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Polymer Composites
This unit of work addresses aspects of the following syllabus outcomes:
A student:
H 1.2. differentiates between properties of materials and justifies the selection of materials, components and processes in engineering.
Extract from Stage 6 Engineering Studies Syllabus, © Board of Studies, NSW, 1999.
By studying the following unit and visiting the links, students will learn to analyse some of the structure, properties, uses and appropriateness of materials in aeronautical engineering applications.
The assistance of Professor A Crosky, Aeronautical Engineering Department, University of New South Wales in the preparation of this unit is gratefully acknowledged.
Go to Dr. Robinson's article on
Aircraft Materials 
at the University of Limerick web site.
View the picture of the advanced Eurofighter in Figure 3 to gain an appreciation of the amount of composite materials that are used in modern fighter aircraft.
Polymers
Polymers can be thermoplastics or thermosetting resins (thermosets). Thermoplastics are fully polymerised materials that are solid at room temperature but soften at high temperature. Thermosetting resins are partially polymerised resins that are cured by heat or by using a curing agent (catalyst). When cured they form a heavily cross-linked solid resin that cannot be reshaped.
In modern aircraft polymers are used for:
- parts of the airframe
- window protection
- lightly stressed parts
- interior trim
- electrical insulators.
Aircraft windows require both transparency and acceptable mechanical properties due to the pressurised nature of the aircraft. Usually the window consists of three layers:
- a glass load bearing layer
- a back-up glass load carrying layer in case the first layer cracks
- a plastic shield on the inside to prevent scratching by passengers.
These plastic shields are often acrylics, e.g. perspex, although polycarbonates are also used.
Describe why it is important to protect the glass window layers in an aircraft from scratching.
Composites
Polymer components subject to reasonably low loads are commonly made from polyethylene or polypropylene mouldings or extrusions. However, where the loads are more significant it is necessary to use composites of various types.
Composite Materials In Aircraft - Source: FireNet International web site, http://www.fire.org.uk/aviation/library/comp.htm, viewed 18 July 2003
Composites consist of two or more materials combined to give a material with properties distinct from the original constituents. They may be naturally occurring, or they may be synthetic.
Why does a composite have more desirable properties than either of its constituent materials on their own?
Select six examples of composites and divide them into natural and synthetic composites. List them in the table below. An example of a natural composite is timber which contains cellulose and lignin. One common synthetic composite is glass reinforced polymer (GRP).
| Natural composites | Synthetic composites |
|---|---|
A very significant proportion of polymers are used as composites.
Composites can be designed to produce a material with desired combinations of properties such as stiffness, strength and density. It is this flexibility to ”tailor-make” a material with desired properties that makes polymer composites so important to the aircraft industry.
Typically, composites consist of a matrix material and a reinforcing material. The matrix and reinforcing materials may be metals, ceramics or polymers, but the composites used in airframe components are fibre reinforced polymer (FRP) matrix composites.
Airframe composites have the advantages of:
- high specific strength and stiffness
- tailored directional properties
- non-corroding in salt environments
- excellent fatigue resistance
- dimensional stability
- reduced number of parts required (compared to metal components).
However, these composites are susceptible to:
- impact damage
- moisture pick-up
- lightning strikes
- extremes of temperature.
irframe composites also have a relatively high cost, do not yield plastically in regions of high stress concentration and are subject to random property variation due to the nature of composite manufacturing processes.
The use of advanced composites in airframe construction has increased substantially over the past few decades. They are used as floor beams, doors, aerodynamic fairings and for control surfaces, such as rudders, elevators and ailerons, due to their low weight and high stiffness. The drawing below gives some examples of the use of polymers in aircraft construction.
Source: Boeing Aircraft Company (n.d.) Seattle.
Reinforcing material (fibres)
Reinforcing materials for polymer matrix composites are often referred to as fibres and they include (in order of increasing cost):
- E-glass
- aramid, e.g. Kevlar, see Figure 1
- carbon
- alumina
- silicon carbide
- boron.
The properties of the fibres are shown in Figure 2. These fibre materials all have high specific strength and stiffness imparting high strength and stiffness to the composite.
Figure 1 Structure of aramid (Kevlar)
Source: Middleton, D.H. (1990) Composite materials in aircraft structures.
Longman scientific and technical: London.
Figure 2 Properties of common reinforcing fibres.
Source: Middleton, D.H. (1990) Composite materials in aircraft structures.
Longman Scientific and Technical: London.
The matrix
The purpose of the matrix in a polymer composite is to:
- support the reinforcing fibres in the required position
- transfer load between the fibres
- increase the toughness of the composite
- protect the fibres from damage.
While the longitudinal tensile properties are dominated by the fibres, the properties of shear, compression and transverse tension are dominated by the matrix properties.
Matrix polymers can be thermoplastics or thermosets. Thermoplastics are fully polymerised materials that are solid at room temperature but may be melted and shaped at higher temperatures (100-300°C).
Thermosetting resins consist of a base resin and a catalyst (curing) agent. When the resin and catalyst are mixed together they react to form a heavily cross-linked solid resin that cannot be reshaped once set.
- Log onto the web site at http://www.azom.com
- Type into the box called Keyword Search then word Composite.
- Now, scroll down to Aircraft Components Damaged By Rain Erosion. Open this link by clicking on it.
- Read the article then answer the following:
- What areas of the aircraft are prone to airborne erosion?
- What two factors influence the rate of erosion?
- In the case of laminates, the interface of the layers can be subject to shear stresses. What problem does erosion rain cause here?
Further relevant information on this topic:
A to Z of Materials - Azom.com
A worldwide composite search engine
Another site on composite materials
New materials for next generation commercial transports, particularly chapters 3 and 4. This book may be accessed on the web
http://books.nap.edu/books/0309053900/html
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