Applications Of Fracture Mechanics To The Durability Of Bonded Composite Joints

The increased numbers of bonded composite components and bonded repairs of cracked structures make knowledge of adhesive bonding crucial to aircraft design and life extension. This report covers an effort which focused on using fracture mechanics to evaluate the Mode I fracture and fatigue properties of several adhesively bonded aerospace material systems. The research concentrated on bond line cracking rather than fatigue crack growth in composite or metal adherends. Particular attention was paid to the environmental durability of bonded systems in use or intended for use on transport, fighter, and supersonic aircraft. Analysis was performed using closed-form solutions as well as finite element analyses. Results were discussed with respect to their relevance and applicability to bonded joint design. Key results include the identification of significant degradation in some varieties of bonded joints subjected to long-term isothermal exposure under hot/wet conditions. The degradation was manifested by decreased fracture toughness and fatigue threshold levels. Finite element analyses were performed on specimens with dissimilar adherends having complex geometries and thermally induced stresses in the bond line. A case study in which finite element analyses were used to relate the experimental results from this program with those of an independent project is also included. The analyses highlight the importance of using tapered adherends to avoid fatigue failures and show that typical aircraft skin stresses are below experimentally obtained threshold values for the specimen geometries investigated. Results of this study emphasize that the environmental durability of adhesively bonded joints is a key issue which must be considered by aerospace design engineers.