---
title: "Material properties for selection"
subject: "Machine Design I"
chapter: 2
key: "semester_6/machine_design_i:material-properties-selection"
source: https://www.bctengineer.com/q/semester_6/machine_design_i/material-properties-selection
site: BCT Engineering
---

# Material properties for selection

Machine Design I · Chapter 2: Materials Selection

- Asked in: year not given, 6 marks
- Video: [Material Properties 101](https://www.youtube.com/watch?v=BHZALtqAjeM) — Real Engineering
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/machine_design_i/material-properties-selection
- Whole chapter: https://www.bctengineer.com/md/semester_6/machine_design_i/chapter-2

## Question

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.

## 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.

## Answer

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

1. List the functions and the loading of the part (static, impact, fatigue, temperature, environment).
2. Convert these into required properties, for example "yield strength not less than 300 MPa, corrosion resistant, weldable".
3. Screen out materials that fail any essential requirement.
4. 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.
5. Check manufacturing feasibility, availability and cost of the finished part, not only the cost per kg.
6. 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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which group lists yield strength, modulus, hardness, toughness and fatigue limit?
   A. Mechanical properties
   B. Physical properties
   C. Manufacturing properties
   D. Economic properties
   Answer: A. These govern strength, stiffness and safety against fracture and fatigue. (Key point: Mechanical properties)

2. Castability, machinability, weldability and heat-treatability are important because they
   A. Decide the weight of the part
   B. Decide how easily and cheaply the part can be made
   C. Decide its corrosion life
   D. Decide the thermal expansion
   Answer: B. They belong to the manufacturing group. (Key point: Manufacturing properties)

3. After converting loading into required properties, what is done next?
   A. Select the cheapest per kg material
   B. Order the material and test later
   C. Screen out materials that fail an essential requirement, then rank the rest
   D. Choose the strongest material available
   Answer: C. Screening then ranking by a performance index or weighted table follows. (Key point: Selection procedure)

4. Why is C45 steel chosen for a transmission shaft rather than cast iron?
   A. Cast iron has a higher fatigue limit
   B. C45 has a lower density
   C. Cast iron cannot be machined
   D. Good strength, fatigue resistance, machinability and low cost; cast iron is brittle and weak in tension
   Answer: D. Cast iron's brittleness and weak tension make it unsuitable. (Key point: Shaft example)

## Related in Machine Design I

- [Required rating of a tapered roller bearing](https://www.bctengineer.com/question.md?key=semester_6%2Fmachine_design_i%3Atapered-roller-bearing-capacity) — Machine Design I, chapter 6
- [Weighted decision matrix for drive selection](https://www.bctengineer.com/question.md?key=semester_6%2Fmachine_design_i%3Aweighted-decision-matrix-drive) — Machine Design I, chapter 1
- [Cost comparison of materials for a tie rod](https://www.bctengineer.com/question.md?key=semester_6%2Fmachine_design_i%3Amaterial-cost-tie-rod) — Machine Design I, chapter 2

## Related in other subjects

- [Physical and mechanical properties of wood](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Aphysical-mechanical-properties-wood) — Civil Engineering Materials, chapter 7
- [Important properties of civil engineering materials](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Aimportant-properties-civil-materials) — Civil Engineering Materials, chapter 1, asked 8×
- [Mechanical and physical causes of deterioration](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Amechanical-physical-causes-deterioration) — Concrete Technology and Masonry Structure, chapter 6, asked 6×
- [Physical meaning of equilibrium and its application](https://www.bctengineer.com/question.md?key=semester_1%2Fapplied_mechanics%3Aphysical-meaning-equilibrium-structural) — Applied Mechanics, chapter 2