Abstract
The hygroscopic behavior of wood has a strong influence on its mechanical performance, yet the moisture gradients within the growth ring structure have not been sufficiently investigated. The main challenge is that moisture variations are coupled with strong sample deformation, which complicates the spatial referencing of moist and dry states. In this work, neutron imaging (NI) for the detection of water and digital image correlation (DIC) for the detection of local deformation were combined to calculate the local gravimetric moisture content (MCgrav) and the volumetric moisture content (MCvol) within single growth rings. Specimens of Norway spruce [Picea abies (L.) Karst.] were exposed to an adsorption-desorption cycle, with relative humidity (RH) steps varying from 0% (oven dry) up to 95% RH. After each acclimatization step, neutron transmission and DIC images were acquired. The local deformations determined by DIC were used to assign the corresponding dry density in the undeformed state to the compartment in a moist state and thus to calculate its MC by NI. No significant MC gradients could be found between earlywood (EW) and latewood (LW) within ±0.5% accuracy. However, strong density gradients between EW and LW can be directly correlated with MCvol. It appears that the MC in the cell wall is constant regardless of the particular growth ring position.
The first author financed by the Swiss National Science Foundation (Grant No. 125184). The Building Physics group at EMPA is acknowledged for using the conditioning device. Furthermore, Ms. V. Krackler and Mr. F. Michel are gratefully acknowledged for helping with the experiments at Paul Scherrer Institut, and Dr. Falk Wittel for his contribution in the data evaluation.
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