Materials and their properties
Design & Technology · Key Stage 3 · The School
The main families
METALS: ferrous (contain iron, magnetic, mostly rust) and non-ferrous (aluminium, copper, brass). POLYMERS: thermoplastics soften on reheating and can be reshaped; thermosets set once and char instead. TIMBERS: hardwood from broadleaved trees, softwood from conifers — the names describe the tree, not the hardness. Plus composites, textiles and ceramics.
Properties that matter
STRENGTH resists force. HARDNESS resists scratching. TOUGHNESS resists sudden impact. DUCTILITY allows drawing into wire. MALLEABILITY allows hammering into shape. Also density, corrosion resistance, conductivity, and cost. Hard and tough are opposites more often than not: glass is hard and shatters; rubber is soft and absorbs.
Choosing properly
A material is chosen against a set of requirements, not on price alone. A cheap material that corrodes, fails or cannot be recycled costs more across the product's life. Add sustainability to the list — where it came from, how much energy it took, and what happens at the end.
HARDNESS (resists scratching): glass high, steel high, rubber low. STRENGTH (resists breaking under load): steel high, glass moderate, rubber low. TOUGHNESS (resists a sudden shock): steel high, rubber high, glass very low. Notice glass is hard and not tough, and rubber is tough and not hard. Any material chosen on 'strong' alone is being chosen on one of three different questions.
A material can pass every test and still fail in use, because a test applies one load once. FATIGUE is failure from small loads repeated — bend a paperclip gently thirty times and it snaps, though one bend does nothing. CREEP is slow deformation under a constant load, which is why a plastic shelf sags over a year and a steel one does not. CORROSION removes material while nothing is loading it at all. Ask how long, how often and in what conditions.