By the end of this topic, you should be able to...
explain why combining materials can create composite materials more suitable for a specific purpose or context using an example.
Guiding Question
How do material properties and classifications aid material selection for a specified manufacturing process?
💡 Did You Know? The frame of the bike ridden by Tour de France winners, the fuselage of the Boeing 787, and the blade of a wind turbine share one material in common — carbon fibre reinforced polymer. It is stronger than steel, lighter than aluminium, and it did not exist in nature. Nobody dug it out of the ground or grew it. It was engineered — by combining two ordinary materials whose individual weaknesses cancel each other out. Composites are not a compromise between two materials. They are a deliberate upgrade on both.
Why Combining Materials Creates Better Properties
Here is the core idea:
Every material has a weakness. Composites are designed to hide it.
The Problem with Single Materials
Take carbon fibre on its own. It is extraordinarily strong when you pull it — but snap a single fibre and it shatters instantly. It is brittle. You cannot mould it into a shape. It is essentially useless as a structural material by itself.
Now take epoxy resin on its own. It can be moulded into any shape, bonds well to other surfaces, and is reasonably tough. But it is soft, flexible, and weak under load. On its own — also useless for anything structural.
What Happens When You Combine Them
Embed thousands of carbon fibres into liquid epoxy resin, cure it under heat and pressure, and something remarkable happens:
Property | Carbon Fibre Alone | Epoxy Resin Alone | CFRP Composite |
Tensile strength | Very high | Low | Very high ✓ |
Mouldability | None | Good | Good ✓ |
Brittleness | Very brittle | Moderate | Significantly reduced ✓ |
Density | Low | Low | Low ✓ |
The matrix holds the fibres in place, transfers load between them through the interphase, and stops cracks propagating. The reinforcement carries the structural load the matrix never could alone.
Neither material achieves this alone. Together, they eliminate each other's critical weaknesses.
The Key Principle
A composite is not an average of its constituent materials — it is a strategic combination where each material is doing the specific job it is best at, simultaneously.
This is why the Boeing 787 Dreamliner uses CFRP for 50% of its structure by weight. The alternative — aluminium — is heavier, fatigues under repeated pressurisation cycles, and corrodes. CFRP does none of these things. The 787 carries more passengers, uses less fuel, and requires less maintenance directly because of that material decision.
The Four-Step Exam Argument
When asked to explain why a composite is more suitable than a single material, always structure the response this way:
Identify the weakness of Material A in the specific context
Identify the weakness of Material B in the specific context
Explain how combining them resolves both weaknesses
Link this to the specific design requirement — weight, strength, cost, formability, durability
Practice Questions
Question 1 — Command Term: Describe
Describe the role of the matrix and the reinforcement in a fibre-reinforced composite material.
What this requires: State what each component is and what each one does within the composite structure. No evaluation required — accurate description of both phases and their functions.
Question 2 — Command Term: Explain
Explain why carbon fibre reinforced polymer (CFRP) is more suitable than aluminium alloy for the construction of a high-performance bicycle frame.
What this requires: Go beyond description — give reasons with evidence. Use specific properties of both materials. Apply the four-step argument structure above. Reference specific strength, stiffness, fatigue resistance, and density as relevant properties.
Question 3 — Command Term: Explain
Explain how combining two materials, each with individual limitations, can produce a composite material with properties superior to either constituent material. Use a named example in your response.
What this requires:Â Identify named constituent materials, state each material's specific weakness in isolation, explain the mechanism by which combination resolves those weaknesses, and link to a real application context.
Question 4 — Command Term: Compare
Compare the properties of glass fibre reinforced polymer (GFRP) and carbon fibre reinforced polymer (CFRP) for use in wind turbine blade construction.
What this requires: A balanced treatment of both materials against the same set of criteria — strength, stiffness, cost, density, fatigue performance. Do not describe one and then the other — directly compare them criterion by criterion. Reach a conclusion.
Question 5 — Command Term: Evaluate
Evaluate the use of composite materials in the aerospace industry, considering both the advantages and limitations of their use.
What this requires: The highest-demand command term. Present a sustained, evidence-based argument considering advantages (specific strength, corrosion resistance, design flexibility) against genuine limitations (cost, repairability, recycling difficulty, anisotropic behaviour). Reach a justified conclusion — do not simply list points on both sides.
Sources
Ashby, M.F. (2011). Materials Selection in Mechanical Design. 4th edn. Butterworth-Heinemann, Oxford.
Ashby, M.F. and Jones, D.R.H. (2012). Engineering Materials 1: An Introduction to Properties, Applications and Design. 4th edn. Butterworth-Heinemann, Oxford.
Berthelot, J.M. (1999). Composite Materials: Mechanical Behaviour and Structural Analysis. Springer, New York.
Boeing Commercial Airplanes (2014). 787 Dreamliner: By Design. Boeing, Seattle. Available at: www.boeing.com/commercial/787family/787-8.page
Callister, W.D. and Rethwisch, D.G. (2018). Materials Science and Engineering: An Introduction. 10th edn. Wiley, New York.
International Baccalaureate Organization (2024). Design Technology Guide: Diploma Programme. First Assessment 2027. IBO, Geneva.
Linking Questions
Why is a good understanding of material properties important when designing structural systems? (A3.2)
When do the physical properties of materials restrict the ability to use certain prototyping techniques? (A2.2)
How do the properties of a material influence the choice of manufacturing techniques for a product? (A4.1)
How can the characteristics of a material limit the effectiveness of modelling and prototyping as designs are developed? (B2.2)
How important is an understanding of the mechanical properties of a material when considering structural and mechanical systems, and their applications? (A3.2, A3.3, B3.2, B3.3)
Which classifications of properties are important when developing electronic systems and their applications? (A3.4, B3.4)
How could the continued development of biodegradable materials influence designers’ ability to address aspects of design for sustainability and design for a circular economy? (C2.1, C2.2)
Why is a thorough understanding of materials key for effective product analysis and evaluation of products? (C3.1)
How do design decisions related to the properties of materials and components impact a product’s life-cycle analysis? (C3.2)