Multi-scale investigation of the mechanical properties of Loblolly pine wood at elevated temperature

The micromechanics of wood cell walls and the bending properties of wood were investigated by using nanoindentation and macromechanical test machine at elevated temperature, respectively. The microstructure of wood at the temperatures ranged from 20 to 160 degrees C was also observed by optical microscope and scanning electron microscope. Results indicated that the increased temperature accompanying with the moisture release affected the mechanical behavior of wood on both cell-wall level and macroscale. The cell wall's elastic modulus (E-r) and hardness (H) decreased as temperature increased to 80 degrees C due to the softening of wood polymers under hot and humid condition. On the contrary, both the E-r and H increased when the temperature was above 100 degrees C ascribed to the massive release of moisture. The increased mechanical properties of the thick-wall cells made a major contribution to the improved bending properties of wood at the macroscale. Furthermore, the cell walls at 80 degrees C own the highest indentation creep ratio (C-IT), that is, wood are easier to creep under a constant load. In hence, the mechanical stability of timber in constructions at about 80 degrees C shall be paid more attention.