Experimental characterization of radiative transfer in semi-transparent composite materials with rough boundaries

This study deals with the analysis of the propagation of thermal radiation within absorbing and scattering composite materials with rough boundaries. The two-phase system (resin matrix and fibers reinforcement) is treated as an equivalent homogeneous medium characterized by volumetric radiative properties, namely extinction coefficient, scattering albedo, and phase function whereas the interaction of the radiation with the medium boundaries is modeled with boundary scattering properties. The aim is to determine these volumetric and boundary scattering properties by an inverse analysis for parameter identification. It consists of minimizing the sum of the squared difference between calculated and measured bidirectional and normal-hemispherical reflectances and transmittances. The Gauss Newton algorithm is employed for solving this nonlinear least squares problem. The experimental data are obtained by using a visible and near-infrared spectrophotometer equipped with a goniometric system enabling measurements in different scattering directions around a sample between 0.4 and 2.5 mu m. The collision-based Monte Carlo method is employed to assess the theoretical values by solving the Radiative Transfer Equation (RTE) along with boundary conditions designed for samples with rough surfaces. The proposed approach is proved to be well appropriate for determining the radiative properties of the rough composite samples. More interestingly, this study demonstrates that the scattering phase function can be modeled by the Henyey and Greenstein approximation and the boundary scattering distributions can be modeled by a Gaussian function and/or a cosine function. (c) 2020 Elsevier Ltd. All rights reserved.