Moisture content dependence of anisotropic vibrational properties of wood at quasi equilibrium: analytical review and multi-trajectories experiments

Abstract

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 softwoods, 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 tanδR, 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.

Transient destabilisation in anisotropic vibrational properties of wood when changing humidity

“…The possible sources of errors, with this actual device and setup, have been quantified (Brémaud et al 2012;Brémaud and Gril 2020). Experimental errors in E′/γ due to frequencies were negligibly small, but those due to thickness could have a noticeable effect, despite precise machining.…”

“…Thus, after initially recording the homogeneity over specimens, dimensions were always measured at a same position for the monitoring of changes through time. The precision of measurements of tanδ by bandwidth increases with acquisition time (Brémaud et al 2012, Brémaud andGril 2020). Here, an acquisition time of 20 s allowed fine monitoring of immediate changes through time, with a resolution ≤1.5% (i.e.…”

“…Relative errors between 10 repetitions gave a coefficient of variation (standard deviation/ average) ≤1% and a range of variation ([maximumminimum]/ average) ≤3% for both R and L tanδ. Specimens were manipulated with tweezers instead of fingers, and vibration tests for monitoring through time were conducted inside a climatic equipment (Brémaud and Gril 2020).…”

“…These transient effects, due to the changes in moisture, are superimposed to the effect of the moisture content (MC) itself on physical-mechanical properties, that can be expressed by their EMC dependence, once that equilibrium is reached. For wood vibrational properties, E′/γ decreases, and tanδ increases, with increasing EMC (Obataya et al 1998), and the degree of EMC dependence is different in the longitudinal (L) and radial (R) directions of wood (Brémaud and Gril 2020). In addition, transient effects of changing moisture can concern, simultaneously, different domains of wood behaviour, e.g.…”

“…A series of experiments of monitoring changes in moisture and vibrational properties through time after step-change in conditions was undertaken, covering a wide span of humidity steps in ad-and de-sorption, measured in L and R directions, on a softwood (spruce) and a hardwood (maple). The quasi-equilibrium moisture dependence, explored in a related article by the authors (Brémaud and Gril 2020), was subtracted from transient changes, and some data analysis methods were proposed to quantify both the amplitude, and time of occurrence, of maximum destabilisation, and to evaluate if properties were approaching an "equilibrium" through the slow process of physical ageing.…”