落叶松板材常规干燥过程的动态黏弹性特性

Dynamic visco-elastic characteristics of larch timber during conventional drying process

为了研究木材常规干燥过程黏弹性应变的发展模式与相对数量级,并为准确区分黏弹性应变与机械吸附应变提供理论依据,该文在实验室条件下对50 mm厚兴安落叶松板材进行常规干燥,使用切片法测定沿厚度方向的横纹弦向干缩应变、弹性应变、黏弹性应变的一维分布情况与变化趋势。基于高聚物与复合材料黏弹性理论,重点定性分析了木材干燥过程中干燥介质温度、蠕变恢复时间、干燥阶段等因子对木材厚度方向不同位置黏弹性应变特性的影响。结果表明:在环境平衡含水率保持在特定水平下,干燥介质温度水平与测定的拉伸弹性应变极值间具有一定正相关性;木材干燥过程中黏弹性应变的发展模式与弹性应变类似,经充分恢复后黏弹性应变的数值略大于弹性应变;木材表层、芯层在干燥过程中具有不同的黏弹性演化模式,干燥温度对木材表芯层黏弹性应变转换有一定影响,这种作用主要是由于不同温度条件下木材含水率梯度所导致的;根据试验测定的木材表层、芯层在3、62、4 h 3个应力释放时间内所达到的黏弹性应变极值,分别给出了干燥过程各阶段落叶松板材表层与芯层黏弹性应变恢复时间的推荐值。

In order to investigate the development mode and relative magnitude of visco-elastic strain during wood conventional drying, and also to provide theoretical proof for discrimination between visco-elastic strain and mechano-sorptive creep strain, larch （ Larix gmelinii ） timber of 50 mm thickness was dried by conventional experiment. The one dimension distribution and change of transverse-tangential strains, including shrinkage strain, elastic strain and visco-elastic strain, were measured along wood thickness direction by slicing method. The influences of drying temperature, creep recovery time and drying stage on visco-elastic strain characteristics were analyzed quantitatively by applying visco-elasticity theory of polymers and composites. The results of the experiment showed that, with equilibrium moisture content of 8.0% -8.5%, there was positive correlation between temperature and elastic strain under tensile state;during wood conventional drying, the development trend of visco-elastic strain was similar to that of elastic strain, and after sufficient recovery of strain, the values of visco-elastic strain were greater than those of elastic strain to some extent;there were remarkable differences between surface and core position in visco-elastic strain development trends during the whole wood drying process, and temperature might affect the direction-change time of visco-elastic strain due to moisture content gradients; according to the maximum values of visco-elastic strain, which were measured on wood surface and core under stress relaxation time of 3,6 and 24 hrs, the recommended recovery time of visco-elastic strain was provided.