The dimensional stability of paper products is a well-known problem, affecting multiple engineering applications. The macroscopic response of paper to moisture variations is governed by complex mechanisms originating in the material at all length-scales down to the fiber-level. Therefore, a recently-developed method, based on Global Digital Height Correlation of surface topographies is here exploited to measure the full-field hygro-expansion of single fibers, i. e. a surface strain tensor map over the full field of view is obtained as function of time. From the strain field, the longitudinal and transverse hygro-expansion and principle strains can be calculated. Long- and intermediate-duration dynamic tests are conducted on softwood and hardwood fibers. A large spread in the softwood fiber’s transverse and longitudinal hygro-expansion coefficient ratio was found, while hardwood fibers behave more consistently. Computing the principle strain ratios reduces this spread, as it takes into account the variations of the deformation direction, which is directly affected by the micro-fibril angle (MFA). Furthermore, long-duration tests allow identification of the half-times at which the fibers equilibrate. Finally, the determined major strain angles for all fibers are consistent with the MFA ranges reported in the literature.

中文翻译：

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单根针叶木和阔叶木浆纤维的全场湿胀特性

纸制品的尺寸稳定性是一个众所周知的问题，影响着多种工程应用。纸张对水分变化的宏观响应受到复杂机制的控制，该机制源自材料的所有长度尺度，直至纤维水平。因此，最近开发的一种基于表面形貌的全局数字高度相关性的方法在这里被用来测量单纤维的全场湿度膨胀，即。e. 作为时间的函数，获得了整个视场上的表面应变张量图。从应变场，可以计算纵向和横向的湿膨胀和主应变。对软木和硬木纤维进行长期和中期动态测试。发现软木纤维的横向和纵向吸湿膨胀系数比有很大差异，而硬木纤维的表现更一致。计算主应变比可以减少这种扩散，因为它考虑了变形方向的变化，而变形方向的变化直接受微纤维角 (MFA) 的影响。此外，长时间测试可以识别纤维平衡的半衰期。最后，确定的所有纤维的主要应变角与文献中报道的 MFA 范围一致。长时间测试可以识别纤维平衡的半衰期。最后，确定的所有纤维的主要应变角与文献中报道的 MFA 范围一致。长时间测试可以识别纤维平衡的半衰期。最后，确定的所有纤维的主要应变角与文献中报道的 MFA 范围一致。