Study On The Effect Of Low Temperature On Mechanical Properties Of Birch Wood

Temperature is one of the important factors which influences the mechanical properties of wood. Reports are mainly concentrated in the field of high temperature, but low temperature effects(including in low temperature environment and after the low temperature treatment) on the mechanical properties has not been systematically studied. Even that the mechanism of low temperature with different water state is not clear. Thus, systematical study on wood mechanical properties in low temperature environment and after the low temperature treatment and the effect of temperature and moisture content(MC) on the properties would have important theoretical significance and practical value for a scientific guidance in the the safe use of wood and wood modification at low temperatures.In this study, mechanical properties of birch(Betula platyphylla) wood with 5 different moisture content(MC) levels in two situations, ie., in the low temperature environment and after the low temperature treatment, was analyzed from 0 °C to-196 °C. X-ray diffraction(XRD) was used to analyze the crystalline structure both in room temperature after the low temperature treatment and in low temperature environment. Cryogenic scanning electron microscopy(Cryo-SEM) was used to explore the morphological characteristics in low temperature environment, while Dynamic mechanical analysis(DMA) was used to evaluate the dynamic viscoelastic properties before and after the low temperature treatment. The conclusions are as follows:(1) Modulus of rupture(MOR) and modulus of elasticity(MOE) of specimens increased with the decreasing temperature from 0 °C to-196 °C. For the water-saturated, green, fiber-saturated, and air-dried specimen, when the temperature dropped from 20 °C to-160 °C, significant differences in MOR and MOE were observed. Increase in MOR was 464%, 329%, 189%, and 92%,while 274%, 171%, 133%, and 66% in MOE, respectively; when the temperature dropped below-160 °C, no significant difference was observed in MOR of these specimens with 4 MCs. For the oven-dried specimen, MOR and MOE increased by 20% and 26%, but no significant difference was observed. When the temperature dropped from 20 °C to-160 °C, the ductility ratio reduced significantly with a decrease of 87.1%, 81.2%, 81.1%, 79.7% and 71.1%, for water-saturated, green, fiber-saturated, air-dried, and oven- dried specimen, respectively; however. when temperature dropped below-160 °C, no significant difference was observed in the ductility ratio at any MC level.(2) Thermal expansion phenomenon occurred in low temperature environment as well. The value of the linear thermal expansion coefficient(λ) in the radial direction showed positive correlation with temperature and MC. λ of the water-saturated, air-dried and oven-dried specimen were 21.37×10-6, 14.8×10-6 and 14.97×10-6 °C-1 at-150 °C, while increased by 227%, 287%, and 293% from-150 °C to 20 °C. Similarly, wood shrinks at low temperatures. The relative shrinkage(ΔL/L) in wood increased with decreasing temperature from 0 °C to-150 °C. The higher the MC, the larger the ΔL/L was. ΔL/L for the water-saturated specimen was the largest one at-150 °C with the value of 0.65×10-2. Considering that mechanical strength of wood would increase with the shrinkage, MOR in the low temperature environment was corrected according to the λ, which would provide a method for the forecast of actual MOR at low temperature according to the value at room temperature.(3) MOE and MOR of wood increased after the low temperature treatment. Wood has a better durability at low temperatures compared to the concrete. MOE and MOR of water-saturated and green wood specimen both increased comparing to the untreated one after 1 cycle of low-to-room temperature treatment, ie., treated for 48 h at-20°C,-70°C,-100°C,-160°C, and-196°C, respectively. The largest increases in MOE of water-saturated and green wood specimens were 7.4% and 4.6%, while 7.4% and 6.8% in MOR, respectively. Variance analysis showed that three were no significant variations after the treatment on MOE and MOR of both water-saturated and green wood specimens at any temperature level. At the same time, no significant difference was observed between specimens treatmed at different temperatures. After the low temperature treatment, ductility ratio increased significantly when the treatment temperature varied from-20 to-100 °C, with a largest increase of 95.4% and 64.4%. There was no significant increase in the ductility ratio when the treatment temperature dropped below-100 °C. MOE of water-saturated, green, air-dried, and oven-dried specimens increased by 6.8、6.2、4.8 and 3.0%, respectively. However, the effect of low temperature treatment on MOE was not significant.(4) XRD, Cryo-SEM, and DMA were used to analyze the effect of low temperature on dynamic viscoelastic properties, crystalline structure and microstructure, showing that the properties of hemicellulose and cellulose was not influenced by low temperature; meanwhile, no significant difference was found in the crystallinity. The mechanism of low temperature on the mechanical property of wood includes three aspects. Firstly, water froze at low temperature(icicle formed by free water was observed by Cryo-SEM in water-saturated specimen at-180 °C), which increased the mechanical property of wood; secondly, the reducing distance of molecules as decreasing temperature enhanced the intermolecular forces of specimens, which caused the increase in wood mechanical property.