Module 14: Materials & Engineering

Steel, concrete, wood — what things are made of

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 material but extremely brittle — hit it with a hammer and it shatters. Steel is less hard but far tougher.

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

1/3 the weight of steel

Strength-to-weight

~100–270 MPa

Weaker than steel, but per kg it competes well

Key property

Corrosion-resistant

Forms a natural oxide layer — no rust

Why aluminium: Weight matters above all else — aircraft, cars, bikes, drink cans, laptops. A Boeing 747 is ~80% aluminium by weight. Costs more than steel to produce (energy-intensive smelting) but infinitely recyclable at 5% of the original energy cost. Discovered in its pure form in 1825 — so rare it was more valuable than gold. Napoleon served guests on aluminium plates while the lower classes used gold ones.

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 concrete 2,000 years ago — their Pantheon dome, unreinforced, still stands. Modern reinforced concrete was invented 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 (787 Dreamliner is 50% carbon fibre), high-end bikes, sports equipment, spacecraft. Cannot be welded; must be bonded with adhesives. Brittle — fails suddenly with no warning deformation. Not recyclable easily. The future of carbon fibre is bringing its cost down 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

~7 MPa (practical)

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

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 (safety glass) is heated and rapidly cooled, creating compression on the surface 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) uses ion exchange to achieve the same effect chemically.

Polymers (plastics) the shape-shifters

Density range

~900–1,400 kg/m³

Lighter than all metals. Some 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; ~91% has never been recycled.

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

Specific strength (MPa per density) — 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 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 is as strong per gram as carbon fibre — the challenge is producing it in industrial quantities. Spiders are territorial and cannibalistic, making farming impossible. Scientists are trying to synthesise it using bacteria.

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 crystalline and 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 and weight while retaining ~90% of the stiffness. Genius efficiency.

Why arches don't need mortar

Redirecting tension into compression

An arch converts all downward loads into compression forces 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.

Part F · test yourself

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 superior for some applications — it actually got stronger over centuries due to a chemical reaction with seawater, which modern concrete does not do.

3. A racing cyclist is choosing between aluminium and carbon fibre frame. 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 the 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 expands by about 23 millionths of a metre per metre per degree Celsius (coefficient of thermal expansion = 23×10⁻⁶/°C). A 300-metre power line spanning a gap will expand by roughly 300 × 23×10⁻⁶ × 40°C temperature increase ≈ 0.28 metres over a 40°C summer day. 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. In extreme heat waves, lines sag more than expected and sometimes short circuit or touch trees — causing blackouts.

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 the material, determines how strong the egg feels.