Characterization of carbon fibers: coefficient of thermal expansion and microstructure

Abstract

The focus of the research is to develop a consistent and repeatable method to evaluate the coefficient of thermal expansion (CTE) of carbon fibers at high temperatures. Accurate measurement of the CTE of carbon fibers is essential to understand and develop optimal processing procedures as well as computational simulations to predict properties and allowables for fiber-reinforced composites. The mismatch between the coefficient of thermal expansion of the fiber and the matrix has a profound impact on the development of residual stresses and the subsequent damage initiation and progression, potentially diminishing the performance of composite structures. In situ transmission electron microscopy (TEM) is selected to perform the experimental work on account of the high resolution and the capability of evaluating both the longitudinal and transverse CTE. The orthotropy in the CTE is tested by rotating the fibers through 45° about their axis. The method is validated by testing standard tungsten filaments of known CTE. Additionally, the microstructure of the fibers is studied in a field emission scanning electron microscope as well as through selected area diffraction patterns in a TEM to observe presence of any potential orthotropy. The pitch based P55 fiber revealed a cylindrically orthotropic microstructure, but the PAN based IM7 and T1000 fibers did not reveal any orthotropy. Finite element models of hexagonally arranged IM7 fibers in a 977 epoxy matrix are developed using PATRAN and analyzed using the commercial FEA code ABAQUS 6.4. The fiber properties were considered temperature independent where as the matrix properties were varied linearly with temperature. The lamina properties evaluated from the finite element modeling are in agreement with the experimental results in literature within 10% in the temperature range of room temperature to the stress free temperature of the epoxy, however at cryogenic temperatures the difference is greater. The residual stresses developed during processing of the composite indicated a potential location for fiber matrix debonding to be in the matrix dominant regions.