Adhesively bonded repairs offer many advantages over mechanically fastened repairs in terms of mechanical and aerodynamic performance, as well as weight. Bonded repairs are often the only repair method when the parts are too thin for bolted repair, or when other requirements, such as external shape and sensors for stealth, are to be maintained.
In spite of positive experience of bonded repair on secondary and tertiary aircraft structures, as well as its successful applications on some primary structures of military platforms, such as F/A 18 and MIG29, there are significant challenges in using bonded repairs to restore and maintain structural integrity of primary structures – in particular with regard to certification. There exist certification approaches to bonded joints on primary structures, but they are limited to specific parts and platform. An accepted general certification approach to allowing for bonded repair on primary structures could lead to increased availability of military aircraft, maximized performance and mission capability, and reduced cost of ownership. Such general certification strategies are expected to comprise the following tools at the component and the platform level:
• Robust and efficient repair technologies;
• Numerical tools for composites damage assessment and repairs;
• Testing and evaluation for repair’s ability to restore structural integrity;
• Repair durability and damage tolerance for continued airworthiness.
A recent AVT activity, AVT-266 –“Use of Bonded Joints in Military Applications” showcased the recent progress in adhesive bonding, including bonded repair, among the participating NATO nations. Furthermore, some approaches were discussed beyond today’s certification recommendations, like integrated risk-based approaches or integrated health monitoring technologies including sensing systems.
The most promising technology to comply with the airworthiness certification requirements is considered to be the application of design features into the bondline that are capable to stop or slow down crack propagation, such as to maintain limit load (LL) capability. Designing with design features in the wake of crack containment will serve the certification of bonded repairs, as well as bonding initial design. Therefore, this ET will mainly concentrate on this approach.
Furthermore, SHM technology for the in-service monitoring of the patch will be used to validate the effect of the design features. Also, the use of traveler patches around the bondline, the so-called Bonded Repair Coupon (BRC) method will be investigated for substantiation of the initial bondline strength. The use of the BRC method will also be used for investigation of the bondline ageing. Large repair of composite military platforms for damaged structures with a residual strength below limited load (i.e. patch off) will be possible with the “design feature” certification approach.
The main scientific objectives of the ET are:
• To identify suitable mechanical test specimens and methods to determine crack propagation in bonded joints (initial design and repair).
• To investigate analysing methods (such as the finite element method) that are able to predict crack propagation in adhesive bonded repairs
• Survey of design features and present state of the art regarding the design feature certification approach
• Determine supporting technology to substantiate the initial bondline strength and bondline degradation due to ageing.