Engineering Materials and Material Properties
Material questions link composition and structure to properties and to the test used. Know ferrous versus non-ferrous metals, typical carbon ranges and where each is used.
Ferrous and non-ferrous
- Cast iron (about 2–4% C): good compressive strength and damping, brittle in tension; used for machine beds and housings.
- Mild steel (about 0.15–0.3% C): ductile, weldable and machinable; general fabrication.
- Medium and high carbon steel (0.4–1.2% C): higher hardness and wear resistance after heat treatment, lower ductility; tools, springs and dies.
- Aluminium: light, corrosion resistant, good conductor, low strength unless alloyed. Copper: electrical and thermal conductivity. Brass (Cu-Zn): machinability. Bronze (Cu-Sn): bearings and corrosion resistance.
Properties and tests
Strength, hardness, ductility (percentage elongation), toughness (energy to fracture), brittleness, elasticity, machinability and weldability. Brinell, Rockwell and Vickers measure hardness; the tensile test gives yield strength, ultimate strength and elongation; Charpy or Izod gives impact toughness. Hardness and toughness are not the same: a hard material can be brittle.
Worked example
A 12 mm diameter mild steel bar carries 20 kN in tension. Area = π/4 × 12² = 113.1 mm². Stress = 20,000 ÷ 113.1 = 176.8 N/mm² (MPa). If yield strength is 250 MPa, factor of safety = 250 ÷ 176.8 = 1.41. The bar is safe, but the margin is modest, so a shock load needs checking.
Traps
- Using radius instead of diameter in πd²/4.
- Confusing hardness with toughness.
- Claiming cast iron is strong in tension.
- Forgetting that 1 N/mm² equals 1 MPa.