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This lesson covers the physical and mechanical properties that influence material selection and working, as required by AQA GCSE D&T (8552), Section 3.2.5. Understanding material properties is fundamental to the entire D&T course — they determine how a material behaves when forces are applied, how it can be shaped and joined, and ultimately whether it is suitable for a given product.
Physical properties describe a material's behaviour in response to physical phenomena such as heat, electricity, and light. They do not involve applying a force to the material.
| Property | Definition | How It Affects Design | Example |
|---|---|---|---|
| Density | Mass per unit volume (kg/m cubed) | Lightweight materials are preferred for portable products; dense materials for stability | Aluminium (2,700 kg/m cubed) is three times lighter than steel (7,800 kg/m cubed), making it ideal for aircraft |
| Electrical conductivity | Ability to conduct electric current | Conductors for wiring; insulators for safety | Copper is used for electrical wiring; PVC insulates the cable |
| Thermal conductivity | Ability to conduct heat | Conductors for cookware; insulators for handles | Aluminium saucepan body conducts heat; Bakelite handle insulates |
| Thermal expansion | Degree to which a material expands when heated | Must be accounted for in structures and assemblies | Railway tracks have expansion gaps to prevent buckling in hot weather |
| Fusibility | Ability to be melted and cast | Materials with lower melting points are easier to cast | Pewter (melting point ~230 degrees C) is easy to cast; steel (~1,500 degrees C) requires industrial furnaces |
| Optical properties | Transparency, translucency, or opacity | Determines suitability for windows, displays, packaging | Acrylic (PMMA) is transparent and used for display cases; MDF is opaque |
| Absorbency | Ability to absorb moisture | Affects durability, weight, and comfort | Cotton absorbs moisture (comfortable clothing); HDPE does not absorb water (suitable for outdoor use) |
AQA Exam Tip: Physical properties are often confused with mechanical properties. The key distinction: physical properties relate to heat, electricity, light, and density (no forces involved). Mechanical properties relate to how a material responds to applied forces. The exam may ask you to classify a property — make sure you know which category each belongs to.
Mechanical properties describe how a material responds when forces are applied to it. These are the properties most relevant to structural and product design.
| Property | Definition | Test Method | Example |
|---|---|---|---|
| Strength | Ability to withstand a force without breaking | Tensile test, compression test | High-carbon steel is used for drill bits because of its high strength |
| Hardness | Resistance to scratching, denting, or wear | Rockwell, Vickers, or Brinell hardness test | Tungsten carbide cutting tools are extremely hard and resist wear |
| Toughness | Ability to absorb energy from impact without fracturing | Charpy or Izod impact test | Mild steel is tough — it absorbs impact in car crumple zones without shattering |
| Elasticity | Ability to return to original shape after deformation | Stress-strain test (elastic region) | Natural rubber returns to shape after stretching — used for elastic bands |
| Plasticity | Ability to be permanently deformed without fracturing | Stress-strain test (plastic region) | Copper is highly plastic — it can be drawn into thin wire |
| Ductility | Ability to be stretched into wire (a form of plasticity under tension) | Tensile test (elongation at break) | Gold is the most ductile metal — 1 g can be drawn into 3 km of wire |
| Malleability | Ability to be hammered or pressed into shape (plasticity under compression) | Compression test, forming trials | Aluminium foil is made by rolling aluminium into very thin sheets |
| Brittleness | Tendency to fracture suddenly with little plastic deformation | Impact test | Glass and cast iron are brittle — they shatter without warning |
| Stiffness | Resistance to bending or deformation under load | Young's modulus measurement | Steel beams are used in construction because of their high stiffness |
Students often confuse related properties. Here are the key distinctions:
| Commonly Confused | Key Difference |
|---|---|
| Strength vs Hardness | Strength is resistance to breaking; hardness is resistance to surface scratching. A material can be strong but not hard (e.g. nylon rope — strong in tension but easily scratched) |
| Strength vs Stiffness | Strength is maximum force before failure; stiffness is resistance to deformation. Glass is very stiff but not strong (breaks at low force); rubber is not stiff but quite strong |
| Toughness vs Hardness | Toughness is resistance to impact (energy absorption); hardness is surface resistance. A diamond is extremely hard but relatively brittle (not tough) |
| Toughness vs Strength | Toughness measures energy absorption before fracture; strength measures maximum force. A tough material can absorb a sudden blow; a strong material resists a steady pull |
| Ductility vs Malleability | Ductility is stretching into wire (tension); malleability is hammering into shape (compression). Lead is very malleable but not very ductile |
AQA Exam Tip: The exam frequently asks you to distinguish between similar properties. Learn the precise definitions and be ready to explain the difference with examples. For instance: "Hardness is resistance to surface scratching and wear, whereas toughness is resistance to impact and fracture. A ceramic tile is very hard (scratch-resistant) but not tough (it shatters if dropped)."
| Product | Key Property Needed | Material Chosen | Why |
|---|---|---|---|
| Surgical scalpel blade | Hardness (holds a sharp edge) | Stainless steel or ceramic | Resists dulling during repeated use |
| Car spring | Elasticity | Spring steel | Must return to shape after compression |
| Copper plumbing pipe | Malleability, corrosion resistance | Copper | Can be bent into shape; resists water corrosion |
| Safety helmet liner | Toughness (absorbs impact) | Expanded polystyrene (EPS) | Crushes on impact, absorbing energy to protect the head |
| Bridge cable | Tensile strength | High-tensile steel wire | Must support enormous loads without breaking |
| Kitchen worktop | Hardness, stain resistance | Granite or quartz composite | Resists scratching from knives and staining from food |
| Trampoline mat | Elasticity, strength | Woven polypropylene | Stretches under load and returns to shape; strong enough for repeated use |
Material properties also determine how a material can be worked (cut, shaped, joined, finished):
| Property | Influence on Working | Example |
|---|---|---|
| High hardness | Difficult to cut and machine; requires harder cutting tools | Hardened steel needs carbide or diamond cutting tools |
| High ductility | Easy to form by drawing, bending, pressing | Copper can be drawn into fine wire |
| High malleability | Easy to form by hammering, rolling, pressing | Gold leaf is made by hammering gold to less than 0.001 mm thick |
| High brittleness | Cannot be bent or formed; must be cast or machined | Cast iron is shaped by casting in a mould, not by bending |
| Thermoplasticity | Can be reheated and reshaped multiple times | Acrylic sheet is heated in a strip heater and bent to shape |
| Fusibility (low melting point) | Easy to cast in moulds | Pewter can be cast in school workshops using simple moulds |
Material properties can be modified through various treatments:
| Treatment | Effect | Material | Application |
|---|---|---|---|
| Hardening and tempering | Increases hardness and strength (then reduces brittleness) | High-carbon steel | Making chisels, drill bits, springs |
| Annealing | Softens metal, relieves internal stresses | Copper, steel, aluminium | Preparing metal for further forming after work hardening |
| Case hardening | Creates a hard outer surface with a tough core | Mild steel | Gears, camshafts — hard surface resists wear; tough core resists impact |
| Work hardening | Repeated bending or hammering increases hardness (but also brittleness) | Most metals | Unintentional during forming — may require annealing to reverse |
AQA Exam Tip: Heat treatment is a topic that bridges material properties and manufacturing processes. If asked how to make a steel chisel blade hard enough to cut other metals, the answer is: heat to cherry red, quench in water or oil (hardening), then reheat to a lower temperature and cool slowly (tempering). This gives a hard cutting edge that is not too brittle.