Research Articles

The influence of sapwood and heartwood on many mechanical properties has been studied; however, their influence on fracture mechanics remains unclear. This study investigated the influence of sapwood and heartwood on the toughness mechanism of Cryptomeria japonica in mode-I radial-longitudinal (RL) and tangential-longitudinal (TL) systems. A single-end-notched bending test was performed to obtain the fracture toughness of four specimen types: RSW, RHW, TSW, and THW (where R = RL system, T = TL system, SW = sapwood, and HW = heartwood). Thereafter, X-ray computed tomography and scanning electron microscopy (SEM) analyses were performed to evaluate the fracture at the microscale level. For all properties evaluated, the RL system showed the highest values, and within the same system, sapwood had the highest values. However, the statistical analysis concluded that fracture toughness was the only property with a significant difference for all specimen types. Microscale analyses using SEM revealed that the higher toughness mechanism values found in the sapwood specimens were related to higher proportions of cell fracture than those in other specimen types. This implies that the toughness mechanism may depend on the fracture of microstructural tissues.