Stress and strain

1. Material Science and Technology
Unit-1 Lecture-02
Saumy Agarwal
Asst. Professor (MED), BTKIT

2. Mechanical Properties of Materials
• The mechanical property of a material is the
response of material (or deformation) to an
applied load or force.
• Important mechanical properties are strength,
hardness, ductility, and stiffness.

3. • Load applied can be
i. Tensile
ii. Compressive
iii. Shear
• The magnitude of load applied can be
i. Constant with time (Static Load)
ii. Vary with Time (Fluctuating Load)
Types of Loads

4. Deformation
• When a body is subjected to an external load
then it may-
i. Translate
ii. Rotate
iii. Deform
• Deformation can be either change in shape
(distortion) or change in size (dilatation).
• It can also be either temporary (elastic) or
permanent (plastic).

5. Stress and Strain
• When a load is applied on a body, it tries to
resist the deformation (reason?).
• This resistance by body against the deformation
is known as stress (Stress=Load/Area).
• Strain is ratio of change in dimension of the
body to the original dimension of the body.
• Strain is a dimensional-less quantity and
denoted in percentage.

6. Tensile Test
• The tension test is done obtain the stress-strain
relationship of the given material.
• It is also done to gain the knowledge of the
mechanical properties of material under axial
loading.
• Dimension of the specimen-
i. Gauge Length= 50 mm
ii. Diameter= 12.8 mm

7. Source: Google images
Fig: Standard tensile test specimen

8. Stress-Strain diagram
• The result of a tension test is plotted on stress-strain
diagram.

9. • Hooke’s law: Upto the proportional limit, the
stress (σ) is proportional to the strain (ε). The
proportionality constant is called modulus of
elasticity or Young’s modulus (N/m2).
𝜎 = 𝐸𝜀
𝜎 =
𝑃
𝐴
𝜀 =
∆𝐿
𝐿
=
𝐿 𝑓 − 𝐿 𝑜
𝐿 𝑜

10. Ductile Materials
• They show high plastic deformation before
failure (formation of neck).
• They are strong in tension but weak in shear.
• They have high toughness.
• When strain is more than 15% then the material
is said to be ductile.
• Examples- Mild steel, aluminium, copper,
rubber etc.

11. Brittle Materials
• They show very low plastic deformation before
failure (sudden failure)
• They are weak in tension but strong in shear.
• They have high strength to failure.
• When strain is less than 5% then the material is
said to be brittle.
• Examples- Cast iron, cement, concrete etc.

12. Fig: Stress- strain diagram of ductile and brittle material

13. Thank you