- Practice · 6 marks
Material properties for selection
Short answer
Material selection requires mechanical (strength, stiffness, fatigue), physical (density, thermal), chemical (corrosion), manufacturing (machinability, weldability) and economic (cost, availability) properties. Selection involves listing functional requirements, screening out unsuitable materials, ranking by performance indices or weighted properties, and confirming with testing.
What information on material properties is required when selecting a material for a machine part? Explain the important mechanical, physical and manufacturing-related properties and describe how a material is finally selected for a given application.
Material selection is choosing, from the many available materials, the one that satisfies the functional requirements of a part at the lowest overall cost. It needs reliable property data from handbooks, standards (IS, ASTM, BS), manufacturers' catalogues and test reports.
Information required
| Group | Properties | Why it matters |
|---|---|---|
| Mechanical | Ultimate and yield strength, modulus of elasticity, hardness, ductility (percentage elongation), toughness (impact value), fatigue (endurance) limit, creep strength | Strength, stiffness, safety against fracture and fatigue |
| Physical | Density, melting point, thermal conductivity, coefficient of thermal expansion, electrical conductivity, magnetic behaviour | Weight, heat flow, thermal stress, special uses |
| Chemical | Corrosion and oxidation resistance, resistance to chemicals | Life in the working environment |
| Manufacturing | Castability, machinability, weldability, formability, heat-treatability | Ease and cost of making the part |
| Economic | Price per kg, availability, standard sizes, scrap value | Total cost of the product |
How the selection is made
- List the functions and the loading of the part (static, impact, fatigue, temperature, environment).
- Convert these into required properties, for example "yield strength not less than 300 MPa, corrosion resistant, weldable".
- Screen out materials that fail any essential requirement.
- Rank the remaining materials using a performance index (for example strength/density for light parts, strength/cost for low cost) or a weighted property table.
- Check manufacturing feasibility, availability and cost of the finished part, not only the cost per kg.
- Select the material, and confirm with testing or experience for critical parts.
Example: for a transmission shaft, medium carbon steel (C45) is chosen because it has good strength, fatigue resistance, machinability and low cost; cast iron is rejected because it is brittle and weak in tension.
✓ Answer checked by Claude Opus 5.5 (high effort)
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