Select a material by translating the product specification into measurable property requirements, then comparing data against them. The best choice satisfies essential constraints and balances performance, manufacture, cost, and environmental impact.
In B3.1 Material selection, identify the product's function, operating environment, expected loads, lifetime, and manufacturing process. Classify the required properties before comparing candidates through data sheets, databases, or physical testing under realistic service conditions.
| Property category | What it describes | Examples used in selection |
|---|---|---|
| Physical properties | Characteristics measurable without changing chemical identity | Density, thermal conductivity, electrical conductivity, melting point, thermal expansion, and optical properties |
| Chemical properties | How the material reacts with substances or its environment | Corrosion resistance, oxidation resistance, chemical stability, flammability, and resistance to acids or alkalis |
| Mechanical properties | How the material behaves under forces | Tensile strength, compressive strength, stiffness, toughness, hardness, ductility, and fatigue resistance |
For example, an outdoor bicycle frame requires low density, corrosion resistance, high stiffness, adequate strength, and fatigue resistance. Aluminium alloy offers low density and good corrosion resistance, but its section dimensions must ensure stiffness and fatigue performance. Carbon-fibre composite offers a better stiffness-to-mass ratio, but cost, repairability, manufacturing requirements, and end-of-life recovery may make it less suitable.
Use this process:
- Define essential constraints, such as maximum mass or minimum tensile strength.
- Rank desirable criteria by priority.
- Eliminate materials that fail an essential constraint.
- Compare quantitative data using consistent units and test conditions.
- Prototype or test candidates before making a justified choice.
A common misconception is that strength, stiffness, and hardness are interchangeable. Strength is resistance to failure, stiffness is resistance to elastic deformation, and hardness is resistance to indentation or scratching.
In an IB response, link each property directly to the product's function and context. Examiners expect justified comparison using specific properties, not unsupported claims such as “metal is stronger” or “plastic is lighter.”