Classification of materials

Name the four main families of engineering materials and write an example of a material from each one, stating a characteristic property for which it is used.

State which family of materials each of these objects belongs to and justify it with a property: (a) a steel saucepan, (b) a PET water bottle, (c) a porcelain plate and (d) a carbon fibre bicycle helmet.

The classification tree in the figure organises engineering materials into families, with an example of each, but one branch is covered with a question mark, as shown in the figure.

State which family is missing, write two examples of materials from that family, and describe a property that characterises it.

Name three characteristic properties of metals (electrical and thermal conductivity, lustre, malleability, etc.) and, for each one, an everyday object that takes advantage of it.

Explain what an alloy is and give two examples (steel, brass, bronze, etc.), stating why industry uses alloys instead of pure metals in most applications.

Metals conduct electricity and heat well, whereas ceramic materials are insulators and withstand very high temperatures. Relate these differences in behaviour to the type of internal structure of each family, and give an application for each property mentioned.

Polymers are divided into thermoplastics and thermosets. Explain the key difference (what happens when reheated) and give an example of an object made of each type.

Name two typical properties of ceramic materials (hardness, heat resistance, brittleness, insulation) and two everyday objects made from them that rely on those properties.

Glass is an amorphous solid (without an ordered crystalline structure) obtained by melting sand. Reason which material family it belongs to based on its behaviour (hard, brittle, insulating, resistant to heat and chemicals) and not based on its commercial name.

Explain what the density of a material is and why it is a critical property when choosing the material for a drone frame, comparing it with what would matter for a gym weight.

Explain what a composite material is and why they are manufactured by combining two or more materials. Describe the case of glass fibre: what the resin provides and what the fibres provide within the composite.

In the construction of a cantilever balcony subjected to its own weight and the weight of people at its edge, an inexperienced worker has placed the rebar at the bottom of the concrete slab instead of at the top.

1. a) Analyze the stress distribution in the cantilever beam: justify what type of mechanical stress (tension or compression) the upper face experiences and which the lower face experiences when the balcony is loaded.
2. b) Predict what will happen upon stripping the formwork: explain why and where the concrete will crack, and why the steel bars located at the bottom will not prevent catastrophic collapse.
3. c) Beyond resisting opposing mechanical stresses, justify why reinforced concrete only works thermally because both materials share a virtually identical coefficient of linear thermal expansion (around 12·10⁻⁶ K⁻¹). What would happen in summer and winter if their coefficients were very disparate?