Multi-scale study of the mechanical response at carbon fiber/matrix interphase under moisture-saturated conditions: Insights from molecular dynamics and finite element simulations

The moisture‑saturated carbon fiber/matrix interphase exhibits intricate multi‑scale physical gradients that hinder the separation of its inherent properties from moisture‑driven behaviors, limiting direct insight and comprehensive mechanical characterization. To address this, a multi-scale numerical analytical method integrating molecular dynamics (MD) and finite element (FE) simulations was developed to investigate the degradation of mechanical properties and multi-scale damage mechanisms in carbon fiber/matrix interphase under moisture‑saturated conditions. The finite-thickness interphase was homogenized via an exponential gradient model incorporating moisture‑induced degradation coefficients from MD simulations. The critical cohesive element parameters were calibrated through a coupled experimental-computational approach, simultaneously validating the reliability of the analysis method. The results showed that moisture saturation reduced interphase debonding strength by 8.57 %. At the molecular scale, the weakening of non-bonded interactions alongside the strengthening of hydrogen bonding serves as the primary driving mechanism for strength degradation. Uncontrolled molecular slippage and enhanced diffusion of water molecules induced local debonding, which propagated along weak interfacial paths at the microscale and culminated in observable failure. This comprehensive methodology elucidates multi-scale moisture‑induced damage processes and will provide valuable guidance for designing more moisture‑resistant composites.