Metal atoms release their outer electrons into a shared "sea" of delocalised electrons surrounding positive metal ions. This gives metals their characteristic properties: good electrical and thermal conductivity (free electrons carry charge and heat), ductility (ion layers can slide past each other without breaking the bond), and a lustrous, opaque appearance.

Electrons transfer completely from a metal atom to a non-metal atom, forming oppositely charged ions held together by strong electrostatic attraction. Ionic solids are hard and brittle — displacing one layer of ions causes like charges to repel, fracturing the material — and are usually electrical insulators in the solid state.

Atoms share pairs of electrons to form strong, directional bonds. This produces very hard, high-melting-point solids such as diamond, and also forms the backbone chain of every polymer — the carbon-carbon bonds along a polymer chain are covalent, even though the chains themselves are held to each other much more weakly.

Weak, temporary attractions between molecules or polymer chains. They are far weaker than metallic, ionic or covalent bonds, which is why polymers soften and melt at much lower temperatures than metals or ceramics — heating the material only needs to overcome these weak inter-chain forces, not break the covalent backbone.

Crystalline, metallically bonded. Sub-divided by crystal structure (BCC / FCC / HCP) which controls ductility, and by alloying, which controls strength, corrosion resistance and hardness. Good conductors of heat and electricity.

Thermoplastics (e.g. polyethylene, nylon) are linear or branched chains held together by weak Van der Waals forces — they soften on heating and can be re-melted and re-shaped. Thermosets (e.g. epoxy, bakelite) form permanent covalent cross-links between chains during curing — they char and decompose rather than melt, and cannot be reshaped. The proportion of ordered, crystalline regions within a thermoplastic (its crystallinity) increases stiffness, strength and density but reduces impact toughness and optical clarity.

A combination of a matrix (usually a polymer, metal or ceramic that binds the structure and transfers load) and a reinforcement (fibres or particles that carry most of the load). Fibre composites such as carbon-fibre-reinforced polymer (CFRP) give very high strength and stiffness for very low weight; particulate composites such as concrete are cheaper and easier to form into shape.

Crystalline compounds (usually a metal bonded to a non-metal, e.g. aluminium oxide, silicon carbide) held together by strong ionic and/or covalent bonds with almost no free electrons or slip planes available. This gives very high hardness, very high compressive strength, and excellent resistance to heat and chemical attack — but almost no ductility, so ceramics are brittle and fail suddenly rather than yielding. Used for cutting tool tips, refractory linings and electrical insulators.

Mass per unit volume. A key driver of the strength-to-weight and stiffness-to-weight ratios that make aluminium alloys and composites attractive for aerospace structures over denser steels.

The temperature at which a material changes from solid to liquid, set by the strength of its atomic or molecular bonding. Governs maximum safe service temperature and casting/joining processes.

How readily a material transfers heat through itself. Metals conduct heat well due to free electrons; polymers and ceramics are generally poor conductors, making them useful as insulators.

The fractional change in dimensions per degree of temperature change. Mismatched expansion between joined materials creates thermal stress — a key consideration in bonded or riveted assemblies.

Strength is the maximum stress a material can withstand — yield strength marks the onset of permanent (plastic) deformation, ultimate tensile strength the maximum reached before fracture. Stiffness is measured by the Young's Modulus, E — the gradient of the elastic (straight-line) portion of the graph. A stiffer material deforms less under a given load.

Ductility is the percentage elongation at fracture; toughness is the total area under the stress-strain curve, representing the energy absorbed before failure.

Fatigue describes failure under repeated cyclic loading, well below the static strength, plotted on an S-N curve. Hardness measures resistance to localised surface indentation or scratching.

When a real material is loaded and then unloaded, the unloading curve does not always exactly retrace the loading curve. The small loop this forms represents mechanical energy converted to heat inside the material through internal friction. Rubber shows a large, fat hysteresis loop — useful in vibration mounts and dampers. Spring steel shows a very narrow loop — almost all the stored elastic energy is returned, exactly what is needed in a suspension spring.

Metals conduct electricity well because their delocalised electrons move freely under an applied field; insulators have tightly bound electrons and a large energy gap, giving very high resistivity.

A measure of how easily a material's internal charges polarise in response to an applied electric field — important for selecting dielectric materials used in capacitors and insulation.

The maximum electric field an insulating material can withstand before it breaks down and begins to conduct — a key design limit for cable and transformer insulation.

µ = B ÷ H describes how strongly a material becomes magnetised for a given applied field H. Ferromagnetic materials (iron, nickel, cobalt and their alloys) have very high permeability.

When a ferromagnetic material is taken through a full cycle of magnetisation and demagnetisation, the flux density B lags behind the applied field H, tracing the loop shown in Fig 15. Soft magnetic materials (e.g. silicon steel, used in transformer and motor cores) have a narrow loop, low coercivity and low hysteresis loss. Hard magnetic materials (e.g. alnico, ferrite magnets) have a wide loop, high coercivity and high remanence — exactly what is needed to make a permanent magnet that resists demagnetisation.

Iron reacts with oxygen and moisture to form hydrated iron oxide (rust), which is porous and flakes away, continually exposing fresh metal beneath. Stainless steel and aluminium instead form a thin, adherent, self-healing oxide layer (passivation) that seals the surface and dramatically slows further attack.

When two dissimilar metals are in electrical contact in the presence of an electrolyte (e.g. rainwater), the more reactive (less noble) metal becomes the anode and corrodes preferentially, while the less reactive (more noble) metal is protected. This is why steel bolts in an aluminium bracket, or a copper pipe joined directly to a steel pipe, corrode faster than expected.

Solvent & chemical attack: some solvents can penetrate between polymer chains, causing swelling, softening or dissolving (e.g. many hydrocarbons attack polyethylene). Thermosets, with their rigid cross-linked structure, generally resist solvents better than thermoplastics.

UV & ozone attack: ultraviolet light and ozone can break polymer backbone bonds, causing surface cracking (crazing) and a gradual loss of strength and flexibility — the reason outdoor plastics fade and become brittle over time.

Flammability: most polymers are combustible to some degree; thermosets tend to char rather than melt and drip, which is one reason they are often chosen where fire resistance matters (e.g. electrical enclosures).

Tensile test — a sample is stretched to fracture to find Young's Modulus, yield strength, UTS and ductility. Hardness test (Brinell, Vickers, Rockwell) — an indenter is pressed into the surface under a known load to find resistance to indentation. Impact test (Charpy/Izod) — a notched sample is struck by a swinging pendulum to measure the energy absorbed before fracture, indicating toughness. Fatigue test — a sample is cycled repeatedly to find how many cycles it survives at a given stress. Creep test — a sample is held under constant load at elevated temperature to measure slow deformation over time.

Visual inspection — the simplest NDT method, looking for obvious surface defects. Dye penetrant — a coloured or fluorescent liquid seeps into surface-breaking cracks and is drawn back out by a developer, making them visible. Magnetic particle inspection — a ferromagnetic part is magnetised and iron particles cluster at surface/near-surface cracks where the magnetic field leaks. Ultrasonic testing — high-frequency sound pulses reflect off internal flaws, revealing their depth and size. Radiography — X-rays or gamma rays pass through the part onto film or a detector, showing internal defects such as porosity or inclusions as density differences.

Uniform corrosion — even attack across the whole surface (general rusting). Pitting corrosion — localised attack producing small deep holes, often under a damaged passive layer. Crevice corrosion — accelerated attack in narrow gaps (under washers, gaskets) where oxygen and electrolyte become trapped. Galvanic corrosion — accelerated attack where dissimilar metals are joined. Stress corrosion cracking — cracks that grow under the combined action of tensile stress and a specific corrosive environment, sometimes with almost no visible warning before failure. Fatigue — progressive cracking from repeated cyclic loading, starting at a stress concentration.

UV/ozone degradation — sunlight and ozone break polymer backbone bonds, causing surface crazing and embrittlement. Hydrolysis — water molecules attack susceptible chemical bonds (e.g. in some polyesters), reducing molecular weight and strength. Chemical/solvent attack — solvents swell, soften or dissolve susceptible polymers. Thermal degradation — excessive heat breaks polymer chains, reducing mechanical properties even without melting. Creep — because polymer chains can slide past one another over time, polymers are especially prone to slow deformation under sustained load, even at room temperature.

Ceramics resist chemical and electrochemical attack almost completely, so they rarely "degrade" the way metals or polymers do. Their real service risks are mechanical: brittle fracture — sudden, catastrophic cracking with little or no plastic warning, usually starting from a small pre-existing surface flaw. Thermal shock — a rapid temperature change creates internal stresses faster than the rigid structure can accommodate, cracking the part even though the material itself has not chemically changed.

Plotted as stress against strain during one loading-unloading cycle. The loading and unloading curves do not retrace each other exactly — the small gap between them (the loop) represents mechanical energy converted to heat.

Plotted as flux density (B) against magnetic field strength (H) as a ferromagnetic material is magnetised and demagnetised. The loop area represents energy dissipated as heat per cycle.

Natural rubber — a very large loop; most of the input energy is dissipated as heat each cycle, which is exactly why it is chosen for vibration-damping engine mounts. Silicone rubber — a moderate loop, giving useful damping while staying more stable across a wider temperature range than natural rubber. Spring steel — a very narrow, almost imperceptible loop; nearly all the input energy is returned elastically each cycle, which is essential in a return spring or a clock mechanism where energy loss would be wasteful.

Silicon (electrical) steel — a narrow loop, engineered specifically to minimise hysteresis loss, which is why it is the standard core material in transformers and motors. Ferrite — an even narrower loop with low electrical conductivity, chosen for high-frequency inductor cores where eddy-current loss must also be minimised. Alnico / neodymium (NdFeB) — a very wide loop with high coercivity; once magnetised, these hold their magnetism strongly, which is exactly the point of a permanent magnet.

What must the component actually do? Carry load, insulate, conduct, resist wear, look a certain way — the function drives every other criterion.

Temperature range, humidity, chemical exposure, UV exposure, and the type of loading (static, cyclic, impact) the component will see in service.

How readily the material can be cast, formed, machined, welded, bonded or fastened into the finished assembly with the processes available.

Raw material cost, processing cost, and whether the material and the required grades/sizes are reliably available in the quantities needed.

Embodied energy, recyclability, and the material's impact across its whole life cycle, from extraction through to end-of-life disposal or recycling.

Corrosion resistance and chemical stability alongside density, conductivity, strength, stiffness and toughness — covered in detail earlier in this unit.