Characterization of CFRP interface properties with varying fiber surface roughness using AFM measurements and finite element modeling

The microscale surface roughness and surface chemical bonding characteristics of carbon fibers (CFs) vary significantly depending on manufacturing and post-processing conditions, critically influencing the overall mechanical properties of CFRP composites. A comprehensive investigation of both chemical bonding and mechanical interlocking mechanisms at fiber–matrix interface is therefore essential for accurate characterization of interfacial behavior. In this study, a novel modeling approach is developed for the first time by directly incorporating AFM images of CF surfaces, which represent the actual surface topography, into the finite element simulation to systematically investigate interfacial behavior in longitudinal, transverse, and normal directions relative to fibers, which are difficult to evaluate from typical experiments. Three types of CFs with different surface roughness including de-sized CFs, heat-treated CFs, and plasma-treated CFs, are investigated in the interface modeling. Additionally, an idealized CF with a smooth surface was also included to isolate and evaluate the influence of surface roughness itself. Simulations reveal that under normal loading, chemical bonding is the sole interaction at the interface, and interface properties are proportional to CF surface areas. Under transverse and longitudinal loading, both chemical bonding and mechanical interlocking coexist: chemical bonding dominates the early stage of debonding, while mechanical interlocking becomes the primary load transfer mechanism as debonding progresses. It was discovered that the contribution of chemical bonding on interfacial response is weak under influence of surface roughness of CF. This study demonstrated that appropriate numerical characterizations are essential for accurately predicting the properties of composites prior to homogenization analysis of CFRP.