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Topic 2 of 6 · Form 5

Materials science and selection

Metals (ferrous, non-ferrous, alloys), polymers (thermoplastics, thermosets, elastomers), woods and manufactured boards, ceramics and glass, composites (GRP, carbon fibre, reinforced concrete), and smart materials (shape-memory alloys, thermochromic and photochromic materials, piezoelectric materials).

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Material categories

Metals (ferrous, non-ferrous, alloys), polymers (thermoplastics, thermosets, elastomers), woods and manufactured boards, ceramics and glass, composites (GRP, carbon fibre, reinforced concrete), and smart materials (shape-memory alloys, thermochromic and photochromic materials, piezoelectric materials).

Properties

Mechanical (strength in tension, compression, shear; hardness; toughness; elasticity; plasticity; ductility; malleability), physical (density, thermal and electrical conductivity, melting point) and chemical (corrosion resistance).

Stress, strain and Young's modulus

Stress σ = F/A; strain ε = ΔL/L; E = σ/ε. The tensile test gives a stress–strain graph: elastic region, yield point, ultimate tensile strength and fracture.

Worked example: A 2 m steel bar with cross-sectional area 50 mm² carries a load of 5 kN. Find the stress, and the extension if E = 200 GPa.

σ = 5 000 ÷ 50 × 10⁻⁶ = 100 MPa; ε = 100 × 10⁶ ÷ 200 × 10⁹ = 5 × 10⁻⁴; ΔL = 5 × 10⁻⁴ × 2 000 mm = 1.0 mm

Testing

Tensile, hardness (Brinell, Vickers, Rockwell), impact (Charpy, Izod) and non-destructive tests (ultrasonic, X-ray, dye penetrant).

Selection

Match properties to function, manufacturing method, cost, availability and environmental impact; use material selection charts.

Exam tip: Justify a material choice with at least two relevant properties and one manufacturing or cost reason.

Check yourself

  1. Distinguish between toughness and hardness.
  2. Name one smart material and its use.
  3. What does the gradient of the elastic region of a stress–strain graph represent?
Show answers
  1. Toughness is resistance to fracture/impact; hardness is resistance to scratching and indentation.
  2. e.g. shape-memory alloy in spectacle frames, or thermochromic ink on kettles.
  3. Young's modulus (stiffness).
Next topicManufacturing processes

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