Abstract:

Objective: This study aims to explore the influence of subzero low temperatures on the impact toughness of wood with different humidity levels, determine the critical temperature and humidity conditions for the transition of wood from ductile to brittle fracture, construct the relationship model between impact toughness and brittleness values, clarify the generation law of impact brittle fractures in frozen wood, so as to provide a theoretical basis for the mechanical stability control and failure risk prevention of wood in engineering applications under cold-humid environments. Method: In this paper, impact tests and bending tests were conducted on poplar (Populus ussuriensis) and larch (Larix gmelinii) wood specimens (300 mm × 20 mm × 20 mm) with different humidity levels (over-dried wood, fiber-saturated wood, and water-saturated wood) in a continuous temperature range (20 ℃ to –196 ℃) to obtain dynamic load impact toughness and static load brittleness value of wood. The mathematical relationship model between the wood's impact toughness and brittleness values was established by the statistical analysis method. Result: 1) As the temperature dropped from 20 ℃ to –196 ℃, the morphology of fracture surface of poplar and larch wood transformed from tough failure characteristics to brittle failure characteristics. The higher the water content, the more pronounced the brittle failure morphology. 2) In the static load experiment, with the decrease of temperature, the plastic deformation stage of load-displacement curves of the moist poplar and larch gradually disappeared at four key temperature points (20 ℃, 0 ℃, –40 ℃ and –196 ℃), and the phenomenon of ductile fracture changed to brittle fracture. The brittle fracture of oven-dry wood occurred earlier. 3) With the decrease of temperature, the impact toughness of wood gradually decreased, and the brittleness value gradually increased. Compared with 20 ℃, the impact toughness of wood with higher moisture content decreased more and the brittleness value increased more at sub-zero temperature. When the temperature dropped to –196 ℃, the impact toughness of poplar and larch satiated wood decreased by 69.7% and 62.5%, respectively, while the brittleness value increased from 7.4% and 14.2% to 72.2% and 78.0%, respectively. 4) The impact toughness and brittleness values of poplar and larch wood at the three humidity levels showed a linear trend with the decrease of temperature, and the inflection point appeared in the range of –40 ℃ to –60 ℃. The impact toughness of wood had a power function relationship with brittleness value, a linear relationship with the logarithm of brittleness value. There were differences in these relationships depending on the water content. The best goodness of fit of the mathematical model between them was 0.96. Conclusion: With the decrease of subzero temperature, the resistance to ductile fracture of wood decreased and the risk of brittle fracture increased when subjected to impact. The higher the moisture content, the higher the probability of brittle fracture. Between –40 ℃ and –60 ℃ is the critical temperature range for wood to undergo tough-brittle transformation. The dynamic load toughness of wood can be effectively predicted by the static load brittleness of wood at low temperature below zero.

Key words: impact toughness, brittle failure, transition to brittleness, sub-zero temperature, transition temperature, Populus ussuriensis, Larix gmelinii