From elasticity to creep: orthotropic moisture-dependent rheology of Norway spruce

In the past, numerous simulations of the long-term performance of wood components undergoing moisture changes have exhibited unresolved discrepancies with reality, pointing to misconceptions about the underlying wood’s physical nature. Those are due to an incomplete and inconsistent experimental picture regarding orthotropy, moisture dependence, and the interconnection of distinct rheological mechanisms. This study, conducted on a single, homogeneous Norway spruce stem, provides a comprehensive insight into the elastic, viscoelastic, plastic, and hygroresponsive behavior in the three main anatomical directions. All data are described by generally accepted models, with moisture-scaling functions provided for each parameter. The described campaigns extend well beyond the minimum required tests for orthotropic bodies. This enables a quantitative assessment of the loss of accuracy due to symmetry assumptions, such as orthotropy and compression-tension symmetry. The completeness of the data allows for identifying oversimplifications in state-of-the-art rheological models, as well as for finding opportunities to reduce future testing effort.