Low-pressure effect on pyrolysis-induced deformation behavior in beech wood

• The deformation behavior of wood during the initial phase of fire was examined. • Four successive deformation stages were observed and hypothesized to describe the periodic pyrolysis-induced deformation. • The extent of deformation was closely related to the fiber orientation. • The deformation behavior exhibited a strong similarity to the temperature difference within the sample. • The influence of ambient pressure is less significant compared to that of thickness. The shrinkage and deformation of the wood influence the cracking and smoldering or flaming combustion dynamics, yet scientific understanding remains limited. To gain deeper insight into the deformation behavior of wood, this study investigated disk-shaped wood samples (60 mm in diameter, 5–15 mm in thickness) cut either parallel or perpendicular to the grain under environmental pressures of 5, 30, 60, and 90 kPa in a nitrogen atmosphere. Through synchronized dual-view imaging and temperature monitoring, it was found that the wood disks exhibited approximately axisymmetric deformation, the extent of which was closely related to the fiber or grain orientation. Samples cut perpendicular to the grain showed more pronounced periodic pyrolysis-induced deformation in the front view, which could be summarized into four stages: (Ⅰ) drying shrinkage to form a U-shape, (Ⅱ) thermal expansion to form a ∩ -shape due to external radiation, (Ⅲ) strong pyrolysis shrinkage to reform a U-shape, and (Ⅳ) an original flat shape driven by downward heat transfer. An increase in wood sample thickness results in a deceleration of the deformation rate, with stage IV deformation nearly disappearing in 15 mm samples. Thermal stress analysis further confirms that thinner samples (5 mm) exhibit a larger internal temperature gradient, resulting in a higher susceptibility to cracking during deformation. Additionally, deformation behavior is significantly influenced by ambient pressure; elevated pressure reduces the extent of deformation in stages III and IV. At 90 kPa, stage IV deformation vanishes, and the shrinkage rate decreases to nearly zero as pressure increases. Overall, the influence of ambient pressure is less significant compared to the effect of thickness. This study helps us understand the pyrolysis-induced deformation behavior of wood in fire, offering guidance for fire-resistant design and risk assessment of wood-based materials.