Physical vs mechanical properties

To select and test materials you must describe their properties precisely. Split them into two families:

• Physical properties — inherent, tested without applying a shaping force: density, electrical and thermal conductivity, fusibility (melting behaviour), corrosion / degradation resistance.
• Mechanical (working) properties — the response to an applied force: strength, hardness, toughness, ductility, malleability, elasticity/plasticity and stiffness.

The five forces on a structure

Loads act on materials as five basic forces. Good design routes each force into a material that resists it well.

• Tension — pulling apart (a tow rope, a suspension cable).
• Compression — squashing (a table leg, a brick pillar).
• Shear — opposing forces sliding across a section (a rivet, scissors cutting).
• Torsion — twisting (a drive shaft, turning a screwdriver).
• Bending — a combination: the top surface is in compression, the bottom in tension (a shelf, a diving board).

Match force to example

Tap a force, then a product example where it dominates.

Tensile and compressive strength

Strength is the ability to withstand force without breaking. It is direction-specific:

• Tensile strength — resistance to being pulled apart (steel cable, spider silk).
• Compressive strength — resistance to being crushed (concrete, cast iron).

Key idea: a material can be strong in one mode and weak in another — concrete is excellent in compression but poor in tension, which is why it is reinforced with steel bars (strong in tension).

Hardness, toughness, brittleness

• Hardness — resistance to scratching, indentation or abrasion (measured by Vickers, Brinell or Rockwell tests). Hardened steel files and drill bits are hard.
• Toughness — ability to absorb impact/shock without fracturing (measured by an Izod or Charpy impact test). Mild steel and polypropylene are tough.
• Brittleness — the opposite of toughness: fractures suddenly with little deformation (cast iron, glass, ceramics).

Tough or brittle?

Ductility and malleability

• Ductility — can be drawn out into a wire / stretched without breaking. Copper is highly ductile (electrical wire).
• Malleability — can be hammered, rolled or pressed into a sheet / new shape without cracking. Aluminium and gold are malleable.

Don’t confuse them: ductility is about pulling into a wire; malleability is about spreading into a sheet. Most ductile metals are also malleable, but the tests differ.

Wire or sheet?

Elasticity, plasticity, stiffness

• Elasticity — returns to its original shape after the load is removed (a spring, an elastic band).
• Plasticity — retains a new, permanent shape after the load is removed (modelling clay, a bent paperclip).
• Stiffness — resistance to elastic bending or deflection (quantified by the Young’s modulus). A stiff shelf sags less under load.

Elastic behaviour

Stress and strain

When a force acts over an area, engineers describe the internal load as stress:

Strain is the fractional change in length (extension ÷ original length). A stiff material shows little strain for a given stress.

Your turn — calculate stress

Factor of safety

Structures are never worked at their breaking point. The factor of safety compares the stress a material can take with the stress it actually carries:

A higher factor of safety means a bigger margin before failure — but usually more material, mass and cost.

Your turn — factor of safety

Physical or mechanical?

Tap a property, then tap whether it is physical or mechanical.

The big ideas to know

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