A3.1 — Material classification and properties

Key concepts

Every product is made of materials, and the choice of material shapes everything — how it looks, how it performs, how long it lasts, how it is manufactured, and how it ends its life. Understanding material categories and their properties is foundational to design.

The five material categories

Metals — strong, stiff, tough, conductive. Typically crystalline structures with mobile electrons. Ductile (can be drawn into wire) and malleable (can be hammered into shape). Examples: steel, aluminium, copper, titanium.

Polymers — long-chain molecules, usually less dense and less stiff than metals but tough and easily shaped. Split into thermoplastics (soften when heated, can be reshaped — PET, polypropylene, ABS) and thermosets (cure irreversibly — epoxy, phenolic).

Ceramics — crystalline inorganic solids, very hard and stiff, excellent thermal/electrical insulators, but brittle. Examples: alumina, porcelain, glass (technically an amorphous ceramic).

Composites — two or more materials combined to achieve properties neither has alone. Usually a fibre (strength) in a matrix (binds and transfers load). Examples: CFRP (carbon fibre in epoxy), GRP/fibreglass, concrete (aggregate in cement).

Natural materials — harvested rather than synthesised. Includes timber, bamboo, leather, cotton, wool, cork. Renewable in principle but can be over-extracted.

Key properties

Designers describe materials using mechanical, thermal, electrical, and aesthetic properties.

Mechanical: - Tensile strength — resistance to being pulled apart - Compressive strength — resistance to being crushed - Hardness — resistance to surface indentation - Toughness — ability to absorb energy before fracturing - Elasticity — ability to return to shape after load is removed - Plasticity — ability to deform permanently without breaking - Stiffness — resistance to elastic deformation (Young's modulus) - Density — mass per unit volume

Thermal: thermal conductivity, thermal expansion, melting/softening point.

Electrical: conductivity (or resistivity), dielectric strength.

Aesthetic: colour, finish, texture, transparency, smell.

Stress-strain curves

When a tensile force is applied to a material, it stretches. Plotting stress (force / area) against strain (extension / original length) gives the stress-strain curve:

1. Elastic region — straight line; material returns to original shape when load is removed. The slope here is Young's modulus (stiffness).
2. Yield point — beyond this, deformation becomes permanent (plastic).
3. Plastic region — material continues to deform; dislocations move through the crystal structure.
4. Ultimate tensile strength (UTS) — peak stress reached.
5. Fracture — material breaks.

Products in normal use should operate in the elastic region only. Manufacturing often uses the plastic region deliberately (rolling, drawing, forging).

Testing

• Tensile testing machine — measures force vs extension directly.
• Hardness testers — Vickers, Brinell, Rockwell (press an indenter into the surface and measure the mark).
• Impact tests — Charpy and Izod drop a weighted pendulum onto a notched sample to measure toughness.

Smart and modern materials

Shape memory alloys (e.g. nitinol) — deform at room temperature then return to a pre-set shape when heated above a transition temperature. Used in stents, eyeglass frames, robotic actuators.

Thermochromic materials — change colour with temperature. Used in baby bath thermometers, mood rings, fever indicators.

Photochromic materials — change colour with light. Used in reactive sunglasses.

Piezoelectric materials — generate electricity when deformed, or deform when electricity is applied. Used in sensors, igniters, ultrasound.

Self-healing polymers — repair scratches autonomously when heated. Emerging in coatings.

Case studies

Apple Unibody MacBook — CNC machined from a single aluminium billet. Aluminium chosen for low density, high thermal conductivity (heat dissipation), ability to be anodised for finish, and full recyclability. Material decision drove manufacturing process.

Boeing 787 Dreamliner — 50% composite (mostly CFRP) by weight. Composites chosen for high strength-to-weight ratio, corrosion resistance, and fatigue performance, enabling longer range and fewer maintenance cycles. Illustrates how material selection drives product architecture.

Corning Gorilla Glass — chemically strengthened alkali-aluminosilicate glass. High scratch resistance, impact tolerance, thin profile. Enabled the smartphone industry by allowing durable touchscreens at glass thinness that used to be impossible.

Bamboo bicycle frames — bamboo has exceptional strength-to-weight ratio and natural damping (vibration absorption). Renewable, aesthetically distinctive. Example of re-evaluating traditional materials with modern engineering.

Glossary

• Metal — crystalline material typically strong, stiff, and conductive.
• Polymer — long-chain molecular material, lightweight and easily shaped.
• Thermoplastic — polymer that softens when heated and can be reshaped.
• Thermoset — polymer that cures irreversibly and cannot be remelted.
• Ceramic — hard, brittle, insulating crystalline inorganic material.
• Composite — material combining two or more constituents with distinct phases.
• Tensile strength — maximum stress a material can withstand while being stretched.
• Hardness — a material's resistance to surface indentation or scratching.
• Toughness — the energy a material can absorb before fracturing.
• Elasticity — the ability to return to original shape after deformation.
• Young's modulus — the ratio of stress to strain in the elastic region (stiffness).
• Yield point — the stress at which permanent deformation begins.
• Ultimate tensile strength (UTS) — the peak stress reached before fracture.
• Shape memory alloy — a metal that returns to a pre-set shape when heated.
• Smart material — a material that responds to an external stimulus in a controlled way.

Check your understanding

1. Name the five material categories and give two examples of each.

Metals — steel, aluminium. Polymers — polypropylene, ABS. Ceramics — porcelain, alumina. Composites — CFRP, fibreglass. Natural materials — timber, leather.

2. Using a stress-strain curve, explain the difference between elastic and plastic deformation.

The elastic region is the initial straight-line portion of the curve, where strain increases linearly with stress. In this region the material returns to its original dimensions when the load is removed. Beyond the yield point, the curve flattens and the material enters plastic deformation — bonds have permanently rearranged and the deformation remains when load is removed. Products in use must operate in the elastic region; manufacturing processes often exploit the plastic region to shape material.

3. Describe two smart materials, explaining their stimulus and response, and give an application for each.

Shape memory alloys (e.g. nitinol) deform at room temperature but return to a pre-set shape when heated above a transition temperature; used in self-opening arterial stents. Thermochromic pigments change colour in response to temperature; used in baby bath thermometers to indicate safe water temperature. Each material converts one form of energy input (thermal) into a visible or mechanical response.

4. A product designer must choose between CFRP and aluminium for a bicycle frame. Identify three properties that matter for this decision and explain how each material compares.

Strength-to-weight ratio: CFRP is significantly higher, so frames can be lighter for the same stiffness. Fatigue resistance: aluminium fails progressively under repeated loading; CFRP resists fatigue well but fails catastrophically if damaged. Manufacturability and cost: aluminium frames can be extruded and welded relatively cheaply; CFRP requires moulds, layup, and curing, increasing unit cost and limiting design iteration. The choice depends on whether performance or cost/reparability is the priority.

Teacher's notes — additional examples and activities

Natural vs manmade materials

• Natural — from plants, animals, earth; not made by humans (timber, leather, stone, wool)
• Manmade — created by humans (polymers, steel alloys, ceramics)
• Both categories — timbers and textiles can be either (natural cotton vs synthetic polyester; natural oak vs engineered MDF)

Quick identification: wood with a natural grain is natural timber; a textile that burns and melts is synthetic (natural fibres char rather than melt).

Timber defects

Key terms for describing wood problems: - Warping — distortion from uneven drying - Bowing — warping along the face length - Cupping — warping across the face width - Twisting — ends in different planes - Knots — reduce strength; caused by branch growth

Common in Paper 1 multiple choice and material-selection justification questions.

Manufactured boards

MDF, plywood, particle board — made by gluing waste wood layers or fibres into consistent sheets. Standard thicknesses: 3, 6, 9, 12, 15, 18, 20, 25 mm. Used in furniture (IKEA, Memphis group) because consistency and large sheet sizes suit mass production.

Thermoplastics vs thermosets — molecular structure

• Thermoplastics — molecules in long chains with few cross-links. Chains move apart when heated, allowing reshaping. Can be re-melted repeatedly (recycling-friendly).
• Thermosets — heavily cross-linked rigid structure. Soften once on first heating, cure irreversibly. Cannot be reshaped.

Plastic memory — thermoplastics return toward their original shape when reheated, unless damaged by overheating or overstretching. A formed shape becomes flat again when reheated.

Injection moulding — the four-step process

1. Plastic granules are fed into a hopper
2. A heater melts the granules in a heated tube
3. A rotating screw forces molten plastic into a mould
4. The mould opens and the product is ejected

This is worth memorising — a common Paper 1 topic.

Physical / mechanical / chemical properties

• Physical — observable without changing the substance (mass, density, melting point, conductivity). "Stuff we can see."
• Mechanical — response to applied forces (tensile strength, hardness, elasticity). See main chapter.
• Chemical — how the material reacts with others (flammability, corrosion resistance). "Stuff we can't always see."

Why a saucepan uses multiple materials

A classic applied question. A typical pan uses: - Steel/aluminium base — high thermal conductivity to heat food evenly - Teflon (PTFE) coating — low friction, non-stick, chemically inert - Stainless steel or plastic handle — low thermal conductivity (insulator) so the user can grip while hot

Each material chosen for a specific physical property.

Smart materials — worth knowing by name

• Shape memory alloys (nitinol) — return to pre-set shape when heated
• Thermochromic pigments — change colour with temperature
• Photochromic materials — darken in UV light (reactive sunglasses)
• Piezoelectric materials — generate voltage when deformed
• Self-healing polymers — repair scratches autonomously

Examiners reward specific smart-material examples by name.