Experimental assessment and numerical analysis of a shear specimen for wood adhesive fiber composites

This study investigated the shear strength of fiber reinforced polymer (FRP)-wood bonds based on the experimental results of a specific type of test specimen developed for this composition by directly comparing the strengths of bondline and solid wood of Picea abies Karst. species. Four types of fibers were used, three synthetic (Vectran, Carbon 300 and Carbon 600) and one natural (Sisal), bonded with two adhesives (polyurethane and epoxy). Carbon 300 and Sisal fibers showed better structural compatibility when bonded with epoxy resin, while Vectran and Carbon 600 presented similar compatibility when glued with epoxy or polyurethane resins. Numerical analysis was carried out in order to understand if the experimental procedure can influence the shear strength results for the different composites. It allowed assessing the stress distribution in the shear plane in terms of tangential and orthogonal stresses to the bond plane for different FRP elastic moduli and glue line thicknesses. The study observed high stress concentrations at the edges of the specimen and lower stresses in the middle of the shear area, both influenced by the elastic modulus and thickness. Numerical results showed that the presence of normal stress peaks increased as stiffness decreased and thickness increased. This occurrence may explain the better strength values observed experimentally for the high stiffness fiber-reinforced assemblies compared to pure adhesives or Sisal, which have low stiffness and high thickness.