Flexural properties
The load–displacement curves of two softwoods and two hardwoods are presented in Fig. 3. There were large differences in the flexural stiffness between the rift-sawn softwoods and hardwoods species. A real-time flexural test was exemplified in Additional file 2: Fig. S2. The flexural displacements of the hardwoods were similar for all grain patterns, whereas the displacements of the softwoods were larger for rift-sawn than for other grain patterns. The curves of the flat- and quarter-sawn specimens consisted mostly of elastic regions, and the specimens fractured after a short plastic region. By contrast, the rift-sawn specimens not only had a long elastic region with a smaller flexural load than the other patterns but also had a longer plastic region. The rift-sawn specimens of Chamaecyparis obtusa and Cryptomeria japonica had a displacement of more than 15 mm—about five times higher than the other patterns.
The differences in the MOE and MOR of the species tested were consistent with their density differences. For a given species, however, the differences in flexural behavior among the different grain patterns did not correlate with density. Overall, the flexural modulus of the quarter-sawn specimens was the highest in each species, and differed for the flat- and rift-sawn specimens depending on the species. We attribute the difference in flexural modulus among different grain patterns in a given species to differences in the arrangement and orientation of wood cells, and the difference between soft- and hardwood to their anatomical differences [13].
The rift-sawn softwood boards had significantly lower MOE than other patterns (Fig. 4). For Chamaecyparis obtusa, the density of the rift-sawn specimens was slightly higher than those of the other specimens. However, the MOE of the rift-sawn specimens was 49% and 25% of that of the flat- and quarter-sawn specimens, respectively. For Cryptomeria japonica, the MOE of the rift-sawn specimens was only 19% and 13% of that of the flat- and quarter-sawn specimens, respectively. A lower MOE means that the material is easier to bend. Thus, in thin wood materials, such as the kokera board, the end-grain pattern determines the flexural performance.
Tracheid deformation
Figure 5 shows the rate of change of the T/R ratio and diagonal ratio of Chamaecyparis obtusa as functions of the flexural displacement. For flat- and quarter-sawn specimens that fractured at flexural displacements of approximately 3 mm, the rate of change is only presented for displacements of 0 and 2 mm. Because the end-grain orientations of the flat- and quarter-sawn specimens were orthogonal to each other, their rates of change were reversed.
Among the end-grain patterns tested, the quarter-sawn wood showed the highest rate of change of the T/R ratio in both the compression and tension parts of the specimen at a flexural displacement of 2 mm. The flexural failure of solid wood propagates inward from the initial crack in the tension part of the specimen. Therefore, the results presented in Fig. 5b show that approximately 10% of the rate of change of the T/R ratio of the tracheids causes the failure of specimens in all grain patterns.
The diagonal ratio was a better indicator than the T/R ratio to explain the tracheid deformation in the rift-sawn wood. The rate of change of the diagonal ratio of the tracheids in the rift-sawn specimens linearly increased or decreased, exceeding 20% at a flexural displacement of 15 mm. The linear increase or decrease in the rate of change means the tracheid gradually deformed without strong resistance against compression and tension stresses induced by the flexural load. In addition, we assume that the deformation is due to the deformation of the corner cell walls of the tracheid, rather than compression and tension on the radial and tangential walls.
The cell walls of the flat- and quarter-sawn specimens subjected to the compression and tension generated under the flexural load were slightly rounded (Fig. 6a and b). By contrast, in the rift-sawn specimen, only slight deformation of the cell wall was observed, even though the cells became increasingly deformed with increasing displacement (Fig. 6c). Deformation of the radial and tangential walls was observed only at a displacement of 15 mm, just before the specimen failure. For Cryptomeria japonica, the change in the shape of the tracheids under the flexural load was almost identical to that of the tracheids of Chamaecyparis obtusa as shown in Fig. 6.