Influence of temperature and moisture content on bark/wood shear strength of black spruce and balsam fir logs

The effects of temperature (T) and moisture content (MC) on bark/wood shear strength (BWSS) were studied. Fifteen stems of black spruce (Picea mariana(Mill.) B.S.P.) and balsam fir (Abies balsamea(L.) Mill.) logs were selected and cross-cut into three 1.25 m log sections, corresponding to bottom (1.3 m), middle (3.8 m), and top (6.3 m) positions. BWSS was measured at five temperatures, ranging from 10 to - 30 degrees C, and at five levels of sapwood moisture contents (SMC), obtained by air-drying, from green state to near to the fiber saturation point (FSP). Bark and sapwood properties, MC and basic density (BD) were also determined. Temperature had a significant effect on BWSS for both species. This property was similar between 10 and 0 degrees C, and significantly increased as temperature decreased below 0 degrees C. However, the influence of temperature on BWSS depended on SMC and it varied between the two species. For black spruce, for each temperature between 0 and - 20 degrees C, BWSS showed similar values for SMC between 157 and 62%. At - 30 degrees C, BWSS showed a tendency to increase with SMC. For balsam fir, the BWSS increase due to the decrease in temperature was more important as the SMC increased. BWSS increased as SMC approached the FSP of wood for both species. Among the studied covariates, inner bark MC and inner and outer bark BDs significantly affected BWSS. Inner bark MC and SMC affected BWSS similarly. Multiple regressions were developed for prediction purposes, which explained 68 and 69% of BWSS variability for black spruce and balsam fir, respectively.