Thermal Diffusivity in Situ Measurements of Carbon/Carbon Composite Reinforcements
The diffusivity of directional reinforced composites (3-D C/C) was measured using the flash method. Problems related to the heat losses and the pulse duration were considered. The variation in the diffusivity determined at various moments during the heating confirms the numerical simulations: instead of being a nearly constant quantity as it is true for a homogeneous medium, the apparent diffusivity for these materials is a monotonic function that decreases with time and depends on the sample thickness.
An original method is used which changes nothing in the usual measurement techniques but permits to find the homogenized diffusivity for the whole composite, the in situ axial diffusivity of the parallel reinforcement with controlled heat flux, and the diffusivity of an equivalent matrix representing the actual matrix with the transverse reinforcements. This method, using an optical measurement of the mean temperature on the rear surface of the sample, is compared by numerical simulation with the method determining the diffusivity of the reinforcement from the temperature-time history by a point measurement on the reinforcement alone. Results of this comparison show that the intrinsic precision of the proposed method is better.
The reinforcement axial diffusivity of several 3-D C/C materials was found to vary from 2.6 x 10-4 to 3-1 x 10-4 m2 s-1, values three times greater than the diffusivities of the equivalent homogeneous materials, which can be determined on very thick samples or calculated from steady-state measurements of the thermal conductivity and specific heat.
KeywordsThermal Contact Resistance Armco Iron Transverse Reinforcement Actual Matrix Rear Face
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