Revealing interphase-driven stiffness and stability in hybrid CNT/CF polymer composites via multiscale FEM

A finite element-based multiscale modeling framework is developed to investigate the effect of the interphase on the mechanical behavior of carbon nanotube/carbon fiber-reinforced polyimide (CNT/CF-RP) composites. The modeling approach consists of two hierarchical steps: (i) nanoscale dispersion of CNTs within the polymer matrix, where an interphase region is explicitly incorporated to represent van der Waals interactions, and (ii) microscale distribution of carbon fibers within the equivalent CNT-reinforced polymer. Random orientations of CNTs and CFs are considered to ensure realistic representation of the composite morphology. The equivalent elastic properties obtained from the multiscale model are employed to analyze the bending, buckling, and free vibration responses of thick composite plates under various boundary conditions. Results reveal that incorporating the interphase increases the effective elastic modulus by up to ≈5%, indicating its significant role in stress transfer across the CNT–polymer interface. Hybrid reinforcement combining CNTs and CFs leads to a nearly 200% improvement in stiffness relative to the pure matrix, with notable enhancements in critical buckling loads and natural frequencies. The proposed framework effectively bridges nanoscale interfacial phenomena with macroscale structural performance, providing valuable guidance for the design and optimization of next-generation lightweight, high-strength hybrid composites for aerospace and advanced engineering applications.