Direct measurement of strain distribution along a wood bond line.: Part 2:: Effects of adhesive penetration on strain distribution

The effect of adhesive penetration into wood on the distribution of strain along a mechanically strained bond line was evaluated. With this goal, spruce wood specimens were bonded with phenol-resorcinol-formaldehyde (PRF) and one-component polyurethane resin (PUR). Penetration of the adhesives into cell cavities was observed in a zone extending over several 100 mu m from the bond line. Electronic speckle pattern interferometry was used to record the elastic deformations caused by mechanical tests in transverse compression. By differentiating 2D displacement maps, the strain distribution was calculated. Both adhesives provided an increase in the stiffness of the adhesive-wood interface due to penetration into cell cavities. Compression strain was much lower in the bond line area than in the wood. Shear strain was also low in the bond line and increased gradually with increasing distance. No change was observed in wood unaffected by the adhesive. The effects were more pronounced for PRF resin than for PUR.