Science > Physics > Elasticity > Classification of Materials
In this article, we shall study the deformation in the body due to the application of deforming force and classification of materials on this behaviour.
Effects of Applied Force on a Body:
- If the forces acting on a body are unbalanced i.e. the resultant force acting on the body is not zero, then the body sets into translational motion.
- If the forces acting on a body are balanced but the net moment is not zero i.e. the resultant force acting on the body is zero but the resultant moment is not zero, then the body sets into rotational motion.
- If the forces on the body are balanced and there is no net moment for the body then the force may produce a change in size, shape or both in the body.
Deformation and Deforming Force:
The forces applied to a body can produce a change in shape or size or both the shape and size of the body. Such a change is called deformation. The forces which produce deformation in the body are called deforming forces.
Characteristics of Deformation:
- The extent of deformation is always directly proportional to the deforming force.
- Due to deforming force the length of wire (chord) or volume of a body or shape of body changes.
- When deformation takes place, the body is said to be strained or in a deformed state.
- In the deformed state, the applied force is numerically equal to internal elastic restoring force within a body but in the opposite direction.
- Deformation produced in a body is due to change in relative positions of the molecules within a body, due to applied deforming force.
Classification of Material on the Basis of Deformation:
Every engineering structure is subjected to external loading of various kinds (normal load, shear load or mixed load such as bending, torsion, etc.). When a substantial load is applied on the material, it automatically tends to deform. Depending upon the response of the material towards the deforming force, the materials are classified into three types: elastic material, plastic material, and rigid material. Corresponding material properties are elasticity, plasticity, and rigidity respectively.
Elasticity:
The property by virtue of which material bodies regain their original dimensions (size, shape or both) after removal of deforming force is called elasticity. The material exhibiting elasticity is called elastic material and the body is called the elastic body. e.g. Rubber, Steel, Aluminum, Sponge etc.
As solids have a definite shape and definite volume, therefore, they alone possess elasticity of shape as well as volume. Liquids have a definite volume but an indefinite shape. Hence, they possess volume elasticity. Gases possess volume elasticity.
When the body is stretched the inter-atomic spacing increases and when it is compressed the inter-atomic spacing decreases. In both cases, internal forces are created in the body which tend to restore the atoms back to their original positions. Such internal forces are called internal elastic forces or restoring forces. The magnitude of restoring or internal elastic force is the same as the applied force. These restoring forces are responsible for the elastic property of the body.
Characteristics of Elasticity:
- Elasticity is the property by virtue of which material bodies regain their original dimensions (size, shape or both) after removal of deforming force (external force).
- When the body is stretched the inter-atomic spacing increases and when it is compressed the inter-atomic spacing decreases. In both cases, internal restoring forces are created in the body which tend to restore the atoms back to their original positions.
- The deforming force required is small.
- The amount of elastic deformation is very small.
- Within elastic limit the Hooke’s law is applicable.
- Within the elastic limit, the stress-strain graph is a straight line.
- Energy absorbed by the material during elastic deformation is called a module of resilience.
- Elastic deformation Occurs before plastic deformation.
- The mechanical and metallurgical properties of the solid material remain unaltered even after deformation.
- It is important concept in designing mechanical and civil structures, frames and equipment.
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Plasticity:
When a body is acted upon by deforming forces the shape and/or size of the body changes. But, if the deforming forces are removed, the body retains its new shape and size. Such body is called a plastic body and the property is called plasticity. e.g. Plaster of Paris, Clay, Mud, Plastic, etc. show plasticity.
In plastic bodies, no internal elastic force or restoring force is produced. If produced they are negligible. Hence they cannot regain original shape but in the absence of restoring force, they retain their new shape.
A body which can easily be deformed when a very small deforming force is applied and which does not regains its original shape or size or both but retains its new shape is called a perfectly plastic body. No body in the universe is a perfectly plastic body. Wall putty can be considered as a perfectly plastic body.
Characteristics of Plasticity:
- Plasticity is the property by virtue of which material bodies retain their new dimensions (size, shape or both) even after removal of deforming force (external force).
- Due to no or negligible internal elastic force or restoring force they cannot regain original shape but retain their new shape.
- The deforming force required is large.
- The amount of elastic deformation is very large.
- Hooke’s law is not applicable.
- In the plastic region, the stress-strain graph is non-linear.
- Total energy absorbed by the material during the elastic and plastic deformation region is called module of toughness.
- Plastic deformation occurs only after the elastic deformation.
- Many properties of the solid material change after plastic deformation.
- It is important concept in sheet metal working, moulding, rolling, forging, riveting, etc.
Rigidity:
When a body is acted upon by deforming forces, the shape and size of the body do not get altered, whatever may be the magnitude of deforming forces. Such body is called a rigid body and the property is known as rigidity.
In rigid bodies, the internal force of attraction is so high that there is no relative motion between two particles of the body. Hence there is no change in the shape of the body.
In practice, it is not possible for us to get a perfectly rigid body. But large blocks of metal, stones can be considered rigid bodies.
Characteristics of Rigidity:
- Rigidity is the property by virtue of which material bodies do not undergo a change in their original dimensions (size, shape or both) whatever be the magnitude of the deforming force (external force).
- In this case, the distances between the constituent particles do not change.
- However, maybe the large force is applied there is no change in the shape of the body.
- Hooke’s law is not applicable.
- As there is no change in the shape of a rigid body, no energy of deformation is in it.
- There is a stress in rigid material but no strain.
To prove that within the elastic limit, Young’s modulus of the material of wire is the stress required to double the length of wire.
As the length is doubled, change in length = l = 2L – L = L
Strain = l / L = L/L = 1
Now, Young’s modulus of elasticity is given by
Y = Stress / Strain = Stress/1 = Stress
Hence within the elastic limit, Young’s modulus of the material of wire is the stress required to double the length of wire.
Distinguishing Between Elasticity and Plasticity:
| Elasticity | Plasticity |
| The body exhibiting elasticity regains its shape or size after the removal of the external force. | The body exhibiting plasticity retains its new shape or size after the removal of the external force. |
| There is a temporary change in dimensions of the body on the application of the deforming force. | There is a permanent change in dimensions of the body on the application of the deforming force. |
| The internal restoring force is set up inside the body. | No Internal restoring force is set up inside the body. Or they are very negligible. |
| The ratio of stress to corresponding strain produced is constant. | The ratio of stress to the corresponding strain produced is not constant. |
| materials exhibiting elasticity: Steel, rubber, etc | Materials exhibiting plasticity: PVC, plaster of Paris, wax, etc. |
Scientific Reasons:
Pressure and stress have the same dimensions but are two different physical quantities.
Both the pressure and the stress have the same dimensions [L-1M1T-2], but the pressure is a scalar quantity while stress is a tensor quantity. Hence they are two different physical quantities.
Steel is more elastic than a rubber.
The elastic properties depend on Young’s modulus of elasticity. Greater the value of Young’s modulus, the more the elastic material is. The value of Young’s modulus of elasticity is more for steel than that for rubber. Hence steel is more elastic than rubber.
Next Topic: Concept of Longitudinal Stress, Strain, and Young’s Modulus of Elasticity