47th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference
1 - 4 May 2006, Newport, Rhode Island

AIAA 2006-1678

Micromechanics Modeling of Functionally Graded
Interphase Regions in Carbon Nanotube-Polymer
Composites

Gary D. Seidel and Dimitris C. Lagoudas
Texas A&M University, College Station, TX, 77843-3141, USA

Sarah Jane V. Frankland
National Institute of Aerospace, Hampton, VA, 23666-1399, USA

Thomas S. Gates
NASA Langley Research Center, Hampton, VA, 23681, USA

The effective elastic properties of a unidirectional carbon fiber/epoxy lamina in which
the carbon fibers are coated with single-walled carbon nanotubes are modeled herein
through the use of a multi-scale method involving the molecular dynamics/equivalent continuum
and micromechanics methods. The specific lamina representative volume element
studied consists of a carbon fiber surrounded by a region of epoxy containing a radially
varying concentration of carbon nanotubes which is then embedded in the pure epoxy
matrix. The variable concentration of carbon nanotubes surrounding the carbon fiber
results in a functionally graded interphase region as the properties of the interphase region
vary according to the carbon nanotube volume fraction. Molecular dynamics and
equivalent continuum methods are used to assess the local effective properties of the carbon
nanotube/epoxy comprising the interphase region. Micromechanics in the form of
the Mori-Tanaka method are then applied to obtain the global effective properties of the
graded interphase region wherein the carbon nanotubes are randomly oriented. Finally,
the multi-layer composite cylinders micromechanics approach is used to obtain the effective
lamina properties from the lamina representative volume element. It was found that
even very small quantities of carbon nanotubes (0.36% of lamina by volume) coating the
surface of the carbon fibers in the lamina can have a significant effect (8% increase) on
the transverse properties of the lamina (E22, v23, G23 and G12) with almost no affect on the
lamina properties in the fiber direction (E11 and v12).