Introduction to Composite Materials

Composite materials consist of two of more individual materials (such as glass and plastic) that are bonded together to produce a material that has better properties than the either of the component materials.  This means they are heterogeneous; the two or more materials can usually be determined without magnification.

One of their more interesting features is that they are anisotropic, this means that their properties depend on the direction in which they are measured. For example, they can have very high tensile strength when pulled in one direction, and comparatively low tensile strength when pulled in another.

Composites come in many forms, from fibres or particulates bonded together by a matrix material to sandwich structures. In day – to – day usage people tend to mean fibres bonded by a matrix when talking about “composites” – for example carbon fibre (Or Carbon Fibre Reinforced Plastic, CFRP, to give its proper names – but in engineering the term is used to mean any two (or more) materials bonded together to produce  third material.

Applications range from concrete used in civil engineering to carbon fibre composites, high strength, low density materials, used for aerospace applications.

Although composites have been around for a very long time, there has been an extensive growth in their use in recent decades. The table gives a comparison between two modern aircraft – the Boeing 777 and 787 – in terms of the materials used in their construction.

As can be seen, the usage of composite has grown considerably, and components and structures that were typically made from aluminium alloys are now being manufactured from composite materials. The fuselage sections of the 787 as – a close up of which is shown in the figure – are constructed in what was when built the largest autoclave of all time (an autoclave is a large, pressurised oven).

Figure 1 A disassembled carbon fiber fuselage section of the Boeing Dreamliner 787. [Wikimedia]

This second image shows the development of composite usage in commercial aerospace since 1980 – as can be seen composite materials have grown from virtually nothing to the dominant position in less than 40 years, an amazing rate of growth.

Figure 2 Development of composite materials in commercial aerospace since 1980.
Public domain, available from SEC Emblem U.S. Securities and Exchange Commission

Properties of Composites

The properties of composite materials are infinitely variable, and can be altered to meet a desired specification. The properties exhibited by any composite will depend on:

• The type of reinforcing material – carbon, glass, aramid, boron, ceramic, steel.
• The geometry of the reinforcement – short fibres, continuous fibres, particulates.
• The structure of the composite - orientation and weave of the fibres (uniaxial, biaxial, woven, knitted, braided), sandwich structure, laminated.
• The type of matrix materials – plastic, metal, ceramic.

As mentioned in the introduction, composite materials exhibit anisotropy, this means that the (mechanical) properties depend on the direction in which they are measured.  In contrast, metals are considered to be isotropic, the properties are the same whichever direction they are measured in. This anisotropy means that depending how composite materials are arranged their properties can be tailored to the requirements.

Figure 5 A close up of Carbon Fibre mat, showing the direction of the fibres. [Pixabay]

Carbon Fibre has high tensile strength when pulled axially (in the direction of the fibres) and fairly low tensile strength when pulled radially (across the direction of the fibres).

This is why the fibres in this image run in two directions, ensuring strength both up and down and left and right.

Fibre Materials

Carbon Fibres

Carbon fibre reinforced plastics (CFRP) are made from fibres of carbon surrounded by a plastic matrix, usually a thermoset plastic such as epoxy. Different types of carbon fibres are available, often termed high modulus or high strength.  Fibres are usually continuous, i.e. of infinite length until cut.

Figure 6 The use of Carbon Fibre is becoming increasingly common in the automotive industry. [Pixabay]

The fibres are often uniaxial, and the material is made up of different layers with the fibres in different directions, ensuring some strength in all directions at the expense of dropping some strength in one direction. Carbon fibre can be woven into cloth that makes handling easier.

Three of the most common arrangements – uniaxial fibres, plain weave and twill weave are shown in the images.

© images courtesy of Gareth Bradley, used with permission.

Aramids (Kevlar)

When compared to plain carbon fibres – and most other materials – aramid fibres have very high strength and impact resistance. Aramid fibre composites have a tensile strength and fracture toughness comparable to those of carbon fibres composites. Aramid fibre composites are used where high impact resistance is required. Carbon and aramid fibres may be mixed together to give the benefits of both materials.

Figure 10 UK Police officer wearing a "Bullet Proof Vest" - more properly a stab vest - one of the best known uses of Aramid (Kevlar)
[Wikimedia / CC BY-SA 3.0]

Glass Fibres

Glass Fibre Reinforced Plastic – better known as fibre glass – is similar to CFRP, except glass fibres rather than carbon fibres are used.

Glass fibres have a higher density and lower modulus and strength than carbon fibres, but are less expensive. They are electrical insulators and therefore do not result in galvanic corrosion when in contact with metals.

Figure 11 Close up of CFRP. [Pixabay]

Ceramic Fibres

Ceramic fibres - silicon carbide and aluminium oxide are the only examples at present – are used for high temperature applications such as turbine blades, heat shields. They are considered exotic materials and only used in high value applications such as aircraft engines.

They are often used in conjunction with a ceramic matrix to increase the toughness of the ceramic, making them more impact resistant. They may also be used to reinforce metals such as titanium and aluminium to increase the strength, stiffness or other properties.

Comparison of Fibre Properties

The following table gives a comparison of the 3 main types of fibre (Carbon, Aramid and Glass) showing which of the three is the best (and worst) for each property with A being the best and C being the worst.

Matrix Materials

The matrix is the material that surrounds the fibres, transferring loads onto the fibres, acting almost like an adhesive, gluing the fibres together.  Matrices are commonly plastics, usually thermosets, although thermoplastics, metals and ceramics are also used.

In the case of thermosetting matrices, the fibres are impregnated with the liquid matrix, known as a resin, and then pressure and/or heat are applied resulting in it curing to form a solid that is bonded to the fibres.

Epoxy

Epoxy matrices are generally used for high performance composites such as CFRP, for several reasons:

• Compared to polyester they result in higher strength materials.
• They tend to be inert, making them corrosion resistant and resistant to chemical reactions.
• They consist of a resin and hardener the types and composition of which can be altered to change the curing time and temperature.

Figure 12 Epoxy resin is a material that can be used in many applications on its own, in addition to its use as a matrix material. [Pixabay]

Polyester

Conversely, polyester matrices are predominantly used with glass fibres for non-structural applications as the resulting composites are of lower strength. Their main benefit over epoxies is their lower cost.

Figure 13 Despite their poor reputation as a clothing material, polyesters are a widely used material in GFRP. [Pixabay]

Sandwich (Honeycomb) Structures

A specific subset of composite, sandwich composites consist of different layers of material bonded together, often stiff and strong outer skins and a low density core. In this instance, the result is a material that has high bending stiffness, but low density. By increasing the thickness of the core the bending stiffness can be increased with little increase in weight.

It is possible (though not common) to make a sandwich structure using composite materials for both the skin and the core – a composite of composites. In this case Skin materials are made from epoxy resins reinforced with carbon, aramid or glass fibres. The number of layers and the fibre direction will influence the properties.

It is more common to use metals for the skin.

Figure 14 The structure of GLARE clearly showing the honeycomb core. [Wikimedia]

Core materials are generally of low density and include balsa wood, polyvinyl-chloride (PVC) and polyurethane foams. Honeycomb cores are usually made from aluminium, glass reinforced plastic or aramid fibre reinforced paper. Honeycomb cores are the lightest cores and result in good compressive and shear strengths.

A comparison of solid laminate, thin core sandwich panel and thick core sandwich panel is given in the table.

Reinforced Concrete

Possibly the most widely used composite material of all is reinforced concrete – that is concrete that contains steel rods embedded within it.

Concrete has a high compressive strength (15 – 30+ MPa), but a low tensile strength. To counteract the deficiency in the tensile strength reinforcement may be added in areas that will undergo tensile loading to resist cracking.  The tensile strength is still about 1/10 that of its compressive strength (for reinforced concrete). The reinforcement is often in the form of steel reinforcing bars (rebar), although polymers or other materials, or a combination of the two, may be used.

The reinforcement may be used to apply a compressive load to the concrete, resulting in a tensile load in the reinforcement; this is known as pre-stressed concrete. The coefficient of thermal expansion for steel is comparable to that of concrete, minimising the effect of internal stresses due to thermal expansion/contraction. The alkaline nature of concrete means the rate of corrosion of the steel reinforcement is reduced. Good bonding, and hence load transfer to the reinforcement, occurs due to the cement part of the concrete.

Figure 15 Reinforced concrete showing the rebar reinforcement. This particular piece of reinforced
concrete has been exposed to salt water for over 50 years, showing its use in hostile environments. [Wikimedia]

Some of the advantages and disadvantages of reinforced concrete are shown in the next two images.