Aeronautical and space applications require the use of composite materials that can withstand very high heat flows with a limited ablation rate. These materials are commonly carbon fibre reinforced polymers (CFRP). During a heating step, the material is progressively transformed into carbon through a pyrolysis process. The resulting carbon/carbon composite will undergo ablation, in the considered applications. In order to qualify and quantify these phenomena, it is necessary to have test facilities in which samples can be submitted to high flux conditions as close as possible to those of the application.

The objective of this work is to design a test method under high thermal flux, fully instrumented, to get: (i) an accurate control and measurement of the flux received by the sample surface and (ii) the response of the sample. Working in such controlled conditions allows to identify and quantify the pyrolysis mechanisms of a polymer under high heat fluxes (>100 MW/m²).

The considered material is a bio-sourced polymer, used as a matrix in composites reinforced with C fibres. To get this level of quantification in the description of pyrolysis, the resin alone and the composite are tested and thus, the role of fibres can be highlighted.
To get these ultra-fast high flux conditions, the heating source is an Ytterbium fibre laser, emitting at 1070 nm for a continuous power of 2000W. Such an approach based on the use of laser heating was already applied to study single-phase materials under extreme conditions, [1-3]. The heating system is instrumented with infrared and visible thermal cameras, and infrared pyrometers, in order to get the surface temperature with its homogeneity. The temperature measurements are done on both faces, front and back, to obtain data usable in a calculation of thermal diffusivity, during the different steps of pyrolysis. A direct and indirect heating method has been used, to improve the homogeneity of the temperature over the sample front surface. Further, the accessible temperature levels and the control of heating rates will be detailed.
To validate the methodology, the initial work is carried out on materials under low fluxes to obtain low heating rates similar to those of more traditional methods (thermo-gravimetric analyses). Differences in the progression of a pyrolysis front inside our materials is discussed in function of the heating system: non-homogeneous (mainly across the thickness) under a laser and quasi-homogeneous in a TGA.

[1] L. Gallais, T. Vidal, E. Lescoute, Y. Pontillon, and J.-L. Rullier, (2021). High power continuous wave laser heating of graphite in a high temperature range up to 3800 K. Journal of Applied Physics, 129(4), 043102.

[2] P. Combis, P. Cormont, L. Gallais, D. Hebert, L. Robin, and J.-L. Rullier, (2012). Evaluation of
the fused silica thermal conductivity by comparing infrared thermometry measurements with two-dimensional simulations. Appl. Phys. Lett. 101, 211908.

[3] S. Elhadj, M. J. Matthews, and S. T. Yang, (2014). Combined infrared thermal imaging and laser heating for the study of materials thermophysical and processing properties at high temperatures. Crit. Rev. Solid State Mater. Sci. 39, 175-196.