Physical and mechanical working properties of materials - strength, hardness, toughness, ductility, malleability, elasticity, plasticity, density, durability, electrical and thermal conductivity - and how they govern selection and processing.

What this dot point is asking

WJEC wants you to define the working properties of materials precisely and use them to explain why a material is or is not suitable for a job. Properties split into two groups: physical properties, which a material has by its nature, and mechanical properties, which describe how it responds to forces. Exam questions almost always ask you to contrast two properties (toughness versus hardness is a perennial favourite) or to justify a choice from a product's mechanical demands.

The answer

Physical properties

• Density - mass per unit volume. Low density (aluminium, balsa, foams) matters where weight is a penalty, as in aircraft and packaging.
• Electrical conductivity - how well a material carries current. Metals (copper, aluminium) conduct; polymers and ceramics insulate, which is why plugs combine the two.
• Thermal conductivity - how well a material carries heat. Metals conduct heat (saucepan base); polymers and timber resist it (handle, worktop).
• Durability - the ability to withstand wear, weathering and use over time, including resistance to corrosion (rusting of ferrous metals) and to moisture (swelling of untreated timber and boards).

Mechanical properties

• Strength - resistance to a load without breaking. WJEC distinguishes tensile (pulling), compressive (squashing), shear (sliding) and bending strength.
• Hardness - resistance to abrasion, scratching and indentation at the surface (files, drill bits, worktops).
• Toughness - the energy absorbed before fracture; a tough material resists sudden impact and shock (hammer heads, helmets).
• Ductility - the ability to be drawn out into a wire under tension without breaking (copper wire).
• Malleability - the ability to be permanently shaped by compression, such as hammering or rolling, without cracking (lead, gold, aluminium foil).
• Elasticity - the ability to return to original shape after a deforming force is removed (springs, elastane).
• Plasticity - the ability to keep a new shape permanently after a force is removed; the basis of forming processes.

How properties govern processing

A material's properties decide not only whether it suits the product but how it can be made. Malleable, ductile metals can be pressed, drawn and forged; brittle materials must be cast or machined. Thermoplastics with good plasticity at moulding temperature can be vacuum formed or injection moulded. The same property that makes a material useful in service often dictates the manufacturing route.

Examples in context

Example 1. A diving board. It needs high elasticity so it flexes under the diver and springs back, plus toughness so repeated flexing does not cause it to fracture. Glass-reinforced plastic combines both, which a single metal would struggle to do.

Example 2. Aluminium drink cans. The metal must be ductile and malleable to be drawn and ironed into a thin seamless wall, light (low density) to cut transport cost, and corrosion-resistant so the contents are not tainted. The forming process is only possible because aluminium has those mechanical properties.

Try this

Q1. Define ductility and name a product that depends on it. [2 marks]

• Cue. The ability to be drawn into a wire under tension without breaking; copper wiring or telephone cable.

Q2. Explain why a high-carbon steel chisel is hardened and then tempered rather than left fully hard. [3 marks]

• Cue. Hardening gives a hard, wear-resistant edge but leaves the steel brittle; tempering trades a little hardness for toughness so the edge does not chip or the tool snap in use.