Mechanical properties of clear wood from Norway spruce

Abstract

This thesis reports mechanical properties of clear wood from Norway spruce, comprising each orthotropic material direction and plane over the complete loading range till failure. The material properties are quantified in a set of linear, nonlinear and failure parameters. In addition, statistical distributions and inter-parametric correlations are presented. Several quantities have hardly been studied for Norway spruce earlier, and are also scarcely documented for spruce softwood in general.

The properties were determined by means of experimental tests in conjunction with numerical analyses. In order to obtain accurate and applicable results suitable for input in numerical simulations, the tests were mostly based on non-standardized procedures. Normal stress behaviour was investigated by means of compressive and tensile tests, whereas shear properties were based on the Arcan method. Constant climatic conditions and loading rates were applied and no effects from variation in humidity, temperature, size, loading rate or load duration were studied. The wood comprises graded and ungraded material from the spruce subspecies Picea Abies (L.) Karst ., with provenance Southern Norway. The use of material from the outer part of wooden stems enabled suitable specimens of various categories with material axes complying with a Cartesian coordinate system. Video extensometry was used for contact free strain measurements. All combinations of load directions and orthotropic measurement planes were tested. The 12 linear elastic orthotropic parameters were based on both loading and unloading, and were, in general, found to correspond relatively well with values reported for other spruce species. Characterization of the upper stress ranges was also emphasized, as accurate predictions of ultimate deformations and capacities are crucial in many analyses. Nonlinearity and ductility were, as normally assumed, found in compression, especially transversely. A more peculiar finding was the observation of a varying degree of nonlinearity in tension, and particularly in shear. The nonlinearity was adapted to bilinear models for each stress component, and for the case of shear, to exponential Voce models. The failure parameters were adapted to the ultimate stress and the Tsai-Wu failure criteria. In order to remove configuration and measurement effects, potentially distorting the material parameters, numerical FEM models were used to modify nominal values.

The relatively large quantity of parametric observations enabled investigation of statistical distributions for each material parameter. Moreover, correlations between values determined from the same test could be estimated. Hence, the work constitutes a basis for deterministic and probabilistic numerical analyses of spruce softwood on the macro scale level (0.1–1.0 m), suitable for general three-dimensional studies of details and joints in timber constructions.