Materials: Properties, Structure and Selection
Classify, then select
Organise materials into metals (ferrous and non-ferrous), polymers (thermoplastics and thermosets), ceramics and glass, composites, and timber (hardwood, softwood, manufactured board). For each, know the link between structure and behaviour: thermoplastic chains slide on reheating, thermoset chains cross-link permanently, composites transfer load between fibre and matrix, timber is anisotropic because of grain direction.
Properties to name precisely
Distinguish mechanical (tensile, compressive, shear, hardness, toughness, elasticity, fatigue), physical (density, thermal and electrical conductivity, thermal expansion, moisture movement), and working/processing properties (machinability, weldability, mouldability, gluability). Marks are lost when a student writes "strong" where "high tensile strength but brittle, so poor toughness" is required.
Traps that appear in questions
- Strength versus toughness versus hardness. Glass is hard and strong in compression but brittle; toughness is energy absorbed before fracture.
- Stiffness versus strength. Stiffness is resistance to deflection (modulus); a stiff material can still fail at low load.
- Ignoring grain or anisotropy. Timber and rolled metals behave differently along and across direction.
- Forgetting corrosion and degradation. A material choice without a protection strategy is incomplete.
- Confusing cost per kilogram with cost per component. A cheaper material may need more processing.
Worked example
A bracket must be light, stiff, and resist outdoor corrosion. Rule of thumb: write the requirement as a property, then screen. Light and stiff points to aluminium alloy; corrosion resistance points to anodising or a 5000/6000-series alloy; if stiffness is critical, remember aluminium has roughly one-third the modulus of steel, so section shape (a folded or ribbed profile) may matter more than the alloy. State the trade-off explicitly - that is where the marks sit.