Page 173 - WCS - Electrical

WORKSHOP CALCULATION & SCIENCE - CITS

E.g.: 1 When brake is applied the brake rod is under tensile stress. 2 During tightening of bolt or nut. 3 Belt driving
the fan. 4 Crane rope (When rope is pulling)
Compressive stress
: When a material is subjected to two equal and opposite axial pushes, the material tends to decrease in length.
The resistance offered against the decrease in length is called compressive stress. The corresponding strain is
called compressive strain.
Eg. 1 Compressive stress on connecting rod on the first part of power stroke 2 Compressive stress on push rod
during valve opening 3 Clutch lining when the clutch is engaged
Shear stress
When a material is subjected to two equal and opposite forces acting tangentially across the resisting section, the
body tends to be sheared off across the cross section. The stress included is called shear stress. It is represented.
The corresponding strain is called shear strain.

Eg. Rivets , Gudgeon Pin ,Spring shackle pin , Brake rod rivets , Chassis rivets , Fly wheel holding bolts, Swivel
pins, Gear box shaft , Axle shaft
Torsional stress

When a shaft is subjected to the action of two equal and opposite couples acting in parallel planes, then the shaft
is said to in torsion. The stress set up by the torsion is known as torsional shear stress.
Eg. Rear axle , Crank shaft , Coil springs , Propeller shaft , Starter motor armature shaft

Generally in any industry the material used are elastic in nature. Hence if a material is subjected to an external
load, it undergoes deformation. During the deformation process the material will offer a resistance against the
deformation. In case if the material fails to put up full resistance to the external load, the deformation continues
until rupture takes place. Hence it is important to have a considerable knowledge about the materials and their
properties for designing and fabricating.
Strain
When an external forces acting on a material, there is a change in its dimension and shape. The deformation is
called strain. Thus, strain is the ratio between the change in dimension of a material to its original dimension. It
has no unit. It is represented by(e)Epsilon
Linear or Longitudinal strain
It is the ratio between the change in length of the material to its original length.

Lateral Strain
It is the ratio between change in cross sectional area of material to its original area.
Volumetric Strain
It is the ratio between change in volume of material to its original volume.
Hooke’s law:Robert Hooke discovered a relationship between stress and strain. According to Hooke’s law stress
is proportional to strain within elastic limit.
Stress versus Strain Curve
- The stress-strain curve represents the relationship between stress and strain for a given material under specific
conditions.
- It typically consists of several regions
1 Elastic Region: Where stress and strain are linearly related according to Hooke’s Law, and the material
returns to its original shape upon removal of stress.
2 Yield Point: The point at which the material begins to deform plastically, experiencing permanent deformation.
3 Plastic Region: Where further stress leads to significant permanent deformation without an increase in stress.
4 Fracture Point: The point at which the material ultimately fails due to excessive stress.

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CITS : WCS - Electrical - Exercise 16