COMPOSITE MATERIALS

engineering

uk

7.5 COMPOSITE MATERIALS

As previously stated with Plastics, the main reason for utilising composite

materials, in aerospace structures, is to reduce weight, which has a direct benefit in lowering operating costs. Composites also provide further benefits in their ability to be easily formed, comparatively lower production costs, resistance to corrosion and reduced maintenance costs.

The principal types of composite materials are those involving fibrous elements which may be used as strands, or be woven into fine ‘tapes’ and ‘cloths’ (or coarser ‘mats’), held in a suitable resin matrix and formed into the required shapes

7.5.1 GLASS FIBRE REINFORCED PLASTIC (GFRP)

The first man-made fibre, glass can be spun into cloth and used for fire-proof curtains or (when extremely pure glass is used), made into fibres which are able to transmit light over long distances.

The ultimate tensile strength of undamaged, very small diameter glass fibres is extremely high, although the strength is reduced significantly if the fibres are slightly damaged.

In its structural use it is often merely referred to as glass fibre or fibreglass, when glass fibres (in various forms) are bonded together by appropriate resins.

When moulded with resin, the resulting composite is, also, of considerably lower strength but, nevertheless, good GFRP structures are stronger than mild steel and, on a simple strength-for-weight basis, can be comparable to high tensile steel if the fibre form and lay-up is near optimum. It is however, considerably less stiff than steel or even aluminium.

A graphic example of GFRP flexibility is the enormous deflection, which takes place in the pole during a pole vault. As the glass fibres are about a hundred times stronger than the resin, it is obviously necessary to get as much fibre packed into the moulding as possible.

Non-structural items may be made from, or include, a percentage of chopped strand mat, (i.e. glass fibres in a random, non- woven state) but, where considerable strength is required, uni-directional glass cloth is used.

To provide all round strength, sheets of uni-directional cloth can be layed up at 90º to each other, in a similar manner to the grain in plywood. Sometimes such sheets are used as facings for an internal honeycomb of plastic-impregnated paper, to give a very efficient structure in terms of strength, stiffness and weight.

JAR 66 CATEGORY B1

The glass fibre sheet material can be supplied with cloth already impregnated with resin and partially cured (‘Pre-preg’), in which case it is necessary to keep the material in refrigerated storage. Resin curing is usually done at elevated temperatures (120C - 170ºC), with the GRP component in its mould and, often, under pressure, in an autoclave.

The main reasons for using GFRP are:

 in instances where metal cannot be used (e.g. for radar domes or other non-electrical conducting applications)

 the ease and low cost of producing very complex shapes

 to provide good strength/weight ratio

 its ability to produce selected directional strength.

The main disadvantage of glass fibre is that it lacks stiffness and, as such, is not suitable for applications subject to high structural loadings.

7.5.1.1 Ceramic Fibres

Made by firing clay or other non-metallic materials, ceramic fibres are a form of glass fibre, used in high-temperature applications. They can be used at temperatures up to 1650C and are suited for use around engine and exhaust systems. Ceramic fibres are heavy (and expensive) and are only used where no other materials are suitable.

7.5.2 CARBON FIBRE REINFORCED PLASTIC (CFRP)

CFRP (also referred to as ‘Graphite’) is a composite material, which was primarily developed to retain (or improve upon) the high strength-to-weight ratio

characteristics exhibited by GFRP, but with very much greater stiffness values.

Carbon fibres are very stiff and, when formed into a composite, the Young's Modulus (‘E’) value can be higher than steel. CFRP is not only six times stiffer than GFRP but is also over 50% stronger. It also has twice the strength of high-strength aluminium alloy and three times the stiffness.

Carbon fibres are typically less than 0.01 mm (0.0004 in) in diameter and are produced by subjecting a fine thread of a suitable nylon-type plastic to a very high temperature (to decompose the polymer), and driving off all of the elements with the exception of carbon. The carbon thread is then stretched, at white heat

(2000C-3000ºC), to develop strength. Unfortunately, the process is complex and very costly.

Nevertheless, where the high cost can be justified, CFRP can offer considerable weight savings over conventional materials. CFRP components are generally made from ‘Pre-preg’ sheet (fibres impregnated with resin and a hardener, which only require heat and pressure to cure). Some specialist items are made by a

JAR 66 CATEGORY B1

laborious, but ideal, process called ‘Filament Winding’, in which a carbon fibre string is wound over a former in the shape of the workpiece whilst bonded with resin.

Because of CFRP's high stiffness modulus, it is also used extensively to stiffen GFRP or aluminium alloy structures.

A material known as Carbon-Carbon (where the resin is also graphitised), is used for the rotors and stators on brake units. It offers a significant weight saving, as well as high efficiency, due to the fact that it dissipates the heat generated very quickly.

Replacing 40% of an aluminium alloy structure by CFRP would result in a 40%

saving in total structural weight and CFRP is used on such items as the wings, horizontal (and vertical) stabilisers, forward fuselages and spoilers of many aircraft.

The use of composites, in the manufacture of helicopter rotor blades, has led to significant increases in their life and, in some cases, they may have an unlimited life span (subject to damage). The modern blade is highly complex and may be comprised of CFRP, GFRP, stainless steel, a honeycomb core and a foam filling.

7.5.3 ARAMID FIBRE REINFORCED PLASTIC (AFRP)

The aramid fibres are closely related to the nylon-type of synthetic fibres and are well known for their superior toughness, strength-to-weight characteristics and heat-resistance. Tyres, reinforced with aramid fibres are comparable to those reinforced with steel cords.

Better known under its trade name – Kevlar –in cloth form, it is a soft, yellow, organic fibre that is extremely light, strong and tough. Its great impact-resistance makes it useful in areas, which are liable to be struck by debris, as experienced in areas around engine reverse-thrust buckets. Kevlar is used to manufacture bullet-proof jackets and, also, as a reinforcement, in aircraft fuel tanks.

7.5.4 GENERAL INFORMATION

A sheet of fibre reinforced material is ‘anisotropic’, - which means its properties depend on the direction of the fibres. Random direction fibres would result in a much lower strength than uni-directional fibres, laying parallel to the applied load.

However, the strength (and stiffness) of a uni-directional lay-up would be very low, with the applied load at 90º to the fibres, as this is primarily a test of the resin (hence the usual practice of placing alternate layers at 90º to each other).

Due to small variations in the size of the individual fibres, and the final quality of the finished component (which can be affected by careless handling, variations in cleanliness or lay-up, voids, pressures, temperatures, etc), there will, inevitably, be a greater scatter on final strength than on a conventional, metallic component.

JAR 66 CATEGORY B1 MODULE 6 MATERIALS AND

HARDWARE

engineering

uk

It has already been stated that composites usually have good internal damping characteristics and are less prone to vibration resonances. Where high strength, combined with stiffness is required, then a CFRP is used but, when lesser levels of stiffness are necessary, then GFRP or AFRP are used.

Composites have very low elongation properties and toughness. Aluminium alloy has a typical elongation-to-fracture value of 11%, whereas composites range from 3% for GFRP to 0.5% for CFRP.

The maximum operating temperatures, for GFRP, CFRP and Kevlar composites, depend, to some extent, on the actual adhesives used, but are, generally, in the range 220C-250ºC.

Some composites, such as carbon fibre in a carbon matrix, have very high permissible operating temperatures (around 3000ºC), and are used for high-energy braking applications and as thermal barriers for space vehicles).

Boron, Tungsten, Silicon Carbide and Quartz may also be used to provide fibres for high-temperature composites

7.5.5 LAMINATED, SANDWICH AND MONOLITHIC STRUCTURES

Laminated plastics consist of layers of synthetic resin-impregnated fibres (or other, coated, fillers), which are bonded together (usually heated and under pressure), to form a single laminate or sheet of composite material. Plastic laminates are used to ‘face’ other structural materials, in order to;

 provide a more durable surface to a softer (less expensive) material

 enhance the surface appearance (colour, porosity, smoothness etc.)

 increase the strength and rigidity of many non-metallic structures

 produce other desirable surface characteristics such as when acid- or corrosion- resistance, non-conductivity, non-magnetisability or the ease of keeping a surface clean is required

To provide a light-weight structure, which possesses strength and rigidity, one of several structural materials, is sandwiched between two laminated composites.

The sandwiched material (the core) may be made of a solid material, such as wood, or a series of thin corrugations of a material, which are joined and placed end-on (in the form of the cells of a honeycomb), within the laminates.

Where wood is used, as the core material, it usually consists of low-density balsa wood, which has been cut across the grain and sandwiched between two layers of reinforced resin (or a metal). This construction makes an extremely light, yet strong material, which can be used as floor panels, wall panels and, occasionally, aircraft skins.

JAR 66 CATEGORY B1 MODULE 6 MATERIALS AND

HARDWARE

engineering

uk

The cellular core, used for laminated honeycomb material, may be made from resin-impregnated paper, or from one of the many fibre cloths. The core is formed or shaped and then bonded between two face sheets of resin-impregnated cloth.

The finished sandwich structure is very rigid, has a high strength-to-weight ratio, and is transparent to electromagnetic (radar/radio) waves, making it ideal for radomes of all kinds.

Metal honeycomb cores (made from light alloy or stainless steel), are also sandwiched between two face sheets of fibre-reinforced resins. On other occasions the metal honeycombs may be found sandwiched between sheets of light alloy, stainless steel or titanium. This type of core is referred to as ‘metal-faced honeycomb’ and is used where abrasion- and heat-resistance is important or when sound-absorption qualities are desired.

In monolithic structures, angle sections (‘Top Hat’, ‘U’, ‘I’ and ‘Z’), frames ribs and stringers are fashioned from similar materials to the outer layers of the sandwich structure, then covered with the appropriate surface ‘skin’, before the stronger, metallic spars and hinges are attached, Such a structure can save many kilograms (or pounds) in the weight of the flying control surfaces (or the fin structure) of a large aircraft.