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It has been previously reported that the average properties of carbon nanotube-coated carbon fiber/polyester multiscale composites critically depend on the length and density of nanotubes on the fiber surface. In this paper the effect of nanotube length and density on the interfacial properties of the carbon nanotube-coated carbon fiber–polymer interface has been studied using shear lag and a cohesive zone model. The latter model incorporates frictional sliding after complete debonding between the fiber and matrix and has been developed to quantify the effect of nanotube coating on various interfacial characterizing parameters. Our numerical results indicate that fibers with an optimal coverage and length of nanotubes significantly increase the interfacial strength and friction between the fiber and polymer. However, they also embrittle the interface compared with bare fibers.

Under compressive loads, single-walled carbon nanotubes (SWNTs) are known to behave like cylindrical shells at small aspect ratios and undergo a shell to column transition at somewhat larger aspect ratios. At even larger aspect ratios SWNTs are capable of undergoing large elastic deformations without suffering localized plastic kinks. In this work, we show that a very long SWNT can be modeled as a Kirchhoff elastica with a small initial twist. The elastic properties of the nanotubes, required to model them as elastica, are obtained from MD simulations on short SWNTs. The total twist on particular straight nanotube depends on its length, diameter and chirality and though small, affects the post buckling deformation of long nanotubes significantly. The modeling of a long SWNT as a twisted elastica may be effectively used to search for their possible folded and coiled equilibrium configurations under various ambient conditions.