A3.1 Material Classification and Properties | DP Design

This topic is mostly about learning to be precise, and precision is what separates a design justification from an opinion. "This material is strong" is not a statement anyone can check. Strong in tension or in compression? Stiff, or tough? Hard, or just heavy? Materials science has spent a couple of centuries building words that each mean exactly one thing, and once you have them you can say what you actually mean and then defend it.

It is tempting to treat this as a vocabulary list you memorise for Paper 1 and then forget. Resist that, because every material decision you make for the rest of the course runs through here. B3.1 asks you to choose a material and justify it, A3.2 and B3.2 ask you to predict how it behaves under load, and C3.2 asks what happens to it after the product dies. Get the properties straight now and those topics become mostly arithmetic. Skip them and you will be guessing for two years.

Students must be able toExplain how and why materials are classified and discuss the advantages of classifying materials in terms of physical, chemical and mechanical properties.

Classification systems allow designers and engineers to organise the vast range of available materials into manageable groups, making comparison and selection practical. The first recorded material classification system is attributed to Aristotle (384–322 BCE), who grouped matter by elemental properties. Modern classification follows three broad property categories:

Physical properties: measurable characteristics that do not involve a chemical reaction. Density, thermal expansion, thermal conductivity, melting point, electrical conductivity and resistivity.
Chemical properties: how a material interacts with other substances. Corrosion resistance, reactivity with food, hygroscopy, flammability.
Mechanical properties: how a material responds to applied forces. Strength, stiffness, toughness, hardness, malleability, elasticity, plasticity, ductility.

Advantages of classification:

Enables systematic comparison across large numbers of candidate materials using databases and selection tools (e.g., CES EduPack)
Allows designers to specify performance requirements in measurable terms that suppliers and manufacturers can match
Supports decision-making across the full product lifecycle, from material extraction and processing to end-of-life recycling or biodegradation
Provides a shared technical vocabulary that bridges design, engineering, and manufacturing disciplines

By Lekritz - Own work, CC BY-SA 4.0, Wikipedia

Category	What it measures	Involves a chemical change?	Examples
Physical	What the material is	No	Density, thermal expansion, thermal conductivity, melting point, electrical conductivity/resistivity
Chemical	What the material does when it meets other substances	Yes	Corrosion resistance, reactivity with food, hygroscopy, flammability
Mechanical	What the material can withstand under force	No	Strength, stiffness, toughness, hardness, ductility

Material property categories are the foundation for selection charts and decision-making throughout DP Design. See A3.2 Structural Systems for how mechanical properties determine structural performance, and A4.1 Manufacturing Techniques for how physical and chemical properties constrain which processes a material can go through.

Classification diagram

Metals: ferrous and non-ferrous

Hover or focus an example for a quick note, or click it for more detail on how it gets used in design technology.

Metals

Ferrous

Contains iron, which gives good strength but leaves it prone to rusting unless treated, coated or alloyed against corrosion.

Non-ferrous

Contains no iron, so it resists corrosion far better than ferrous metals. Usually lighter, but often more costly to produce.

Classification diagram

Composites: reinforcement and matrix

Hover or focus an example for a quick note, or click it for more detail on how it gets used in design technology.

Composites

Reinforcement

The strong, stiff phase running through a composite, usually fibres or particles that carry most of the load.

Matrix

The binder that holds the reinforcement in place, transferring load between fibres or particles and protecting them.

Students must be able toDiscuss frame, shell, solid and combination structures, and how they are used in the design of products. Understand that materials are classified into natural and human-made categories.

Materials are grouped by their origin into natural (found in or derived from nature) and human-made (synthesised or significantly processed by people):

Timbers: natural. Softwoods (pine, spruce, cedar) from coniferous trees; hardwoods (oak, teak, mahogany) from deciduous trees. Engineered wood (plywood, MDF, LVL) is human-made from timber by-products.
Polymers: largely human-made. Thermoplastics (polyethylene, polypropylene, ABS, PET, all of which can be remelted and reshaped); thermosets (epoxy, polyester resin, permanently cross-linked and unable to be remelted); elastomers (rubber, silicone, flexible and able to return to shape).
Metals: processed from natural ores. Ferrous (contain iron: mild steel, stainless steel, cast iron) and non-ferrous (aluminium, copper, titanium, brass, bronze).
Glass: human-made from natural silica (SiO₂) with additives. Soda-lime glass, borosilicate glass, optical glass, toughened and laminated safety glass.
Textiles: natural fibres (cotton, wool, silk, linen) and human-made fibres (nylon, polyester, Kevlar, carbon fibre).
Composites: human-made combinations of two or more materials (covered in 3.1.7).
Smart materials: human-made materials with properties that respond to environmental stimuli (covered in 3.1.8).
Biomaterials / biodegradable materials: natural or human-made materials that break down biologically (covered in 3.1.9).

Structural forms and their material implications: Product structures are often classified as frame, shell, solid, or combination, and the material choice is directly linked to the structural form:

Frame structures: load-bearing members (rods, beams, tubes) connected at joints; requires materials with high strength-to-weight ratio (steel, aluminium, carbon fibre)
Shell structures: thin curved surfaces carry loads through their geometry; requires materials that can be formed into thin sheets (sheet steel, fibreglass, carbon fibre)
Solid (mass) structures: material fills the entire volume; suitable where compressive strength is primary (concrete, stone, cast iron)
Combination structures: use two or more structural forms together (e.g., a car body combining a steel frame with sheet metal shell panels)

Frame, shell, solid and combination structures are explored in much greater depth in A3.2 Structural Systems, including how to analyse the forces each form is best suited to carry.

Students must be able toEvaluate physical, chemical and mechanical properties to ensure selection of the most appropriate material for a specific purpose.

No single material excels in all properties. Material selection is a multi-criteria optimisation problem: the designer must identify the properties most critical to the product's function, environment of use, manufacturing process, cost constraints, and end-of-life requirements, then find the material (or combination of materials) that best satisfies those criteria simultaneously.

Key considerations in material selection:

Functional requirements: what the material must do (support loads, conduct electricity, insulate heat, resist chemicals, flex without breaking)
Manufacturing requirements: how the material will be shaped (casting, machining, forming, moulding, additive manufacturing); some processes require specific material characteristics (e.g., die casting requires materials with low melting points)
Environmental conditions of use: temperature range, exposure to moisture, chemicals, UV radiation, or biological agents; outdoor use demands corrosion resistance; food contact demands chemical inertness
Aesthetic characteristics: surface finish quality, colour, translucency/opacity, texture, perceived quality ("feel" and "weight" that communicate value to users); materials communicate brand identity beyond their function
Cost and availability: raw material cost, processing cost, lead time; exotic materials with excellent properties may be impractical for high-volume production
Sustainability: embodied energy, recyclability, biodegradability, source (renewable vs. finite), and life-cycle environmental impact

In practice, designers use material selection charts (Ashby charts) to plot two properties simultaneously (e.g., strength vs. density), allowing rapid visual comparison of material families and identification of candidates for more detailed evaluation.

Concept: Ashby Charts

An Ashby chart is a scatter plot with one material property on each axis (commonly logarithmic scales), such as Young's modulus against density. Each material family (metals, polymers, ceramics, composites, foams) occupies its own cluster or "bubble" on the chart rather than a single point, because properties vary within a family depending on processing and composition.

This connects to the property categories introduced earlier in this topic: a designer who has already weighed up the functional, manufacturing, environmental, aesthetic, cost, and sustainability requirements for a product can draw a target zone on the chart (for example, "stiffness above 50 GPa and density below 3000 kg/m³") and immediately see which material families fall inside it. This turns a verbal list of requirements into a visual shortlist, narrowing dozens of candidate materials down to a handful worth investigating in detail.

Ashby chart plotting Young's modulus against density for different material families

Sustainability as a selection criterion is developed fully in C2.1 Design for Sustainability and C2.2 Circular Economy.

Students must be able toExplain density, thermal expansion, thermal conductivity, melting point, electrical resistivity and electrical conductivity.

Physical properties describe what a material is. They can be measured without causing a chemical reaction or permanently altering the material's identity.

Density (ρ): mass per unit volume (kg/m³ or g/cm³). A material's density determines structural weight and influences shipping cost, handling, and user experience. Aluminium (2700 kg/m³) is roughly one-third the density of steel (7800 kg/m³), making it attractive for weight-sensitive applications like aircraft and bicycle frames.
Thermal expansion (coefficient of thermal expansion, α): the fractional change in length per degree of temperature change (K⁻¹). Engineering components operating across temperature ranges must accommodate differential expansion. For mild steel, α ≈ 11 × 10⁻⁶ K⁻¹, meaning a 1-metre steel rod expands by 0.011 mm for every 1°C rise. Bridges need expansion joints; pipelines need flexible connectors.
Thermal conductivity (k): the rate at which heat flows through a material (W/m·K). High thermal conductivity (copper, aluminium) is needed in heat exchangers and cooking pans. Low thermal conductivity (wood, expanded polystyrene, aerogel) is required for thermal insulation in buildings and packaging.
Melting point (T_m): the temperature at which a solid transitions to a liquid. A high melting point (tungsten: 3422°C) is needed for materials used in high-temperature environments. A low melting point is required for casting and soldering processes. Thermoplastics soften progressively rather than having a sharp melting point.
Electrical conductivity (σ) and electrical resistivity (ρ_e): conductivity is the ease with which electrons flow through a material (S/m); resistivity is its reciprocal (Ω·m). Conductors (copper, aluminium, silver) have very low resistivity; insulators (rubber, ceramic, glass) have very high resistivity; semiconductors (silicon, germanium) fall in between and can be tuned by doping.

Students must be able toExplain corrosion resistance, reactivity (food safe), hygroscopy and flammability.

Chemical properties describe what a material does when it encounters other substances. They involve chemical reactions that alter the material's composition.

Corrosion resistance: the ability to resist degradation by chemical reactions with the environment (oxygen, moisture, acids, salts). Stainless steel and aluminium resist corrosion through passivation: they spontaneously form a nanometre-thick chromium oxide (stainless steel) or aluminium oxide (aluminium) surface layer that is stable and adherent, blocking further oxidation of the underlying metal. This self-healing layer reforms if scratched. Iron and mild steel do not passivate; instead they form loose, flaky iron oxide (rust) that continues to expose fresh metal, causing progressive degradation.
Reactivity with food (food safety): materials used in food contact applications must not release chemicals into food. Three relevant reactions are: migration (plasticisers or monomers diffusing from packaging into food), oxidation (fats and oils reacting with oxygen, accelerated by some metals), and hydrolysis (water breaking down polymer chains). Stainless steel, glass, HDPE, and food-grade polypropylene are used in food contact applications because they are chemically inert under normal conditions. Regulations (e.g., EU 10/2011, FDA CFR Title 21) specify migration limits.
Hygroscopy: the tendency to absorb and retain moisture from the surrounding environment. Hygroscopic materials (wood, nylon, PVA, silica gel, salt) swell, weaken, or lose dimensional stability when they absorb water. Hydrophobic materials (wax, polyethylene, PTFE) repel water. Hygroscopicity is critical for packaging (a hygroscopic package protecting a moisture-sensitive product would be counterproductive), storage, and applications where dimensional accuracy is essential.
Flammability: the ease with which a material ignites and sustains combustion. Measured parameters include the ignition temperature, rate of flame spread, and heat release rate. Materials are classified as flammable, combustible, or non-combustible. Designers specify flame-retardant additives or inherently non-flammable materials (ceramics, metals, glass) for products used near ignition sources or required to meet fire safety standards.

Students must be able toExplain tensile and compressive strength, stiffness, toughness, hardness, malleability, elasticity, plasticity and ductility.

Mechanical properties describe what a material can withstand when forces are applied.

Tensile strength (Ultimate Tensile Strength, UTS): the maximum stress a material can withstand before fracturing when pulled. Measured in MPa (megapascals). Steel structural members, cables, and tensioned fasteners require high UTS.
Compressive strength: the maximum stress a material can withstand when compressed (squeezed). Concrete is strong in compression (≈ 20–40 MPa) but weak in tension; steel is strong in both (hence reinforced concrete combines the two).
Stiffness (Young's modulus, E): resistance to elastic deformation under load; the ratio of stress to strain in the elastic region (GPa). A high Young's modulus means a material deflects very little under load. Steel (E ≈ 200 GPa) is far stiffer than rubber (E ≈ 0.01–0.1 GPa).
Toughness: the energy absorbed per unit volume before fracture; the area under the full stress-strain curve. Toughness combines strength and ductility: a material can be tough by being strong (like high-strength steel) or by stretching a long way before breaking (like rubber). Measured by impact tests: Charpy (notched bar struck from behind) and Izod (notched bar struck from the side).
Hardness: resistance to surface indentation or scratching. Tested by multiple methods: Brinell (steel/WC ball, best for castings); Rockwell (steel ball or diamond cone, measures depth); Vickers (diamond pyramid, standard for metals and ceramics); Knoop (elongated pyramid, for thin or brittle materials); Durometer (0–100 scale for polymers); Janka (half-ball penetration, for wood); Pencil test (for coatings); Shore Scleroscope (rebound height test).
Malleability: the ability to be permanently deformed (plastically deformed) by compressive forces (hammering, rolling) without fracturing. Gold is the most malleable metal: it can be hammered into gold leaf 0.1 μm thick.
Elasticity: the ability to return to original shape after the applied force is removed (elastic deformation). Occurs below the elastic limit (yield point). A rubber band is highly elastic.
Plasticity: the ability to permanently deform without fracturing after the elastic limit is exceeded. Essential for forming processes (forging, rolling, drawing). Opposite of brittleness.
Ductility: a specific form of plasticity: the ability to be drawn into wire or stretched into thin sections under tensile (pulling) forces without fracturing. Copper is highly ductile: it can be drawn into fine electrical wire.

Hardness test	Indenter	Best suited to
Brinell	Steel or tungsten carbide ball	Castings and inhomogeneous structures
Rockwell	Steel ball or diamond cone	Quick general-purpose metal testing
Vickers	Diamond pyramid	Metals and ceramics needing comparable values across materials
Knoop	Elongated diamond pyramid	Thin sections or brittle materials
Durometer	Steel rod (0-100 scale)	Polymers and elastomers
Janka	Steel ball (half embedded)	Wood

Mechanical properties feed directly into structural calculations. See A3.2 Structural Systems for how stress, strain and Young's Modulus are used to size structural members and apply safety factors.

Students must be able toExplain why combining materials can create composite materials more suitable for a specific purpose or context, using an example.

Composite materials combine two or more constituents, a matrix (binder/continuous phase) and a reinforcement (dispersed phase), to produce a material with properties superior to either constituent alone. The three main categories are:

1. Particle-reinforced composites: Hard particles distributed in a softer matrix. The particles resist deformation and wear; the matrix transfers loads and holds particles in place.

Concrete: Gravel or crushed stone particles in a cement paste matrix. High compressive strength. Combined with steel rebar (reinforced concrete), it also gains tensile strength.
Cemented carbide (tungsten carbide + cobalt binder): WC particles provide extreme hardness for cutting tools; the cobalt matrix provides toughness to prevent brittle fracture of the tool edge.

2. Fibre-reinforced composites: Fibres embedded in a matrix (typically epoxy resin). Fibres are excellent in tension but cannot resist compression or shear without the matrix. The matrix glues fibres together, transfers load between them, and prevents buckling.

Carbon fibre reinforced plastic (CFRP): Carbon fibres in an epoxy matrix. Used in the Boeing 787 Dreamliner (over 50% composite by weight), high-performance sports equipment, and racing cars. Lighter than aluminium, stiffer and stronger than steel for the same mass.
Fibreglass (GFRP): Glass fibres in polyester or epoxy matrix. Used in boat hulls, car body panels, bathroom fixtures. Lower performance than CFRP but much cheaper.

3. Laminar (layered) composites: Layers of different materials bonded together.

Plywood: Thin wood veneers glued with alternating grain directions, giving balanced strength in both directions and resistance to splitting. Far more stable than solid timber.
Laminated glass: Two glass panes with a polyvinyl butyral (PVB) interlayer. When broken, the PVB holds the fragments together, preventing dangerous flying shards. Used in car windscreens and building façades.
Cardboard: Corrugated paper sandwiched between flat paper liners, forming a structural sandwich panel that uses minimal material but achieves high bending stiffness for its weight.
Sailcloth: Polyester, carbon or Aramid fibres laminated with polymer film in multiple orientations to withstand wind loads from all directions.

CFRP and fibreglass are also common in rapid prototyping and product manufacture. See A2.2 Prototyping Techniques and A4.1 Manufacturing Techniques for how composite parts are actually produced.

Students must be able toExplain how materials can be selected to react to external stimuli, including piezoelectricity, shape memory, photochromicity, magneto-rheostatic, electro-rheostatic and thermoelectricity.

Smart materials respond to a change in their environment (mechanical stress, temperature, light, electric or magnetic field) by significantly and reversibly changing one or more of their properties. This allows products that adapt to conditions without complex external control systems.

Piezoelectric materials: generate an electric charge when mechanically stressed (direct effect), and conversely change shape when an electric current is applied (inverse effect). Applications: ultrasonic transducers for non-destructive testing (a piezoelectric probe sends sound waves into a pipe; returning echoes stress the crystal, generating a voltage that reveals internal flaws); pressure sensors; sonar; medical ultrasound imaging; loudspeakers and microphones.
Shape memory alloys (SMAs): return to a pre-programmed shape when heated after being plastically deformed at a lower temperature. Nitinol (nickel-titanium alloy, discovered at the Naval Ordnance Laboratory) is the most common SMA. Applications: coronary stents (inserted compressed in martensite phase at room temperature, expand to programmed diameter at body temperature in austenite phase, opening blocked arteries without surgery); eyeglass frames that spring back to shape after bending; orthodontic wires that apply continuous gentle force as they return to shape. Pseudo-elasticity (superelasticity): some SMAs return to shape immediately on stress removal at constant temperature (no heating required).
Photochromic materials: darken proportionally to UV light exposure through a reversible photochemical reaction. Applications: photochromic (transition) lenses, where glass lenses contain silver chloride (AgCl) molecules; UV exposure causes a reversible oxidation-reduction reaction forming colloidal silver that absorbs up to 80% of incident light; the lens clears when UV exposure decreases. Polymer lenses use organic photochromic dyes with similar reversible reactions.
Electro-rheostatic (ER) fluids: suspensions of particles that form chain-like structures and become semi-solid (increase in viscosity by several orders of magnitude) when exposed to a strong electric field. The change is nearly instantaneous and fully reversible.
Magneto-rheostatic (MR) fluids: suspensions of iron particles in oil that stiffen dramatically when exposed to a magnetic field. Applications: automotive suspension systems (2002 Cadillac Seville STS; second-generation Audi TT), where turning up the magnetic field thickens the fluid almost instantly, firming up the ride through corners and relaxing again for a comfortable cruise; the technology also shows up in architecture, where Tokyo's National Museum of Emerging Science uses MR dampers throughout the structure to soak up sway caused by typhoon-strength wind and earthquake tremors.
Thermoelectric materials: generate a voltage across a temperature gradient (Seebeck effect), enabling conversion of waste heat to electricity. The reverse (Peltier effect) uses electricity to create a temperature difference for solid-state cooling (no moving parts). Applications: industrial waste heat recovery; wearable sensors powered by body heat; solid-state refrigerators and CPU coolers.

Product Spotlight

NASA's Shape-Memory Tyre

A tyre with no air to lose and a memory of its own shape.

Read spotlight →

Students must be able toExplain how biomaterials are a key part of a circular economy and can be used by designers to design out waste.

Biodegradable materials are broken down by microorganisms (bacteria, fungi, algae) into water, carbon dioxide or methane, minerals, and organic matter: non-toxic substances that can re-enter natural cycles. This contrasts with conventional synthetic plastics, which persist in the environment for hundreds of years.

Examples include natural materials (wood, cotton, wool, paper, food) and engineered biodegradable polymers such as PLA (polylactic acid), derived from corn or sugarcane starch, which is used for biodegradable packaging, cutlery, and medical sutures.

Biodegradable materials and the circular economy: The circular economy's biological cycle depends on biodegradable materials re-entering natural systems safely after use. Designers who specify biodegradable materials are:

Designing out waste: products that biodegrade do not accumulate as persistent pollution in landfills or oceans; they return material value to the soil as nutrients
Closing the loop: materials flow: product → use → composting/biodegradation → soil nutrients → new plant growth → new material → product again
Reducing dependency on finite feedstocks: biodegradable polymers sourced from renewable crops (corn, sugarcane, cassava) reduce reliance on fossil-fuel-derived plastics

Design considerations for biodegradability:

The end-of-life pathway must match the material: some biodegradable plastics (PLA) require industrial composting conditions (high temperature and specific microbial environment) and will not break down in a home compost pile or a landfill within a reasonable timeframe
Products labelled "biodegradable" must be accompanied by clear instructions for correct disposal; otherwise the benefit is not realised
Biodegradable materials must still satisfy functional requirements during the product's useful life: premature degradation from heat, moisture, or UV exposure is a design failure
Using biodegradable materials alongside non-biodegradable components (e.g., a biodegradable package with a plastic window) creates end-of-life complications that defeat the purpose

The circular economy framework introduced here is explored in full in C2.1 Design for Sustainability and C2.2 Circular Economy.

Discussion

Is "compostable" a lie?

A PLA cup stamped "compostable" almost never breaks down in a home compost bin, a landfill, or the ocean within any timeframe that matters. It needs an industrial composting facility: sustained heat above 55°C and a specific microbial mix that most cities don't actually operate at scale. Most "compostable" packaging ends up landfilled anyway, where it behaves close to ordinary plastic.

Is a manufacturer who prints "compostable" on the packaging, technically true but practically misleading, guilty of greenwashing? Where's the line between a genuine design-for-end-of-life decision and a marketing claim that shifts responsibility onto a disposal system that doesn't exist where the product is actually sold?

Ten questions covering all nine learning objectives, from classification and material selection through to composites, smart materials and biodegradable materials. Select one answer per question, then click "Check all answers" to see your score and the explanations.

Q1 · 3.1.1 Classifying Materials

Corrosion resistance is classified as a chemical property rather than a physical one because it:

Physical properties such as density, melting point and thermal conductivity describe what a material is and can be measured without changing it. Chemical properties describe what a material does when it meets another substance, and the reaction alters its composition. Mechanical properties describe what it can withstand under force.

Q2 · 3.1.2 Material Categories

Which statement about the three polymer groups is correct?

Thermoplastics such as polyethylene, polypropylene, ABS and PET soften on heating and can be reshaped, which is what makes them recyclable by remelting. Thermosets such as epoxy and polyester resin cross-link permanently during curing and cannot be remelted. Elastomers such as rubber and silicone are flexible and do return to shape.

Q3 · 3.1.3 Material Selection

A designer plots stiffness against density on an Ashby chart and marks out the region above 50 GPa and below 3000 kg/m³. The purpose of doing this is to:

Each material family occupies a bubble on the chart rather than a single point, because properties vary with processing and composition. Drawing a target zone shows at a glance which families fall inside it, reducing dozens of candidates to a handful worth investigating in detail. Cost, processing and sustainability are then weighed separately.

Q4 · 3.1.4 Physical Properties

A steel rod is heated from 20°C to 300°C. Which physical property determines how much it will grow in length?

The coefficient of thermal expansion gives the fractional change in length per degree of temperature change, so it predicts how far the rod grows. For mild steel it is about 11 × 10⁻⁶ K⁻¹, which is why bridges need expansion joints and pipelines need flexible connectors. Thermal conductivity describes how fast heat travels through the material, not how much it expands.

Q5 · 3.1.5 Chemical Properties

Which statement best explains why stainless steel resists corrosion while mild steel does not?

This is passivation. Chromium reacts with oxygen to form a chromium oxide layer only nanometres thick, which blocks further oxidation and reforms when damaged. Mild steel does not passivate: it forms loose, flaky rust that keeps exposing fresh metal, so degradation continues.

Q6 · 3.1.6 Mechanical Properties

A material with a high Young's modulus is characterised by:

Young's modulus is the ratio of stress to strain in the elastic region, so a high value means the material resists elastic deformation. Steel is around 200 GPa while rubber is under 0.1 GPa. Stiffness is not the same as strength or toughness: a very stiff material can still be brittle and fracture at a low load.

Q7 · 3.1.6 Mechanical Properties

Which hardness test uses a steel or tungsten carbide ball and is best suited to castings and other inhomogeneous structures?

Brinell presses a ball into the surface and leaves a comparatively wide indentation, which averages out local variation in the structure. Vickers uses a diamond pyramid and suits metals and ceramics that need comparable values, Durometer is for polymers and elastomers, and Janka is used for timber.

Q8 · 3.1.7 Composites

Which of the following is a particle-reinforced composite?

Concrete is hard particles distributed through a softer matrix, which is the definition of a particle-reinforced composite, as is cemented carbide. CFRP is fibre-reinforced, while plywood and laminated glass are laminar composites built from bonded layers.

Q9 · 3.1.8 Smart Materials

A coronary stent is inserted in a compressed form and opens to a pre-set diameter once it reaches body temperature. The material responsible is:

Shape memory alloys such as Nitinol return to a pre-programmed shape when heated after being deformed at a lower temperature, which is what lets a stent be delivered through a catheter and expand inside the artery. Piezoelectric materials respond to mechanical stress or voltage, photochromic materials to UV light, and magneto-rheostatic fluids to a magnetic field.

Q10 · 3.1.9 Biodegradable Materials

A cup moulded from PLA is labelled "compostable". Which statement is correct?

PLA is derived from crop starch and does biodegrade, but only under sustained high temperature and the microbial conditions of an industrial composting facility. Where those facilities do not exist, the product is landfilled and the environmental benefit is not realised, which is why a biodegradable specification has to be matched to a real end-of-life pathway and clear disposal instructions.

Every Paper 2 question is attached to a product. Nothing here can be answered from memory alone: read the case study first, then answer the parts in order. The tariff tells you how many creditable points to make, and the command term tells you what kind of point counts. Write your answer before you open either panel, then mark yourself against the markscheme rather than against the example.

Question 1 · A3.1 · SL and HL6 marks

Case study

Cricket bats are made from the timber of the cricket bat willow, a variety of white willow grown for the purpose. A tree is felled at about fifteen years, cut into rounds, split into clefts, and air dried for a year before shaping.

The bat is struck by a hard ball at speeds above 140 km/h. Before use the face is compressed with a mallet over several hours, a process called knocking in.

Table 1: Properties of cricket bat willow compared with two alternatives

Property	Willow	Ash	Aluminium alloy
Density (kg/m³)	380	690	2700
Young's modulus (GPa)	7	12	69
Toughness	High	High	High
Hardness	Low	Medium	High

(a) State the classification of cricket bat willow by source or origin. [1]

(b) Describe why the low density of willow matters to a batter, see Table 1. [2]

(c) Explain why the low hardness of willow is an advantage in a cricket bat rather than a defect, see Table 1. [3]

Example answer

(a) A natural material, specifically a hardwood timber.

(b) At 380 kg/m³ willow is roughly half the density of ash, so a bat of the same size is about half the mass. The batter has to accelerate that mass through a swing in a fraction of a second and then stop it, so a lighter bat can be swung faster and redirected late as the ball deviates, which is what decides whether a shot connects.

(c) Hardness is resistance to indentation, and in a bat some indentation is exactly what is wanted. A soft face deforms locally when the ball strikes, and that deformation spreads the impact over a larger area and a longer time, which lowers the peak force reaching the batter's hands. Knocking in exploits this deliberately: compressing the surface with a mallet forms a dense skin over fibres that remain compliant beneath, giving a face that will not split while still yielding on impact. A hard material would instead return energy sharply and transmit shock into the wrists, and because willow is low in hardness but high in toughness it absorbs the blow by deforming rather than cracking, which is the combination that lets a bat survive thousands of impacts.

Markscheme

(a) • Natural material ✓
• Timber / hardwood ✓

Award [1] for the correct classification by source or origin up to [1 max].

(b) Physical properties include aspects of a material that can be measured and observed without it changing in any way.
• At 380 kg/m³ willow is about half the density of ash, so a bat of equal size is about half the mass ✓
• A lighter bat is accelerated faster through the swing for the same effort ✓
• A lighter bat can be redirected late as the ball deviates ✓
• Lower mass reduces fatigue over a long innings ✓
• A large hitting face can be provided without the bat becoming unmanageable ✓
• Bat mass is regulated in play, so low density buys volume within the mass allowed ✓

Award [1] for each detail, leading to an account of why low density matters to a batter, up to [2 max]. Credit responses that quote a value from Table 1.

(c) Mechanical properties include aspects of a material affected by the application of a force. Hardness is resistance to indentation or abrasion; toughness is the ability to absorb energy without fracturing.
• A soft face deforms locally at the point of impact ✓
• Local deformation spreads the impact over a larger area and a longer time ✓
• Spreading the impact lowers the peak force transmitted to the hands and wrists ✓
• Knocking in compresses the surface into a dense skin over fibres that stay compliant beneath ✓
• The compressed skin resists splitting while the material below still yields on impact ✓
• A harder face would return energy sharply and transmit shock into the wrists ✓
• Willow is low in hardness but high in toughness, so it deforms rather than cracks ✓
• Aluminium alloy at high hardness and 69 GPa would be unforgiving on a mistimed shot ✓
• The trade-off is a face that marks and dents in use, which is accepted as normal wear ✓

Award [1] for each relevant reason / cause explaining why low hardness is an advantage in a cricket bat up to [3 max]. Award a maximum of [2] where the response does not distinguish hardness from toughness.

Question 2 · A3.1 · SL and HL6 marks

Case study

A fire door is fitted with an intumescent seal, a strip set into a groove in the door edge. In normal use it is a thin, hard, inert strip and the door swings freely against a 3 mm gap.

Above about 200 °C the strip expands to many times its original volume and fills the gap, sealing the door into its frame so that smoke and flame cannot pass. The expansion is irreversible: once triggered the seal is spent and must be replaced.

Table 2: Behaviour of the intumescent seal

Condition	Seal state	Gap
Ambient, 20 °C	Rigid strip, 2 mm thick	3 mm
Fire, above 200 °C	Expanded char, up to 30 mm	0 mm
After cooling	Remains expanded, friable	0 mm

(a) State the classification of the intumescent seal as a material type. [1]

(b) Outline why the seal's response must be irreversible for it to work as a fire barrier, see Table 2. [2]

(c) Analyse the requirement for a 3 mm gap around a door that must nevertheless close completely in a fire, see Table 2. [3]

Example answer

(a) A smart material.

(b) A fire door must hold back smoke for thirty minutes or more, and the air at the door edge does not stay above 200 °C throughout, so a seal that contracted as the local temperature fluctuated would open and close the gap during the fire. The expanded char also has to survive the door being hosed and cooled while people are still evacuating behind it, which a reversible material would not do.

(c) The two requirements pull in opposite directions and the seal resolves them by changing state. A door needs clearance to function at all: a timber door swells and shrinks with humidity, the frame moves as the building settles, and a door fitted with no gap would bind and be left propped open, which defeats the whole purpose. So the gap is a functional necessity in the ninety-nine per cent of the door's life when there is no fire. In a fire the same gap becomes the failure path, because smoke passes through a 3 mm slot readily and smoke kills before flame does. A fixed material cannot satisfy both conditions, since anything sized to seal the gap would prevent the door closing. The intumescent strip works because its geometry is not fixed: it is thin and hard while the door is in use and expands fifteen-fold only when the condition that requires sealing is present. The material's response is triggered by the hazard itself, so the door needs no detection system, no power and no moving parts to seal.

Markscheme

(a) • Smart material ✓

Award [1] for the correct classification up to [1 max]. Accept thermally responsive or reactive material.

(b) Smart materials have one or more properties that change significantly in response to changes in their environment.
• A fire door must hold back smoke for thirty minutes or more ✓
• Temperature at the door edge fluctuates during a fire rather than staying above 200 °C ✓
• A reversible seal would contract and reopen the gap whenever the local temperature dropped ✓
• The char must survive the door being hosed and cooled while evacuation continues ✓
• An irreversible change means the seal cannot fail back to its open state ✓
• A spent seal is visible evidence that the door has been exposed and must be replaced ✓

Award [1] for each relevant brief point on why irreversibility is required up to [2 max].

(c) Smart materials respond to a change in their environment, which allows one component to satisfy conflicting requirements.
The gap is necessary:
• A door needs clearance to swing without binding ✓
• Timber swells and shrinks with humidity, so the gap absorbs dimensional change ✓
• Building movement and settlement alter the frame over time ✓
• A binding door is propped open by users, which defeats the fire door entirely ✓
The gap is the hazard:
• Smoke passes readily through a 3 mm slot ✓
• Smoke inhalation causes most fire deaths, so the smoke path matters more than the flame path ✓
• The gap runs the full perimeter, so the total open area is substantial ✓
The resolution:
• No fixed-geometry material can satisfy both, since anything sized to seal would stop the door closing ✓
• The intumescent strip changes geometry, thin in use and expanded fifteen-fold in fire ✓
• The trigger is the hazard itself, so no detector, power supply or mechanism is needed ✓
• A passive response cannot fail through lack of maintenance or power ✓
• Expansion to 30 mm against a 3 mm gap gives a large margin for a door that has warped ✓

Award [1] for each distinct guiding element / structure identified in the conflict between the two requirements up to [3 max]. Award a maximum of [2] where the response addresses only one side of the conflict.

Question 3 · A3.1 · SL and HL10 marks

Case study · part 1

An espresso machine forces water at 93 °C and nine times atmospheric pressure through a bed of ground coffee. The coffee sits in a basket held in a portafilter, a handled fitting that twists into the machine's group head and is removed and knocked out after every shot.

Portafilters are normally made from brass, often chrome plated. A café makes around 300 shots a day.

(a) Identify two mechanical properties required of the portafilter body. [2]

Case study · part 2

The portafilter is left locked into the machine between shots so that it stays hot. If it cools, the first water through it drops in temperature and the shot is under-extracted and sour.

Table 3: Properties of three candidate portafilter materials

Property	Brass	Stainless steel	Aluminium alloy
Density (kg/m³)	8500	7900	2700
Specific heat capacity (J/kg K)	380	500	900
Thermal conductivity (W/m K)	120	16	170
Corrosion resistance in hot water	Good, leaches with acid	Excellent	Poor without anodising

(b) Outline why brass holds its temperature well between shots, see Table 3. [2]

Case study · part 3

Coffee is mildly acidic. Brass is an alloy of copper and zinc, and prolonged contact with hot acidic water can leach zinc from the surface. Chrome plating prevents this, but the plating wears at the edges where the portafilter is knocked against a bar to empty it.

(c) Describe the chemical property of brass that requires it to be plated, see Table 3. [2]

Case study · part 4

A manufacturer proposes replacing brass with stainless steel, unplated, in a portafilter of the same external dimensions. The handle is walnut on both versions.

(d) Explain the consequences of changing from plated brass to unplated stainless steel, see Table 3. [4]

Example answer

(a) Strength, to contain nine atmospheres of pressure, and toughness, to survive being knocked against a bar 300 times a day.

(b) Brass is dense at 8500 kg/m³, so a portafilter of a given size is heavy and holds a large mass of metal. Although its specific heat capacity is the lowest of the three, that mass means the total heat stored is high, and a large store of heat cools slowly, so the metal is still near brewing temperature when the next shot is pulled.

(c) Brass is an alloy of copper and zinc, and zinc is the more reactive of the two, so in prolonged contact with hot acidic water it is preferentially attacked and dissolves out of the surface. That reactivity leaves the remaining copper structure weak and porous and puts zinc into the drink, which is why a barrier layer of chrome is plated over the brass.

(d) The change fixes one problem and creates several others.

Corrosion improves outright. Stainless steel is excellent in hot water and does not leach, so the plating becomes unnecessary. That removes the failure the plated brass has, where chrome wears through at the knock-out edge and exposes brass exactly where the acid and abrasion are worst, and it removes a plating stage from manufacture.

Thermal behaviour changes in two ways that partly cancel. Stainless steel is slightly less dense at 7900 kg/m³, but its specific heat capacity is 500 J/kg K against brass at 380, so for the same volume it actually stores more heat and would hold temperature at least as well between shots. Against that, its thermal conductivity is 16 W/m K against 120, roughly seven times lower, so heat moves through it slowly. The portafilter will take far longer to come up to temperature from cold at the start of service, and once hot it will be unevenly heated, warm where it contacts the group head and cooler at the spouts. A café can work around this by leaving the machine on, but the first shots of the day will be inconsistent.

There are secondary effects. Stainless steel is harder to machine than brass, so unit cost rises even with the plating removed. The unplated surface will look and feel different from chrome, which matters on a component the customer sees. On balance the change is worth making for durability and for removing the leaching risk, provided the machine is left on, but it is not a straight substitution.

Markscheme

(a) Mechanical properties include aspects of a material affected by the application of a force.
• Strength ✓
• Toughness ✓
• Hardness ✓
• Stiffness ✓
• Fatigue resistance ✓
• Ductility ✓

Award [1] for each relevant mechanical property up to [2 max]. Do not credit density, thermal conductivity or corrosion resistance, which are not mechanical properties.

(b) Physical properties include density, thermal conductivity and specific heat capacity.
• Brass is dense at 8500 kg/m³, so a portafilter of a given size holds a large mass of metal ✓
• Heat stored is the product of mass and specific heat capacity, so mass compensates for the lower capacity ✓
• A large store of heat falls in temperature slowly for a given rate of heat loss ✓
• The metal is therefore still near brewing temperature when the next shot is pulled ✓
• High thermal conductivity at 120 W/m K keeps the whole body at an even temperature ✓
• Leaving it locked in the group head lets the machine continue to supply heat ✓

Award [1] for each relevant brief point on why brass holds temperature well up to [2 max]. Credit responses that reason from density and specific heat capacity together.

(c) Chemical properties include aspects of a material that lead to it chemically reacting with another.
• Brass is an alloy of copper and zinc ✓
• Zinc is the more reactive of the two constituents ✓
• Hot acidic water preferentially attacks and dissolves the zinc from the surface ✓
• This is dezincification, a form of selective corrosion ✓
• The remaining copper structure is left weak and porous ✓
• Zinc enters the drink, which is a contamination and food safety issue ✓
• Chrome plating provides a barrier that prevents contact between brass and the coffee ✓

Award [1] for each detail, leading to an account of the chemical property requiring plating, up to [2 max]. The response must refer to reactivity or corrosion for full marks.

(d) Identifying the most suitable material is a complex task involving physical, chemical and mechanical properties and aesthetic characteristics.
Corrosion, improved:
• Stainless steel has excellent corrosion resistance in hot water and does not leach ✓
• Plating becomes unnecessary, removing a manufacturing stage ✓
• Removes the failure mode where chrome wears at the knock-out edge and exposes brass ✓
• The wear point is where acid and abrasion are worst, so this is the plated design's weakest feature ✓
• No zinc can enter the drink, removing a food safety concern ✓
Heat storage, comparable or better:
• Density falls slightly, 7900 against 8500 kg/m³ ✓
• Specific heat capacity rises from 380 to 500 J/kg K ✓
• For the same volume the steel body stores more heat, so it holds temperature at least as well ✓
Heat transfer, worse:
• Thermal conductivity falls from 120 to 16 W/m K, roughly seven times lower ✓
• The portafilter takes much longer to reach working temperature from cold ✓
• Heat spreads unevenly, warm at the group head and cooler at the spouts ✓
• First shots of the day will be inconsistent unless the machine is left on ✓
Secondary effects:
• Stainless steel is harder to machine than brass, raising unit cost despite the plating saving ✓
• The unplated surface differs in appearance and feel from chrome on a visible component ✓
• Slightly lower mass alters the balance against the walnut handle ✓
Judgment:
• Worth making for durability and safety provided the machine stays on ✓
• Not a straight substitution, because conductivity changes the warm-up behaviour ✓

Award [1] for each relevant detail / reason / cause relating to the consequences of the material change up to [4 max]. Award a maximum of [3] where the response addresses only corrosion. Credit responses that distinguish heat storage from heat transfer.

MatWeb material property database

matweb.com

Free searchable database of density, Young’s modulus, tensile strength and thermal conductivity for thousands of metals, polymers, ceramics and composites. Use it when you need a real number for a real material.

The Efficient Engineer, YouTube channel

youtube.com/c/TheEfficientEngineer

Animated explanations of stress strain curves, Young’s modulus, toughness and hardness testing. The clearest free videos on the mechanical properties in 3.1.6.

Boeing 787 Dreamliner, Wikipedia

en.wikipedia.org/wiki/Boeing_787_Dreamliner

The airframe is roughly half carbon fibre composite by weight. Covers why Boeing chose composites over aluminium, and the production problems that came with the decision.

Indentation hardness, Wikipedia

en.wikipedia.org/wiki/Indentation_hardness

Brinell, Rockwell and Vickers side by side: indenter shape, applied load, and how each number is arrived at. Supports the hardness testing in 3.1.6.

Nickel titanium (Nitinol), Wikipedia

en.wikipedia.org/wiki/Nickel_titanium

The shape memory alloy behind stents and orthodontic wires, including its accidental discovery at the Naval Ordnance Laboratory. Supports 3.1.8.

Ultrasonic testing, Wikipedia

en.wikipedia.org/wiki/Ultrasonic_testing

How pulse echo testing finds a flaw inside a part without cutting it open, and how a piezoelectric crystal turns the returned echo into a voltage.

Photochromic lens, Wikipedia

en.wikipedia.org/wiki/Photochromic_lens

The reversible silver chloride reaction in glass lenses and the organic dyes used in plastic ones. A smart material you can buy in a shop.

Magnetorheological fluid, Wikipedia

en.wikipedia.org/wiki/Magnetorheological_fluid

A fluid that stiffens within milliseconds in a magnetic field, used in adaptive car suspension and building dampers. Supports 3.1.8.

Linking Questions

Why is a good understanding of material properties important when designing structural systems? (A3.2)
When do the physical properties of materials restrict the ability to use certain prototyping techniques? (A2.2)
How do the properties of a material influence the choice of manufacturing techniques for a product? (A4.1)
How could the continued development of biodegradable materials influence designers' ability to address sustainability and circular economy? (C2.1) (C2.2)
Why is a thorough understanding of materials key for effective product analysis and evaluation? (C3.1)
How do design decisions related to materials impact a product's life-cycle analysis? (C3.2)

Material Classification & Properties | A3.1

Course Notes

Materials are classified based on their physical, chemical and mechanical properties.

Metals: ferrous and non-ferrous

Composites: reinforcement and matrix

Materials are classified according to their source or origin: natural and human-made, including timbers, polymers, metals, glass, textiles, composites, smart materials and biomaterials.

Identifying the most suitable material for a product is a complex task, involving physical, chemical and mechanical properties and aesthetic characteristics.

Concept: Ashby Charts

Physical properties include aspects of a material that can be measured and observed without it changing in any way.

Chemical properties include aspects of a material that lead to it chemically reacting with another.

Mechanical properties include aspects of a material affected by the application of a force.

Composites consist of two or more materials combined to enhance their properties.

Smart materials are materials that have one or more properties that can be significantly changed in response to changes in their environment.

NASA's Shape-Memory Tyre

Biodegradable materials break down in the environment after disposal or at the end of their useful life.

Quiz

Paper 2 Practice

References & Further Reading