Unconventional flash technique for the identification of multilayer thermal diffusivity tensors

This paper presents a novel non-intrusive technique for the thermal characterization of opaque multilayer materials that consists in the simultaneous estimation of the thermal diffusivities tensors of each constituting layers that may be isotropic or orthotropic. The present identification method is based on the resolution of an inverse heat conduction problem consisting in the minimization of the least square objective function constructed with the temperature measurements and the prediction of a pseudo-analytical model. The pseudo-analytical model relies on the quadrupoles formalism that proves its capability to treat multi-layered materials and to mimic the experimental method investigated in this work. This latter, inspired from the flash method, relies on a unique and non-intrusive experiment, in terms of excitation and measurements. One of the sample surfaces is locally and shortly heated by means of a CO2 laser, while the resulting transient temperature field is continuously recorded during the cooling period by an IR camera. The minimization procedure invokes a PSO algorithm that has been proved to be convenient for such complex problems that deal with a non-linearity and a large number of unknown parameters, including those related to the excitation. The overall identification method is validated using an isotropic opaque monolayer material of known properties. The identification method is then performed on an actual two layers sample. The accuracy and robustness is discussed depending on the face of excitation and measurement leading to two distinct experimental configurations. A strong emphasis is placed on the sensitivity analysis in order to test the feasibility of the estimation for both configurations.