Investigation on the char crack growth of densified wood

Abstract

Densified wood (DW) is a novel engineering material with superior mechanical performance, but how its compact structure affects its char cracking behavior under fire remains unknown. This work experimentally and theoretically investigates the crack pattern and crack width of DW under radiative heating (Q_r). Longitudinal tensile facture-induced crack (L-crack) dominates the crack pattern due to its lower crack resistance (C_t). Delignification largely reduces the C_t of tangential tensile facture-induced crack (T-crack) resulting in more T-cracks observed on DW. Although DW has a lower C_t than natural wood (NW), its compact structure increases the cohesion among fibers, which increases the difficulty of the crack propagation. The crack width (w_c) growth rate increases with the increased Q_r and decreased wood density (ρ). Based on that, a dimensionless empirical correlation is proposed with an R2 of 0.853 to the measurement. Afterward, an analytical model, which considers the displacement due to the crack propagation in the T-L plane (w_T-L) and the L-R plane (w_R-L) and the displacement due to the shrinkage strain (w_s), is proposed to describe the crack width growth of L-crack. w_T-L and w_s are combined to dominate w_c. Both w_T-L and w_R-L decelerate with the increased ρ. The calculated w_c well-fits to the current and previous measurements indicating good accuracy and applicability. This work provide new insight into the understanding of the charring cracking of wooden materials and bio-materials.