Elasticity and stress strain curve

1. 10+2 PHYSICS
ELASTICITY & STRESS-STRAIN CURVE
KAJAL SARMAH
BE(Aeronautical),MTech(Mechanical)

2. What is Elasticity?

3. • Elasticity is a property of a material. When an
external force is applied in a material, its
molecular orientation gets disturbed.
Molecules get displaced from their mean
position but their tendency is to come back to
original position and for that they offer
resistance. This tendency of materials is
Elasticity.

4. STRESS & STRAIN
• It is law of nature that change is always
resisted. On application of external force, they
offer resistive force to avoid displacement of
molecules. This resistance force per unit area
is Stress. Its unit is N/m2.
• So when external force is applied, dimension
of materials is changed. Change in dimension
divided by Original dimension is termed as
Strain. It is unit less.

5. Stress or Strain, what is developed
first?
• Now when an external force is applied,
molecules of materials fight with the force to
avoid deformation. But eventually a time will
come when molecules deformation starts. At
that situation Resistance force comes into play
to resist the effect of external force.
• That is why Strain comes first, then after
Stress.

6. HOOK’S LAW
• Within elastic limit of a body, stress is
proportional to strain and strain is
proportional to stress.
• If we remove sign of proportionality by equal
sign, we have to add a constant.
• This constant is Young modulus as it is named
after the scientist Young.

7. Stress-Strain curve

8. Proportional limit and Elastic limit
• Although they both sound same, they are
different. For some materials they are same.
• Proportional limit is the limit up to which
stress and strain is proportional to each other.
• Elastic limit is the limit beyond which if
force(stress) is applied, the body will not come
to its original shape and size.

9. Permanent set
• It is the phenomenon just before plastic
behaviour of material starts.
• It is the amount by which material is stressed
beyond its elastic limit and fails to return its
original dimension.

10. Fracture
• It is physical separation of an object/material
in to 2 or multiple segment under the action
of stress(force).
• It always occurs after the yield point except for
brittle materials.
• Brittle materials do not have well defined yield
point.
• Sometimes brittle materials may have fracture
even before yield point.

11. Comparison of brittle and ductile
materials