Variances in the measurement of thermal diffusivity on coarse-weave carbon-carbon composites in terms of fiber-fraction involvement

Abstract

A geometrical analysis was performed on the sensitivity of the fiber-fraction parameter applicable to the sighting area of a detector for thermal diffusivity measurements using the flash technique on a coarse-weave C-C composite. The percentage variation in the detector viewing-area fiber fraction was examined as a function of viewing diameter for the axial and radial cases of a proprietary material, billet 304. Suggestions are made for optimization of the viewing diameter for thermal diffusivity measurements. The uncertainty limits caused by small displacements of the spot center or by uncertainties in the spot diameter are derived for viewing diameters of 0.1 and 0.5 in. Selected thermal diffusivity data obtained from room temperature to 1300 K using a 0.1 in. diameter spot on three different types of proprietary coarse-weave carbon-carbon composites (100, 200, and 300 series) are presented. From the analysis of these data, it is concluded that the thermal diffusivity shows a strong dependence upon fiber fraction at room temperature and that this dependence decreases considerably at higher temperatures. Typical thermal diffusivity results obtained in the radial direction for billet 206 as a function of temperature using both 0.10 and 0.37 in. diameter spots are analyzed in view of the conclusions drawn in the geometric analysis study. Finally, suggestions are made concerning the characterization necessary in reporting data on the thermal diffusivity of coarse-weave carbon-carbon composites.