Damage mechanisms in composite bonded joints under fatigue loading

In the present paper, the fatigue damage mechanisms in composite bonded joints are analysed and discussed, with particular emphasis on the influence of layer orientation at the adhesive-adherend interface, corner geometry at the end of the overlap area and the stacking sequence. Results indicate that the corner geometry at the end of the bonded area as well as the length of the overlap have a significant influence on the fatigue strength of the joints, while the layer orientation at the adhesive/adherend interface was seen to have a lesser influence on the fatigue performance. The evolution of fatigue damage, consisting in its essential features of crack initiation followed by propagation up to a critical length, is investigated by means of optical and scanning electron microscopy and by monitoring the stiffness of the tested joints. As a result, it is seen that a 45° oriented layer at the adhesive-interface makes crack paths much more complicated with respect to 0° oriented interface joints, with an increase in the crack propagation resistance. Moreover, measurements of the evolution of axial stiffness are promising in order to develop a simplified technique to assess the crack propagation during fatigue life.