A practical guide to composites: What are composites?
Modern composites offer an enormous range of engineering possibilities, but what actually makes a material a composite? Ross Mitchell strips the subject back to basics, explaining how matrix and reinforcement work together, and how cost, working environment, and intended service life all shape the choices and compromises behind a finished product.
A composite combines two or more materials into something better than either one alone. Reinforced concrete is a classic example. The steel and concrete cover each other's weaknesses, with concrete taking the compression and steel taking the tension. The same goes for mud combined with straw to make adobe bricks.
So why isn't a plastic or a metal alloy a composite material?
Once blended and processed, the plastic ingredients become uniform throughout, the same everywhere you sample it. An alloy such as brass, a blend of copper and zinc, behaves in a similar way. Those materials have a relationship like an old married couple with only one bank account. A composite is not like that; it is more like a team with the members having separate bank accounts bringing their own individual contribution.
So, what separates a composite from a blend like brass?
A material only earns the name composite if it meets three conditions.
The components stay separate rather than merging. Each has a distinct role: the reinforcement makes up a real share of the material, not just a trace addition, and carries the strength, while the matrix holds everything together, gives the part its shape, and transfers load through to the reinforcement. And together, the combination achieves properties neither could manage alone.
Because a composite keeps its ingredients separate rather than merging them, an engineer can choose each one deliberately. Change the reinforcement and you can change performance. Glass fibre is affordable and strong enough for most jobs. Carbon fibre can easily cost ten times as much as conventional glass fibre reinforcement, but where stiffness, strength and weight are critical, the performance can justify the cost. Flax and other natural fibres are often assumed to be the compromise choice, trading strength for a lighter footprint, but some have properties that rival or beat glass fibres, while being more sustainable to produce. The same basic partnership, but different reinforcements for different jobs.
The same choice applies to the matrix. Thermoplastics soften when heated and can be reshaped, useful where a part might need to be reworked or recycled. Thermosets, once cured, stay cured and cannot be melted back down.
Within thermosets there are also lots of options, but we will focus on three basic resin families. Polyesters are an affordable choice and suitable for many applications. Epoxies sit at the opposite end of the scale, costing around five times as much as a basic polyester, but where high performance is required, their superior properties can outweigh the additional cost. They are often paired with high-end reinforcements. Vinyl esters sit somewhere between the two, costing around twice as much as a basic polyester but generally offering better resistance to water /chemicals as well as higher mechanical properties. Of course, this story of “three resin families” is a generalisation. A high-quality polyester formulation can outperform a poor-quality epoxy or vinyl ester. The best choice depends on the application and the outcome we are trying to achieve.
An important point to remember is that the resin, not the fibres, stands between the part and the outside world, holding off water, chemicals and everything else the environment throws at it over years of service. Again, it’s horses for courses — choices and compromises. More on that next time.
My articles in this series will lean toward fibre-reinforced thermoset resins, because that is where my own experience lies. Even within my sheltered corner of the composites world, the manufacturing processes used to build a part or product are just as varied as the materials themselves, with different processes suited to different shapes, production volumes and performance requirements. Underneath all of them sits the same simple partnership: matrix and reinforcement, glue and strength, working separately but together, and neither one carries the part on its own.
Where we put the reinforcement matters just as much as what we make it from. Engineers already do a version of this in steel fabrication, choosing a T bar or an I beam over a flat plate because shaping the section puts material where it can best resist the load. Composites take that same principle further, placing and orienting the reinforcement to suit the load while relying on the resin to hold it all in place and transfer that load properly, rather than relying on the shape of the finished part alone.
This is where composites differ from traditional materials such as steel and timber, which arrive with their material properties already established. In composites, the fabricator combines and processes the matrix and reinforcement, directly influencing the properties of the finished material. We are not just making the part — we are also making the material the part is made from.
Main picture: Illustration developed by the author using ChatGPT
Ross Mitchell explains composites in plain English, helping people understand real concepts in simple terms. A respected shipwright and boatbuilder with broad experience in composites, timber and metal fabrication, from surfboards to naval ships, he spent years as a vocational trainer in boatbuilding and composites before returning to industry. He is now R&D Coordinator at Compass Pools Australia, where he builds prototypes, tests assumptions, and validates results under ACM CRC Project APN018 (Product and Process Optimisation). As this role is industry-based, articles may credit Compass Pools' own research, or specific products and suppliers, where they're directly relevant to the topic. He remains close to the production floor, translating theory into practice.
A practical guide to composites is a series explaining composites in simple terms for a curious audience. Series articles including the one above can be re-published online or in print under Creative Commons (see CC BY 4.0 terms here.) Please include a backlink to the original (for online articles) and credit the author and their organisation, as well as mention that the article was originally published at ACM CRC's website. It's free to contribute. Editorial guidelines are available here. [INSERT LINK]
