Exercises: 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.

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.

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.

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.

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.

Choose the most suitable family of materials for each component and justify it with at least two properties: (a) the conductive wire inside an electrical cable, (b) the insulating body of a plug, and (c) the glass in an oven door.

Classify these objects into their material family and justify each with a property: (a) the aluminium heat sink of a processor, (b) a glass window, (c) a tyre (fabric-reinforced rubber), (d) a silicone kitchen spatula, (e) a terracotta plant pot, and (f) a copper wire.

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.

Match each property with the material family that best represents it (metals, polymers, ceramics, composites) and justify each pairing with an example object: (a) the highest thermal and electrical conductivity, (b) the highest brittleness combined with high hardness, (c) the lowest density combined with cheap and easy moulding, (d) the best combination of lightness and strength achieved by joining two materials.

Cars increasingly feature aluminium and polymers in parts that were previously made of steel. Analyse the reasons for this change (weight and fuel consumption, corrosion, design freedom) and the drawbacks it brings (cost, repair of dents, recycling of multi-material components).

Wood combines cellulose fibres with a lignin matrix. Analyse whether it can be considered a natural composite material by comparing its structure to that of glass fibre (reinforcement plus matrix), and state a practical consequence of this fibrous structure (the grain direction when sawing or bending).

A novice designer states: "Since the density of aluminum (2,7 g/cm³) is almost one third that of steel (7,8 g/cm³), if we make the bicycle frame using aluminum tubes of identical dimensions to the steel ones, the frame will weigh one third and provide exactly the same mechanical performance".

1. a) Technically refute this claim: considering that the elastic modulus of steel is 210 GPa and that of aluminum is 70 GPa, predict what will happen to the stiffness and deformation of the aluminum frame when facing the same bumps if the tube dimensions are not redesigned.
2. b) Design the geometric solution: explain how the outer diameter and wall thickness of the aluminum tubes must be modified to match the bending stiffness of steel, and justify why the actual weight savings do not reach 66%.
3. c) From the viewpoint of fatigue behavior under cyclic loading, predict the expected lifespan of both frames knowing that structural steels exhibit a defined fatigue limit, whereas aluminum alloys lack one.

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?

An automotive company is debating two proposals for the fenders and bonnet of a new high-volume electric city car (200 000 units per year):

• Option A: Cold-drawn sheet steel.

• Option B: Carbon-fibre-reinforced thermoset polymer composite (CFRP).

1. a) A manager argues: "The composite will halve the weight of these parts, so it will economically offset its higher material cost by extending the battery range." Technically refute this argument by analysing the speed of the forming process (stamping in seconds versus the curing cycle of thermosets) and the impact of the extra cost on an entry-level vehicle.
2. b) Predict the failure mode under a minor parking impact: compare the plastic deformation of steel with the phenomenon of internal delamination or brittle failure of CFRP, assessing the ease and cost of repair in a standard workshop.
3. c) Justify why recycling the residue at the end of the car's useful life is critical: explain the structural difference between a thermoplastic or a metal (remeltable) and a cross-linked thermoset matrix in the face of material recovery regulations.

You are going to select the material for a travel thermal mug that keeps coffee hot, withstands drops of 1,5 m, and endures daily dishwasher cycles at 70 °C with caustic detergents. Analyse the following proposals:

• Proposal A: Thick-walled glazed stoneware ceramic (5 mm).

• Proposal B: Single-layer medium-walled polypropylene (PP) (3 mm).

• Proposal C: Double-walled austenitic stainless steel with an intermediate vacuum chamber.

1. a) Analyse heat transfer: the thermal conductivity of stainless steel (~15 W/(m·K)) is nearly two orders of magnitude higher than that of PP (~0,2 W/(m·K)). Justify why, despite this, Proposal C retains heat much better than Proposal B, identifying the three heat transfer mechanisms and how the vacuum acts upon them.
2. b) Predict the mechanical outcome of a fall onto a hard pavement for Proposal A compared to Proposal C, justifying the difference based on toughness and the predominant type of atomic bonding in each material.
3. c) Predict what physical and chemical degradation polypropylene (Proposal B) will suffer in the medium term after repeated dishwasher cycles compared to stainless steel, and decide which is the optimal design choice, justifying the decisive criterion.

An engineering firm is evaluating replacing the main steel cables of a long-span suspension bridge with high-strength aramid fibre cables, arguing that their tensile strength per unit mass is four times greater than that of steel and that they are immune to marine corrosion.

1. a) Creep: the cables will withstand permanent stresses of 40 % of their breaking load for more than 80 years. Predict what will happen to the bridge's catenary if the polymer undergoes creep deformation, and explain why the metallic crystalline structure of steel is superior against this phenomenon at room temperature.
2. b) Stiffness and dynamics: the Young's modulus of steel is 210 GPa, whereas that of aramid is around 120 GPa. Justify why a lower elastic modulus in the main cables, even with a higher breaking strength, increases the risk of aeroelastic instability (wind-induced oscillations) in the bridge deck.
3. c) Environmental durability: justify what degradation tests against ultraviolet (UV) solar radiation and moisture absorption the polymer fibres should pass to offer the durability guarantees that steel has already proven.

Graphene is often described as "the miracle two-dimensional material that will replace silicon in processors and copper in electrical transmission cables".

1. a) Justify, based on the structure of graphene (sp² hybridization of carbon in a planar hexagonal lattice and delocalised electrons in pi orbitals), its extraordinary electrical conductivity and in-plane mechanical strength.
2. b) Technically refute the claim that monolayer graphene will replace silicon in the digital transistors of microprocessors in the short term: explain what the bandgap is, why it is essential for a transistor to be able to switch between cut-off and conduction states (logical 0 and 1), and what value it has in pure graphene.
3. c) Explain the obstacles limiting its integration on an industrial scale: compare the quality of graphene obtained by micromechanical exfoliation with that produced by chemical vapour deposition (CVD), and interpret how crystalline defects (wrinkles, grain boundaries) affect the actual performance of the material.

An organising committee for a three-day music festival is debating the environmental impact of cups for 40 000 attendees and is considering three options:

• Option 1: Disposable PLA (polylactic acid, "compostable bioplastic") cups. Manufacturing emissions: 30 g CO₂/cup.

• Option 2: Reusable polypropylene (PP) cups with a €1 deposit. Manufacturing emissions: 120 g CO₂/cup; washing and transport: 10 g CO₂/cycle.

• Option 3: Disposable 100% recycled PET (rPET) cups. Manufacturing emissions: 25 g CO₂/cup.

1. a) Interpret the data and calculate from how many reuse cycles (uses and washes) Option 2 becomes more environmentally favourable in terms of carbon footprint than consuming Option 1 PLA cups.
2. b) A sponsor states: "PLA is the perfect option because, if attendees throw them on the ground or they end up in a landfill, they will disappear and turn into compost in just a few weeks." Refute this statement by justifying the actual conditions required for the industrial composting of PLA (temperatures above 58 °C and controlled humidity), and predict what problem it causes if accidentally mixed into the yellow recycling bin with conventional PET.
3. c) Knowing that at large-scale events the actual return rate of reusable cups is only around 60 % (users take them home or throw them in the rubbish), predict whether the reusable cup will achieve its ecological goal compared to Option 3 and design an organisational measure to ensure that the model is sustainable.

The urban planning department of a coastal town commissions you to specify the materials for a new model of outdoor public bench exposed to salty sea breezes, intense solar radiation, and the risk of vandalism (graffiti, impacts, and burns).

1. a) Load-bearing structure: a team member proposes uncoated cast iron because it is economical and heavy. Refute this proposal by predicting what type of failure due to corrosion and brittleness it will suffer under impacts, and advocate for a suitable metallic alternative (such as hot-dip galvanised steel with a powder coat finish or anodised aluminium).
2. b) Seat and backrest: compare slats made of recycled high-density polyethylene (HDPE) with slats made of autoclave-treated pine wood. Predict how each material will respond to continuous solar exposure (UV degradation) and to the recurrent use of aggressive solvents (acetone) to clean graffiti.
3. c) Galvanic corrosion in fasteners: if an aluminium structure is bolted using standard zinc-plated carbon steel bolts in direct contact under salty moisture, predict which of the two metals will act as the anode and corrode at an accelerated rate. Justify how you should prevent this failure through material selection or electrical insulation at the joints.