Direct measurement of strain distribution along a wood bond line. Part 1: Shear strain concentration in a lap joint specimen by means of electronic speckle pattern interferometry

Phenol-resorcinol-formaldehyde (PRF) and one-component polyurethane (PUR) resins were used to manufacture single lap joint samples corresponding to EN 302-1. 3D electronic speckle pattern interferometry (ESPI), which enables measurement of spatial displacement of less than 0.1 mu m, was used during tensile shear experiments to observe full-field in-plane and out-of-plane deformation of the lap joint samples and to detect strain concentrations in the vicinity of glue lines. Finite element analysis was performed to validate ESPI measurements. In general, ESPI measurements showed that in a lap joint experiment a very small volume of material close to the ends of the overlapping area is highly strained. ESPI and finite element analysis pointed out that PUR glue lines are characterised by much higher shear deformations than PRF glue lines, especially at the ends of the overlapping area. However, due to the lower Young's modulus of PUR resin compared to PRF, higher shear strains but lower shear stress concentrations can be expected in PUR, which was confirmed by the FE model.