Anisotropic shrinkage is a typically feature in wood, which is of critical importance in wood drying. In this study, the shrinkage strains over each growth ring were determined by a full-field strain measurement system during moisture content (MC) loss. Color maps were used to visualize the full-field distribution of displacement and shrinkage strain under different MC conditions. The variation of tangential and radial shrinkage strain from pith to bark, as well as the anisotropic shrinkage in heartwood and sapwood were studied. Both of the displacement and strain values increased as the MC decreased. From pith to bark, the tangential strains were higher at two poles as compared to the center, showing a parabolic distribution below fiber saturation point. While for radial shrinkage strain, a minor difference was observed except for the MC of 10%. An intersection between tangential and radial shrinkage ratio curve was observed at the MC of 28%. Both expansion and shrinkage in tangential direction were larger than radial counterparts, and the transformation from expansion to shrinkage occurred at the MC region of 32–28%. In addition, the shrinkage in heartwood was larger than sapwood, whereas anisotropic shrinkage in sapwood was more pronounced as compared to heartwood.

中文翻译：

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木材水分减少过程中收缩应变的全场跟踪和分析

各向异性收缩是木材的典型特征，这在木材干燥中至关重要。在这项研究中，通过全场应变测量系统在水分含量（MC）损失期间确定每个生长环上的收缩应变。彩色图用于可视化在不同MC条件下位移和收缩应变的全场分布。研究了从髓到树皮的切向和径向收缩应变的变化，以及心材和边材的各向异性收缩。位移和应变值都随MC的减小而增加。从中心到树皮，在两个极点处的切向应变都比中心点高，这表明纤维饱和点以下的抛物线分布。对于径向收缩应变，除MC为10％以外，观察到很小的差异。在MC为28％时观察到切向收缩率与径向收缩率曲线之间的交点。切向方向上的膨胀和收缩都大于径向切向，并且从膨胀到收缩的转变发生在32％到28％的MC区域。此外，心材的收缩比边材大，而边材的各向异性收缩比心材更明显。