Effects of high temperature kiln drying on the practical performances of Japanese cedar wood (Cryptomeria japonica) II: Changes in mechanical properties due to heating

Japanese cedar wood specimens were steamed at 80°, 100°, and 120°C over 14 days, and their equilibrium moisture content (M) at 20°C and 60% relative humidity, longitudinal dynamic Young’s modulus (E), bending strength (σ _max), and breaking strain (ε _max) were compared with those of unheated specimens. Steaming for a longer duration at a higher temperature resulted in a greater reduction in M, σ _max, and ε _max. The E of wood was slightly enhanced by steaming at 100°C for 1–4 days and 120°C for 1–2 days, and thereafter it decreased. The slight increase in the E of sapwood was attributable to the reduction in hygroscopicity, while sufficient explanation was not given for a greater increase in the heartwood stiffness. Irrespective of the steaming temperature, the correlations between M and the mechanical properties of steamed wood were expressed in terms of simple curves. M values above 8% indicated a slight reduction in E and s max, whereas M values below 8% indicated a marked decrease in the mechanical performances. In addition, the e max decreased almost linearly with a decrease in the value of M. These results suggest that hygroscopicity measurement enables the evaluation of degradation in the mechanical performances of wood caused by steaming at high temperatures.