Physical Properties & Selecting Materials for Engineered Products
What Are Physical Properties?
Every material has a set of physical properties — measurable characteristics that describe how it behaves under real-world conditions. When an engineer selects a material for a product, they match the properties of available materials to the demands the product will face. Get the match wrong and the product fails; get it right and the product works reliably throughout its designed lifetime.
The specification lists ten physical properties you must be able to describe, distinguish and apply. This lesson covers all ten, then shows how they combine when selecting materials for four specific products: mobile phones, home security alarms, bicycles and children’s play area equipment.
The ten physical properties are: tensile strength, compressive strength, hardness, toughness, malleability, ductility, conductivity, corrosion resistance, environmental degradation and elasticity. You need to describe each one precisely — they are not synonyms.
Strength: Tensile and Compressive
Tensile strength is a material’s ability to resist a pulling (tensile) force without breaking. A steel cable supporting a suspension bridge must have high tensile strength — the force in the cable is always a pull. Steel and most metals have high tensile strength; ceramics and cast iron have relatively low tensile strength because they are brittle.
Compressive strength is a material’s ability to resist a squeezing (compressive) force without deforming or cracking. Concrete has high compressive strength, which is why it is used for columns and foundations. In contrast, concrete has low tensile strength — steel reinforcing bars are embedded to compensate for this weakness.
Hardness is a material’s resistance to surface indentation or scratching. A hard material like tool steel resists the cutting forces of a drill bit; a glass screen is hard enough to resist scratching by keys. However, hardness and toughness are different properties — a hard material is not necessarily tough.
Toughness is a material’s ability to absorb energy and resist fracture under impact. Glass is hard but not tough — it resists scratching but shatters under impact. Mild steel is both hard enough and tough: it can absorb an impact without cracking. A bicycle frame material must be tough so that a sudden bump or collision does not cause the frame to fracture suddenly.
Exam tip on this distinction: if a question asks why a product needs toughness rather than just hardness, your answer must mention impact energy absorption and resistance to fracture — not just resistance to scratching.
Malleability and Ductility
Malleability is the ability of a material to be shaped by compressive forces — rolled into sheets, hammered into shapes, or pressed into moulds — without cracking. Copper and aluminium are both malleable, making them suitable for pressing into complex shapes such as car body panels or electrical connectors.
Ductility is the ability of a material to be stretched into a wire or rod without breaking. Copper is highly ductile: it can be drawn into the thin wires used inside electrical cables. A material can be malleable without being ductile, and vice versa, so the exam expects you to use the correct term for the correct process.
Conductivity: Electrical and Thermal
Electrical conductivity is the ease with which a material allows an electric current to flow through it. Copper is an excellent electrical conductor — it is the most widely used material for wiring. Plastics are electrical insulators: they have very low conductivity and are used to coat cables and encase electrical components to prevent electric shock.
Thermal conductivity is the ease with which a material allows heat to flow through it. Metals generally conduct heat well; plastics and ceramics are poor thermal conductors. These two conductivities often move in the same direction (metals conduct both electricity and heat), but this is not always the case, so treat them as separate properties.
Copper is used for electrical wiring because it has very high electrical conductivity — it allows current to flow with very low resistance. Its ductility means it can be drawn into thin wire. Both properties must be stated when justifying copper as a wiring material.
Corrosion Resistance and Environmental Degradation
Corrosion resistance is a material’s ability to resist chemical attack by substances in its environment, most commonly water, oxygen, acids or salts. Stainless steel is chosen for outdoor structures partly because its chromium content creates a protective oxide layer that resists corrosion. Mild steel, by contrast, corrodes readily in damp air unless it is painted, plated or coated.
Environmental degradation is the broader process by which materials deteriorate due to environmental exposure over time. UV light breaks down many plastics, causing them to become brittle and discolour. Temperature cycling can fatigue metals. Engineers working on outdoor products — such as play area equipment — must account for all the environmental factors the product will face over its working life.
Elasticity
Elasticity is the ability of a material to return to its original shape after a deforming force is removed. A spring steel returns to its shape after being compressed or stretched. Rubber is also highly elastic. When a material is stretched beyond its elastic limit, it deforms permanently — this is called plastic deformation. Engineers must ensure that materials in springs and flexible components operate within their elastic limit in normal service.
Mobile phone casing and screen: The casing must be hard (to resist scratches from keys and coins), tough (to survive drops), and have low electrical conductivity (to be safe to handle). The screen must also be hard and have good optical clarity; some glass formulations are engineered to be tougher than standard glass.
Home security alarm housing and sensor: The outer casing must have good corrosion resistance and resist environmental degradation (UV, damp) because it may be mounted outside. Electrical components inside require materials with high electrical conductivity for wiring and low electrical conductivity for the casing — to prevent short circuits. The casing must be tough enough to resist deliberate tampering.
Bicycle frame: The frame must have high tensile strength (pedalling and rider weight create pulling forces in the frame tubes), good toughness (to survive kerb drops and collisions without fracturing), and low density so that the bike remains light. Aluminium alloy frames balance these requirements: aluminium is not as strong as steel, but it is much lighter, so larger-diameter tubes are used to compensate. Good corrosion resistance is also important for a product used outdoors in wet conditions.
Children’s play area equipment: Safety is the primary concern. Materials must have high toughness (impact from children climbing and falling), good corrosion resistance (outdoor environment, rain), resistance to environmental degradation (UV light degrades plastics over time), and smooth surfaces to prevent injuries. Rubber impact surfaces beneath equipment also rely on high elasticity to absorb landing impacts safely.