ASSESSMENT OF A THREE DIMENSIONAL FIBRE ORIENTATION MODEL FOR TIMBER

Wood is an orthotropic material with very different properties along and across fibres, and every board has its own pattern of knots and fibre deviations. Therefore, detailed knowledge of the three dimensional fibre orientation of individual boards would enable more accurate assessment of properties like stiffness, strength and shape stability. This paper presents a method for modelling 3D fibre orientation of side boards of Norway spruce. The method is based on dot laser scanning and utilization of the tracheid effect, and it is verified by a comparison between strain fields calculated on the basis of the fibre orientation model and corresponding strains determined using digital image correlation (DIC) technique. By means of the method, it is possible to identify knots and to reproduce the fibre orientation in clear wood in the vicinity of knots. Fibre orientation models of side boards including traversing edge knots were established and integrated in finite element board models used for simulation of four point bending tests. The same boards were also tested in laboratory and displacement fields of the wide faces were recorded at different load levels using DIC technique. Comparisons of strain fields from measurements and simulations showed close agreement, regarding both strain patterns and strain levels. Local strain concentrations caused by very small defects were detected using the models and also found from the laboratory test results. The modelling approach is promising and may be developed further for more advanced assessment of e.g. crack propagation and strength properties in both side boards and centre cuts