Bending–shear domain of Mode-V failure in hardwood dowels: Experimental tests and implications for dynamic grading

Timber joints are still built almost exclusively with steel fasteners, despite the high carbon footprint of steel production. Wooden dowels made from dense hardwoods offer a circular and renewable alternative, but current design codes do not yet cover their failure mechanisms. This study investigates the bending-shear interaction (Mode V) in rotary-cut birch and profiled beech dowels through a combination of dynamic and quasi-static testing. Ninety specimens, in two diameters and three gap-to-diameter ratios, were tested to evaluate the influence of geometry and wood directionality on strength. Free-free modal tests provided axial, bending, and shear moduli, which proved to have limited correlation with the measured shear strength, highlighting the practical limits of dynamic grading for dowel performance prediction. In contrast, quasi-static double-shear tests revealed two purely geometric parameters, the unsupported span and loading direction (radial versus tangential), that govern the shear strength of hardwood dowels. A minimalist linear model based on these parameters predicted the yield strength of birch and beech dowels with cross-validated errors below 5%. Expressed in normalised stress space, all results follow an almost linear bending-shear failure line, confirming that Mode V is a brittle shear-dominated fracture mechanism, markedly different from the curved interaction surfaces assumed for ductile steel fasteners. The study concludes by proposing a simplified design format for hardwood dowels and outlining the boundaries of applicability for dynamic grading approaches.