The Importance of Failure Mode Identification in Adhesive Bonded Aircraft Structures and Repairs

The performance of adhesive bonded components and repairs in the aviation industry has varied significantly, resulting in a perception that adhesive bonding may not be reliable. The authors suggest that adhesive bond failures require closer scrutiny to accurately assess the causes of these failures as in recent history they have not received the same rigorous investigative scrutiny usually associated with fracture of metallic or mechanically fastened structures. Current poor understanding of adhesive bond failures in some areas of the industry has resulted in some defects being attributed to causes which, under closer investigation, are shown to be totally unrelated to the true cause of the failure. The consequences of incorrect identification of the causes of adhesive bond failure include: • Failure to correct defective bonding processes during manufacture. • Use of inappropriate test methods for qualification of bonding processes. • Continued use of inappropriate certification test methods for bonded structures. This paper will present various failure mechanisms and their most probable causes together with case studies on how to identify the causes of bond failure from post-failure evidence. In most cases failures are directly related to processes used for initial production of the bond and are unrelated to the service loads. The authors recommend that adhesive bond failures be treated in the same rigorous manner as applied to metallic failures to ensure that the technology is applied correctly by the use of proper design, certification and production methodologies.