Cumulative Fatigue Disbond of Adhesive Joints and its Detection using Thermal wave Imaging

Adhesively bonded structures have found increased acceptance in both military and commercial aircraft. Reliability of these aircraft structures is of paramount importance. This issue is especially important for aging aircraft, which are very often in service beyond their stipulated lifetime. Joints are exposed to various combinations of cumulative mechanical fatigue, thermal fatigue and environmental conditions. In addition, voids inherent in the adhesive joint cause sites of stress concentration and coalesce to form disbonds when the joints are loaded. Simultaneous presence of these disbonds in the same structural element will result in a loss of its load bearing strength. Moisture can also attack the primary bonds between the substrate and the adhesive decreasing the joint’s interfacial strength. Periodic cycling and environmental factors lead to disbond propagation and premature failure of the joints. The effect of cumulative cycling on disbonding and its propagation in adhesive joints is an important subject, which has been the focus of this study