Abstract
The change in absorptivity during the degradation process of materials is discussed, and its influence as one of the involved parameters in the degradation models is studied. Three materials with very different behaviors are used for the demonstration of its role: a carbon composite material, which is opaque, almost grey, a plywood slab, which is opaque and spectral-dependent and a clear PMMA slab, which is semitransparent. Data are analyzed for virgin and degraded materials at different steps of thermal degradation. It is seen that absorptivity and emissivity often reach high values in the range of 0.90–0.95 with a near-grey behavior after significant thermal aggression, but depending on the materials of interest, some significant evolution may be first observed, especially during the early stages of the degradation. Supplementary inaccuracy can come from the heterogeneity of the incident flux on the slab. As a whole, discrepancies up to 20% can be observed on the absorbed flux depending on the degradation time, mainly because of the spectral variations of the absorption and up to 10% more, depending on the position on the slab. Simple models with a constant and unique value of absorptivity may then lead to inaccuracies in the evaluation of the radiative flux absorption, with possible consequences on the pyrolysis analysis, especially for properties related to the early step of the degradation process, like the time to ignition, for example.
Highlights
Thermal degradation involves radiative transfer as one of the key heat transfer modes, since experimental facilities generally use a strong radiative source to heat the material surface: a high temperature lamp like in the fire propagation apparatus (FPA) or a high temperature coil in a cone calorimeter
As a summary, when introducing a constant absorptivity in a degradation model, two error sources should be kept in mind: (i) for non-grey samples, an average value may penalize the accuracy of the real absorbed flux, depending on the used radiative source; and (ii) the absorptivity currently reaches values as high as 0.90 to 0.95, but its value may be significantly smaller at the early stage of the degradation process because of the spectral dependence or after a significant degradation duration because of sample regression for materials with in-depth absorption, such that a unique value does not provide the true absorption history of the material
The previous section showed that the knowledge of the absorptivity values and their evolutions as the degradation process goes on directly affect the prediction of the radiative flux absorption
Summary
Thermal degradation involves radiative transfer as one of the key heat transfer modes, since experimental facilities generally use a strong radiative source to heat the material surface: a high temperature lamp like in the fire propagation apparatus (FPA) or a high temperature coil in a cone calorimeter. Absorptivity and emissivity become key properties for evaluating the energy absorbed by the surface or the radiative heat loss by the sample They should be known in order to evaluate the true absorbed flux and to evaluate the deviation between the degradation test under calibrated source and the material behavior under fire conditions. Roos [8] presented an extended study on various materials, showing the complexity of the absorptivity evaluation They concluded that its average value varies with the heat source temperature, which is a well-known consequence of a non-grey behavior. The underlying idea was first to illustrate through examples how much a single value of absorptivity can result in an inaccurate evaluation of the absorbed fluxes, depending on material type, time and position In this frame, three samples with expected different behaviors were selected: a carbon composite (near black), plywood (non-grey) and PMMA (participating medium). Simulations were conducted to evaluate the true discrepancies in the incident fluxes received near the center and at the edges of a tested sample
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