Measurements of the mechanical properties of larch at elevated and high temperature under nitrogen conditions

The mechanical characteristics of wood members in a fire are reduced as a result of strength loss of the outermost charring layer and strength reduction of the secondary outer layer in high temperature zones. The charring layer has strong oxygen isolation effects. The main mechanical characteristics of larch wood, including modulus of elasticity (MOE), parallel-to-grain compressive ( f c ), tensile ( f t ), bending strengths ( f b ), and ductility coefficient, were determined under oxygen-free conditions at higher temperatures. Fourier transform-infrared (FT-IR) and scanning electron microscopies (SEM) were used to analyze chemical composition and microscopic changes of wood. The results suggested that hemicellulose softening resulted in decreases in mechanical properties of wood between 20 and 70 °C. From 70 to 190 °C, FT-IR spectroscopy provided evidence that increases in MOE and f c, with relatively minor changes in f b and f t, were associated with cellulose crystallinity and lignin cross-linking/condensation. The use of SEM further indicated degradation of wood composition.