Materials: properties, structure, testing and selection
What the exam tests
Materials questions in the Basque PAU Technology & Engineering II paper typically ask you to link structure to properties, select a material for a stated service condition, and interpret a mechanical test. You need numbers as well as names: density, Young's modulus, yield strength, toughness and cost.
Structure and properties
- Crystalline metals: BCC (ferrite), FCC (austenite, aluminium), HCP (magnesium, titanium). Slip is easiest in FCC, which explains ductility; HCP metals are less ductile at room temperature.
- Strengthening: grain refinement (Hall–Petch), solid-solution alloying, cold work (dislocations multiply), precipitation hardening (ageing), and martensitic transformation.
- Polymers: amorphous vs semi-crystalline; glass transition temperature Tg controls the service limit. Ceramics: ionic/covalent bonding, high hardness, low toughness.
- Composites: fibre/matrix load transfer; properties are anisotropic.
Testing and selection
Tensile test gives an engineering stress–strain curve: proportional limit, yield strength (0.2% offset), ultimate tensile strength, elongation at fracture. Young's modulus is the initial slope. Hardness (Brinell HB, Vickers HV, Rockwell HRC) is cheap and roughly correlates with UTS. Charpy impact tests toughness; fatigue tests produce S–N curves; creep matters above about 0.4 Tm. Selection uses Ashby-type charts: plot Young's modulus vs density for stiffness at minimum mass, strength vs density for strength-limited design, and add cost. A material index such as E/ρ or σy/ρ captures the lightweight requirement.
Worked example
A bracket must carry 12 kN in tension with a safety factor of 2. The candidate material has σy = 240 MPa. Required design stress = σy/2 = 120 MPa. Required area A = F/σ = 12,000/120 = 100 mm². If the plate is 5 mm thick, width = 20 mm. Then check stiffness: with E = 210 GPa, L = 200 mm, elongation δ = FL/(AE) = 12,000×200/(100×210,000) = 0.114 mm. If the allowable deflection were 0.05 mm, the section would need to be thicker or a stiffer material chosen.
Traps
- Confusing engineering stress (F/A0) with true stress (F/A).
- Quoting UTS as the design strength; design uses yield for ductile metals.
- Forgetting units: 1 MPa = 1 N/mm².
- Treating hardness as a direct substitute for toughness.
- Ignoring corrosion or temperature limits when selecting.