3.50445 · Section 3: Specialist option: Resistant Materials

Smart, modern and composite materials

Key learnings

Smart materials and the four examples
A smart material changes a property in response to a stimulus, and the change is reversible. Know all four. Thermochromic changes colour with temperature (mood spoons). Shape memory alloy (SMA), e.g. Nitinol, is deformed cold but springs back when heated (spectacle frames). Shape memory polymer (SMP) is the plastic version. Polymorph is modern: granules that soften in hot water and set hard on cooling.
Composite = reinforcement + matrix
A composite combines two or more materials so the result beats either alone, and the parts keep their own identities — which separates it from an alloy (a single new metal). Every composite has two parts: a reinforcement (fibres that carry the load) held in a matrix (resin that binds them, transfers load, protects them and sets the shape). The three named 0445 composites are all fibre-in-resin: GRP, CFRP and Kevlar (aramid).
The three composites, by property
Link each composite's stand-out property to the job. GRP (glass-reinforced plastic): glass fibres in resin — strong, light, corrosion-proof, cheap. CFRP (carbon-fibre-reinforced plastic): carbon fibres in resin — highest strength-to-weight, very stiff, but costly (racing bikes, aircraft). Kevlar (aramid): very high tensile strength, cut- and impact-resistant (body armour). Name each with its property and matching product.

Common mistakes

Drawn from real examiner reports.

Thermochromic material not known

Examiners report thermochromic materials are "relatively new" and repeatedly missed. Lock it: a thermochromic pigment changes colour when its temperature changes (reversible). Examples: a mug that reveals a colour when hot, a battery tester, a forehead thermometer. If a product's colour signals its temperature, thermochromic is the answer.

s23 P33 Q10 · w23 P31/P33 Q9 — thermochromic material ("relatively new") repeatedly missed

Composite vs alloy trap

Treating "composite" and "alloy" as the same idea is a recurring trap. A composite joins materials that keep their own identities — in GRP you still see the glass fibres and the resin. An alloy mixes a metal with other elements into one new metal (brass = copper + zinc). So GRP and Kevlar are composites; brass and steel are alloys.

Shape memory alloy misdescribed

Shape memory alloy (SMA) is got wrong on three points. First, it is a metal alloy (Nitinol), not a plastic — the plastic version is shape memory polymer. Second, the trigger is heat: deformed while cool, it springs back when heated ("when cooled" is backwards). Third, the change is reversible. Link it to the job: spectacle frames.

SMA vs SMP (metal vs plastic)

Shape memory alloy and shape memory polymer both recover a remembered shape when heated, so the trigger is not the difference — the material is. An SMA is a metal alloy (Nitinol), used where a springy metal is wanted (frames). An SMP is a plastic (self-fitting mouthguards). Calling an SMA a "smart plastic" loses the mark.

Thermochromic vs photochromic

These two are easily swapped. Thermochromic changes colour with temperature (a mug or forehead thermometer). Photochromic changes colour with light (sunglasses that darken in sunlight). The stimulus is the whole difference: heat versus light. If colour tracks how hot it is, it's thermochromic; if brightness, photochromic.

Smart-material change is reversible

A testable error: thinking a smart material changes once and stays changed. Its defining feature is that the response is reversible. A thermochromic mug turns dark again as it cools; an SMA frame recovers its shape whenever warmed. Test: if the change reverses when the stimulus goes, it is smart.

Exam tips

Name material, property, then the use
Smart and composite marks want a chain, not a word. Name the specific material — SMA, GRP, CFRP, Kevlar — never "a smart material". State the stimulus or property, then link it to the product's demand — the "so that" step.
Match the command word
Read the command word. Describe wants what the material does. Explain wants why it suits the job. Evaluate wants a judgement against an alternative — CFRP vs aluminium: lighter, stiffer, but dearer. Answering below caps the marks.
Finish with "so that"
For an application mark, do not stop at the property — add the "so that". "CFRP for a racing-bike frame so that it is light yet stiff." "GRP for a hull so that it will not rot and moulds in one piece." The "so that" clause is where the AO2 mark lives.

Revision notes