Scott Blair Fractional-Type Viscoelastic Behavior of Clear Spruce Wood: Influence of Compression Wood on Power-Law Stiffness Parameters.

In this paper, we investigate the influence of intrinsic compositional parameters on the viscoelastic compliance by employing three-point bending creep tests on clear, i.e., defect-free, spruce samples with a dimension of 15 × 15 × 280 mm(3). In addition to the regular samples, a prominent wood variation was investigated: so-called compression wood, stemming from an adaptive response of the growing tree to maintain structural stability. Tests were conducted at constant ambient conditions: isothermal at 20 degrees Celsius and at a relative humidity of 65 percent. These conditions were also employed during sample conditioning, leading to an equilibrium moisture content of the specimens of approximately 12 percent. Hence, so-called basic creep properties were investigated. Furthermore, we show that the experimentally observed compliance can be exceptionally well-modeled by a Scott Blair fractional-type element, with the latter calibrated by a mere number of two independent material parameters. This allows to render rather explicit dependencies of these parameters with respect to the dry density and the volumetric content of the compression wood. There, the quasi-instantaneous stiffness of the employed Scott Blair element is an increasing function of the dry density. While this primary dependency is also observed for compression wood, the quasi-instantaneous stiffness is significantly smaller over the investigated density range.