3. INTRODUCTION
Energy methods are widely used to
obtain solutions to elasticity problems
and determine deflection of structures.
3
• Like deflections of joint on a truss or
points on a beam or shaft.
In energy method, strain energy is
associated with it. Hence it is also
known as Strain energy method.
4. P
Strain energy is the energy stored in the material due to
deformation under external load.
STRAIN ENERGY
5. P
Strain energy is the energy stored in the material due to
deformation under external load.
STRAIN ENERGY
11. 1 1
STRAIN
ENERGY
CALCULATION
UNDER AXIAL LOADING
Strain Energy [ U ] = × ∆l × P
Since , ∆l =
Strain Energy [ U ] = × × P
Strain Energy [ U ] =
P = Force applied
l = Length of the
bar
E = Young's
Modulus
A = Area of the
bar
Where ,
J or N-m
14. 1 4
STRAIN
ENERGY
CALCULATION
UNDER TORSION
Strain Energy [ U ] = × θ × Τ
Since , θ =
Strain Energy [ U ] = × × Τ
Strain Energy [ U ] = J or N-m
Τ = Applied Torque
l = Length of the
bar
G = Modulus of
Rigidity
J = Polar moment of
inertia
Where ,
18. 1 8
STRAIN
ENERGY
CALCULATION
UNDER BENDING
Strain Energy [ U ] = ×
𝑙
𝑅
× M
Since ,
𝑀
𝐼
=
𝐸
𝑅
Strain Energy [ U ] = × × M
Strain Energy [ U ] =
Since , 𝑙 = 𝑅𝜃
𝑀𝑙
𝐸𝐼
𝑀2
𝑙
2𝐸𝐼
This Equation is valid only for “ PURE BENDING
J or N-m
19. 1 9
STRAIN
ENERGY
CALCULATION
UNDER BENDING
dx
Suppose a bending stress W is acting on the bar.
Let , on the dx portion bending moment is M.
Strain Energy [ dU ] =
𝑀2
𝑑𝑥
2𝐸𝐼
l
W
Strain Energy [ U ] =
0
𝑙
𝑀2𝑑𝑥
2𝐸𝐼
For the whole bar
20. 2 0
EXERCISE
3KN
5KN
2 m 1 m
Find the strain energy (in N-m ) stored in the beam.
EI = 𝟏𝟎𝟒
.
x
O B
A
𝑀𝐴𝐵 = −3𝑥
Moment for the section AB will be __
Where x = 0 to 1
Similarly , Moment for the section OA will be __
𝑀𝑂𝐴= −3(𝑥 + 1) − 5𝑥
𝑀𝑂𝐴= −8𝑥 − 3
Where x = 0 to 2
x
21. 2 1
EXERCISE
3KN
5KN
2 m 1 m
Find the strain energy (in N-m ) stored in the beam.
EI = 𝟏𝟎𝟒
.
x
O B
A
x
Here the total strain energy U will be ___
𝑈 = 𝑈𝑂𝐴+ 𝑈𝐴𝐵
𝑈 =
𝑀𝑂𝐴
2
𝑑𝑥
2𝐸𝐼
+
𝑀𝐴𝐵
2
𝑑𝑥
2𝐸𝐼
𝑈 =
0
2
(−8𝑥 − 3)2 𝑑𝑥
2𝐸𝐼
+
0
1
(−3𝑥)2 𝑑𝑥
2𝐸𝐼
22. 2 2
EXERCISE
3KN
5KN
2 m 1 m
Find the strain energy (in N-m ) stored in the beam.
EI = 𝟏𝟎𝟒
.
x
O B
A
x
𝑈 =
1
2𝐸𝐼
[ 0
2
64𝑥2 + 9 + 48𝑥 𝑑𝑥 + 0
1
9𝑥2 dx ]
𝑈 =
1
2×104[
64×8
3
+ 9 × 2 + (48 × 2)]
𝑈 = 14.38 𝑁 − 𝑚