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.
KeywordsGraphite ZnSe Biot Ruby
Unable to display preview. Download preview PDF.
- 1.D.L. Balageas and A.M. Luc, Nonstationary thermal behavior of directional reinforced composites. Limit of application of thermal property homogenization, AIAA Paper 83–1471 (1983)Google Scholar
- 2.W. Fritz, W. Huttner, K. Maier and R. Brandt, Thermophysical properties of carbon-fibre/carbon matrix composites at high temperatures, Rev. Int. Hts Temp. Refr. 16: 350 (1979).Google Scholar
- 3.R. Taylor and R. Procter, Measurement of thermal diffusivity of carbon fibre carbon composites from 300-3000 K,Univ. Manchester/UMIST(U.K.), Report AF0SR-TR-81-0638 (1981).Google Scholar
- 4.R.E. Taylor, H. Groot and R. Shoemaker, Thermophysical properties of fine weave C/C composites, AIAA 81–1103 (1981).Google Scholar
- 6.A.M. Luc and D. Balageas, Comportement thermique des composites à renforcement orienté soumis à des flux impulsionnels, to be published in High-Temp. High Pressure (1984).Google Scholar
- 7.D.L. Balageas, Determination par méthode flash des propriétés thermiques des constituants dfun composite à renforcement orienté, idem.Google Scholar
- 9.M. Assouline, Private communication.Google Scholar
- 10.R.E. Taylor and L.M. Clark III, Finite pulse time effects in flash diffusivity method, High Temp. High Pres. 6: 65 (1974)Google Scholar
- 11.N. Coulon, Private communication.Google Scholar
- 14.A.M. Luc and D.L. Balageas, Le problème de la definition de la mesure de la diffusivité thermique des matériaux composites a renforcement, ONERA TP 1981–30 (1981).Google Scholar
- 15.H.J. Lee and R.E. Taylor, Thermophysical properties of carbon/graphite fibres and MOD-3 fibre reinforced graphite, Carbon 13 (1975).Google Scholar
- 16.M.L. Minges AFML-TR-74-96 (1975).Google Scholar
- 17.R.E. Taylor, Thermal diffusivity of composites, TPRL 253 (1981).Google Scholar