The Incredible Properties of Composite Materials

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The development of composite materials  over the last few decades has completely  

transformed how some of the most advanced  engineering problems out there can be solved.  

It's allowed the development of materials  with unique thermal properties that can  

better handle the blistering temperatures  of atmospheric re-entry, for example.  

And has pushed the limits of jet engine  design through the use of lightweight  

fan blades that have carefully  tailored mechanical properties.  

But what exactly are composite materials, and  what makes them so special? Let's find out.  

A composite is really just any material made from  two or more distinct constituent materials. They  

can be found in nature - wood is just one  example of a natural composite material.  

But they can also be engineered, where  different materials are carefully combined  

to develop all sorts of incredible and exotic  composites that have mechanical, electrical,  

thermal or even magnetic properties that have  been tailored to suit a specific application.  

In most composites, one material, called the  dispersed phase, is contained within another,  

called the matrix phase. The ability to  carefully select each phase to optimise  

the properties of the material for a specific  application is what makes composites so powerful.  

The dispersed phase is usually what provides  the desirable material properties, like high  

strength or improved ductility, and is usually  either a ceramic or a metal. Composites are often  

categorised based on the form of the dispersed  material. This is a particle-reinforced composite,  

but they can also be fiber-reinforced,  either with short, or with continuous fibers.  

The matrix material is used to form a  mechanical and chemical bond with the elements  

of the dispersed phase, and allows loads to be  transferred between them. It holds everything  

together, and it protects the dispersed phase from  the environment. Composites are also categorised  

based on the type of matrix material, which  can be a polymer, a ceramic, or even a metal.  

Probably the most widely used composite  materials in engineering applications are the  

fiber-reinforced, polymer-matrix composites. This  category of composites includes Glass Reinforced  

Polymers, also called GRP or Fiberglass, and  Carbon Fiber Reinforced Polymers, or CFRP.  

These composites usually have an epoxy  matrix, which is a thermosetting polymer,  

and the dispersed material is glass or  carbon fibers, which make up around 60%  

of the material by volume. The most basic form  of fiber reinforcement is unidirectional tape,  

which has all of the fibers  running in the same direction.  

The individual fibers are grouped together into  bundles, which are held together with stitching  

or using a chemical binder. In the case of  carbon fibers these bundles are called tows.  

Each tow usually contains anywhere from 3  thousand to 24 thousand individual fibers.  

A typical fiber is around 10 microns in diameter,  which is ten times thinner than a human hair.  

Any fiber-reinforced material that has fibers  all running in the same direction will be highly  

anisotropic - its material properties will be  different in different directions. If you apply  

a load along the axis of the fibers, the material  will be much stronger and stiffer than if you  

apply it perpendicular to the axis, because the  load is taken by the stronger and stiffer fibers  

instead of by the matrix. This can be a good  thing. If you know that your material will be  

loaded mainly in one direction you can orient the  fibers to make it very strong in that particular  

direction. In pressure vessels for example, fibers  can be aligned mostly in the hoop direction,  

because the hoop stress is the largest  stress when the vessel is pressurised.  

In most cases though you need good strength  and stiffness in several directions at the  

same time. In the case of this pressure  vessel there will be axial stresses too,  

so we'll also need some reinforcement in the axial  direction, either with axial or helical fibers.  

This is why components made from fiber-reinforced  materials are built up by stacking multiple layers  

that have different fiber orientations.  Each layer is called a lamina, or a ply,  

and the stack is called the laminate. In this  laminate the 0 degree layer provides strength and  

stiffness in the axial direction. The 90 degree  layer provides it in the transverse direction.  

And the 45 degree layers provide it in  the shear directions.

If enough layers are stacked with the correct  orientations, the laminate can have very similar  

properties in all of the in-plane directions.  This is called a "quasi-isotropic" laminate.  

Fibers can also be arranged in weave patterns,  

which have fibers running  in two different directions.  

There are hundreds of possible weave patterns  - this is a plain weave, but the twill weave  

pattern is also commonly used. There are slight  differences in how different patterns behave. A  

twill weave is more flexible and will conform  more easily to a curved surface, for example.  

Weave patterns have good stiffness and strength  along the two fiber axes but they're weak at 45  

degrees, so they should be layered in different  orientations if quasi-isotropic properties are  

needed. Once the laminate structure has been  defined, the different fiber layers need to  

be assembled and combined with the polymer  matrix to create the final composite part.  

One way of doing this is the wet layup method,  where fiber layers are built up in a mould,  

and the resin is applied to each  layer using a roller or a brush.  

The number of plies and ply orientation  are carefully selected to achieve the  

required properties. An alternative  method involves the use of "pre-preg",  

tapes or sheets of fibers that have been  pre-impregnated in a partially cured epoxy resin,  

meaning they can be applied to the mold  without needing any additional resin.  

The laminate can then be vacuum bagged to  ensure it conforms well with the mould and  

to remove any voids, and it will then need to  cure. The polymer matrix is usually a thermoset,  

a polymer that irreversibly hardens when  heated, in which case curing is done at  

elevated temperatures in an oven.

Filament winding is another manufacturing method where a machine is used to

wind unidirectional tape that has  been impregnated with resin around a mandrel.  

Once complete the mandrel can either be left in  place or removed, and the structure is cured.  

Other methods like injection moulding can be used  for composites reinforced with short fibers, since  

the orientation of the fibers can be arbitrary.  So why are fiber-reinforced materials so special?  

To find out, let's look at their mechanical  properties. This graph shows tensile strength  

on the vertical axis, and Young's modulus,  which represents the stiffness of a material,  

on the horizontal axis. Let's plot a few common  engineering materials - titanium alloys, aluminum  

alloys, mild steel and high strength steel. Next  we can add carbon-fiber reinforced polymers.  

A plain weave carbon fiber material has a tensile  strength of around 600 Megapascals, and a Young's  

modulus similar to Aluminum, although the exact  properties will depend on a number of factors,  

including the type of polymer matrix that's used  and the layup configuration. A unidirectional  

carbon fiber material is much stronger than the  plain weave, and has higher stiffness as well,  

although remember that this is only true if  the load is applied along the fiber axis.  

These materials correspond to  standard carbon fiber grades,  

but there are also high strength, high  modulus and ultra-high modulus variants.  

We can also plot glass-fiber reinforced polymers,  which have lower stiffness but very good tensile  

strength. E-glass and S-glass refer to different  glass fiber compositions that are optimised  

for different applications. E-glass is the most  commonly used type and was originally developed  

for electrical insulation applications, and  S-glass was developed for structural applications  

and has improved strength. The really amazing  thing about these fiber-reinforced composites only  

becomes apparent when considering their mass. If  we plot specific strength and specific stiffness  

on this graph, by dividing by the material  density, it's clear that the composites far  

outperform traditional materials. The unbelievable  strength-to-weight and stiffness-to-weight ratios  

of CFRP materials are why they're so commonly used  in industries where weight reduction is critical,  

like aerospace, the automotive industry,  and even in sports like cycling and sailing.  

Glass fiber-reinforced composites  have lower stiffness than CFRP,  

but excellent strength properties on a per-weight  basis, and are much more cost effective than CFRP.  

They're often used in wind turbine blades and in  the construction of boats, where light weight,  

high strength and low cost are critical  parameters. The impressive strength of  

fiber-reinforced composites is in large part due  to the small diameter of the reinforcing fibers.  

The strength of a fiber, like any material, is  limited by the presence of defects within its  

microstructure, from which cracks can form  and grow to failure. The larger a fiber is,  

the more likely it is that it will contain more  defects, and that the defects will be larger.  

This means that if you take two fiber  bundles with the same cross-sectional area,  

but different fiber diameters, the bundle with the  smaller fibers will be stronger. Not only that,  

but in the bundle of smaller fibers, failure of a  single fiber can occur without hugely increasing  

the load on the remaining fibers. And the smaller  the fibers the larger the surface area between the  

fibers and the matrix, which means better load  transfer between the two. The result is that the  

strength of a fiber-reinforced material increases  significantly as the fiber diameter reduces.  

The main thing limiting the use of ever  thinner fibers is manufacturing constraints.  

Fiber-reinforced polymer matrix composites  aren't only used for their good specific  

strength and specific stiffness. They  have many other useful properties  

that provide advantages over traditional  materials like steel and aluminum alloys.  

They have excellent internal damping properties,  which can be useful for applications involving  

dynamic loads, and they have good corrosion  resistance the polymer matrix does a great  

job of protecting the reinforcing fibers from the  environment. They also have interesting thermal  

properties - they're usually relatively poor  conductors of heat, and have very low thermal  

expansion coefficients compared to metals, which  can be useful for applications requiring good  

dimensional stability over a wide range of  temperatures. But they also have drawbacks.  

The cost is one - they're significantly more  expensive than using standard metals. They can  

also be difficult to design with, because their  highly anisotropic nature and complex and varied  

failure modes make it difficult to accurately  model their behaviour and to predict failure  

And integration of fiber-reinforced parts into  a larger assembly isn't always straightforward  

welding isn't an option, and although mechanical  fasteners can be used, they tend not to perform  

as well as they do in metals, so fiber-reinforced  polymers are usually bonded to other parts using  

adhesives. Another drawback is the brittleness  of these materials. Let's compare stress-strain  

curves for a few fiber-reinforced polymers  alongside steel and an aluminum alloy.  

Fibers tend to be made from materials like glass  and carbon because they have high strength and  

stiffness, but they're also very brittle. This  means the resulting composite material is also  

quite brittle - CFRP in particular will fail at  very low strains compared to steel and aluminum  

alloys. A well known fiber-reinforcement  we haven't mentioned yet is Kevlar,  

a type of Aramid fiber. Kevlar-reinforced  polymers are stiffer and stronger than GRP,  

more ductile than CFRP, and lighter than  both. This makes them ideal materials  

for applications where excellent impact  resistance is required, like in body armor.  

Another issue with materials that have a  polymer matrix is that above temperatures  

not much higher than 100 or 200 degrees Celsius  the polymer will typically start breaking down,  

limiting the maximum temperatures the composites  can be used at to well below the level of metals.  

If you're working with extremely high  temperatures, you'll have to turn to  

ceramic materials, like Alumina, Silicon Carbide,  and Silicon Nitride, because they have very high  

melting points, much higher than metals and  polymers. They can withstand temperatures  

upwards of 1000 degrees Celsius. Ceramics have  other properties that make them useful at these  

high temperatures, including high thermal  shock resistance and low thermal expansion  

coefficients. Plus they have high strength and  high stiffness. Carbon has similar properties  

to these ceramics with a melting point above 3000  degrees it can handle extremely high temperatures.  

But all of these materials are very brittle.  They fracture suddenly at very low strains,  

which limits how useful they are. And this  is where once again the use of composites  

can make a big difference. Adding Silicon Carbide  fibers to a Silicon Carbide matrix, for example,  

results in a material with significantly increased  toughness. To see how this works let's compare  

two ceramic materials, with and without fiber  reinforcement. Both contain an initial crack.  

When a load is applied, the crack in the pure  ceramic propagates very quickly, resulting  

in failure of the material. In the composite  though the fibers bridge any cracks that form  

in the matrix, which prevents them from growing,  increasing the overall toughness of the material.  

Unlike polymer-matrix composites, where  the aim is to have a strong bond between  

the matrix and fibers, so that loads  can be transferred between the two,  

in ceramic-matrix composites the fibers are coated  to allow them to slide somewhat within the matrix,  

so that cracks in the matrix don't overstress  the fibers. The resulting composite is extremely  

resistant to temperature without being too  brittle. Composites with a silicon carbide  

matrix and silicon carbide fibers are used in  high temperature jet engine turbine blades.  

And carbon-carbon composites have  applications in spacecraft heat  

shields to protect from the extremely high  temperatures during atmospheric re-entry.  

They're also used in the braking systems of  some aircraft and even in high performance cars.  

Composites with a metal matrix are usually used  to try and improve the strength or stiffness  

of a metal, which often involves incorporating  carbon fibers into an aluminum or titanium matrix.  

But sometimes the goal is to modify other  properties of the metal. One example of this is  

the use of magnesium in biomedical engineering.  Magnesium is a very promising metal for use in  

implants designed to heal bone fractures,  because it's lightweight and has excellent  

biocompatibility. Another advantage it has over  commonly used metals like titanium is that it  

biodegrades in the body, so a second surgery  isn't needed to remove the implant once the  

injury has healed. But it has quite low strength,  and biodegrades too quickly to be all that useful.  

Researchers have found that by replacing  pure magnesium with a composite that has a  

magnesium matrix and a dispersed phase of ceramic  particles, the degradation rate can be controlled  

and the material strength and other properties are  greatly improved. It's really quite incredible.  

Particle-reinforced materials can be developed  for all sorts of different applications.  

Designing an electronic component that  dissipates a lot of power? A heat spreader  

made from a composite with a copper matrix  and diamond particles will have higher thermal  

conductivity than standard materials, allowing  you to dissipate the heat more effectively.  

The composite also allows the thermal expansion  coefficient of the heat spreader to be tailored  

to match the properties of the chip die, which  helps avoid high shear stresses between the two.  

Concrete is an example of a much more  common particle-reinforced material.  

The matrix phase is cement, and the dispersed  phase is aggregate, a mixture of sand and  

crushed stone. The cement binds everything  together, and the aggregate improves strength,  

and has the added advantage of being less costly  than the cement. A more recent development is the  

use of engineering cementitious composites. These  composites incorporate short randomly-oriented  

polymer fibers into a concrete matrix to  obtain a material that has the properties  

of concrete but is also ductile, which is  why it's sometimes called bendable concrete.  

This just illustrates that the possibilities  for innovation using advanced composites really  

are endless. There are so many different ways  materials can be combined to obtain something  

useful. A final category of composites is  sandwich composites, where a lightweight  

core material is sandwiched between thin skin  layers made of a stronger and stiffer material.  

The lightweight core is typically a foam or  honeycomb structure, and the skin layers are  

either metals, like aluminum, or a composite,  like CFRP. The layers are bonded together using  

an adhesive, and the result is a lightweight  structure that has high bending stiffness.  

Under loading the sandwich composite  behaves in a similar way to an I-beam.  

The outer layers carry bending loads, like the  flanges of an I beam, one side being in tension  

and the other in compression. The core is like the  web it carries shear loads, but also increases the  

distance between the outer layers, increasing  the second moment of area of the cross-section.  

Inserts are incorporated into the panel  to allow the use of threaded fasteners.  

Honeycomb panels are used extensively in  satellites as structural panels to which  

instruments and communication  equipment can be attached.  

There's no doubt that the study of composites  is an exciting and constantly evolving field  

in materials science, that opens up  new opportunities for innovation.  

Whether for fun or for professional projects,  having an understanding of the different  

composite types will help you design and build  stronger, lighter, and better performing products.  

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And that's it for this look at composite  materials. Thanks for watching!