Effects of systematic variation of wood adherend bending stiffness on fracture properties: Part 1. Influence of grain angle

When wood beams are bonded to form double cantilever beam (DCB) specimens, the resulting fracture properties are often quite scattered. Random variations of properties of wood are usually considered as the reason for the data scatter, but there are also morphological aspects that can possibly be accounted for. The present paper focuses on these morphological aspects and, in particular, an analytical model has been developed for evaluating how the orientation and stiffness of the layers of beams of constant cross section influences the stiffness variation of the beam along its length. Wood in DCBs is a common example of a material with oriented layers resulting from the alternating earlywood (EW) and latewood (LW). Part of the paper is dedicated to Douglas fir (Pseudotsuga menziesii), where the variability of equivalent elastic stiffness is found to be on the order of ±6–8% in bonded DCBs, depending on grain orientation. Other representative cases of bonded wood beams are also presented, where the stiffness variability is on the order of ±15–20%. Part 2 of this paper will evaluate how these levels of elastic stiffness variations influence the measured critical strain energy release rate, .