Growth stress of Zelkova serrata and its reduction by heat treatment

Zelkova is one of the tree species that is known to have a very high level of growth stress. During sawing, yield decreases as a result of heart crack and distortion of boards owing to growth stress release. Therefore, the study of growth stress in zelkova is considered to be important for its effective utilization. In this study, 6 erect trees with diameters at breast height ranging from 25 to 45 cm were felled. Surface growth stress of zelkova was measured by the groove method, using resistant strain gauges. Released surface growth stress of trunks at various tree heights, and also the residual growth stress distribution over the radial positions from pith to bark, were measured. The steaming method was used in an attempt to decrease growth stress, and the effects of different steaming temperatures and duration times on the decrease of growth stress were examined. There was no obvious relationship between surface growth stress and trunk height. The residual growth stress was tensile for the outer part of the tree trunk and compressive for the inner part, yielding a V-shaped stress distribution. Both the surface growth stress and the internal residual stress were reduced by heat treatment. The reduction of longitudinal growth stress increased with increased heating temperature, and with increased heating time. With a 110℃ steam heating temperature and 24 to 48 hour exposure time, over 86﹪of the surface growth stress was released. If 85 or 95℃ steam heating temperature and 24 hour exposure time is used, only 50% of the growth stress can be reduced. However, a heating temperature below 100℃ may be most suitable for practical applications.

榉树（Zelkova）是已知生长应力（growth stress）水平极高的树种之一。在锯切过程中，因生长应力释放引发的板材心裂与翘曲会导致出材率下降。因此，针对榉树生长应力的研究对其高效利用具有重要意义。本研究选取6株胸高直径介于25~45 cm的直立榉树进行伐倒。采用开槽法（groove method）结合电阻应变片（resistant strain gauges），对榉树的表面生长应力进行测定；同时测定了不同树干高度处的表面释放生长应力，以及从髓心到树皮的径向位置上的残余生长应力分布。为降低生长应力，本研究采用蒸汽处理法，并考察了不同蒸汽温度与保温时长对生长应力消减的影响。研究发现，表面生长应力与树干高度之间无显著关联。树干外层的残余生长应力表现为拉应力，内层则为压应力，整体呈现V形应力分布特征。热处理可同时降低表面生长应力与内部残余应力，纵向生长应力（longitudinal growth stress）的消减幅度随加热温度升高与保温时长延长而增大。当蒸汽加热温度为110℃、保温时长24~48 h时，表面生长应力的释放率可超过86%。若采用85℃或95℃的蒸汽加热温度并保温24 h，生长应力的消减率仅为50%。但从实际应用角度考量，100℃以下的加热温度或许更为适宜。