Influence of Density and Equilibrium Moisture Content on the Hardness Anisotropy of Wood

The aim of this study was to investigate the anisotropic characteristics of Brinell hardness for six species (softwood, Chinese fir [Cunninghamia lanceolata (Lamb.) Hook.], Red pine [Pinus koraiensis Sieb. et Zucc.], Mongolian scotch pine [Pinus sylvestris L. var. mongolica Litv.]; hardwood, Manchurian walnut [Juglans mandshurica Maxim.], Asian white birch [Betula platyphylla Suk.], Mongolian oak [Quercus mongolica Fisch. et Turcz.]) with the equilibrium moisture content (EMC) obtained at four relative humidity (RH) levels (20 degrees C; 50%, 65%, 85%, and 95% RH). The results showed that the cross section of specimens presented higher Brinell hardness and lower elastic recovery than those tested on radial and tangential surfaces. Ovendried density was significantly positively correlated with Brinell hardness. With the exception of softwood on a tangential surface (r = 0.02), there was a statistically significant positive correlation of elastic recovery against ovendried density (at the 0.01 level). We found that a general increase in EMC significantly lowered the Brinell hardness for six species, irrespective of grain orientation. The decreased extent of relative Brinell hardness (HB/HB0) was highest for cross section samples, compared with radial and tangential surfaces. For three softwood species, the decreased extent of HB/HB0 on the radial surface was higher than that on the tangential surface, whereas the opposite relationship was found in three hardwood species. The results indicated that rays were probably the main factor controlling transverse anisotropy in hardwood, while the interaction of earlywood and latewood was more important for softwood. In addition, elastic recovery anisotropy was probably owing to different failure behavior in three directions, and it was difficult to establish a direct influence of EMC on elastic recovery.