absorption and desorption. The model was based on a modified two-phase diffusion theory, incorporating molecular-level analyses of water-polymer interactions and polymer networks. Water transport and moisture-induced deformation mechanisms were comprehensively interpreted using gravimetric analysis, nuclear magnetic resonance (NMR), positron annihilation lifetime spectroscopy (PALS), and optical fiber sensing. The results showed that bound water diffused preferentially during absorption, while free water transport dominated during desorption. Even after long-term desorption, some water remained strongly bound to hydrophilic sites in the epoxy matrix, leading to irreversibility in the system. The modified two-phase diffusion models successfully captured this behavior. The nonlinear strain behavior was then formulated using a piecewise linear function model, which accounted for both volume expansion due to plasticization and contraction due to anti-plasticization, depending on the bound water concentration. The model reproduced the strain hysteresis observed in the experiment, confirming that the hysteresis resulted from the differences in water diffusion behavior during absorption and desorption. Finally, the model’s prediction was compared with the long-term deformation measurement of a composite tube, validating its accuracy in predicting moisture-induced deformation in a practical structure.