Evaluation of end-check propagation based on mode I fracture toughness of sugi (Cryptomeria japonica)

The relation between crack propagation based on fracture mechanics and end-check propagation during drying was evaluated in this study. Corresponding to the direction of end-check propagation, the mode I fracture toughness of air-dried sugi specimens in TR, TL, and intermediate systems was examined by single-edge-notched bending tests. The occurrence and propagation of end checks on sugi (Cryptomeria japonica D. Don) blocks during drying were observed at the scale of the annual rings. It was found that the critical stress intensity factor (KIC) decreased as the crack propagation changed from TL to TR. The value of KIC in the TR system was significantly lower than that in the TL system. As a measure of fracture energy, the area under the load-crack opening displacement curve in the TR system was more than twice that in the TL and intermediate systems. These results indicate that cracks perpendicular to the tangential direction initiate radially with ease, and then crack arrest occurs to prevent growing. This finding provides a consistent interpretation of the end-check propagation observed during drying as follows: tiny end checks, as an analog of TR cracks, occur easily and selectively in latewood or transition wood and propagate toward the pith during drying. When there is no corresponding secondary check in the forward latewood, the checks are arrested and do not propagate further.