COMPOSITE REPAIR: ENSURING ROBUST BONDING PERFORMANCE
In-flight damage from lightning and bird strike are more common than most of us realize. While in-flight damage from such events is expected, other events can also occur while the aircraft is parked at the gate. A bump from a catering truck, a tossed piece of luggage, even the dropping of a tool on a component can cause enough damage requiring repair.
For airlines, the benefits composites provide from their strength to weight ratio, light-weighting and corrosion resistance fully justifies and encourages the acceptance of newer materials in the airframe. Incremental improvements in weight reduction improve the aircrafts’ fuel efficiency, which in turn reduces the airlines operating cost while safeguarding against volatility in fuel costs.
“OEM’s are meeting the needs of its airline customers by moving towards more composite airframes and parts.”
OEM’s are meeting the needs of its airline customers by moving towards more composite airframes and parts. Inherent benefits of composite parts over aluminum parts come with some additional costs and considerations for the airline. The extent of the damage to aluminum parts can be more visible than damage to composite parts. Undue stress often goes beyond the visible impact on a composite part due to hidden ply damage between layers or across surfaces. As a result, repair technicians are required to inspect and expand the area for repair to safeguard against further propagation, which can lead to a more catastrophic event during service.
Due to the complex configuration of composite parts, consisting of multiple fiber plies, resin, adhesive, honeycomb or foam core consolidated to create highly contoured structures, technical feasibility and material availability will drive the decision to repair or replace a damaged component. If no ready spare replacement for the composite part is available, the airline operator must decide the safest, quickest and most economical path to addressing the composite damage.
For all these reasons, standardized repair practices and readily access to repair materials are critical for an airline operator to improve its on-time performance and fleet utilization. As a result, advanced technologies have been developed to meet the challenge of these repair scenarios. These include film adhesives and pastes that enhance aircraft performance and safety, while meeting stringent OEM requirements for full-scale production and aftermarket repair.
The Repair Process
Whether it is a damaged wing skin, nacelle or cargo door, the options are to perform a repair or order a replacement part if a spare is not readily available. As the latter would keep the aircraft on the ground for a longer period, the repair route might be the best solution, especially for composite repairs that are smaller but still necessary.
Though up to 50% of a modern-day aircraft’s primary and secondary structures may be comprised of composite materials, the basics of a repair assessment and procedure follow the same methodology no matter where they are employed.
- An inspection to determine the degree and extent of damage
- Removing the damaged area
- Treating any contaminated material
- Preparation of the repair area
- Replacing the damaged area
- Final inspection to detect any additional de-laminations or inclusions
- Surface finish restoration
A Standardized Solution
Best practices in how to approach such repairs prompted an effort toward standardization in requirements for repair technicians and their training. That effort has been led by collaboration between key material suppliers, service providers, airline end-users and OEM’s. Consolidation and harmonization of this activity takes place through industry trade consortiums and working groups.
A strong example of this industry collaboration is the characterization of the leading structural film adhesive, LOCTITE EA 9695 AERO, in out of autoclave applications. LOCTITE EA 9695 AERO is ideal for repair applications due to its multiple features and benefits.
- Dual cure at 250°F/121°C and 350°F/177°C for OEM’s and as low as 180°F/82°C for repair
- Autoclave for production and out-of-autoclave capable for repair
- Comparable performance in bonding either aluminum and composites substrates
- 60-day long open time in room-temperature controlled shop environments
- Leading solution for ease of global procurement
This epoxy-based film adhesive tested well for co-cure and pre-cured laminates and demonstrated exceptional environmental resistance. Its ability to cure at lower temperatures made it suitable for repair of composite structures, and its low flow characteristics minimize prepreg resin intermingling. Its consistent performance characteristics within a broad range of cure profiles allowed for a variety of structural component repairs in either in-shop autoclave or on-wing vacuum bag with heat blanket applications. As a result, a variety of OEM’s, Airlines and MRO’s agreed that this was a reliable aerospace grade structural film adhesive for both metal and composite repair. OEM and industry specifications have been written with LOCTITE EA 9695 AERO, thus making it an industry-wide and ready solution.
Additional Challenges and Solutions
The LOCTITE AERO line of structural bonding film and paste adhesives comprises a wide product range that provides performance within a wide temperature range in a variety of forms, viscosities and areal weights for both metal and composite substrates.
