A Composites Matrix for Lighter, Leaner Aerospace Parts

By Richard Leibfried

The pressure to enhance performance efficiency in commercial and military aircraft is the driving force behind the development of advanced high-performance structural materials, such as composites. Composites are especially attractive to the aerospace industry because of their excellent strength- and stiffness-to-weight ratio, combined with long term durability. They can reduce airframe weight, resulting in better fuel economy and lower operating costs. Because composites can be directly formed into more complex shapes than their metallic counterparts, manufacturers can reduce the number of component parts, facilitating an overall reduction in system cost.

Meeting Complex Design Challenges

VICTREX^® Composites Matrix is successfully meeting the challenges posed by the complex design of modern aircraft. It is based on VICTREX^® PEEK^TM polymer, a semi-crystalline thermoplastic widely regarded as having an excellent combination of performance properties in harsh environments. Victrex Composites Matrix is used in a variety of forms (powder, film, and filament) as a matrix material in the manufacture of advanced thermoplastic composite prepregs made of continuous carbon, glass, ceramic, or aramid continuous fibers.

Victrex polymers are engineered to provide optimum impregnation of the reinforcing fibers and fiber-matrix interface. They are recyclable and are cost-effective to process, either by thermoforming out-of-autoclave or by more innovative techniques, such as thermoplastic film bagging. The material's outstanding mechanical properties and processability make it an excellent substitute for metals and thermosets in a wide range of aerospace applications.

Victrex Polymer Solutions, a division of Victrex plc, partners with leading composite manufacturers to produce a variety of composite forms, including unidirectional (UD) tape and sheet, multi-axial fabric laminates, comingled tow and braid, chopped UD tape, drapable fabrics, pultruded and formed stock shapes, and honeycomb sandwich structures. Victrex also offers an array of technical resources to engineers, both at the processor and end-user level, in order to address specific requirements within the composites area.

Key Properties

Made with Victrex PEEK polymer, Victrex Composites Matrix can withstand continuous operating temperatures of up to 260°C (500°F) in low stress applications, and 120°C (248°F) wet in aerospace structural applications. Key features include excellent thermal stability, low coefficient of thermal expansion, abrasion resistance, and low smoke and toxic gas emission, as well as excellent hydrolysis, corrosion, chemical, and radiation resistance. A combination of high stiffness and low density makes Victrex Composites Matrix suitable for innovative designs requiring the combination and integration of non-metallic and metallic materials.

Victrex Composite Matrix provides numerous benefits. When compared to metals, they offer weight reduction; damage tolerance; chemical and corrosion resistance; and thermal, electrical, and noise insulation. When compared to thermosets, they provide fracture toughness and fatigue strength; hot/wet aging performance; faster, scalable production rates with higher yields; and are recyclable and reworkable.

Applications

Primary and secondary structure applications for Victrex Composites Matrix include fuselage structures; floor cross beams; fastening systems; fuel tank access doors in wings; and door access panels, as well as spars, ribs, stiffeners, thermal protection panels, and bulkheads and fire walls.

Fastening Systems

Fastening systems are an excellent example of how Victrex Composites Matrix can reduce weight, total part count, and assembly labor when joining composite structures in a variety of aircraft applications. Joining composite structures has traditionally been done with standard metal fastening systems that add unwanted weight, and are a remarkably "low-tech" solution in this otherwise high-tech environment.

A composite material made of continuous fiber reinforced Victrex Composites Matrix is used to make truly lightweight fastening systems, bolts, nuts, inserts, and brackets. The fasteners can be bolted traditionally or friction welded directly into other Victrex PEEK molded parts/inserts, composite laminate panels, or even Victrex PEEK polymer-based honeycomb sandwich structures, providing increased impact resistance and fatigue strength. These composites have a static strength comparable to high- tech aluminum alloys and the fatigue properties of titanium alloys. Victrex PEEK fasteners can offer electrically insulating or electro-static dissipative (ESD) qualities in structural systems requiring careful management of lightning strikes and electrical interference.

Engine Brackets

Aircraft engine brackets are an example that demonstrates the excellent dynamic fatigue strength and vibration dampening properties of components made from Victrex Composites Matrix applications. These are structural brackets that support several engine components--including hoses, cables, and printed circuit boards--via integrated captive hardware. The high-performance brackets are 20% lighter than titanium parts while maintaining part stiffness and all existing interfaces. The composite parts reduce system cost through weight savings, as well as cost-effective manufacturing.

Thermal Protection Panels

Victrex Composites Matrix is used in high temperature environments. The thermal protection panel, composed of Victrex Composites Matrix, is installed in the engine hood for use on civil and military helicopters. The composite material was chosen due to its high temperature resistance in working conditions up to 300°C (572°F), along with increased dimensional stability, which offers a strong, reliable component. These panels also provide improved chemical resistance in the engine area, especially around aeronautic fluids, lubricants, and solvents, as well as added insulation properties when combined with other insulating materials.

Processing

In addition to excellent physical and chemical properties, Victrex Composites Matrix raw materials and composite parts have an indefinite shelf life when stored at ambient conditions. When combined with its ability to be produced in a solvent-free working environment, while still offering a very stable melt and a wide processing window [between 360°C (680°F) and 400°C (752°F)], Victrex Composites Matrix is compatible with a wide range of rapid, automated, and economical processes. These processes include compression molding; roll forming; thermoforming and stamping; pultrusion; filament winding and tape laying; autoclave and vacuum bag molding; diaphragm forming; and bladder inflation molding.

The aerospace industry continues to evolve at a rapid pace. As product development cycles shorten and requirements become more stringent, composites will play an increasingly significant role in the design of ever-more complex structures. Victrex Composites Matrix offers a unique range of high strength, stiffness, toughness, and low density properties, ensuring that it can be used in the broadest range of demanding aerospace composite applications while providing advantages over metals and thermosets.

Richard Leibfried is market development manager at Victrex Polymer Solutions.

VICTREX^® is a registered trademark of Victrex Manufacturing Limited. PEEK^TM, APTIV^TM, PEEK-ESD^TM, HT^TM, ST^TM and WG^TM are trademarks of Victrex plc. VICOTE^® is a registered trademark of Victrex plc.

Advantages of VICTREX Composites Matrix Material density, at 1.6 g cm-3, is 40% lighter than aluminum and 67% lighter than titanium Fast manufacturing of finished parts due to thermoplastic behavior Static strength comparable to high-tech aluminum alloys Fatigue properties of titanium alloys Carbon fiber reinforced VICTREX Composites Matrix gives a material a specific flexural strength exceeding that of titanium and aluminum Chemical corrosion and contact corrosion not found in hybrid metal/composite junctions Components dampen structures due to the relative elasticity of VICTREX Composites Matrix Optimal coefficient of thermal expansion, which matches that of composite structures Outstanding shear strength Glass fiber reinforced VICTREX Composites Matrix products are suitable for radome applications due to their radio transparency and in areas requiring electrical insulation