Alternative Materials For Aircraft Wings

For this report I have chosen to research and find an alternative material for aircraft wings, currently the material being on aircraft wings is aluminium alloy series 2000, specifically 2024 as this alloy consists of about 4.5% copper, 1.5% magnesium, 0.6% manganese with traces of other specific elements permitted, and the remainder aluminium. [1] The alternative material must be capable of achieving all the characteristics of aluminium alloy 2024, yet be light in weight and preferably within the cost region of materials currently used.

Aims and objectives

The aim of this report is discuss how alternative materials can be produced to manufacture a new lighter aircraft wing. In order to do this the new material must be capable of withstanding high stress loads, be light in weight and preferably low in cost. Also, manufacturing technique must be looked into with respect to machinability of material, cost of machines, and repairing material.

Current materials in use

Aluminium alloy 2024

This is used in the aviation industry as it has the right properties required to meet the demands. The principle alloying element used is copper. This alloy requires solution heat treatment to obtain its best properties when the alloy has been through the solution heat treatment process the mechanical properties become similar to and sometimes exceed those of mild steel. To further increase the mechanical properties of aluminium alloy 2024, an artificial ageing process is used; this method of treatment increases the yield strength. Once the aluminium alloy has been made, the properties obtained are high strength to weight ratio, as well as good fatigue resistance. Though this is not wieldable and has average machinability. Due to poor corrosion resistance the alloy is usually cladded with pure aluminium, however, this does usually reduce the fatigue resistance of the alloy.

Aluminium alloy 2024 consists of about 4.5% copper, 1.5% magnesium, 0.6% manganese with traces of other specific elements permitted and the remainder is aluminium.

Mechanical properties of aluminium 2024

The mechanical properties of 2024 depend on the temper of the material.

2024-0

This form of alloy has no heat treatment (annealed). It has an ultimate tensile strength of 207-220 MPa more yield strength of 96 MPa. This form of alloy also has an elongation factor of 10-25%.

2024-T3

This form of alloy has been solution treated and strain hardened with the ultimate tensile strength is between 400-427 MPa and yield strength of 369-276 MPa with an elongation of 10-15%.

New materials to consider

Titanium

This metal has a high strength to weight ratio, a relative density of 4.5 which is 60% heavier than aluminium however, it is twice as strong.

Titanium has excellent corrosion resistance properties this is due to the oxide film which forms. Titanium is not normally susceptible to stress, fatigue, intergranular or galvanic corrosion, putting or localised attack. However, under certain circumstances it will burn in air, therefore in order to prevent a reaction with oxygen or nitrogen it may be treated with chloride gas in order to form a protective coating of titanium dioxide.

Titanium normal alloying elements include aluminium, chromium, iron, manganese, molybdenum and vanadium.

Titanium and its alloys are classed in 3 categories:

Alpha (A) – Wieldable, tough, strong both hot and cold and resistant to oxidisation.

Beta (B) – Excellent bend ductility, strong both hot and cold however vulnerable to contamination.

Combined (C) – Combination of alpha and beta with comprised performance, strong cold and warm but weak when hot, excellent forgability, good bendibility moderate contamination resistance. The melting point of titanium is 1668 degrees Celsius and has low thermal conductibility and a low co efficient of expansion. Its high temperature properties are however disappointing.; the ultimate yield strength falls rapidly above 425 degrees Celsius and atmospheric oxygen and nitrogen absorbent above 540 degrees Celsius makes the metal brittle and worthless after a long term exposure. Therefore it is only useful for short durations, high temperatures applications where strength is not important such as air conditioned firewalls.

When working with titanium extra care must be taken when making due to its extreme work hardening properties. E.g. centre drilling should be used prior to drilling as centre punch this material would harden the metal, causing difficulty when drilling.

Composite

A composite material consists of 2 or more different materials whose mechanical properties compliment each other although maintain their separate identities, unlike alloy.

The reason the composite materials are used on an aircraft and their strength to weight ration and corrosion resistance. Reinforced plastics are much lighter than metals. If the metal part can be as much as 25 times heavier than an equivalent composite part, however that composite part must be as strong and durable as the original.

Therefore reinforced plastics must have very good strength, stiffness and impact resistance.

Strength- this is the ability of a material to support a load without breaking.

Stiffness- this is the ability of material to support a load without bending too much.

Impact- this is the ability of a material to withstand resistance impact without shattering.

The types of composites to consider for this project would be,

Glass reinforced plastic

Aramid fibres

Carbon fibres

Glass Reinforced Fibres (GRFs)

Glass Reinforced Fibres are currently used on aircraft for radomes (the fairings which cover radar antennas and must be transparent to radio waves). The fibreglass is used for reinforcement for thermosetting resins in aircraft applications is available as a cloth in many different weights and weaves as a loose of fibreglass. When combined, the fibre and resin GRF.

For applications which require the most strength it is necessary to use uni-directional glass tape. Whereas woven glass cloth has better shaping properties and high strength.

There are many types of Glass Reinforced Fibres used, and the main ones used in the aviations industry are:

A – Glass- standard soda glass has a high alkaline content which absorbs moisture which increases degrading of material and corrosion. This leaks to resins deterioration. The main use for this is for windows.

C – Glass- high resistance to corrosive material. It is normally produced and used only as a surface matt to reduce cost.

D – Glass- with a low di-electric constant this type of glass is used for radomes.

E – Glass- with low alkali content and good resin adhesion properties where used in air conditioning.

Styles of woven fabric

The most common style used is the plain weave where; the warp and wraft threads cross alternately. The strength of woven fabrics in comprised due to the severe ‘pre- buckling’ already present in the fabric. Fibres usually produce their greatest strength when they are perfectly straight. Due to the high frequency of over and under weaving of the threads the strength is reduced; in plain weave. This is where twill weave and satin weave come in as it is high pliable and stronger than the plain weave style.

This table shows a comparison of the properties of common weaves used in aerospace:

Stability – ability of the weave to hold together when cut.

Drape – ability of the cloth to follow a complex shape.

Porosity – an indication of the amount of resin required to thoroughly wet the cloth.

Smoothness – surface finish of the cloth.

Balance – a comparison between the warp and weft direction.

Symmetry – the weave pattern.

Crimp – an indication of the amount or frequency of bend in the yarns. [5]

Aramid fabrics

Aramid fibres also known as ‘Kevlar’ is made from aromatic polyamide, a type plastic similar to nylon.

The properties include;

High tensile strength and resistance to impact of any composite reinforcing fibre.

Stiffer than glass but only half as stiff as carbon fibre.

40% lighter than glass fibre

10x stronger than aluminium

Up to 400% stronger than comparable glass reinforced laminates.

Up to 20% stronger than comparable carbon enforced laminates

Aramid fibres have very high impact strength with the damage confined to small areas. Due to this, Aramid can be used in areas prone to stone and runaway debris damage, so this could be useful for use on aircraft wings.

However, Aramid fibres have lower compression strength than carbon, it absorbs moisture more readily than glass or carbon, also Aramid deteriorates in strong sunlight. Aramid is more difficult to cut, drill, sand then either glass or carbon, it also does not give clean edges.

Aramid fibres do not resist flame well and burn through more quickly than other fibres. Resin adhesion is also lower, delamination being one of the ways in which it absorbs impact energy.

Carbon fibres

Carbon fibres are made from carbon and are black in colour carbon fibres were first used on air conditioning in the 1980’s.

The fibres are manufactures by the controlled heating of POLYCRILONITRILE (PAN), polythene or rayon fibres are pre-oxidised at 200-300 degrees Celsius for 1 hour then carbonised at 1200 degrees the graphitised at 2000 to 3000 degrees Celsius. This removes the hydrogen, nitrogen and oxygen leaving long oriented carbon chains.

The fibres are sometimes surface oxidised, this improves their building characteristics and sized, this then reduces the build up of static electricity and improves bonding.

Carbon fibres are available in forms basic groups and produced at different graphitisation temperature and defined by tensile modules

Standard modules (high strength)

Intermediate modules (high stiffness)

High modulus

Ultra high modulus.

All forms of carbon fibre are stiffer than glass fibres, however only standard modulus is stronger than glass fibres in tension. The higher modulus fibres are very brittle and are not suitable for general aeronautical use.

Aluminium- lithium

This is part of series 8000 of aluminium alloys. Having low density, the lithium reduces the weight of alloy while offering strength which is comparable to series 7000 (also a higher strength aluminium alloy made from zinc) and competes with composite materials. Aluminium lithium also has high specific modulus and excellent fatigue and cryogenic toughness properties.

The disadvantages of aluminium lithium is reduces ductility and fracture toughness in short transverse direction also the need to cold work this alloy to obtain peak properties and accelerate fatigue crack extension rates when cracks are micro structural small.

Analysis

Current material

The current material used for aircraft wings is aluminium. This material is currently used to construct aircraft wings. This is because this material is light in weight, easy to machine, easy to shape also this form of aluminium is easy to machine in order to meet required standards such as high yield strength in ratio with the weight, and also aluminium 2024 also has very good fatigue resistance. However, this form of aluminium alloy has poor corrosion resistance therefore in order to protect against this the alloy is cladded with pure aluminium, this however has a downside to it as it further reduces the fatigue resistance of the alloy.

The advantages of aluminium are:

Light weight

Easily shaped/cast/forge

Good electrical conductor

Good thermal conductor

Easy to machine

The disadvantages of aluminium are:

Expensive to refine (must be done by electrolysis of fused salts)

Poor chemical resistance (acids and base)

Loses strengths when heated

Cladded with 6.25mm of pure aluminium, if that is broken, the material begins to corrode fast.

Alternative materials

The alternative materials that can be used are:

Titanium

This metal has a high strength to weight ratio, a relative density of 4.5 which is 60% heavier than aluminium however it is twice as strong.

Titanium and its alloys are classed in 3 categories:

Alpha (A) – Wieldable, tough, strong both hot and cold and resistant to oxidisation.

Beta (B) – Excellent bend ductility, strong both hot and cold however vulnerable to contamination.

The ideal type of titanium to use on an aircraft wing would be the combined (C) class as it does meet to requirements for an aircraft wings. However, the major drawback for this material is when working with titanium extra care must be taken when making due to its extreme work hardening properties.

The advantages of titanium are:

Lightweight

Strong

Able to withstand high temperatures

Corrosion resistant

The disadvantages of titanium are:

Expensive

Process for forming and joining titanium are complex and expensive

Glass Reinforced Fibres

The fibreglass is used for reinforcement for thermosetting resins in aircraft applications is available as a cloth in many different weights and weaves as a loose of fibreglass. When combined, the fibre and resin Glass Reinforced Fibre.

For applications which require the most strength it is necessary to se uni-directional glass tape. Whereas woven glass cloth has better shaping properties and high strength.

There are many types of Glass Reinforced Fibres used, and the main ones used in the aviations industry are:

A – Glass- standard soda glass has a high alkaline content which absorbs moisture which increases degrading of the material and corrosion. This leaks to resins deterioration. The main use for this is for windows.

C – Glass- high resistance to corrosive materials. It is normally produced and used only as a surface matt to reduce cost.

D – Glass- with a low di-electric constant this type of glass is used for radomes.

E – Glass- with low alkali content and good resin adhesion properties.

Styles of woven fabric

The most common style used is the plain weave where; the warp and wraft threads cross alternately. The strength of woven fabrics in comprised due to the severe ‘pre- buckling’ already present in the fabric. Fibres usually produce their greatest and strength when they are perfectly straight. Due to the high frequency of over and under weaving of the threads the strength in reduced in plain weave. This is where twill weave and satin weave come in as it is high pliable and stronger than the plain weave style.

The advantages of GRF are:

Strength and resistance can be adjusted during the manufacturing

Impact resistance

Lightweight

Heat resistant

Will not corrode

Able to withstand all but the strongest forms of acid and alkali

The disadvantages of GRF are

Easy to damage

Expensive machines required to produce

Requires special storage

Although glass reinforced fibres are very good with respect to the advantages and disadvantages, it may not be the ideal choice to use as a material for an aircraft wing as it would be expensive to produce and store, and also there are many types of glass reinforced fibres but not a particular type could be chosen because there is not a material which is specifically ideal and has all the characteristics required for an aircraft wing.

Aramid

However, Aramid fibres have lower compression strength than carbon, it absorbs moisture more readily than glass or carbon, also Aramid deteriorates in strong sunlight. Aramid is more difficult to cut, drill or sand then either glass or carbon; it also does not give clean edges.

The advantages of Aramid are:

High tensile strength

Impact resistant

Ten times as strong as aluminium

400% stronger than GRF

20% stronger than carbon fibre

The disadvantages of Aramid are:

Low compressive strength then carbon

Absorbs moisture more than glass or carbon fibre

Deteriorates in sunlight

Difficult To Cut, Drill or Sand

Does not give clean cut edges

Aramid is a very good material to use, however it is important that when making aircraft skin the wings must be smooth and easy to machine, due to Aramid not being able to provide these key features, it should be used for this part of the skin, however, Aramid should be considered for the leading edge of the aircraft wings, because it has the characteristics required to withstand stone and runaway debris damage.

Aluminium- Lithium

Having low density, the lithium reduces the weight of alloy while offering strength which is comparable to series 7000 (also a higher strength aluminium alloy made from zinc) and competes with composite materials. Aluminium lithium also has high specific modulus and excellent fatigue and cryogenic toughness properties.

The disadvantages of aluminium lithium is reduces ductility and fracture toughness in short transverse direction also the need to cold work this s alloy to obtain peak properties and accelerate fatigue crack extension rates when cracks are micro structural small.

Fatigue affects materials after long term exposure to cyclic loading using aluminium lithium is stronger than carbon fibre therefore aluminium lithium can withstand fatigue longer.

The advantages of aluminium-lithium are:

10% denser than aluminium 2024

Lightweight

10 – 15% higher modulus than aluminium 2024

Excellent fatigue and cryogenic toughness properties

Higher stiffness

Superior fatigue crack growth resistance

The disadvantages of aluminium-lithium are:

Reduced ductility

Low fracture toughness

Aluminium- lithium is a new concept within the aviation industry which allows the industry to progress in a new direction, this allows an aircraft to be light, efficient yet have the same amount of or more power to transport both passengers and freight. Also, with aircraft being so light it may allow even bigger aircrafts then the currently produced Airbus A380, and Boeing 787 to be produced. Aluminium- lithium the characteristics required for use on aircraft wings and therefore should be considered for this project.

Conclusion

In conclusion aluminium- lithium should be used as it has the properties such as light weight and excellent fatigue and cryogenic toughness properties required for an aircraft wing and carry the weight loaded on to the plane hence enabling aircrafts to be designed on a bigger scale in order to carry more cargo and passengers as is proven by the production of the Airbus A380 which is one the largest planes in production giving a higher power to weight ratio.

The A380’s wing is sized for a maximum take-off weight (MTOW) over 560 tonnes in order to accommodate these future versions, albeit with some strengthening required. The stronger wing (and structure) will be used on the A380-800F freighter. This common design approach sacrifices some fuel efficiency on the A380-800 passenger model, but Airbus estimates that the size of the aircraft, coupled with the advances in technology will provide lower operating costs per passenger than the 747-400 and older 747 variants. [6]

Reference

Books Used;

British Airways Engineering Training Part 66 Module 6- Materials and Hardware. Published: 14/02/2005. Issue: 1.

Introduction to aircraft design. John P. Fielding. ISBN: 0521657229

Aircraft Design Projects: For Engineering Students. Lloyd R. Jenkinson. Dr. Jim Marchman. ISBN: 0750657723

Websites used:

http://www.sciencedaily.com/releases/2007/09/070926094727.htm

http://en.wikipedia.org/wiki/Al-Li

www.soton.ac.uk/~jps7/…/manufacturing/aluminum-lithium.doc

www.keytometals.com/Article58.ht

http://en.wikipedia.org/wiki/Airbus_A380#cite_note-norris_wagner_book-12

Appendix 1

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