Mr. A. SATHIYASEELAN, B.E, LECTURER/AUTOMOBILE ENGINEERING, LAKSHMI AMMAL POLYTECHNIC COLLEGE, K.R.NAGAR, KOVILPATTI-628503 THOOTHUKUDI, TN, INDIA. Mobile No: 7708852537 Email Id: sathiyaauto@lapc.in sathiyahockey@gmail.com

1. STRENGTH OF MATERIALS FOR
DIPLOMA STUDENTS
BILINGUAL E – LECTURE
UNIT-I
Prepared by
A. SATHIYASEELAN
LECTURER/AUTOMOBILE,
LAKSHMI AMMAL POLYTECHNIC COLLEGE,
K.R.NAGAR, KOVILPATTI

2. Definiton of load & Types
 The system of external force acting on a body
or structure is known as load.
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3. Tensile, Compressive &
Shearing load
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 Tensile load: It acts axially i.e. along the axis of
the body and
has pulling tendency. It produces some degree of
elongation or extension.
 Compressive load: It acts axially and it has
pushing tendency. It produces some degree of
contraction.
 Shearing load: It consists of equal and parallel
opposite forces acting on surfaces of the body.
These forces tend to cause the sliding of the
surfaces.

4. Shearing & Twisting or Torsional
load
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 Bending load: It is produced by two couples
and produces certain degree of curvature.
 Twisting or Torsional load: It is produced by
two couples applied at opposite ends. It
causes certain degree of twist in the members.

5. Stress
Resistance offered by the material per unit
cross- sectional area is called STRESS
 Tensile and compressive forces are called direct
forces
 Stress is the force per unit area upon which it
acts.
Stress (s )= Load (P)/Area (A)
Units is = N / m2
( σ is called as Sigma)
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6. Strain
The deformation per unit length of the body
is called strain.
 Strain (e) = change in length/original length
 Strain (e) = 𝛿l/l
 Strain has no unit’s since it is a ratio of length
to length
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7. Classification of force system
 Tensile stress
 Compressive stress
 Shear stress
 Bending stress
 Torsional stress
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8. Tensile stress
When a body is subjected to tensile load (also
called axial pull), the length of the body tends
to increase.
Tensile load = Tensile load/Area
or
Axial pull/ Area
Units is = N /mm2
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9. Compressive stress
When the body is subjected to compressive
force or axial push, the length of the body
tends to decrease.
Tensile load = Compressive load/Area
or
Axial push/ Area
Units is = N /mm2
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10. Shear stress
The resistance offered by the body per unit
area against the shear off called shear stress.
Tensile load = Tangential force/Area
Units is = N /mm2
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11. Stress - Strain Relationship
Diagram
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12.  A – Proportionality
 B – Elastic limit
 C – Upper yield point
 D – Lower yield point
 E – Ultimate point
 F – Fracture pint (or) Breaking point
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13. Elastic limit stress
The stress induced in the body when it is
loaded upto elastic limit is called elastic limits
stress.
Elastic limit stress = Load at elastic
limit/Area
Units is = N /mm2
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14. Yield stress
The point at which the strain increases without
increase in load is called yield point.
Yield stress = Load at yield point/Area
Units is = N /mm2
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15. Ultimate stress
The maximum load carried by the specimen is
called ultimate load. The ratio of ultimate load
to cross sectional area is called ultimate
stress.
Ultimate stress = Ultimate point or ultimate
load/Area
Units is = N /mm2
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16. Breaking stress
When the specimen is loaded beyond the
ultimate point, the load on the specimen tends
to decrease and the strain on the specimen
increases. At certain load the specimen may
break.
Breaking stress = Load at breaking point/Area
Units is = N /mm2
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17. Hooke’s Law
It states that when the material is loaded with
in elastic limit, the stress is directly
proportional to strain.
i.e., Stress ∞ Strain
Stress/Strain = Constant
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18. Young’s modulus of Modulus of
elasticity (E)
When the material is loaded within the elastic
limit, the ratio of tensile stress to tensile strain
Or compressive stress to compressive strain is
called Young’s modulus.
Young’s modulus = Tensile stress or
compressive stress/
Tensile strain or Compressive strain
Units is = N /mm2
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19. Working stress
The safe stress , which may be induced in the
material during the working condition is called
working stress.
Working stress = Working load / Area
Units is = N /mm2
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20. Factor of safety(FOS)
It is defined as the ratio of the ultimate stress
to working stress.
Factor of safety = Ultimate stress/Working stress
FOS for steel = 2 to 2.5
FOS for CI = 4 to 5
It has No unit
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21. Load factor
It is defined as the ratio of ultimate load to
working load.
Load factor = Ultimate load / Working load
Units is = KN or N
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22. Linear strain
The ratio of change is dimension to original
dimension of material along the direction of
application of load is called linear strain.
Linear strain = Change in dimension/Original
dimension
It has No unit
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23. Lateral strain
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When load is applied along axial direction, the
dimensions like diameter, breath & thickness of
material also change.
Lateral strain = change in diameter/Original
diameter
= change in Breath/Original Breath
= change in Thickness/Original
Thickness
Units is = mm

24. Relationship between stress, strain
& young’s modulus
Young’s modulus = stress/strain
E = σ/e
E = load per area / change in
length
per original
length
E = σ/e = P/A / 𝛿l/l
E = σ/e = P.l / A. 𝛿l
𝛿l = P.l/A.E
Units is = mm 7/8/2020

25. Poisson’s ratio (1/m)
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When the material is loaded with in the elastic
limit, the ratio of lateral strain to linear strain is
called poisson’s ratio.
 It is denoted by the letter (µ) or (1/m)
 Poisson’s ratio = Lateral strain/Linear strain
 Its value is ranges from 0.25 to 0.35
It has No unit