Molecular interfacial shearing creep behavior of carbon fiber/epoxy matrix interface under moisture condition

During the intended service-life, carbon fiber-reinforced polymer (CFRP) composite is inevitably exposed to moisture and external loading conditions. Here, molecular simulation is used to investigate the moisture effect on interfacial creep responses of carbon fiber/epoxy matrix interface subjected to sustained loading. The molecular model is constructed by bonding epoxy molecule on top of graphite sheets representing the fiber outer-layer. In wet case, water molecules are added inside interface, and graphite sheets are subjected to different shearing load levels to simulate external loadings. Compared with dry case, the threshold stress and energy barrier for the onset of creep failure decrease by 19.8-20.0% and 18.5%, respectively. Strain and stress evolution associated with configurational changes show that shearing deformation is larger and interfacial sliding occurs faster in wet case. It is found that water molecules aggregated near fiber surface and around epoxy functional groups interrupt the molecular interactions and degrade the bonding properties and mechanical responses of fiber/matrix interface, which accelerates the interfacial sliding and debonding process under shearing loading, and hence the interfacial resistance to shearing loading is significantly reduced in wet case. This study provides molecular insights into interfacial degradation under moisture and sustained loading conditions, which form the basis for predicting the interfacial degradation of CFRP composites during the intended service-life.