Composite Selection Guide
A composite material (also called a composition material or shortened to composite, which is the common name) is a material made from two or more constituent materials with significantly different physical or chemical properties that, when combined, produce a material with characteristics different from the individual components. The individual components remain separate and distinct within the finished structure, differentiating composites from mixtures and solid solutions.
The new material may be preferred for many reasons: common examples include materials which are stronger, lighter, or less expensive when compared to traditional materials.
More recently, researchers have also begun to actively include sensing, actuation, computation and communication into composites, which are known as Robotic Materials.
Typical engineered composite materials include:
Reinforced concrete and masonry
Composite wood such as plywood
Reinforced plastics, such as fibre-reinforced polymer or fiberglass
Ceramic matrix composites (composite ceramic and metal matrices)
Metal matrix composites
and other Advanced composite materials
Composite materials are generally used for buildings, bridges, and structures such as boat hulls, swimming pool panels, racing car bodies, shower stalls, bathtubs, storage tanks, imitation granite and cultured marble sinks and counter tops. The most advanced examples perform routinely on spacecraft and aircraft in demanding environments.
What makes a material a composite
Composite materials are formed by combining two or more materials that have quite different properties. The different materials work together to give the composite unique properties, but within the composite you can easily tell the different materials apart – they do not dissolve or blend into each other. Composites exist in nature. A piece of wood is a composite, with long fibers of cellulose (a very complex form of starch) held together by a much weaker substance called lignin. Cellulose is also found in cotton and linen, but it is the binding power of the lignin that makes a piece of timber much stronger than a bundle of cotton fibers.
It's not a new idea
Humans have been using composite materials for thousands of years. Take mud bricks for example. If you try to bend a cake of dried mud, it will break easily but it is strong if you try to squash, or compress it. A piece of straw, on the other hand, has a lot of strength when you try to stretch it but almost none when you crumple it up. When you combine mud and straw in a block, the properties of the two materials are also combined and you get a brick that is strong against both squeezing and tearing or bending. Put more technically, it has both good compressive strength and good tensile strength.
A man reconstructing an ancient mud brick citadel in Iran after it was damaged in an earthquake. The mud bricks are the same materials that were used to construct it around 2,500 years ago.
Another well-known composite is concrete. Here aggregate (small stones or gravel) is bound together by cement. Concrete has good strength under compression, and it can be made stronger under tension by adding metal rods, wires, mesh or cables to the composite (so creating reinforced concrete).
Composites have been made from a form of carbon called graphene combined with the metal copper, producing a material 500 times stronger than copper on its own. Similarly, a composite of graphene and nickel has a strength greater than 180 times of nickel.
As for fiberglass, it’s made from plastic that has been reinforced by filaments or fibers of glass. These filaments can either be bundled together, and woven into a mat, or they are sometimes cut up into short lengths which are randomly oriented in the plastic matrix.
More than strength
Nowadays many composites are made for functions other than simply improved strength or other mechanical properties. Many composites are tailored to be good conductors or insulators of heat or to have certain magnetic properties; properties that are very specific and specialized but also very important and useful. These composites are used in a huge range of electrical devices, including transistors, solar cells, sensors, detectors, diodes and lasers as well as to make anti-corrosive and anti-static surface coatings.
Composites made from metal oxides can also have specific electrical properties and are used to manufacture silicon chips that can be smaller and packed more densely into a computer. This improves the computer’s memory capacity and speed. Oxide composites are also used to create high temperature superconducting properties that are now used in electrical cables.
Making a composite
Most composites are made up of just two materials. One material (the matrix or binder) surrounds and binds together a cluster of fibers or fragments of a much stronger material (the reinforcement). In the case of mud bricks, the two roles are taken by the mud and the straw; in concrete, by the cement and the aggregate; in a piece of wood, by the cellulose and the lignin. In fiberglass, the reinforcement is provided by fine threads or fibers of glass, often woven into a sort of cloth, and the matrix is a plastic.
Examples of various forms of glass reinforcements to be used in the creation of fiberglass.
The threads of glass in fiberglass are very strong under tension but they are also brittle and will snap if bent sharply. The matrix not only holds the fibers together, it also protects them from damage by sharing any stress among them. The matrix is soft enough to be shaped with tools, and can be softened by suitable solvents to allow repairs to be made. Any deformation of a sheet of fiberglass necessarily stretches some of the glass fibers, and they are able to resist this, so even a thin sheet is very strong. It is also quite light, which is an advantage in many applications.
Over recent decades many new composites have been developed, some with very valuable properties. By carefully choosing the reinforcement, the matrix, and the manufacturing process that brings them together, engineers can tailor the properties to meet specific requirements. They can, for example, make the composite sheet very strong in one direction by aligning the fibers that way, but weaker in another direction where strength is not so important. They can also select properties such as resistance to heat, chemicals, and weathering by choosing an appropriate matrix material.
Choosing materials for the matrix
For the matrix, many modern composites use thermosetting or thermosoftening plastics (also called resins). (The use of plastics in the matrix explains the name 'reinforced plastics' commonly given to composites). The plastics are polymers that hold the reinforcement together and help to determine the physical properties of the end product.
Thermosetting plastics are liquid when prepared but harden and become rigid (ie, they cure) when they are heated. The setting process is irreversible, so that these materials do not become soft under high temperatures. These plastics also resist wear and attack by chemicals making them very durable, even when exposed to extreme environments.
Thermosoftening plastics, as the name implies, are hard at low temperatures but soften when they are heated. Although they are less commonly used than thermosetting plastics they do have some advantages, such as greater fracture toughness, long shelf life of the raw material, capacity for recycling and a cleaner, safer workplace because organic solvents are not needed for the hardening process.
Ceramics, carbon and metals are used as the matrix for some highly specialized purposes. For example, ceramics are used when the material is going to be exposed to high temperatures (such as heat exchangers) and carbon is used for products that are exposed to friction and wear (such as bearings and gears).
Choosing materials for the reinforcement
Although glass fibers are by far the most common reinforcement, many advanced composites now use fine fibers of pure carbon. There are two main types of carbon that can be used – graphite and carbon nano tubes. These are both pure carbon, but the carbon atoms are arranged in different crystal configurations. Graphite is a very soft substance (used in ‘lead’pencils) and is made of sheets of carbon atoms arranged in hexagons. The bonds holding the hexagons together are very strong, but the bonds holding the sheets of hexagons together are quite weak, which is what makes graphite soft. Carbon nano tubes are made by taking a single sheet of graphite (known as graphene) and rolling it into a tube. This produces an extremely strong structure. It’s also possible to have tubes made of multiple cylinders – tubes within tubes.
Carbon fiber composites are light and much stronger than glass fibers, but are also more expensive. Of the two, graphite fibers are cheaper and easier to produce than carbon nano tubes. They are used in aircraft structures and in high performance sporting equipment like golf clubs, tennis rackets and rowing boats, and are increasingly being used instead of metals to repair or replace damaged bones.
Even stronger (and more costly) than carbon fibers are threads of boron. Nano tubes of boron nitride have the additional advantage of being much more resistant to heat than carbon fibers. They also possess piezoelectric qualities, which means they can generate electricity when physical pressure is applied to them, such as twisting or stretching.
Polymers can also be used as the reinforcement material in composites. For example, Kevlar, originally developed to replace steel in radial tyres but best known for its use in bullet-proof vests and helmets, is a polymer fiber that is immensely strong and adds toughness to a composite. It is used as the reinforcement in composite products that require lightweight and reliable construction (eg, structural body parts of an aircraft). Even stronger than Kevlar is a substance made from a combination of graphene and carbon nano tubes.
Choosing the manufacturing process
Making an object from a composite material usually involves some form of mold. The reinforcing material is first placed in the mold and then semi-liquid matrix material is sprayed or pumped in to form the object. Pressure may be applied to force out any air bubbles, and the mold is then heated to make the matrix set solid.
The molding process is often done by hand, but automatic processing by machines is becoming more common. One of these methods is called pultrusion (a term derived from the words 'pull' and 'extrusion'). This process is ideal for manufacturing products that are straight and have a constant cross section, such as bridge beams.
In many thin structures with complex shapes, such as curved panels, the composite structure is built up by applying sheets of woven fiber reinforcement, saturated with the plastic matrix material, over an appropriately shaped base mold. When the panel has been built to an appropriate thickness, the matrix material is then cured.
Many new types of composites are not made by the matrix and reinforcement method but by laying down multiple layers of material. The structure of many composites (such as those used in the wing and body panels of aircraft), consists of a honeycomb of plastic sandwiched between two skins of carbon-fiber reinforced composite material.
These sandwich composites combine high strength, and particularly bending stiffness, with low weight. Other methods involve simply laying down several alternating layers of different substances (for example, graphene and metal) to make the composite.
Why use composites?
The greatest advantage of composite materials is strength and stiffness combined with lightness. By choosing an appropriate combination of reinforcement and matrix material, manufacturers can produce properties that exactly fit the requirements for a particular structure for a particular purpose.