A new testing methodology for the assessment of fatigue properties of structural adhesives

The monotonic and fatigue shear behaviour of an epoxy adhesive joint has been studied using a short overlap - thick adherend configuration. A specific in situ bonding procedure has been developed in order to accurately control the initial stress state of the joint before testing. The strength of the joint has been found to be strongly dependent on the strain rate and the joint thickness. This dependency was associated with a change in damage mechanisms from cohesive failure (for small joint thickness or elevated speed) to adhesive failure (for large joint thickness and low speed). This was attributed to changes in peel stresses and/or joint morphology with thickness. Fatigue tests were carried out at imposed strain amplitude. Fatigue logs giving the changes in the shape of the transverse load-displacement loops were at first considered. The latter have been found to be useful to differentiate between the different failure modes (i.e. cohesive and adhesive). In a second step, all the results were summarised in a fatigue map giving the endurance properties and the failure mode of the joint as a function of the joint thickness and the strain level. This method should be generally useful whenever an accurate determination of the contribution of adhesive and interface behaviour is required for the assessment of joint durability.