Latest composites push boundaries of aerospace materials technology
The world’s aerospace industry has always been at the very edge of manufacturing materials and technology development. Companies have worked closely with aircraft manufacturers over many years to develop innovative solutions able to meet the increasingly higher performance standards set by aerospace manufacturers and federal regulators.
This has resulted in the introduction and use of new materials, particularly composites, which provide a range of impressive benefits including: increased strength; lightweight; flexibility; low co-efficient of thermal expansion (CTE); high resistance to corrosion and fatigue; flame retardancy; bromine-free formulations; reduction of noise and vibrational dampening.
The latest advanced composites provide aircraft manufacturers with viable, efficient, sustainable materials for high volume, large scale aircraft component production.
The most commonly used thermosets in structural and semi-structural composites are epoxies. These bring the key characteristics of strength, durability and chemical resistance. They deliver high performance at elevated temperatures (up to 180°C), come in liquid, solid or semi-solid form and are cured by reacting with amines or anhydrides.
Many aerospace applications use these multi-functional epoxies but they must be cured at an elevated temperature, so toughening agents such as thermoplastics are added to maintain flexibility in the composite matrix. However, Huntsman’s AralditeMY0600 is a multi-functional epoxy resin based on meta-aminophenol. Compared to the properties of today's most used
multi-functional epoxy resins, this material is unique in its ability to provide structural matrices with higher modulus and greater toughness, making it ideally suited to aerospace composites applications.
Increasing demand for faster component production rates has driven aerospace manufacturers to look for alternative direct processing technologies that do not require an autoclave.
Resin transfer moulding (RTM) is a simple, cost effective alternative, which greatly reduces production time and delivers lightweight but very tough and durable components. In a one-step process, extremely low viscosity resins such as AralditePY 306, AralditeMY0510 and AralditeMY721 which impregnate the backing material quickly and evenly are used to produce the parts. The mould can be then be heated or not, as the application requires. The flexibility of RTM means it can produce highly complex parts with an accuracy not achievable with traditional stamped metal processes, so eliminating the need for most post-fabrication work.
As part of the European Union’s PreCarBi programme (Pre-impregnated Carbon Binder Yarn), Huntsman Advanced Materials is already working on the next generation of toughened RTM technology.
Composite syntactics
Another area where composites are replacing traditional materials is with the use of syntactic pastes. Epoxy based aerofoil-fill syntactic pastes have been shown to exhibit exceptional noise and vibration dampening characteristics.
For example, Araldite1641A/B, developed in a joint partnership with UK aircraft engine manufacturers, Rolls Royce and the Engineering Department at the UK’s University of Sheffield, was originally designed for this specific role on the Trent900 aero engine. This material has the unique ability to absorb vibrational energy over a broad temperature range without compromising structural integrity.
The key benefits of syntactic pastes over traditional materials are:
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Increased structural dampening capability.
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Equivalent mechanical performance in terms of material stiffness and strength.
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Reduced component weight.
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More efficient manufacturing process and reduced costs.
Syntactic pastes can also be used to seal and close-out honeycomb edges, to enclose inserts in cored parts such as aircraft luggage lockers or, when produced in sheet form, can be used as core or linings.
The benefits of adhesive bonding for aircraft manufacture including weight reduction, cost savings, elimination of stress points and the elimination of mechanical fastenings and drilling.
Structural adhesives evenly distribute stresses over a wide area, act as sealants and also reduce noise and vibration. Nearly all adhesives used in aerospace interior applications must also exhibit the required flame retardancy and FST (fire/smoke/toxicity) resistance, required by federal regulations.
The most commonly used materials are 1 or 2 two component flame retardant paste epoxy adhesives, which offer a good balance of FST and mechanical properties. These materials can be cured at either room temperature or elevated temperatures and exhibit good mechanical properties on both metal and thermoplastic substrates. They have good environmental resistance and show excellent stress crack resistance on thermoplastic substrates.
The EU’s ABiTAS programme (Advanced Bonding Technologies for Aircraft Structures) is now focusing on the development of a new generation of adhesives for use on commercial aircraft to replace film adhesive which allow bond to to bonding of the autoclave.
Adhesive pastes in the form of liquid shims are commonly used for levelling or filling gaps between two component surfaces or parts. Composite aerospace structures are very stiff and often parts cannot be machined closely enough to mate perfectly.
So, liquid shim adhesives are used to fill the small gaps around mechanical fastenings. They cure to form hard, strong fillers that prevent stress concentration at mechanical fastening points.
The tools used for forming composites can be made from a variety of materials including epoxy boards and seamless modelling pastes (SMP).
SMP offers a range of benefits including increased production rate, lower cost, lightweight and easy to handle, easy machinability, good surface finish quality and minimal waste. The most recently developed paste also maintains dimensional stability at higher temperatures making it a viable alternative to traditional toolmaking. For example, Huntsman has recently introduced a High Temperature SMP, RenPaste4633. This cost-effective epoxy SMP has a heat deflection capacity of up to 180¢ªC for high temperature resistant tools.
Maintenance and repair of aircraft is a major cost issue for the aerospace industry. While most aircraft repair shops are familiar with the repair of aluminium skinned aircraft, composite components require a different repair strategy. A range of low and room temperature curing epoxy and polyurethane adhesives and pastes as well as laminating systems are available to cover a broad range of needs to effect quick, reliable infield repairs. For example, Huntsman offer a range of wet laminating resins for repairs including Epocast50-A1/B which is a toughened, flame retardant, low viscosity resin that gives high flexural strength while still being easy to handle on random repairs.
Future developments
The future success of composites in aerospace construction is dependent on the on going development of new, ever more versatile materials and direct processing technologies. Materials suppliers are already working on a range of next generation products including nanocomposites. While there is some way to go to bring these to commercial realisation there is no doubt that nanostructures will have a profound impact on product characteristics and performance delivering ever lighter, stronger and more versatile structures.
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Klaus Ritter, Marketing and Business Develop Manager, Transportation of Huntsman Advanced Materials. www.huntsman.com/advanced_materials
