Evolution of damage zone in adhesive layer of adhesively bonded butt joints under cyclic loading

For rubber-modified adhesives, the damage zone plays an important role in governing adhesive strength. Recently, the damage to the adhesive layer has been investigated by in-situ observation under several load conditions. However, these observations are limited to the behaviour on the surface of the adhesive layer. If an adhesively bonded butt joint with a very thin adherend can be tested, damage in the interior of the adhesive layer will be observed in transmitted light, which facilitates clarification of the fracture mechanism of adhesive joints. In this study, to observe the damage in the interior of the adhesive layer, adhesively bonded butt joint specimens were made from 0.3 mm thick steel plates bonded by a rubber modified adhesive. These specimens were used for the fatigue tests, and the damage to the adhesive layer was examined by use of a microscopic video-camera under transmitted light. Furthermore, elastic-plastic finite element analysis of the butt joint was conducted to simulate the propagation behaviour of the damage zone. Modified Gurson's constitutive equations were applied to the adhesive layer and the rubber particles were considered to be the initial voids. The result for the analytical distributions of the void volume fraction was compared with the observed damage zone. It was found that the observed damage zone coincides with the high volume fraction zone.