In this paper the load transfer mechanism in a carbon nanotube (CNT) reinforced polymeric composite is considered. It is assumed that the polymer matrix is reinforced with single-walled carbon nanotubes and that the continuum model with adjustments is valid in estimating the effective properties of the composite. The existing studies contradict each other with respect to effective load transfer between the matrix and the nanotubes. In this study we show that there is a critical CNT length below which the load transfer is not effective. Thus for an effective load transfer the CNT length must exceed a critical length. To determine the critical length we consider a CNT embedded in a polymer matrix. The polymer/CNT interface is modeled as a distinct layer with elastic properties different than those of the CNT and the matrix. The strain energy change due to the inclusion of a CNT in a polymer matrix is then computed for various interphase stiffnesses using the finite element method. The variation of the strain energy per unit fiber length ΔU/L is plotted versus the aspect ratio of the CNT, L/D. It is observed that ΔU/L first increases steeply with L/D and then reaches a plateau. Since the region of constant ΔU/L is associated with uniform stress distribution, we define the critical CNT length as 90% of the asymptotic value of ΔU/L. It is shown that the load transfer is affected by the nature of the interphase. Next, using a dilute solution the effective moduli of the composite are derived for the cases of both hard and soft interphase. The results indicate that the nature of matrix/CNT interface affects the effective moduli of the composite only slightly.

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