Difficulties in the Theories for Predicting Transverse Thermal Conductivity of Continuous Fiber Composites

A large number of theories of varying degrees of sophistication are available for predicting the transverse thermal conductivity of continuous fiber composites. These theories assume a knowledge of fiber and matrix properties and volume fractions, interphase properties (if required), and fiber architecture. The fiber transverse conductivity is normally difficult to measure directly due to the small fiber diameter. The usual suggested method is to measure matrix and composite conductivity, and infer (“back out”) the fiber conductivity. This paper reviews several pathological conditions that can occur for composites where the fiber conductivity is relatively large compared to the matrix conductivity, with particular application to graphite fibers. These conditions include great sensitivity to measured composite conductivity, to the model used, and to the measured fiber volume fraction, as well as failure to predict fiber values. Some suggestions for improving the results are discussed.