Ensuring tensile performance of oak finger-joints using fiber orientation for cross cutting optimization

Finger-jointing is commonly used in the production of engineered wood products such as glued laminated timber (GLT) and cross-laminated timber (CLT). However, current standards for defining cross-cutting positions are based on visual rules developed for softwoods and are not well adapted to hardwoods such as oak, where significant fiber deviations may occur away from knots. This study proposes a machine-based cross-cutting method relying exclusively on laser-measured fiber orientation data. A 3D fiber orientation model was used to compute a stiffness-based Indicative Property (IP) along oak board profiles. IP thresholds were then established to ensure the tensile strength requirements of two target classes, T11 and T14, for both solid wood and finger-jointed elements. Boards were cross-cut according to an optimization algorithm, finger-jointed, and tested in tension following [EN 14080, 2013. Timber structures – Glued laminated timber and glued solid timber – Requirements]. The resulting characteristic tensile strengths reached 15.6 MPa and 19.4 MPa for the T11 and T14 classes, respectively, confirming the validity of the approach. Failure analyses showed that straighter fiber orientation increases the likelihood of failure occurring in the joint. Equivalent fiber angle thresholds of 14.4∘ (T11) and 12.4∘ (T14) were identified, providing practical grading criteria for future hardwood finger-jointing standards.