2. Deformation of Solids
Stress and Strain
● Tensile and compressive
forces.
● Understand terms of load,
extension, compression
and limit of proportionality.
● Recall and use Hooke’s Law
Elastic and Plastic
Behaviour
● Understand terms of,
elastic deformation,
plastic deformation and
elastic limit.
● Understand graph of work
done.
● Determine the elastic
potential energy of
material deformation.
01 02
3. Tensile and
Compression
❖ The change of shape or size is called
deformation.
❖ Tensile deformation happens when an
object is stretched (a).
❖ Compressive deformation happens when
an object is squeezed/compressed (b).
4. Tensile strength is the amount of load or stress a
material can handle until it stretches or break.
What is MPa?
Megapascal
1 MPa = 1.000.000 Pa
5. Relationship of tensile strength, force and strain.
❖ What F stands for?
➢ Force
❖ What e stands for?
➢ Strain
❖ What is tensile force?
➢ The amount of load or stress a material can
handle until it stretches or breaks
6. Relationship of tensile strength, force and strain.
❖ What is elastic deformation?
➢ Temporary deformation, due to stress. After
the stress is released, shape of the body is
restored.
❖ What is plastic deformation?
➢ Permanent deformation, due to stress. After
the stress is released, shape of the body
elongate, compress, buckle, bend or twist.
7. Relationship of tensile strength, force and strain.
❖ What is breaking point?
➢ The point where the body breaks.
❖ What is elastic behaviour?
➢ Ability of a deformed body to return to its
original shape and size when the force is
released.
8.
9. 1. The extension of the spring is determined
by how much it has increased in length.
2. The limit of proportionality is the point
beyond which Hooke’s law is no longer
true when stretching a material i.e. the
extension is no longer proportional to the
applied load.
3. The point is identified on the graph where
the line is no longer straight and starts
to curve (flattens out).
4. Hooke’s law also applies to compression
as well as extension. The only difference is
that an applied force is now proportional
to the decrease in length.
5. The gradient of this graph is equal to the
spring constant k. This is explored further
in the revision notes “The Spring Constant”
10.
11. Questions
What is meant by the limit of proportionality?
➢ Limit of proportionality is the point up to which the force is proportional
to the extension
Calculate the spring constant for a spring which extends by a
distance of 3.5 cm when a load of 14 N is hung from its end.
k = F / Δx
F = 14 N
Δx = 3.5 cm = 0.035 m
k = 14 N/0.035 m = 400 N/m
12. A steel extends by 1.5 mm when it is under a tensile force of 45
N. Calculate:
a. Spring constant of wire.
b. Tensile force required to extend it by 1.8 mm and
proportionality is not exceeded.
(a)Why do we use Hooke’s Law?
Because proportionality is not exceeded.
k = F / Δx
F = 45 N
Δx = 1.5 mm = 0.0015 m
k = 45 N/0.0015 m = 30.000 N/m
13. A steel extends by 1.5 mm when it is under a tensile force of 45
N. Calculate:
a. Spring constant of wire.
b. Tensile force required to extend it by 1.8 mm and
proportionality is not exceeded.
(b)Why do we use Hooke’s Law?
Because proportionality is not exceeded.
F = k. Δx
k = 30.000 N/m
Δx = 1.8 mm = 0.0018 m
F = 30.000 x 0.0018 = 54 N
14.
15. ● Tensile stress is the applied force per unit cross sectional area of a material.
● The ultimate tensile stress is the maximum force per original cross-
sectional area a wire is able to support before it breaks.
Stress
16. ● Strain is the extension per unit length.
● This is a deformation of solid due to stress in the form of elongation and
contraction.
● Strains is a dimensionless unit because its the ratio of lengths.
Strain
17. Young’s Modulus
❖ The Young Modulus is the measure of the ability of material to
withstand changes in length with an added load (how stiff
materials are).
❖ Modulus Young represents the elasticity of a material.
❖ The Young Modulus is defined as the ratio of stress and strain.
18. Young’s Modulus
❖ Its unit is the as stress: Pa.
❖ Just like the Force-Extension
graph, stress and strain are
directly proportional to one
another for a material exhibiting
elastic behavior.
❖ A stress-strain graph is a straight
line with its gradient equal to
Young Modulus.
20. The heels on a pair of women’s shoes have a radius of .5
cm at the bottom. If 30% of the weight of a woman 480N
is supported by each heel, find the stress on each heel.
25. Spring constant (k1 = k2 = k3) = 200 N/m
extension = 1 cm
what is the magnitude of the weight?
Force that pulls the spring is weight.
Hence Weight = F
26. Spring constant (k1 = k2 = k3 = k4) = 500 N/m
weight (w) = 20 Newton
calculate the extension
