PH8151 Engineering Physics Chapter 1

20 CHAPTER 1. PROPERTIES OF MATTER

are (i) To impart an angular displacement of one end cross – section with respect to the
other endand(ii) To setup shear stresses on any cross section of the bar perpendicular
to its axis.

Torsional stress: The shear stress produced inside a material as a reaction to an externally
applied torque to the end of a shaft about its axis is known as torsional stress.

Torsional deformation and shear strain: If a line a-b is marked on the surface of the
unloaded bar, then after the twisting moment or torqueT has been applied, this line
moves to a-bÕas shown in Figure1.17. The angle“, measured in radians, between ab and
abÕis defined as the shear strain at the surface of the bar or shaft^3. The same definition
will hold at any interior point of the bar.

Angle of Twist: If a shaft of lengthLis subjected to a constant twisting momentTalong
its length, then the angle◊ through which one end of the bar will twist relative to the
other is known is the angle of twist as shown in Figure1.17. (In Figure1.2(d), angle of
twist was denoted by„.)

Torsion is utilised in a large number of engineering devices, of which some examples
are shown in Figure1.18.

Torsion wrench Screw driver

Automobile Suspension

Tightening of a

nut with a wrench

Steam turbine shaft

Figure 1.18: Examples of torsion devices

Learning Resource : Understanding Torsion

Another excellent presentation from The Ecient Engineer

Youtube channel about fundamentals of torsion and its engi-

neering applications.

URL:https://youtu.be/1YTKedLQOa

(^3) Strictly speaking, the shear strain (“) is the tangent of the angle between ab and abÕ.Butwhenthe
angle is small (and expressed in radians), the tangent is nearly equal to the angle itself.
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