Fracture toughness of the surface layer of the boxed-heart square timber of sugi associated with drying check

Fracture toughness tests were performed on the surface layer of boxed-heart square timber of sugi, after and during drying. Experiments were carried out using of notched tensile specimens with single edge. The critical stress intensity factor (K_Ic) in the TR system was investigated to clarify effects of drying temperature on resistance against drying surface check of the boxed-heart square timber of sugi. Timbers dried with temperatures of 20 °C and 80 °C had higher resistance against drying check than dried with temperature of 40 °C and 60 °C. AE cumulative events increased slowly at the occurrence state for the tension tests of the specimens dried with temperature of 20 °C and 80 °C, whose K_Ic values were relatively high. On the other hand, AE cumulative events increased relatively fast for the specimens dried with temperature of 40 °C and 60 °C. Cell-wall break type of micro-fracture occupied the most of fracture in the early wood, when the large amplitude of AE were detected in the early state. Therefore, it was found that the characteristics of AE were closely concerned with K_Ic values and micro-fracture. The K_Ic values of the fracture toughness decrease with drying until moisture content of 20%, because of drying stresses. After then, the K_Ic values increase rapidly with drying under moisture content of 20%. In the drying processes the K_Ic values have a tendency to be higher with lower drying temperature within this experiment conditions.