This study is concerned with the viscoelastic behavior of wood. The time dependency of the orthotropic compliance for beech wood is investigated by performing tensile (Te) and compressive (Co) creep experiments in all wood's orthotropic directions. Time evolution of the creep strain in the axial and lateral directions is recorded using the digital image correlation technique, to determine the time dependent Young's moduli and the Poisson's ratios needed for the calculation of the diagonal and nondiagonal elements of the viscoelastic compliance matrix. The results of this study demonstrate the viscoelastic character of wood, revealing the significant time influence on the mechanical behavior. The unequal time dependency of the Young's moduli and the Poisson's ratios obtained for the individual directions highlights the orthotropic nature of the viscoelastic compliance. Differences between the time dependent behavior for the compliance determined in Te and Co further indicate that the viscoelastic behavior of wood depends on the loading modality. Supported by the unequal evolution of the Te and Co creep strain, the results suggest that the time dependent stress–strain relationship of wood is essentially different in Te and Co. Poisson's ratio values, which are shown to increase with time in Te and decrease in Co, demonstrated this fact. The substantially different time dependency of the nondiagonal elements of the compliance matrix further emphasizes the complexity of the viscoelastic character of wood. Visualized by the time evolution of the corresponding nondiagonal elements ratio, differences between the Te and Co viscoelastic behavior become particularly visible in the time dependency of the orthotropic compliance asymmetry.
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