Accurate and continuous adhesive fracture energy determination using an instrumented wedge test

The wedge test and the related double cantilever beam test are practical methods of assessing structural adhesive fracture energy. In the former, and to a lesser extent the latter, a recognised problem is the difficulty of following the length of the growing crack, required to calculate fracture energy with any accuracy. We present a novel method of measurement of crack length that has the advantages of being accurate and allowing continuous assessment of crack-length evolution during the failure process. It is based on the attachment of a series of strain gauges to the outer surface of one of the beams constituting the adhesive assembly. Surface strain measurements are interpreted directly using simple beam theory. The method has been validated both with adhesive assemblies under failure conditions and by tests undertaken on “artificial” joints, where “bonding” is effected by clamping adherends together.