Module 14: Materials

Steel, concrete, wood: what things are made of and why

Part A · the three properties that define every material

Strength

How much force before it permanently deforms or breaks. Measured in megapascals (MPa): force per area.

Rubber~10 MPa

Concrete (compression)~30 MPa

Aluminium alloy~270 MPa

Structural steel~400 MPa

Carbon fibre composite~1,500 MPa

Stiffness (Young's modulus)

How much it resists being stretched or compressed. A stiff material bends little under load. Measured in GPa.

Rubber~0.01 GPa

Wood (along grain)~10 GPa

Concrete~30 GPa

Aluminium~69 GPa

Steel~200 GPa

Toughness vs brittleness

Tough = absorbs energy before fracture (steel, rubber). Brittle = breaks suddenly with little warning (glass, ceramic, cast iron).

Key insight: High strength ≠ tough. Diamond is the hardest known natural material but extremely brittle: hit it with a hammer and it shatters. Steel is less hard but far tougher.

Stress-strain curves: what they actually look like

Each curve tells the full story of a material from gentle loading to catastrophic failure.

Stress-strain curves for four common materials. Glass fractures with almost no plastic deformation: brittle. Steel has a clear yield point then a long plastic zone before failure: tough. Rubber stretches enormously before breaking.

Part B · the main materials: why we use each one

Steel backbone of civilisation

Density

7,850 kg/m³

Heavy: 7.85× denser than water

Tensile strength

~400–800 MPa

Varies widely by alloy and heat treatment

Melting point

~1,370–1,540°C

Harder to work than aluminium, but much stronger

Why steel: Strong, tough, weldable, and cheap. Used in buildings, bridges, ships, cars, railways, appliances, and tools. The great workhorse. Downside: heavy and rusts without protection (stainless steel adds chromium to prevent this). The world produces ~1.9 billion tonnes of steel per year, more than all other metals combined.

Aluminium the lightweight champion

Density

2,700 kg/m³

About ⅓ the weight of steel

Strength-to-weight

~100–270 MPa

Weaker than steel in absolute terms, but competes well per kg

Key property

Corrosion-resistant

Forms a natural oxide layer: no rust

Why aluminium: Weight matters above all else: aircraft, cars, bikes, drink cans, laptops. Aluminium makes up roughly two-thirds of a classic jetliner's structural weight. Costs more than steel to produce (energy-intensive smelting) but infinitely recyclable at ~5% of the original energy cost. Pure aluminium wasn't isolated until 1825: so rare and new that it was briefly more valuable than gold. Napoleon reportedly served honoured guests on aluminium plates while gold and silver were left for ordinary guests.

Concrete strongest in compression, weakest in tension

Compressive strength

~25–50 MPa

Excellent: handles being squashed

Tensile strength

~3–5 MPa

Very poor: cracks easily when pulled or bent

Why reinforce it?

Steel rebar inside

Steel handles tension; concrete handles compression. Together: reinforced concrete, the world's most-used building material.

Why concrete: Cheap, mouldable into any shape, fire-resistant, and superb under compression. Buildings, dams, roads, bridges. The Romans invented hydraulic concrete 2,000 years ago: their Pantheon dome, unreinforced, still stands because its shape keeps every part in pure compression. Modern reinforced concrete was pioneered in the 1850s. The world uses ~4 billion tonnes per year, more than any other manufactured material.

Carbon fibre (composite) strongest per kg, but expensive

Density

~1,600 kg/m³

Half the weight of aluminium; ¼ of steel

Tensile strength

~1,500–3,000 MPa

3–7× stronger than steel by weight

Cost

~€20–100/kg

vs steel at ~€0.50/kg, 40–200× more expensive

Why carbon fibre: When weight is critical and cost is secondary: F1 cars, aircraft (the Boeing 787 Dreamliner is ~50% carbon fibre by weight), high-end bikes, sports equipment, spacecraft. Cannot be welded; must be bonded with adhesives. Brittle: fails suddenly with no visible warning deformation. Not easily recyclable. The future challenge is driving cost down far enough for mass-market vehicles.

Glass amorphous solid: neither liquid nor crystal

Compressive strength

~700–1,000 MPa

Stronger than steel in compression!

Tensile strength (practical)

~7 MPa

Surface scratches cause catastrophic failure, much weaker in practice than theory suggests.

Young's modulus

~70 GPa

As stiff as aluminium, but brittle

Why glass: Transparent, impermeable, chemically inert, hard. Windows, bottles, phone screens, optical fibres. Tempered glass is heated and rapidly cooled, creating surface compression that requires far more force to crack, and when it does, it shatters into small blunt pieces rather than sharp shards. Gorilla Glass (phone screens) achieves the same effect chemically through ion exchange.

Wood natural composite: underrated by engineers

Density (oak)

~700 kg/m³

About ¼ the weight of steel

Tensile strength (along grain)

~80–120 MPa

Surprisingly strong, but direction-dependent (anisotropic)

Key quirk

Anisotropic

Up to 10× stronger along the grain than across it, entirely unlike metal

Why wood: Cheap, renewable, thermally insulating, workable with simple tools, and beautiful. Still used in construction (timber framing, CLT skyscrapers), furniture, and musical instruments. Wood is a natural composite: long cellulose fibres (strong in tension) embedded in a lignin matrix (holds shape). The same fibre-in-matrix logic as carbon fibre, just grown instead of manufactured. Cross-laminated timber (CLT) stacks layers at 90° to overcome the grain-direction weakness: buildings up to 18 storeys tall have been built from it.

Polymers (plastics) the shape-shifters

Density range

~900–1,400 kg/m³

Lighter than all metals. Some (PE, PP) float in water.

Strength range

~20–100 MPa

Wide range: engineering plastics (nylon, PEEK) approach aluminium

Key advantage

Mouldable + cheap

Injection moulding can produce millions of identical complex parts cheaply

Why plastics: Cheap, light, corrosion-proof, electrically insulating, infinitely mouldable. Packaging, pipes, clothing, electronics, medical devices. The downside: durability that was an engineering triumph is an environmental catastrophe: most plastics persist for 400–1,000 years in the environment. The world produces ~400 million tonnes per year; approximately 91% has never been recycled (Geyer et al., 2017).

Part C · strength-to-weight: the real comparison

Specific strength (kN·m/kg): how strong per kilogram

This is what engineers care about for vehicles and aircraft, not absolute strength, but strength per unit weight.

Carbon fibre composite

~938 kN·m/kg: best practical structural material

Spider silk

~800 kN·m/kg: extraordinary

Titanium alloy

~500 kN·m/kg

Aluminium alloy

~290 kN·m/kg

High-strength steel

~180 kN·m/kg

Structural steel

~50 kN·m/kg

Wood (oak, along grain)

~40 kN·m/kg

Concrete

~10 kN·m/kg

Spider silk rivals carbon fibre per gram: the challenge is producing it at scale. Spiders are territorial and cannibalistic, making farming impossible. Researchers are using genetically modified bacteria and silkworms to synthesise silk proteins.

Material radar: five key properties at a glance

Each axis scored 0–10 relative to the best material for that property. Use the buttons to compare.

Part D · interactive material explorer

You're designing something. Which material fits?

Part E · why things break, bend, or hold

Why bridges sag in the middle

Bending = tension below

The top of a beam under load is in compression (squashed); the bottom is in tension (stretched). Concrete handles compression well but cracks in tension: that's why bridges use steel cables or rebar along the bottom.

Why wine glasses ring but plastic cups don't

Elasticity & damping

Glass is highly elastic: it vibrates for a long time. Plastic is amorphous and damps (absorbs) vibrations quickly. The "ring" test tells you how much energy a material can store and release.

Why rubber bounces but clay doesn't

Elastic vs plastic deformation

Rubber returns to its original shape (elastic). Clay permanently deforms (plastic). Steel has both zones: small forces → elastic (springs back). Large forces → plastic (bent permanently). Engineers design structures to stay in the elastic zone.

Why I-beams are I-shaped

Putting material where it matters

In a beam under load, the top and bottom surfaces carry the most stress. The middle carries almost nothing. Removing the middle (making an I or H shape) saves ~40% of the material while retaining ~90% of the stiffness. Genius efficiency.

Why arches don't need mortar

Redirecting tension into compression

An arch converts downward loads into compression along its curve, and stone/concrete are strong in compression. Roman arches built 2,000 years ago still stand because they're made of dry stones held in pure compression. No glue, no steel, just geometry.

Why planes are thin-walled tubes

Monocoque structure

A hollow tube is far stiffer than a solid rod of the same weight. Aircraft fuselages are thin aluminium (or carbon fibre) shells where the skin itself carries the load, like an eggshell. This is called monocoque design.

I-beam cross-section: stress distribution

Under a bending load, stress is highest at the top and bottom flanges and nearly zero at the neutral axis.

Part F · thermal expansion: materials grow when heated

Why this matters

Every material expands when heated. Engineers must account for this in bridges (expansion joints), power lines (sag), railway tracks (buckling), and even the gaps between concrete pavement slabs. The formula is simple: ΔL = α × L₀ × ΔT, where α is the material's coefficient of thermal expansion (CTE).

Coefficients of thermal expansion (×10⁻⁶ per °C)

PTFE (Teflon)

135 ×10⁻⁶

PVC plastic

80 ×10⁻⁶

Aluminium

23 ×10⁻⁶

Copper

17 ×10⁻⁶

Steel

12 ×10⁻⁶

Concrete

10–12 ×10⁻⁶

Glass (soda-lime)

9 ×10⁻⁶

Carbon fibre (along fibre)

~1 ×10⁻⁶

Invar (Fe–Ni alloy)

~1.5 ×10⁻⁶

Invar (iron-nickel alloy) was engineered specifically to have near-zero thermal expansion: it's used in precision instruments, laser mirrors, and telescope frames where dimensional stability matters more than almost anything else. Carbon fibre's near-zero CTE along the fibre is one of the reasons it's used in spacecraft and satellite structures exposed to extreme temperature swings.

Thermal expansion calculator: ΔL = α × L₀ × ΔT

Material

Original length (metres)

Temperature change (°C)

—

Part G · atomic bonds: why materials behave as they do

Everything we've discussed traces back to how atoms are held together. The type of bond determines whether a material is stiff or flexible, conducts electricity, melts at a high or low temperature, and whether it is ductile or brittle.

Metallic bonds

Examples: Steel, aluminium, copper, gold
Why ductile: Atoms can slide past each other while electrons move freely: the bond doesn't "snap." This is why metals can be bent without breaking.
Why conduct electricity: The free electrons carry charge.

Covalent bonds

Examples: Diamond, silicon, glass, carbon fibre
Why brittle/hard: Electrons are locked between specific atom pairs. Atoms cannot slide: the bond either holds or snaps suddenly. Hence diamond's hardness and brittleness.
Why insulate: No free electrons to carry charge.

Ionic & Van der Waals

Ionic: Ceramics, concrete, salt. Hard and brittle: bonds are directional and strong, but slip causes charge misalignment and catastrophic fracture.
Van der Waals: Polymers: weak attractive forces between polymer chains. This is why plastics are relatively soft and can be shaped by heat.

Part H · environmental cost: it's not just about weight

Embodied carbon: CO₂ emitted to produce 1 kg of material

This is before the material does any work, just the extraction, processing, and manufacturing phase.

Carbon fibre

~30 kg CO₂/kg

Primary aluminium

~16 kg CO₂/kg

Primary steel (BF-BOF)

~18 kg CO₂/kg (but very cheap)

Recycled steel

~1.8 kg CO₂/kg

Recycled aluminium

~0.8 kg CO₂/kg

Concrete

~0.8 kg CO₂/kg

Structural timber

~0.3 kg CO₂/kg

Important nuance: Timber is often carbon-negative when accounting for the CO₂ stored in the wood. Concrete is cheap per kg but the world uses so much (~4 billion tonnes/year) that it accounts for ~8% of global CO₂ emissions, more than aviation. Recycling makes an enormous difference: recycled aluminium costs only ~5% of the energy of primary production.

What happens to plastic ever produced (Geyer et al., 2017)

91%: landfill, environment, or still in use

6%: recycled (often only once)

3%: incinerated

Of all plastic ever produced as of 2015 (~8.3 billion tonnes), only ~9% had been recycled. The fraction has not improved dramatically since. The main barrier is not technology; it's economics: virgin plastic is often cheaper than recycled plastic.

Part I · side-by-side material comparator

Compare two materials head-to-head

Material A

Material B

Part J · Q&A

1. A marketing claim says a phone case is "military-grade aluminium." What does that actually mean?

Very little. "Military-grade" is a marketing term with no universal standard. Aluminium alloys span a huge range: from soft 1000-series (cooking foil) to hard 7000-series aerospace alloys (3× stronger). Most "military-grade" phone cases use 6061 aluminium, which is a solid general-purpose alloy but nothing exotic. The phone case industry uses this phrase because it sounds impressive, not because it meets any specific military specification. The actual drop protection depends far more on the case's geometry and internal padding than on the alloy grade.

2. Why is concrete always reinforced with steel rebar in modern construction, but ancient Roman concrete wasn't?

Because ancient Roman structures were designed to stay in pure compression (arches, domes, vaults) where concrete's weakness in tension doesn't matter. The Pantheon is a dome: every point on it is under compression, so unreinforced concrete works perfectly. Modern buildings use flat floors, horizontal beams, and cantilevers, which create bending forces with tension in some areas. Steel rebar handles that tension. Interestingly, Roman concrete was also chemically remarkable for some applications: certain harbour concrete actually grew stronger over centuries through a reaction with seawater involving aluminous tobermorite crystals, which modern Portland-cement concrete does not replicate.

3. A racing cyclist is choosing between aluminium and carbon fibre. Carbon fibre is 5× more expensive. What are they actually buying?

Primarily weight reduction, and the ability to tune stiffness directionally. A carbon fibre frame might weigh 800g vs 1,400g for aluminium, a 600g saving. On a 70kg rider+bike system, that's less than 1% of total weight. The real advantage is that carbon fibre can be laid in specific orientations: stiff vertically (efficient pedalling) but slightly flexible horizontally (road vibration absorption). Aluminium is isotropic: equally stiff in all directions. For amateur cyclists, the performance difference is measurable in lab conditions and nearly imperceptible in real riding. Most of the speed difference comes from aerodynamics, the wheels, and the rider, not the frame material.

4. Why do overhead power lines sag more in summer than winter?

Thermal expansion. All materials expand when heated. Aluminium has a coefficient of thermal expansion of ~23×10⁻⁶/°C. A 300-metre power line will expand by roughly 300 × 23×10⁻⁶ × 40°C ≈ 0.28 metres over a 40°C temperature rise. Since the attachment points are fixed, the extra length has to go somewhere: it sags. Engineers design towers with enough clearance that sagging lines never touch the ground under normal conditions. In extreme heat waves, lines sag more than expected and sometimes short circuit by touching trees, causing blackouts. This is a major concern for grid operators in regions experiencing increasing peak summer temperatures.

5. Why is an egg so hard to crush when you squeeze it in your palm, but so easy to crack on a bowl edge?

The eggshell is a thin arch: a perfect compression structure. When you squeeze in your palm, the force is distributed evenly over the curved surface, and the shell converts it into compression along its curves (which it handles well). This is the same principle as a Roman arch or an aircraft fuselage. But when you crack it on a bowl edge, you apply a concentrated point load that creates a sharp local bending moment: suddenly one tiny region is in tension, which the brittle calcium carbonate shell cannot handle. The shell cracks there first, and the crack propagates rapidly. Geometry, not material, determines how strong the egg feels.

6. Steel and concrete have remarkably similar thermal expansion coefficients (~10–12 ×10⁻⁶/°C). Is this a coincidence?

No, and it's one of the most fortunate accidents in engineering history. When concrete and steel are bonded together in reinforced concrete, they expand and contract at nearly the same rate as the temperature changes. If they had different expansion rates, the interface would develop large stresses every time the temperature swung, eventually cracking the concrete and debonding the rebar. The fact that their CTEs are so similar (steel ~12, concrete ~10–12 ×10⁻⁶/°C) means they work in near-perfect thermal harmony. This compatibility was not engineered; it was discovered empirically in the 1850s when Joseph Monier and others began embedding iron in concrete. Engineers later confirmed the physical reason and were relieved to find it.

7. Wood is a natural composite. What are its two constituents, and how does this parallel carbon fibre?

Wood consists of long cellulose fibres (strong in tension, similar to carbon fibre strands) embedded in a lignin matrix (the binder that holds everything together, similar to the epoxy resin in carbon fibre composites). In both cases, the fibre provides tensile strength and the matrix transfers load between fibres and provides shape. The key difference: in carbon fibre, engineers choose the fibre orientation for each layer to optimise direction-specific stiffness. In wood, the "fibre orientation" is fixed by how the tree grew, always along the grain, which makes wood very strong along the grain but weak across it. Cross-laminated timber (CLT) solves this by stacking layers at alternating 90° angles, just as engineers do with woven carbon fibre fabrics.