Engineering Materials and Their Properties
Materials are selected by matching properties to function. Know four families: metals (strong, ductile, conductive, crystalline), ceramics (hard, brittle, heat-resistant, insulating), polymers (light, corrosion-resistant, low stiffness, temperature-limited), and composites (tailored, high strength-to-weight, anisotropic).
Key mechanical properties: strength (stress at failure/yield), stiffness (Young's modulus, slope of stress–strain), ductility (strain at fracture, measured by elongation), toughness (energy absorbed before fracture, area under stress–strain curve), hardness (resistance to indentation), fatigue resistance (cycles to failure under repeated load), and creep resistance (deformation under sustained load at high temperature). Also physical/chemical: density, thermal and electrical conductivity, corrosion resistance, melting point.
Traps
Confusing strength with stiffness - a stiff material (ceramic) may be brittle; a ductile steel is tough but not necessarily stiff. Toughness is not the same as hardness. Composites are strong along fibres but weak transverse - anisotropy matters. Polymers degrade under UV and heat; metals corrode; ceramics fail catastrophically without warning.
Worked rule of thumb
Use Ashby-style selection: identify the function (e.g., lightweight beam), objective (minimise mass), constraint (must not yield). For a beam in bending, specific stiffness E/ρ and specific strength σ/ρ drive selection. Example: comparing aluminium (E≈70 GPa, ρ≈2700 kg/m³, E/ρ≈26) with steel (E≈200 GPa, ρ≈7850, E/ρ≈25) - similar specific stiffness, but aluminium is lighter overall.