This article aims at providing a synthetic view of the equilibrium moisture content (EMC) dependence of wood vibrational properties (i.e. dynamic mechanical properties in the audio-frequency range), including specific dynamic modulus of elasticity (E′/γ) and damping coefficient expressing internal friction (tanδ). A series of multi-trajectories experiments was designed to complete an analytical review. Literature indicates that: (1) in longitudinal (L) direction, the EMC dependence of E′/γ shows a very consistent shape (rather linear) between studies, while its shape is non-linear for tanδ and varies significantly between studies; (2) EMC dependence of tanδ is rather well documented in the L direction, in adsorption, for soft-woods, but data covering EMC dependence in both L and other anisotropic directions, and sorption hysteresis, are still scarce. Experiments were conducted on a softwood (spruce) and a hardwood (maple), in L and radial (R) directions, in full adsorption from oven-dry state, full desorption from water-saturated state, and relative humidity (RH) loops without extreme conditioning. Measurements were made at conditions considered "at equilibrium" and some were monitored through time. Results indicated that tanδ was much more (×3) sensitive to EMC differences than E′/γ. R properties, especially tanδ R , were much more (×2-3) sensitive than L properties-resulting in strong increase of anisotropy with increasing EMC. In L direction, differences due to EMC remained moderate compared to the natural variability of wood for E′/γ, while for tanδ the EMC-induced changes were at least equal to natural variability in high-grade spruce. Vibrational properties did exhibit a hysteresis as a function of RH, but very little hysteresis as a function of EMC. The tanδ-EMC relation strongly depended on the actual time of stabilisation after reaching EMC. A related paper will address the transient, out of equilibrium effects of changing moisture conditions on the vibrational properties of wood.

中文翻译：

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准平衡时木材各向异性振动特性的含水量依赖性：分析回顾和多轨迹实验

本文旨在提供木材振动特性（即音频范围内的动态机械特性）的平衡含水量 (EMC) 依赖性的综合视图，包括比动态弹性模量 (E'/γ) 和阻尼系数表示内摩擦（tanδ）。一系列多轨迹实验旨在完成分析审查。文献表明：(1) 在纵向 (L) 方向上，E'/γ 的 EMC 依赖性在研究之间表现出非常一致的形状（相当线性），而其形状对于 tanδ 是非线性的，并且在研究之间变化很大；(2) tanδ 的 EMC 依赖性在 L 方向上有很好的记录，在吸附中，对于软木，但数据涵盖了 L 和其他各向异性方向上的 EMC 依赖性，以及吸附滞后，仍然稀缺。在软木（云杉）和硬木（枫木）上进行了 L 和径向 (R) 方向的实验，从烘箱干燥状态完全吸附，从水饱和状态完全解吸，相对湿度 (RH) 循环没有极端调理。测量是在被认为是“平衡”的条件下进行的，有些是通过时间监测的。结果表明，tanδ 对 EMC 差异的敏感性比 E'/γ 高得多（×3）。R 特性，尤其是 tanδ R ，比 L 特性更敏感（×2-3）-导致各向异性随着 EMC 的增加而强烈增加。在 L 方向，与木材的自然变异性相比，E'/γ 的 EMC 引起的差异保持适度，而对于 tanδ，EMC 引起的变化至少等于高级云杉的自然变异性。振动特性确实表现出作为 RH 函数的滞后，但作为 EMC 函数的滞后非常小。tanδ-EMC 关系很大程度上取决于达到 EMC 后的实际稳定时间。一篇相关的论文将讨论湿度条件变化对木材振动特性的瞬态、非平衡影响。