Effect of Fiber Orientation and Sample Geometry on the Effective Thermal Conductivity of a Uniaxial Carbon Fiber-Reinforced Glass Matrix Composite

From solutions for the thermal conductivity of anisotropic solids, it is pre dicted that the effect of fiber angle on the thermal conductivity of uniaxial fiber-reinforced composites is a function of sample geometry. Experimental data for the effect of fiber an gle on the thermal conductivity of a uniaxial carbon fiber-reinforced aluminoborosilicate glass matrix composite taken from three different sample geometries are presented in sup port of this conclusion. The thermal conductivity of the center portion of samples with a thickness in the direction of heat flow much smaller than their width agreed very well with the corresponding effect predicted for a composite plate of infinite extent. Similar results were obtained for "angled" samples, with a geometry such that the net direction of heat flow was parallel to the fiber direction. In contrast, for samples of rectangular geometry with a thickness of the order of the width, the thermal conductivity data for fiber angles greater than about 45° agreed very well with those predicted for a narrow composite strip with thermally insulated sides. At lower fiber angles, the data for these latter samples were intermediate to those predicted for the infinite composite plate and the insulated strip. The results of this study imply that care needs to be taken in the selection of specimen geom etry when measuring the effect of fiber orientation on thermal conductivity of uniaxial composites, and when using such data in the thermal analysis of composites with different shape and boundary conditions.