Thermal response to fire of a fibre-reinforced sandwich panel: Model formulation, selection of intrinsic properties and experimental validation

A predictive model is formulated for the fire response of a glass reinforced plastic panel, consisting of two glass-fibre/polyester skins and Vermiculux sandwich material (core) in between. Polymer conversion takes place according to a first-order decomposition reaction and an n-order combustion reaction both with an Arrhenius-type dependence on temperature. Intrinsic kinetic parameters have been estimated by re-examination of thermogravimetric data at four heating rates, resulting in activation energies for the two steps of 128 and 150 kJ/mol, respectively. Physical processes are modelled by the unsteady, one-dimensional conservation equations taking into account heat transfer by convection and conduction, convective mass transfer, surface heat transfer, effective thermal conductivity, moisture evaporation, ablation of the heat-exposed surface at a critical temperature and property variation. Simulated process dynamics, using intrinsic values for all the model parameters, are highly influenced by the behaviour of the heat-exposed skin which shows three main regimes: I) very rapid conversion of a thin surface layer (fast heating regime), II) slowing down of the conversion processes following the formation of a thick insulating fibre glass layer (slow heating regime) and III) a new enhancement in the reaction rates as a consequence of surface collapse and ablation (ablation regime). Good agreement is obtained for the predicted and measured temperatures for both a single skin composite plate and a sandwich panel loaded with a hydrocarbon flame.