Mechanical properties of compressed wood in accordance with the compression ratio

The material properties of compressed wood were investigated according to the compression ratio, considering its use as mechanical fasteners. Compressed wood made of Japanese cedar in various compression ratios was provided for material tests such as tension, compression, bending, shearing and bearing. The relationship between stiffness or strength and density were investigated. In order to clarify the influence of the improvement of material properties, test results were standardized by using density and material properties of non-compressed wood. As a result, it was made clear that tension and compression properties parallel to the grain and bending properties improved in proportion to the increment of density regardless of the deformation of the cell wall. Shear elastic modulus and strength on the LT plane increased almost in proportion to the density, but no such improvement was observed in those on the LR plane. Shear elastic modulus on the LT plane and strength on the LR plane showed rather higher values than the ideal relationship with density, and elastic modulus on the LR plane and strength on the LT plane showed rather lower values at low density and then became higher than the ideal relationship. Those tendencies were qualitatively explained by a model of cell wall deformation. MOE and strength on bearing in the T direction increased following a power function with index of 2 and 1.8, respectively. On the other hand, their changes in the R direction were relatively small until the cells of early wood fully turned into compressed cells, and then they increased very drastically. This tendency was very similar to the stress-strain relationship in R-compression of normal softwood, and was explained well by the honeycomb cell model of the deformed compressed cell.