Effects of temperature and moisture content on selected wood mechanical properties involved in the chipping process

The effects of temperature and moisture content on selected mechanical properties associated with the chipping process were evaluated. In chipping, mechanical properties such as shear parallel to the grain, cleavage, and bending are involved. Matched samples of heartwood and sapwood were obtained from freshly harvested logs of black spruce and balsam fir to determine the variation of the studied mechanical properties between −30 and 20 °C, at intervals of 10 °C. Moisture content (MC), basic density (BD), and annual ring width (RW) were measured for each sample. For both wood species, temperature had a significant effect on all mechanical properties under freezing conditions (below 0 °C). This effect was more important for sapwood than for heartwood, which was explained by the difference in MC between these two types of wood. Between 0 and 20 °C, temperature and type of wood did not show any significant effect on the mechanical properties. Multiple regression models were obtained to predict the mechanical properties. These regressions showed that MC was the most important factor to explain the mechanical properties below 0 °C. However, for temperatures of 0 °C and higher, BD was the principal factor to predict the mechanical properties. RW was not a significant factor to predict any mechanical property. Cleavage was the most sensitive one to changes in temperature followed by shear, modulus of rupture, and modulus of elasticity. These results could be of great importance in the chipping process.