Materials, Components and Their Properties
Properties vs characteristics
Properties are measurable behaviours under test: tensile strength, hardness, density, thermal conductivity, electrical resistivity, elastic modulus. Characteristics are broader descriptors: appearance, workability, cost, availability, recyclability. Questions often ask you to justify a selection, so link a property to a function.
Know the families. Metals (ferrous and non-ferrous) offer strength and conductivity; aluminium is light and corrosion-resistant via its oxide layer, steel is strong but needs finishing. Polymers divide into thermoplastics (reformable on heating, e.g. acrylic, HDPE, PLA) and thermosets (cross-linked, cannot be remelted, e.g. epoxy, polyester resin). Composites combine a matrix and reinforcement (glass-fibre reinforced polymer) to gain stiffness-to-weight. Timber is anisotropic: properties differ along and across the grain. Textiles vary by fibre, weave and finish.
Traps
- Saying a material is "strong" without specifying tension, compression, bending or impact.
- Assuming all plastics are recyclable; thermosets generally are not remelted.
- Ignoring surface finish, joining method and fastener compatibility.
Rule of thumb
Match the dominant in-service load and environment to a property, then check secondary requirements (cost, finish, joining, end-of-life).
Worked example: A bicycle rack must resist bending under load and survive rain. Bending stiffness favours a hollow section (higher second moment of area for the same mass). Corrosion resistance favours aluminium or coated steel. Joining favours weldable steel or bolted aluminium. The final choice trades mass against cost and finish.