Materials and their properties
Materials questions test whether you can classify a material, name the property that matters for a given application, and justify a choice against alternatives on more than one criterion.
Classification
- Metals: ferrous (steel, cast iron - magnetic, prone to rust), non-ferrous (aluminium, copper, zinc, titanium), alloys (brass, bronze, stainless steel 304/316).
- Polymers: thermoplastics (PET, HDPE, PP, PS, ABS, PMMA, nylon - remeltable, recycling codes 1–7), thermosets (epoxy, polyester resin, melamine, GRP - cross-linked, cannot be remelted), elastomers (natural and silicone rubber - elastic, difficult to recycle).
- Timbers: softwoods (pine, spruce - fast grown, cheaper, resinous), hardwoods (oak, beech - denser, slower grown), manufactured boards (plywood, MDF, chipboard, OSB - stable and uniform but adhesive-dependent).
- Ceramics and glass: hard, brittle, heat resistant, electrical insulators. Composites: matrix plus reinforcement (GRP, CFRP). Smart materials: shape-memory alloys, thermochromic and photochromic pigments, piezoelectrics.
Properties and how they are measured
Tensile strength and stiffness come from a tensile test (stress–strain curve gives ultimate strength, Young's modulus, elongation). Hardness by Brinell/Rockwell or Mohs. Toughness is energy absorbed before fracture, measured by Izod/Charpy impact. Ductility (drawing into wire) and malleability (hammering into sheet) are both plasticity under different stress states. Also relevant: elasticity, conductivity, density, melting point or glass transition, corrosion resistance, workability and cost - remember cost per unit volume, not only per kilogram.
Worked example
Bicycle frame: 6061-T6 aluminium, 4130 steel or CFRP. Density: steel 7.85, aluminium 2.70, CFRP about 1.6 g/cm³; E: steel 210 GPa, aluminium 70 GPa. Specific stiffness is similar for steel and aluminium, but aluminium's low density allows larger-diameter tubes to recover stiffness at lower mass. Steel welds easily and is repairable; aluminium needs TIG welding and often post-weld heat treatment; CFRP is bonded and anisotropic, strong along the fibres. Race frame → CFRP; cheap durable frame → steel; mid-range → aluminium. The marks come from stating criteria, not from naming a favourite.
Traps
- Using "strong" for hardness, stiffness and toughness interchangeably.
- Confusing ductility with malleability, and thermoplastic with thermoset.
- Assuming aluminium does not corrode - it passivates but suffers galvanic corrosion against steel.
- Ignoring timber's anisotropy: properties differ along and across the grain.