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 g overning adhesive strength. Recently, the damage to the adhesive layer has been investigated by in-situ observation under several load conditions. How ever, these observations are limited to the behaviour on the surface of the adhesive layer. If an adhesively bonded butt joint with a very thin adhere nd can be tested, damage in the interior of the adhesive layer will be obse rved in transmitted light, which facilitates clarification of the fracture mechanism of adhesive joints. In this study, to observe the damage in the i nterior of the adhesive layer, adhesively bonded butt joint specimens were made from 0.3 mm thick steel plates bonded by a rubber modified adhesive. T hese specimens were used for the fatigue tests, and the damage to the adhes ive layer was examined by use of a microscopic video-camera under transmitt ed light. Furthermore, elastic-plastic finite element analysis of the butt joint was conducted to simulate the propagation behaviour of the damage zon e. Modified Gurson's constitutive equations were applied to the adhesive la yer and the rubber particles were considered to be the initial voids. The r esult for the analytical distributions of the void volume fraction was comp ared with the observed damage zone. It was found that the observed damage z one coincides with the high volume fraction zone.