白桦树盘生长特性对干燥开裂的影响

Effect of White Birch Disk Characteristics on Drying Check Behaviors

以白桦树盘为对象,研究分别采用恒温40、60℃,相对湿度渐降的干燥工艺干燥时,树盘干燥开裂的特点(产生时期、部位、形貌),进而研究树盘几何生长特性(偏心率、圆度、横截面积)对干燥开裂的影响。结果表明:树木不同部位、不同径级、不同圆度的树盘,在相同干燥工艺下开裂初期差异很大。开裂初期均值,在40℃干燥工艺下对应树盘含水率43.9%,60℃干燥工艺下对应树盘含水率45.6%,温度高时平均开裂期超前;最初开裂产生位置大多为心边材交界处,60℃工艺时有向内扩张趋势,并伴有心裂产生;60℃干燥工艺下无论开裂总长度还是平均开裂宽度都要较40℃工艺时的开裂值大,但差异不显著;60℃工艺时的V-裂数量明显高于40℃时;圆度与橫截面积是影响开裂总长度、开裂平均宽度的主要影响因素,而V-裂主要受后者影响。横截面积越大、形状越不规则,树盘越易产生干燥开裂,揭示树盘按几何生长特征分选干燥的必要性。

This paper was carried out to study the characteristics of white birch （Betula platyphylla） disk drying checks（such as the position, MC of check formation, morphology）under stepwise decreasing relative humidity and constant temperature of 40 and 60 ℃ drying conditions. On that basis, the effects of geometrical characteristics of tree disk, namely, growth eccentricity（GE）, roundness deviation（RD）and cross-sectional area（S）on drying checks were investigated. The results showed that under the same drying condition, the early formation of checks within disks with different position, diameter and roundness deviation（RD）had a significant difference. Under 40 ℃ and 60 ℃ drying conditions, the early formation of checks’ average MC value was 43.9% and 45.6%, respectively. The occurrence of checks was earlier under 60 ℃ than 40 ℃ drying condition. Also, the early formation of checks’ position was on the border between heartwood and sapwood. Under 60 ℃ drying condition, the position of checks was slightly expansion to inter with heart checks emerging. Although check total length（CTL）and check average width（CAW）under 60 ℃ were greater than 40 ℃ drying condition, there was no significant difference under two drying conditions. The number of V-checks was under 60 ℃ more than 40 ℃ drying condition, obviously. The roundness deviation（RD）and cross-sectional area（S）were the main factors that effected check total length and V-check. The disks with larger cross-sectional area（S）and more irregular shape were prone to triggering drying checks during the drying process. Therefore, it was necessary that tree disks were sorted by geometrical characteristics prior to drying.