Material classification

By their physical, chemical and mechanical properties.

Measure it and put it back. If the material is unchanged the property is physical; if it reacted and was altered, it is chemical.

It lets a designer eliminate candidates systematically, and it lets unlike materials be compared on the same measured terms using published data rather than testing everything.

As natural (timbers, textiles, biomaterials) or human-made (metals, polymers, glass, composites, smart materials).

One that carries its loads through a skeleton of linear members, with the spaces between open or non-structural. A bicycle, a roof truss, a tent.

One where the outer skin itself carries the load, usually because it is curved. It encloses and protects while using very little material for the volume it contains.

Because real products use each structure where its advantage matters — a car has a frame chassis, shell body panels and solid brake discs.

Because the requirements conflict — light fights heat-resistant, warm-looking fights damp-proof — so no candidate satisfies every one and the designer must decide which matter most.

Physical, chemical and mechanical properties, plus aesthetic characteristics. The first three are measured; aesthetics are judged against the brief.

Safety requirements — food contact, flammability, electrical insulation — anything that would make the product fail in use, and anything a standard requires.

One that can be measured or observed without the material changing in any way.

Density, thermal expansion, thermal conductivity, melting point, electrical resistivity and electrical conductivity.

Mass per unit volume, in kg/m³ or g/cm³. It decides how heavy a product is for its size.

Bridges need expansion joints; a metal lid on a glass jar loosens under hot water; two bonded materials with different expansion rates warp or crack.

An aspect of a material that leads to it chemically reacting with another substance — so the material is altered by the encounter.

Corrosion resistance, reactivity (food safe), hygroscopy and flammability.

Its chromium reacts with oxygen to form a thin, self-repairing oxide film. Scratch it and the film reforms, so corrosion cannot get started.

The material absorbs moisture from the air. Timber swells and warps, nylon softens and cardboard loses strength — which is why some polymers must be dried before moulding.

An aspect of a material affected by the application of a force.

Strength is the force needed to break it; stiffness is how much it bends before then. A diving board is strong and deliberately not stiff.

Hardness resists scratching and wear at the surface; toughness is the energy absorbed before fracture. Glass is very hard and not at all tough.

Malleability is being hammered or rolled into SHEET; ductility is being drawn out into WIRE.

Elastic deformation returns to the original shape when the load is removed; plastic deformation stays. A bent paperclip has deformed plastically.

Two or more materials combined to enhance their properties — a matrix that gives shape and a reinforcement that carries the load.

The matrix gives shape, surface and protection, and transfers load into the fibres so they cannot buckle. The reinforcement carries the load and stops the matrix cracking.

GRP: glass fibre in polyester resin. Carbon fibre: carbon fibre in epoxy. Plywood: cross-bonded veneers in adhesive. Reinforced concrete: steel bars in concrete.

They cost more and are labour-intensive to lay up, they fail suddenly rather than bending first, and they are very hard to recycle because the constituents cannot be separated.

A material with one or more properties that can be significantly changed in response to changes in its environment.

Piezoelectricity, shape memory, photochromicity, magneto-rheostatic, electro-rheostatic and thermoelectricity.

A squeeze or vibration produces a voltage — and it runs backwards too: apply a voltage and the material moves. Used in gas-lighter sparks and ultrasound probes.

Because it replaces a mechanism. The material is both sensor and actuator, so there are fewer parts, no wiring and much less to wear out.

One that breaks down in the environment after disposal, or at the end of its useful life, into substances that rejoin natural cycles — water, carbon dioxide and biomass.

Heat, moisture and oxygen. Those exist in an industrial composter and not in a sealed landfill or the sea, so the label alone guarantees nothing.

They close the biological loop: grow the feedstock, make the product, use it, compost it — and its nutrients grow the next crop. Waste is designed out rather than managed.

For a durable product that will be collected and recycled properly. Biomaterials win where collection is unrealistic — packaging, single-use items, things used outdoors.