Deformation of wood slice in fire : interactions between heterogeneous chemistry and thermomechanical stress

Abstract

Wood is a common flammable material in the building fire and the dominant fuel in the wildland fire. In this work, disc wood slices were examined under irradiation to characterize the smoldering burning and the corresponding deformation behaviors. Due to interactions between chemical reactions and thermomechanical stresses, four successive deformation stages were observed and hypothesized: (I) drying shrinkage to ∪ shape, (II) irradiation-driven thermal expansion to ∩ shape, (III) pyrolysis shrinkage to ∪ shape, and (IV) oxidation-driven thermal expansion to ∩ shape. For these 5-15 mm thick samples, the degree and occurrence of these deformation stages are sensitive to the aspect ratio (i.e. D/δ). Increasing the slice thickness decreases the deformation in the first three stages but increases the deformation of the fourth stage. These experimental observations are qualitatively reproduced by a 2-D finite-element numerical model, coupling 3-step heterogeneous kinetics with a thermomechanical solver. Modeling results further verified the underlying heterogeneous processes (dehydration, pyrolysis, and oxidation) and thermomechanical stresses (thermal expansion and pre-stress) for each deformation stage. This study helps understand the influence of burning processes on the deformation of wood and the failure of timber structures.