Residual stresses in adhesively bonded wood determined by a bilayer flexion reporter system

The application of adhesives in modern timber engineering often introduces moisture into the wood, leading to permanent residual stresses after hardening. This paper proposes a novel approach to assess these residual stresses by using wooden bilayers as a reporter system. For thin bilayers, moisture-induced stresses lead to pronounced visible flexion that can be used to identify the stress-driving parameters of the adhesive’s gelation process. These parameters depend solely on the wood/adhesive combination and are inversely determined by fitting a finite element method model on the experimentally obtained flexion state. In a subsequent step, the determined parameters are used to calculate the residual stresses in the adhesive bondline of cross-laminated timber plates, emphasizing this approach’s scale independence and general applicability to larger scale structures. All combinations of European beech and Norway spruce with the adhesives Melamine–Urea–Formaldehyde (MUF), Phenol–Resorcinol–Formaldehyde (PRF), and Polyurethane (PUR) were investigated.