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 degrees, 100 degrees, and 120 degrees C over 14 days, and their equilibrium moisture content (M) at 20 degrees C and 60% relative humidity, longitudinal dynamic Young's modulus (E), bending strength (sigma (max)), and breaking strain (epsilon (max)) were compared with those of unheated specimens. Steaming for a longer duration at a higher temperature resulted in a greater reduction in M, sigma (max), and epsilon (max). The E of wood was slightly enhanced by steaming at 100 degrees C for 1-4 days and 120 degrees 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 sigma(max), whereas M values below 8% indicated a marked decrease in the mechanical performances. In addition, the epsilon(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.