Effect of freeze and freeze-thaw pretreatment on the impact bending behavior of glubam and spruce-pine-fir

This study investigates the dynamic impact bending behavior of glued laminated bamboo (glubam) including thick-strip (G1), thin-strip (G2) and spruce-pine-fir (SPF) panels under cryogenic conditions. Specimens at oven-dried and air-dried moisture contents were subjected to freeze-only pretreatments, ranging from 20℃ to -196℃ or five freeze-thaw cycles (FTC). Pendulum impact bending tests were performed along two loading directions (X‑ and Z‑axes) to evaluate the dynamic modulus of rupture (MOR), the modulus of elasticity (MOE) and impact toughness. The tests were supplemented by microstructural observations using SEM and numerical simulations using ABAQUS analyses. A low-temperature strengthening was observed, with MOR and MOE peaking between -50℃ to -80℃, followed by a sharp decline at the ultralow temperature of -196℃ caused by severe embrittlement. The FTC pretreatments degraded mechanical properties due to cumulative microstructural damage. Moisture content played a dual role. Ice formation from free water provided a reinforcing effect in freeze-only specimens, while it generated internal microstructural damage. A divergence between static and dynamic cryogenic responses was identified.