374 G.A.Schoeppner,G.P.Tandon and K.V.Pochiraju the fiber axis direction.The mechanical (stiffness and strength)and thermal (coefficient of thermal expansion [52]and thermal conductivity [51]) properties of these oriented PAN-based fibers are highly anisotropic in nature.Typical density values of these fibers range between 1.75 and 1.86g cmand a porosity of approximately 15%with most of the porosity appearing in needle-shaped forms with axes of the needles parallel to the fiber axis [77].Therefore,it follows that the diffusivity of the fibers will likely be anisotropic in nature.Unfortunately,experimental measures of carbon fiber diffusivity have not been obtained. Numerous researchers have attributed the anisotropic diffusivity of unidirectional laminates to the preferential oxidation along the fiber-matrix interface or along the interphase region.However,the interphase region is a very small-volume percentage of the composite and a very small area fraction of the surface area.It is questionable that the diffusion of oxygen through and subsequent oxidation of the interphase region by itself can have such a significant effect on the composite oxidation process.More likely,the rapid oxidation rate along the fiber length indicates that,in addition to diffusion of oxygen from the specimen surface to the oxidation front,the interphase region has a supplemental oxygen path or source.Two possible mechanisms or scenarios for the supplemental oxygen source to the interphase are being investigated.Firstly,if oxidation in the interphase region leads to early fiber-matrix interface debonds,the debonds provide a pathway for oxygen to penetrate deeper into the composite.Knowing that there are residual curing stresses at the fiber-matrix interface,it is likely that such a scenario would entail the interface debond propagating along with the oxidation front.The second scenario is that the oxygen diffuses into the composite along the fiber and to the fiber-matrix interface at a rate much greater than through the neat resin or interphase.Based on the aniso- tropic oxidation behavior of unidirectional composites,the axial diffusivity of the fibers would have to be much greater than the diffusivity of the resin.Unfortunately,quantitative measures of the diffusivity of carbon fibers are lacking.Although one might anticipate that the bulk diffusivity of carbon is representative of that of the PAN fibers,this would only be the case if the morphology of the fibers and bulk carbon match,which in general is not the case. For unit cell modeling of the diffusion behavior of unidirectional com- posites,the diffusivity of the resin,the interphase,and the fiber constituents must be defined.Only the diffusivity of the resin phase can be defined with any certainty due to lack of experimental data on the diffusivity of thethe fiber axis direction. The mechanical (stiffness and strength) and thermal (coefficient of thermal expansion [52] and thermal conductivity [51]) properties of these oriented PAN-based fibers are highly anisotropic in nature. Typical density values of these fibers range between 1.75 and 1.86 g cm−3 and a porosity of approximately 15% with most of the porosity appearing in needle-shaped forms with axes of the needles parallel to the fiber axis [77]. Therefore, it follows that the diffusivity of the fibers will likely be anisotropic in nature. Unfortunately, experimental measures of carbon fiber diffusivity have not been obtained. Numerous researchers have attributed the anisotropic diffusivity of unidirectional laminates to the preferential oxidation along the fiber–matrix interface or along the interphase region. However, the interphase region is a very small-volume percentage of the composite and a very small area fraction of the surface area. It is questionable that the diffusion of oxygen through and subsequent oxidation of the interphase region by itself can have such a significant effect on the composite oxidation process. More likely, the rapid oxidation rate along the fiber length indicates that, in addition to diffusion of oxygen from the specimen surface to the oxidation front, the interphase region has a supplemental oxygen path or source. Two possible mechanisms or scenarios for the supplemental oxygen source to the interphase are being investigated. Firstly, if oxidation in the interphase region leads to early fiber–matrix interface debonds, the debonds provide a pathway for oxygen to penetrate deeper into the composite. Knowing that there are residual curing stresses at the fiber–matrix interface, it is likely that such a scenario would entail the interface debond propagating along with the oxidation front. The second scenario is that the oxygen diffuses into the composite along the fiber and to the fiber–matrix interface at a rate much greater than through the neat resin or interphase. Based on the aniso￾tropic oxidation behavior of unidirectional composites, the axial diffusivity of the fibers would have to be much greater than the diffusivity of the resin. Unfortunately, quantitative measures of the diffusivity of carbon fibers are lacking. Although one might anticipate that the bulk diffusivity of carbon is representative of that of the PAN fibers, this would only be the case if the morphology of the fibers and bulk carbon match, which in general is not the case. For unit cell modeling of the diffusion behavior of unidirectional com￾posites, the diffusivity of the resin, the interphase, and the fiber constituents must be defined. Only the diffusivity of the resin phase can be defined with any certainty due to lack of experimental data on the diffusivity of the 374 G.A. Schoeppner, G.P. Tandon and K.V. Pochiraju