Weak interfacial bonds and the long-term durability of bonded repairs to polymer matrix composites
Major technological advances using composite materials in airframe components to improve performance and energy efficiency have been achieved in the last 50 years. For the first time, these materials are being introduced in load-bearing wing and fuselage structures. Durability, repairability, and maintainability are essential elements in the continued airworthiness of these components. Weak interfacial bonds between composite adherends are not detectable with the current inspection methods and may deteriorate in service, possibly leading to the component’s failure. In-service experience with, and interfacial degradation of, bonded repairs to metallic substrates is well documented in the literature; there is a growing need to investigate these mechanisms with composite substrates as well. In-service experience is necessary to understand the durability of the parent construction and the requirements for repair subsequent to damage. In the first phase of this research work, teardown studies were conducted to evaluate the long-term durability of two composite components with service history: a B737 horizontal stabilizer and a Beechcraft Starship aft wing. Results indicate that both structures maintained their structural integrity, with no evidence of detrimental degradation or deterioration. Results also revealed moisture levels near equilibrium in both components. The fundamental objective of second phase of this work was to investigate mechanisms of interfacial degradation in a bonded repair to a composite substrate resulting from a compromised parent interface or a defective interphase, subjected to mechanical loading and the environment. Weak interfacial bonds were investigated using a thermodynamic-based approach to surface analysis followed by a post-repair residual strength evaluation of these repairs. The case studies performed demonstrated that exposure to repeated loading and the environment results in further deterioration of the originally deficient bonds.