Choosing materials
The propertiesThe characteristics of something. In chemistry, chemical properties include the reactions a substance can take part in. Physical properties include colour and boiling point. of a materialsThings that are used, sometimes to make other things. determine whether it is suitable for a given use. For any given use, certain properties will be required. There may be more than one suitable material to choose from, and the advantages and disadvantages of each one must be evaluated.
Question
This table shows some properties of samples of aluminium alloyAn alloy is a mixture of two or more elements, at least one of which is a metal., mild steel and timberWood used in building and making furniture.:
| Alluminium alloy | Mild steel | Timber | |
| Density (g/cm3) | 2.8 | 7.7 | 0.7 |
| Tensile strength (MPa) | 380 | 240 | 20 |
| Compressive strength (MPa) | 380 | 240 | 15 |
| Resistance to moisture | Good - does not need painting | Rusts unless protected | Absorbs water unless protected |
| Density (g/cm3) | |
|---|---|
| Alluminium alloy | 2.8 |
| Mild steel | 7.7 |
| Timber | 0.7 |
| Tensile strength (MPa) | |
|---|---|
| Alluminium alloy | 380 |
| Mild steel | 240 |
| Timber | 20 |
| Compressive strength (MPa) | |
|---|---|
| Alluminium alloy | 380 |
| Mild steel | 240 |
| Timber | 15 |
| Resistance to moisture | |
|---|---|
| Alluminium alloy | Good - does not need painting |
| Mild steel | Rusts unless protected |
| Timber | Absorbs water unless protected |
(a) Identify the weakest metalShiny element that is a good conductor of electricity and heat, and which forms basic oxides. shown in the table, and explain your answer.
(b) All three materials could be used to make the frame for a train carriage. Evaluate these materials for this use.
(a)The mild steel is the weakest metal, although timber is the weakest material of the three. Mild steel has a lower tensile strengthThe tension a material can withstand without breaking. than aluminium, so it stretches more easily when forceA push or a pull. The unit of force is the newton (N). are applied. It has a lower compressive strengthA measure of how well a material resists being crushed when a force is applied. , so it is crushed more easily when forces are applied.
(b)
- Timber has the lowest densityA measure of compactness and the ratio of mass to volume. It is usually measured in kilograms per metre cubed (kg/m3) or grams per centimetre cubed (g/cm3)., so a wooden frame could be the most lightweight. However, timber is the weakest material, so the frame will need to be thicker or use more pieces. This might make the wooden frame heavier than a metal frame.
- Mild steel is almost as strong as the aluminium alloy. However, it is 2.75 times denser than aluminium, so a steel frame is likely to be much heavier. A heavier carriage may reduce the number of carriages the train can pull, or reduce its acceleration so that journey times increase.
- The aluminium alloy is four times denser than timber, but at least 19 times stronger. An aluminium frame is likely to be the most lightweight of the three materials. Aluminium alloy has a good resistance to moisture. Unlike mild steel and timber, it does not need protecting. Overall, aluminium alloy would be the best choice.
