Tensile creep of Norway spruce on the tissue scale

The rheological behavior of wood emerges from complex mechanical interactions occurring across multiple length scales. This behavior is characterized by directional dependence, as well as sensitivity to moisture content, loading time, and the degree of loading. This study focuses on the viscoelastic creep response of Norway spruce (<i>Picea abies</i>) tissues under different moisture levels and loading degrees. Using a custom-designed, fully automated test rack with moisture control, we investigate the uniaxial, moisture-dependent creep compliances across all feasible anatomical directions, as well as of isolated earlywood (EW) and latewood (LW) slices to understand their contribution to the cumulative behavior of the growth ring. The creep response is compared to the moisture dependence of the elastic compliance, revealing nontrivial scaling behavior as a function of moisture content. The results show significant directional dependencies and reveal the critical impact of moisture on deformation mechanisms. The transverse directions involve a complex interaction between bending, determining a more compliant and moisture-sensitive creep response, and cell wall stretching in the softest direction compared to loading in grain. These findings offer valuable insights into the moisture-dependent creep mechanisms of wood slices, highlighting the importance of exploring different orientations and tissues at various moisture content to fully understand the creep behavior at the bulk scale.