Define and distinguish mechanical and physical properties of materials, including strength, hardness, toughness, ductility, elasticity and durability, and relate them to design choices

What this dot point is asking

SEAB wants you to define the mechanical and physical properties of materials, such as strength, hardness, toughness, ductility, malleability, elasticity, density and durability, distinguish them clearly (especially strength versus toughness), and relate each to design choices. Material properties are the language you use to justify why a material suits a job, so precise definitions matter.

The answer

Mechanical versus physical properties

Properties fall into two groups. Mechanical properties describe how a material responds to forces (strength, hardness, toughness, ductility, malleability, elasticity). Physical properties describe other characteristics such as density (mass per volume), thermal and electrical conductivity, and appearance. Both groups influence design choices, but mechanical properties usually decide whether a material can do its structural job.

The key mechanical properties

• Strength. The ability to withstand a force without breaking or permanently deforming. A strong material carries a large load. There are types: tensile (pulling), compressive (squashing) and shear (sliding).
• Hardness. Resistance to scratching, denting and wear. Hard materials keep an edge and resist abrasion.
• Toughness. The ability to absorb impact or shock without breaking. A tough material survives sudden blows.
• Brittleness. The opposite of toughness: a brittle material breaks or shatters suddenly with little bending (e.g. glass, cast iron).
• Ductility. The ability to be drawn into wire or stretched without breaking (e.g. copper).
• Malleability. The ability to be hammered, pressed or rolled into shape without cracking (e.g. aluminium, gold).
• Elasticity. The ability to bend or stretch under load and return to the original shape when the load is removed (e.g. a spring).
• Plasticity. The ability to be permanently changed in shape and keep the new shape.

Strength is not the same as toughness

A common confusion is treating strong and tough as the same. Strength is resisting a load; toughness is absorbing impact. A material can be strong but brittle: cast iron carries heavy static loads (strong) yet cracks under a sharp blow (not tough). Glass is strong under steady pressure but shatters on impact. Mild steel is both strong and tough. Keeping these separate is essential for justified choices.

Physical properties that matter

• Density. Mass per unit volume; low density means lightweight (aluminium), which matters for portable or moving parts.
• Conductivity. Thermal and electrical; copper conducts well (wiring), plastics insulate (handles, casings).
• Durability. Resistance to weathering, corrosion and wear over time; vital for outdoor products.

Relating properties to design choices

Every material choice is a property matched to a requirement. A cutting edge needs hardness; a hammer head needs toughness; a spring needs elasticity; an electrical wire needs conductivity and ductility; a portable frame needs low density and adequate strength. Justifying a choice means naming the property and linking it to what the product must do.

Examples in context

Example 1. A safety helmet. A helmet must absorb a sudden impact and spread the force without shattering, so toughness is the deciding property; a hard but brittle shell would crack and fail to protect. A tough thermoplastic such as ABS or polycarbonate is chosen because it absorbs impact and returns much of its shape, exactly the toughness a helmet needs.

Example 2. A kitchen knife blade. A blade must keep a sharp edge and resist wear from cutting, so hardness is the key property, with enough toughness that it does not chip. Hardened stainless steel suits it: hard enough to hold an edge and resist scratching, corrosion-resistant for food use, and tough enough not to shatter. The property requirements rule out a soft or brittle material.

Try this

• Cue. Define hardness and toughness in one sentence each. Answer: hardness is resistance to scratching, denting and wear; toughness is the ability to absorb impact or shock without breaking.

• Cue. Name the key property for a spring and explain why. Answer: elasticity, because a spring must bend or stretch under load and return to its original shape when the load is removed, repeatedly.

• Cue. Explain why glass is described as strong but brittle. Answer: it withstands a steady load (strong) but cannot absorb impact, so a sudden blow makes it shatter (brittle, low toughness).