Towards Tailored Composite Bonded Repairs (BONDPAIR)

Advanced composites are currently being used in aircraft structures (primary and secondary components) to a significant extent (e.g. 50% by weight in Boeing 787, Airbus A350 and Bombardier C-Series). For these new generation of aircraft structures with large composite components, the existing repair technology must be significantly improved to fabricate robust, reliable and certifiable repairs for restoring damaged (e.g. impact loads) primary structural components. It is important to note that primary aircraft structural components are critical to aircraft structural integrity and safety. The primary components are highly loaded and relatively thick (ranging from ~3 mm to ~25 mm) with several laminae having different fibre orientations, and thus require advanced repair design and fabrication techniques to restore the original structural/operational conditions. In this regard, current repair design and fabrication approaches need to be significantly improved for repairing primary composite aircraft structures.

As mechanically fastened composite repairs are not ideal for polymer composite structures because of the stress concentrations induced by fasteners, bonded scarf repairs, which can provide high joint efficiency and aerodynamic surface finish, offer opportunity to improve composite repair methodology and thus have greater potential for aircraft repairs. The extent of material damage is often uncertain and depends on the source/nature of damage. The amount of material that needs to be removed from the parent component for a scarf repair not only depends on the amount of material damaged but also the geometrical parameters (e.g. patch shape, scarf angle) of the patch designed. A significant amount of undamaged material around the damaged region needs to be machined in order to achieve the designed patch geometry. In addition, the amount of undamaged material removed can have an adverse effect on a component that is designed to take high stresses. To minimize the removal of undamaged material from the component, the design of scarf repairs should be tailored for a given repair condition. On the other hand, novel fabrication techniques need to be developed to achieve complex patch geometries (e.g. accurate machining of parent laminate, curing of a mating patch).

In this context, the proposed research is focused towards tailored composite bonded repairs to primary composite structures using non-conventional scarf patch geometries. The project aims to (a) develop a novel infusion-based repair methodology to fabricate scarf repairs using easy-to-store materials (carbon fibre fabric, thermoplastic veils) without pre-preg and film adhesives, (b) minimize the amount of undamaged material from the parent component by incorporating non-conventional patch geometries for enhancing parent-patch interface stress distribution, and (c) characterising failure behaviour of tailored scarf repairs and comparing with conventional scarf repairs. The ambition of this research project is to develop scientific pathways that will enable future repair technologies, especially tailored for primary composite components in aircraft structures.

National Importance:

Globally it is now scientifically and politically acknowledged that greenhouse gas emissions from different industries have been contributing to climate change. Aircraft emissions are similar to other emissions resulting from fuel combustion, and contribute significantly to global climate change. To combat the environmental threat posed by aviation, the aerospace industry has been aiming to considerably reduce emissions through weight reduction, aerodynamic improvements and new aircraft concepts. For considerable weight reduction, the application of polymer composites in aircraft design is considered globally as one of the key technologies to meet emission targets. Advanced composites, which are usually continuous fibres within a polymer matrix for aerospace applications, provide superior material properties compared with metals and thus enable lighter structural designs to be achieved. In the UK, significant research investments were made in recent years by different funding bodies (e.g. EPSRC CDTs related on composite materials) to develop and improve science and technology around polymer composites. While composite recycling is an important issue, extending the service life of composite structures is also a major element in relation to the UK composites strategy.

Industrial Relevance:

The long-term vision and objectives of the proposed project are strongly driven by the current industry needs in relation to the maintenance, repair and overhaul (MRO) of composite aircraft structures (e.g. Airbus A350, Boeing 787, Bombardier C-Series). Civil aircraft-on-ground (AoG) costs on average $US 100,000 per day per aircraft. Reducing aircraft downtime, improving repair reliability, ensuring repair repeatability are the key requirements for not only the sustainability of airliners and aircraft manufacturers but also the associated composite industry (e.g. materials suppliers). The estimated (Visiongain) world commercial aircraft MRO market in 2013 was $US 49.2 billion. Research activities in academia and industry (e.g. GKN Aerospace UK, DSTO Australia, EADS, GMI Aero France, and all major aircraft manufacturers) are currently focusing on composite repair technologies to address some of these challenges. It is also important to note that there is a growing interest in advanced composites by non-aerospace industries (e.g. automotive, wind energy) in recent years-further emphasising the potential impact of novel and robust composite repair technology.

Industrial Collaboration:

The PI had discussions with Bombardier Aerospace-Belfast (aircraft manufacturer), TFP Global (material supplier) and Signatex (material supplier). During the project, the PI and a newly appointed PDRA will engage with the maintenance and repair department of Bombardier-Belfast. The PI will invite them to the University of Manchester and discuss the progress of the project (6 month review) and plan a visit to Bombardier Belfast. The PI will engage with other relevant industries by exploring the contacts that exist within the Northwest Composite Centre at the University of Manchester.

Academic Collaboration:

The PI had discussions with the Irish Centre for Composites Research (IComp, Limerick) where currently industry-led research projects related to composite repairs in progress. In the UK, the proposed research can generate collaborations with academic researchers focusing on composite manufacturing, machining, adhesive bonding, non-destructive and health monitoring techniques (e.g. composite groups in Bristol, Nottingham, Imperial, Queen's Belfast and Manchester). The PI/PDRA will disseminate the research results and initiate collaborations for developing grant proposals and joint PhD projects. Internationally, the PI has initiated academic collaborations with University of Porto, RMIT Australia and University of Limerick, and this proposed research would further strengthen these collaborations.