Measuring the shear strength ratio of glued joints within the same specimen

Measuring the shear strength of both the glueline and the solid wood with the same test specimen offers the advantages that the comparison is direct and the influence of wood variability is minimized. A new specimen geometry was developed for this purpose, and the test is run with two successive loading steps. Finite element modelling showed that the stress distribution is not significantly influencing the results, whereas the compressive pre-stress on the wood adherents results in a slightly lower resistance, probably mainly caused by micro-cracks that are already present or occur during the first loading step and propagate during the second. The analysis of fracture propagation in mode I revealed that different spruce wood batches have a much different behaviour. The method will be proposed as a standard test for epoxy adhesives used in thick joints. The evaluation criterion developed might be interesting also for other types of structural adhesives.