Structural materials and their uses
Things we build are made from a few families of materials.
- Metals (steel, aluminium, copper): strong, can be bent and cut, carry heat and electricity.
- Wood: light, easy to cut, burns and can rot.
- Plastics: light, do not rust, can melt or crack.
- Ceramics and glass: hard and heat-proof, but brittle (they break).
- Composites (such as fibreglass): two materials working together.
Each material has properties: how strong, how heavy, how easy to shape, how well it carries heat or electricity, and how much it costs. The job decides which properties matter most.
Why make alloys? Working metals and alloys
A pure metal such as copper or iron is made of identical atoms in layers. The layers slide over each other, so pure metals are usually soft and easy to bend.
An alloy is a mixture of a metal with another element, melted together. The extra atoms are a different size, so they jam the layers. Sliding is harder, so an alloy is usually harder and stronger than the pure metal. Mixing can also change the colour, the melting point and how well the metal resists rust.
Common alloys (typical recipes):
- Steel: iron with about 0.2 to 2 percent carbon. Bridges, tools, bolts.
- Brass: copper with about 35 percent zinc. Taps, keys, musical instruments.
- Bronze: copper with about 10 percent tin. Bells, statues, bearings.
- Duralumin: aluminium with a little copper and magnesium. Aircraft parts and bicycle frames.
Mechanisms and machines in production
A machine uses energy to do work. A mechanism is the part inside that changes motion or force.
- Lever: a bar that turns on a pivot, so a small push far from the pivot lifts a big load near it.
- Gears and belts: pass turning motion and change speed or force.
- Crank and cam: change turning into back-and-forth motion.
- Screw: turns a turning push into a strong straight push (a vice, a jack).
In a workshop, machines such as the drill, the lathe, the press and the saw shape metal quickly and accurately. A motor supplies the energy and mechanisms deliver it to the tool. Machines give accuracy and speed; they do not give free energy.
Choosing materials and calculating costs
Step 1: list what the job needs. Strong? Light? Must it carry electricity? Must it be cheap?
Step 2: score the materials. Give each material a score from 1 to 5 for each property, multiply by how important that property is (weight), and add up. The highest total is the best fit. A bike frame needs light (x3), strong (x2), cheap (x1): aluminium wins. A bridge girder needs strong (x3) and cheap (x2): steel wins. House wiring needs conduction: copper wins.
Step 3: work out the cost.
volume = length x width x thickness (for a bar)
mass = volume x density
cost = mass (kg) x price per kg
Typical densities: steel 7.85 g/cm³, aluminium 2.7 g/cm³, brass about 8.5 g/cm³. Remember to buy a bit extra: cutting and filing waste some metal.
Making the product: steps and safety when working metal
From idea to product: 1. Idea and sketch. 2. Technical drawing with sizes. 3. Choose the material. 4. Work out mass and cost. 5. Mark out the metal. 6. Cut, drill, bend, file and join. 7. Finish (smooth, paint or polish). 8. Check the size and quality.
Safety rules:
- Wear goggles: chips and sparks fly.
- Wear gloves for sharp sheet metal, but never near a spinning drill.
- Tie hair back, wear a close-fitting apron, no loose sleeves.
- Clamp the metal; never hold small pieces by hand while drilling or cutting.
- Keep machine guards in place and switch off before you adjust anything.
- Metal can be hot or very sharp. Carry long pieces with a partner.
Try it
In the 3D. At step 2, predict how far the top layer will slide with 8 tin atoms before you press the button. Then check.
At home. Bend a plain metal paper clip once, then bend a thick steel nail the same way (use pliers and safety goggles, ask an adult). Which resists? Also list five metal things at home and write the metal for each: a spoon, a key, a wire, a tap, a coin. Decide which are alloys.
Key formulas and definitions
- Alloy = metal + another element → layers jam → harder
- Steel = iron + a little carbon; brass = copper + zinc; bronze = copper + tin
- Volume of a bar = length x width x thickness
- Mass = volume x density
- Cost = mass (kg) x price per kg
- Material score = sum of (property score x importance)
Worked examples
1. A steel bar is 2 cm x 2 cm x 50 cm. Find its mass. (density of steel 7.85 g/cm³)
Volume = 2 x 2 x 50 = 200 cm³. Mass = 200 x 7.85 = 1570 g = 1.57 kg.
2. The same bar is made of aluminium (2.7 g/cm³). How much lighter is it?
Mass = 200 x 2.7 = 540 g = 0.54 kg. Difference = 1.57 - 0.54 = 1.03 kg lighter.
3. Steel costs 1 unit per kg and aluminium 3 units per kg (illustrative). Find the cost of each 200 cm³ bar.
Steel: 1.57 kg x 1 = 1.57 units. Aluminium: 0.54 kg x 3 = 1.62 units. Almost the same cost, but the aluminium part is much lighter.
4. The same bar in brass (8.5 g/cm³, price 6 units per kg). Find the mass and cost.
Mass = 200 x 8.5 = 1700 g = 1.7 kg. Cost = 1.7 x 6 = 10.2 units.
5. How much copper and tin are in 2 kg of bronze that is 10 percent tin?
Tin = 10% of 2 kg = 0.2 kg. Copper = 2 - 0.2 = 1.8 kg.
6. A job needs 1.8 kg of steel after cutting. About 10 percent of the bought steel is lost as waste. How much should you buy, and what does it cost at 1 unit per kg?
Only 90 percent is used, so 0.9 x B = 1.8, B = 1.8 / 0.9 = 2.0 kg. Cost = 2.0 x 1 = 2.0 units.
7. Score for a bike frame (strong x2, light x3, cheap x1): steel has strong 5, light 2, cheap 4; aluminium has strong 3, light 5, cheap 3. Which is better?
Steel: 5x2 + 2x3 + 4x1 = 10 + 6 + 4 = 20. Aluminium: 3x2 + 5x3 + 3x1 = 6 + 15 + 3 = 24. Aluminium scores higher.
Common mistakes
- Thinking an alloy is a compound. It is a mixture; the atoms are mixed, not chemically bonded into a new substance.
- Mixing up units: density in g/cm³ gives grams. Divide by 1000 to get kg before using a price per kg.
- Choosing the strongest material without thinking about weight and cost.
- Wearing gloves near a spinning drill or holding small pieces by hand instead of clamping.