Material Classification & Properties | A3.1

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

Text Adventure Game

Central Middlezhong Adventure

Help the people of Central Middlezhong to build new structures and demonstrate your knowledge of material classification and selection criteria. →

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)

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.

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.

Interactive Tool

Ashby Chart Game

Plot and compare real materials on logarithmic property charts (strength vs. density, stiffness vs. cost) the same way materials engineers select candidate materials for a design. Try it →

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.

Interactive Tool

Infinite Composites

A materials combination game, in the spirit of Neal Agarwal's Infinite Craft: combine matrix and reinforcement materials to discover new composites and learn a few things along the way. →

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.

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