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Meeting the weight challenges of aircraft construction

02 January 2014

PEEK polymers, reinforced with high-strength carbon fibres for structural components or as prefabricated components, have long been used in aircraft construction.

Thin-walled PEEK tube can be bent and formed, thereby ensuring a high degree of design freedom

Today, the primary focus is on weight reduction and system optimisation. The use of high-performance polymers in the aerospace industry is now a major contributor to reductions in manufacturing, maintenance and fuel costs.

According to Harald Hedderich, aerospace strategic marketing manager for Victrex Polymer Solutions (the supplier of Victrex PEEK polymers for a broad range of industries), removing as little as 45kg from the all-up weight of a long-range aircraft can result in as much as US$10,000 in annual fuel cost savings for the operator.

PEEK (polyetheretherketone, a thermoplastic in the polyaryletherketone family of polymers), while delivering equivalent strength and stiffness, is 70 percent lighter than steel, 55 percent lighter than titanium and 40 percent lighter than aluminium. Despite its low density of 1.3 grams per cubic centimetre, the polymer’s strength and stiffness compare favourably with those of the metals traditionally used in aircraft construction.

Thanks to its outstanding mechanical properties, its ability to perform across a broad range of temperatures and its durability in chemically aggressive environments, PEEK polymers are a natural choice for the aerospace industry. Moreover, injection-moulded PEEK components open up new design possibilities, which not only save weight in the aircraft, but can also simplify assembly and maintenance.

Given these properties, Hedderich believes the material addresses three core challenges that the aircraft industry currently faces: to improve the fuel economy, to reduce maintenance time and downtime, and to achieve lean manufacturing goals.

Because they are lightweight, durable, exceptionally strong, chemically resistant, and inherently flame retardant, Victrex polymers have been used in the aerospace industry for years. In the summer of 2013, aircraft manufacturer Airbus qualified another high performance polyaryletherketone along the entire supply chain: Victrex PEEK 90HMF40.

Reinforced with high-modulus fibres, HMF polymers are based on the 90-series formulation, which has primarily been used to mould thin-walled parts. The thermoplastic is a high flow, easily processed material that provides parts with a high modulus. Victrex claims it delivers several key benefits standard grades cannot, including up to 100 times longer fatigue life and up to 20 percent higher specific strength and stiffness when compared with aluminium 7075-T6 under the same conditions.

Thermoplastic composite manufacturers are beginning to specify Victrex PEEK polymer as the matrix material in glass and carbon-fibre reinforced composites. The US company, Tri-Mack is taking advantage of the benefits of thermoplastic PEEK composites and is using the material to produce brackets for structural components in aircraft.

The manufacturing cycle times for the weight-saving composite component are far shorter than those of thermosets. While it typically takes several hours to produce thermoset composite components, the thermoplastic composite brackets have manufacturing cycle times measured in minutes. Tri-Mack’s Tom Kneath says that speed, paired with the ability to recycle the material for other applications, has taken his company to a whole new level of processing efficiency that isn’t achievable with thermosets.

Thermoplastic composites made from PEEK can provide the chemical and corrosion resistance to jet fuel, hydraulic fluid, de-icing solution, salt, steam, water and other commonly used service fluids that would typically undermine the longevity of metals. PEEK composites can offer between four and five times higher fatigue strength, specific stiffness, and specific strength when compared to that of traditional aviation metals such as aluminium.

Hedderich says these mechanical properties, along with the resulting vibration and noise dampening improvements, make PEEK composites an attractive and viable alternative for engineers looking to replace metals and thermosets. But it’s not all to do with structural components. PEEK is also finding application in the lightweight, assembly-optimised connectors used in avionics systems, as Amphenol PCD’s Tony Padula explains:

“Instead of installing several metal retention clips separately, a one-piece Victrex PEEK retention clip is used to make the process much easier and more efficient for the assembly teams.” The new connectors also weigh 9 percent less than their predecessors. The supplier has also performed tests on these polymers to verify their reliability when exposed to various aerospace environments. Tony Padula again:

“We tested the material in hydraulic fluid at 70°C for 1,000 hours and were pleased with the durability of the material. The thermoplastic connectors not only eliminate the need for anti-corrosion treatments, but are non-conductive as well, improving the electrical integrity of the connector. Electrical integrity is paramount as the ground block connectors are used to secure and fasten electrical wiring that is critical to the aircraft.”

In addition to resin materials, Victrex has branched out into other product formats for aircraft components such as thin APTIV films and durable Victrex Pipes. Working together with the German company, PFW Aerospace AG, for example, Victrex Polymer Solutions has developed and qualified a novel PEEK polymer-based tubing system for the cargo drainage on the Airbus A350 XWB. The novel plastic tube system achieves weight savings of 33 percent over existing stainless steel tubing.

A straight PEEK tube with an outer diameter of 25.4mm and 1mm wall thickness weighs only 100 grams per metre without end-fittings, offering a weight reduction of up to 60 percent compared with an equivalent stainless steel straight tube. The thin-walled PEEK tube can be bent and formed, thereby ensuring a high degree of design freedom. Harald Hedderich summarises:

“The use of Victrex PEEK still has a large cost-reduction potential which has yet to be completely developed. Our goal is to work together with our customers to provide solutions for even the toughest challenges in aircraft construction – from the material development right through to the reliable supply.”

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