1. MECHANICAL PROPERTIES OF MATERIALS
Department of Physics
KL University

2. Objectives
Identification of elastic and plastic behaviour of materials through molecular level
Behaviour of ductile and brittle materials with external load
Measurement of various properties like hardness and toughness of materials

3. Basic Terms Stress Strain Types of Stress Types of Strain Hooke’s law
Elasticity
Plasticity
Ductility
Brittleness
Hardness
Toughness
Fatigue
Creep
Fracture

4. Elasticity Plasticity
Regains to original shape
Deforms permanently

5. Elastic deformation Molecular approach
1. Initial
2. Small load
3. Unload F d
bonds
stretch
return to
initial
Elastic means reversible

6. Plastic deformation Molecular approach
1. Initial
2. Small load
3. Unload p
lanes
still
sheared F d
elastic + plastic
bonds
stretch
& planes
shear
plastic
Plastic means permanent
Process of plastic deformation in crystals is by slip process (motion of dislocation) and in non-crystalline solids, the plastic deformation is by viscous flow mechanism.

8. Stress:
The internally developed forces per unit area of a material due to the application of external force.
Its SI unit is Pascal (or) N/m2

9. Strain Fractional change in the dimensions of a material due to the application of external force.
It has no unit

10. Types of Stress & Strain
Tensile stress – Longitudinal Strain
Compressive Stress – Volume Strain
Shear stress – Shear Strain

11. Tensile Stress – Longitudinal Strain
Tension F W
Equal and opposite forces directed away from each other

12. Compressive Stress Longitudinal strain
Compression F W
Equal and opposite forces directed towards each other

13. Shear Stress – Shear Strain
Tangential force

14. Hooke’s Law Within elastic limit, Stress α Strain Stress = E x Strain
E - Modulus of elasticity, Unit : N/m2

15. Different modulli of elasticity

16. Modulus of elasticity of materials depends on bond strength between atoms, stronger the bond, larger will be the modulus of elasticity.
Values of the modulus of elasticity for ceramic materials are about the same as for metals; for polymers they are lower. These differences are a direct consequence of the different types of atomic bonding in the three materials types.

17. Increase in temperature of material, decreases the modulus of elasticity.

18. Poisson’s Ratio: If lateral strain along x- and y- directions is same
loaded
unloaded
If lateral strain along x- and y- directions is same
(ε x = ε y),material may be isotropic.

19. Material – thin wires – withstands plastic deformation
DUCTILITY
Material – thin wires – withstands plastic deformation

20. Fractures without deformation
BRITTLENESS
Fractures without deformation

21. Hardness:. - withstand plastic deformation or
Hardness: - withstand plastic deformation or indentation produced in the material.
Creep:
- time dependent deformation at
constant load

22. Strength : Ability to withstand loads
Types of strength
1.Yield strength :
Strength beyond which it
exhibits plasticity
2. Tensile strength (or)
Ultimate strength :
Strength at which the material breaks or fractures

23. Tensile Strength TS engineering stress strain engineering strain
• Maximum stress on engineering stress-strain curve. y
strain
Typical response of a metal
F = fracture or
ultimate
strength
Necking acts as stress concentrator
engineering TS
stress
engineering strain
• Metals: TS occurs when noticeable necking starts.
• Polymers: TS occurs when polymer chains are aligned
and about to break.

24. Toughness : Energy absorbed up to fracture
Toughness : Energy absorbed up to fracture. Fatigue: Failure under cyclic (or) repeated stress Fracture: Breakage of a material into separate parts under the action of stress

25. Engineering Stress-Strain Diagram
ultimate
tensile strength 3
necking
Slope=E
Strain
Hardening
yield
strength
Fracture 5 2
Elastic region
slope =Young’s (elastic) modulus
yield strength
Plastic region
ultimate tensile strength
strain hardening
fracture
Plastic
Region
Stress (F/A)
Elastic
Region 4 1
Strain ( ) (DL/Lo)

26. Stress – strain curve – brittle and ductility

27. Engineering and True Stress-Strain
Engineering stress
(σn ) = F/A0
A0 – original area of cross section
Engineering strain
εn = (L-L0)/L0

28. Engineering and True Stress-Strain Curve

29. Hardness Tests Indentation Method
Brinell Hardness Test
Rockwell Hardness Test
Vicker Test

30. VH = 1.854F/d2 , d = (d1+d2)/2