Effect of structural characteristics on charring shrinkage and cracking of densified wood under radiative heatings

Densified wood (DW) is a novel engineering material with excellent mechanical properties. It forms a thermally insulating and hardly-cracked char layer during pyrolysis and combustion. However, the scientific fundamentals regarding how wood structural characteristics (e.g. density) affect its charring behavior remain unknown, which is important for the fire safety of wood. In this work, the charring shrinkage and cracking of NW and DW with various densities (ρ) under heating exposures (Q_r) are investigated and analyzed. Partial crack closure was observed due to the oxidation. As wood ρ increases or Q_r decreases, the number of cracks (N_c) decreases and cracking time (t_c) is delayed because both the surface temperature (T_surf) and altered thermal stress concentrations on the wood surface are reduced. DW presents an earlier mass loss peak and lower peak value due to the delignification. DW presents high fluctuations in T_surf due to the combing effect of thermal inertia and radial rebound. As wood ρ increases, char shrinkage depth at t_c decreases and shrinkage gradient at t_c increases first and then decreases. An empirical correlation is proposed and well-predicts the N_c based on ρ and Q_r. It can be potentially implemented in the combustion model to improve prediction accuracy.