Chapter 2 · 2 hours

Materials Selection

Practice questions

Practice questions and answers

2 exam-style questions on this chapter, written for this site from the official syllabus. We haven’t found past IOE papers for this subject yet; if you have some, share them in the community.

Practice · 6 marks

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.

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

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.

Practice · 6 marks

A tie rod of length 1 m must carry a steady tensile load of 50 kN with a factor of safety of 2 based on yield strength. Compare the following materials on the basis of mass and material cost (assumed prices): mild steel (yield strength 250 MPa, density 7850 kg/m^3, Rs 120/kg); alloy steel 4140 (650 MPa, 7850 kg/m^3, Rs 180/kg); aluminium alloy 6061-T6 (270 MPa, 2700 kg/m^3, Rs 450/kg); titanium alloy Ti-6Al-4V (880 MPa, 4430 kg/m^3, Rs 3000/kg). Which material is cheapest and which is lightest? Comment on the economics of material selection.

Answer

The tie rod is in simple tension, so the required area is $A = \dfrac{P \times FS}{S_y}$ , mass $m = \rho A L$ and cost $= m \times$ price.

Given: $P = 50\ \text{kN}$ , $FS = 2$ , $L = 1\ \text{m}$ . Design load $= 50 \times 2 = 100\ \text{kN}$ .

Area, mass and cost

For mild steel:

\begin{aligned} A &= \frac{100\,000}{250} = 400\ \text{mm}^2 \\ m &= 7850 \times 400\times10^{-6} \times 1 = 3.14\ \text{kg} \\ \text{Cost} &= 3.14 \times 120 = \text{Rs } 377 \end{aligned}

The other materials are worked out in the same way:

Material	$S_y$ (MPa)	A (mm $^2$ )	Mass (kg)	Price (Rs/kg)	Cost (Rs)
Mild steel	250	400.0	3.14	120	377
Alloy steel 4140	650	153.8	1.21	180	217
Al 6061-T6	270	370.4	1.00	450	450
Ti-6Al-4V	880	113.6	0.50	3000	1510

Comment

Cheapest: alloy steel 4140 (about Rs 217), although its price per kg is higher than mild steel, because its higher strength needs much less material.
Lightest: titanium alloy (0.50 kg), but it costs about seven times as much as 4140, so it is justified only where weight is critical (aircraft, aerospace).
Price per kg alone is misleading; the cost should be compared on the basis of the function performed (cost per unit strength, or cost of the finished part). Machining, joining, corrosion protection, availability and the cost of a heavier structure supporting the part must also be included.
For stiffness-limited or buckling-limited parts the comparison would use modulus, and the ranking can change.

Answer: Cheapest = alloy steel 4140 (Rs 217, 1.21 kg); lightest = Ti-6Al-4V (0.50 kg, Rs 1510).

Written from the official syllabus. Questions and answers are written for this site; check them against your class notes.