A 3D model for knots and related fiber deviations in sawn timber for prediction of mechanical properties of boards

Abstract

Increased use of wood has led to complex timber constructions and new types of engineered wood products. In simulations, however, mainly simplified models are used to describe this material with its strongly varying properties. Therefore, reliable prediction tools for mechanical properties of wooden boards are needed. Those varying properties mainly originate from knots and fiber deviations. Thus, we use fiber directions on board surfaces to reconstruct knots within boards. Combined with a fiber deviation model we assess our model with experiments on different levels: fiber directions on surfaces, strain fields and bending stiffness profiles.This model now better describes fiber patterns near knots and knot clusters. Also, we showed that accurate modeling of the pith is important to avoid large regions of incorrect fiber deviations. Furthermore, modified knot stiffness properties were successfully used to consider pre-cracked knots. Finally, we obtained multiple bending stiffness profiles, where we showed that even local effects can be simulated accurately.We anticipate our tool to be a starting point for improving strength grading models, where effects of knot configurations can be studied more easily than with experiments alone. Furthermore, the presented improvements will render the simulation of realistic failure mechanisms in wooden boards more likely. Keywords: Wood, Knots, Fiber course, Effective mechanical properties