Mechanical behavior of silver birch structural timber under compression and tension loading

In modern timber construction, there is growing demand to use a wider diversity of tree species. The birch (Betula) genus is a deciduous hardwood tree with stiff and strong material properties growing abundantly in northern Europe and other temperate and boreal forest regions. The application of birch in the construction industry is, however, not well established due, in part, to a lack of knowledge on the mechanical performance of birch structural timber under various load types. The motivation of this thesis is to further investigate the structural potential of birch timber and develop fundamental knowledge important for predicting and assessing the mechanical quality. Investigations focus on the silver birch (Betula pendula Roth) species, whereby structural timber test specimens are derived from logs of various qualities. The experimental investigations include non-destructive test methods for the measurement of timber characteristics, and mechanical testing under compression and tension loading. From compression investigations, the influence of knot characteristics and clear wood timber characteristics on the compressive strength and stiffness is described together with the statistical dependencies. Qualitative investigations are further performed, by means of digital image correlation and strain field analysis. The results provide insight into the local deformation behaviour around knots and annual rings, along with the related failure mechanisms. A non-destructive laser scattering technique for the determination of grain angle is investigated for accuracy and applicability on silver birch. The laser imaging technique and ellipse detection algorithms are resolved using small-scale clear wood specimens. Comparisons between laser-based grain angle measurements and the fracture angles indicate efficacy of the method for measuring grain direction and predicting strength. The method is further applied to structural timber boards. From tension investigations, statistical representations of the tension strength and stiffness properties are described, including the local stiffness properties measured at knots and defect-free regions along the timber boards. Holistic analysis on the influence of timber characteristics on the mechanical properties are performed, and models for predicting the strength and stiffness properties of birch timber are subsequently developed, based on relevant non-destructive indicators. The models are used to predict the mechanical properties of the full-length timber boards as well as the local knot regions within the timber boards. Results indicate that suitable global and local predictions of the tensile properties are possible and constitute a step towards the further development of material models and quality assessment techniques for birch timber.