Effect of thermal expansion at low temperature on mechanical properties of Birch wood

Thermal expansion phenomenon would occur in woody materials in terms of temperature change as well as moisture content (MC), which will influence the mechanical properties in extreme low temperature condition. In this study, thermal expansion of birch (Betula platyphylla) wood with three levels of MC (water-saturated specimen with a MC of 149.7%, air-dried one with a MC of 9.7% and oven-dried one with a theoretical MC of 0%), was analyzed for temperature ranging from − 150 to + 20 °C. Furthermore, the dimension changes of the birch specimens were measured and the effect of thermal expansion on the mechanical properties in low temperature condition was investigated. The crystallinity of the wood at low temperatures was examined by the X-ray diffraction (XRD). The results showed that all specimens shrank from + 20 to − 150 °C. Water-saturated wood showed more dramatic shrinkage as compared to the other two groups (air/oven-dried). The value of the linear thermal expansion coefficient λ increased with the increasing temperature. At any temperature, λ of the water-saturated specimen was the greatest one among the three groups. The increase of MOR, i.e., the variation between the corrected MOR adjusted according to the λ at low temperature and the uncorrected one, had a linear relationship with temperature, which would provide a method for the forecast of actual MOR at low temperature according to the value at room temperature. XRD analysis indicated that the crystalline structure of wood polymers was not influenced by temperature. Stability and mechanical property offer insight of the performance of wood product used for low temperature applications.