# Elasticity

When an external force is applied to a rigid body there is a change in its length, volume (or) shape. When external forces are removed the body tends to regain its original shape and size. Such a property of a body by virtue of which a body tends to regain its original shape (or) size when external forces are removed is called elasticity.

## Elastic Stress and Strain

### What is Stress?

When the body is deformed by the application of external forces, forces within the body are brought into play. Elastic bodies regain their original shape due to internal restoring forces. The internal forces and external forces are opposite in direction.  If a force F is applied uniformly over a surface of area A then the stress is defined as the force per unit area.

Stress = Force/Area

S.I unit for stress is Nm-2

### Types of Stress

There are three types of stress

• Longitudinal stress
• Volume stress or Bulk Stress
• Tangential stress (or) shear stress

#### Longitudinal Stress

When the stress is normal to the surface area of the body and there is a change in the length of the body it is known as longitudinal stress.

Again it is classified into two types

• Tensile stress
• Compressive stress.

Tensile stress: When longitudinal stress produced due to an increase in the length of the object is known as tensile stress.

Compressive stress: Longitudinal stress produced due to the decrease in length of the object is known as compressive stress.

#### Volume Stress or Bulk Stress

If equal normal forces applied to the body causes a change in volume of the body, the stress is called volume stress.

#### Tangential Stress

When the stress is tangential (or) parallel to the surface of the body is known as Tangential (or) Shear stress. Due to this shape of body changes (or) gets twisted.

### What is a Strain?

A body under stress gets deformed. The fractional change in the dimension of a body produced by the external stress acting on is called strain. The ratio of charge of any dimension to its original dimension is called strain. Since strain is the ratio of two identical physical quantities, it is just a number. It has no unit and dimension.

Strain=\frac{Change\,in\,dimension}{initial\,dimension}

Strain also classified into three types

• Longitudinal strain
• Volume strain
• Shearing strain or tangential strain

#### Longitudinal Strain

The strain under longitudinal stress is called longitudinal strain.

Longitudinal\,strain=\frac{Change\,in\,length\,of\,the\,body}{initial\,length\,of\,the\,body}

=\frac{\Delta L}{L}

#### Volume Strain

The strain caused by the volume stress is called volume strain.

Volume\,strain=\frac{Change\,in\,volume\,of\,the\,body}{\,Original\,volume\,of\,the\,body}

=\frac{\Delta V}{V}

#### Shearing Strain

When a deforming force is applied to a body parallel to its surface its shape (not size) changes this is known as shearing strain. The angle of shear \phi

\tan \phi =\frac{\ell }{L}=\frac{displacement\,of\,upper\,face}{distance\,between\,two\,faces}

### Stress- Strain Curve

1. Proportion limit: The limit in which Hooke’s law is valid and stress is directly proportional to strain.
2. Elastic limit: That maximum stress which on removing the deforming force makes the body recover its original state completely.
3. Lower Yield point: The point beyond the elastic limit at which the length of the wire starts increasing with increasing stress. It is defined as the yield point.
4. Breaking point or Fracture Point: The point when the strain becomes so large that the wire breaks is called the breaking point.

#### Elastic hysteresis

The strain persists even when the stress is removed. This lagging behind of strain is called elastic hysteresis. This is why the values of strain for the same stress are different while increasing the load and while decreasing the load.

## Hooke’s Law

If deformation is small, the stress in a body is proportional to the corresponding strain, this fact is known as Hooke’s law.

Within elastic limit, Stress & strain \Rightarrow \frac{Stress}{Strain}=Constant

This constant is known as modulus of elasticity (or) coefficient of elasticity. The elastic modulus has the same physical unit as stress. It only depends on the type of material used. It is independent of stress and strain. The modulus of elasticity is of three types.

• Young’s modulus of elasticity “y”
• The bulk modulus of elasticity “B”
• Modulus of rigidity

### Young’s modulus of elasticity “y”

Within the elastic limit, the ratio of longitudinal stress and longitudinal strain is called Young’s modulus of elasticity (y).

y=\frac{Longitudinal\,stress}{Longitudinal\,strain}=\frac{\frac{F}{A}}{\frac{\ell }{L}}=\frac{FL}{A\ell }

Within the elastic limit, the force acting upon a unit area of a wire by which the length of wire becomes double is equivalent to Young’s modulus of elasticity of the material of the wire. If L is the length of wire, r – radius and is the increase in the length of wire by suspending a weight (mg) at its one end then young’s modulus of elasticity of the wire becomes,

y=\frac{F/A}{\ell /L}=\frac{FL}{A\ell }=\frac{mgL}{\pi {{r}^{2}}\ell }

(a) The increment of the length of an object by its own weight:

Let a rope of mass M and length (L) is hanged vertically. As the tension of different point on the rope is different, similarly stress as well as the strain will be different at different points.

• Maximum stress at hanging point
• Minimum stress at a lower point

Consider a dx element of rope at x distance from the lower end, then tension;

T=\left( \frac{M}{L} \right)\times g

So stress =\frac{T}{A}=\left( \frac{M}{L} \right)\frac{xg}{A}

Let the increase in length of element dx is dy then

Strain=\frac{Change\,in\,length}{Original\,length}=\frac{\Delta y}{\Delta x}=\frac{dy}{dx}

Now we got stress and strain then young’s modulus of elasticity “y”

y=\frac{Stress}{Strain}=\frac{\left( \frac{M}{L} \right)\frac{xg}{A}}{\frac{dy}{dx}}\Rightarrow \left( \frac{M}{L} \right)\frac{xg}{A}dx=\frac{1}{dy}

The total change in length of the wire is

\frac{Mg}{LA}\int\limits_{o}^{L}{x\,dx}=y\int\limits_{o}^{\Delta l}{dy}

\frac{Mg}{LA}\frac{{{L}^{2}}}{2}=y\Delta \ell

\frac{MgL}{2Ay}=\Delta \ell

(b) Work done in stretching a wire

If we need to stretch a wire, we have to do work against its inter atomic forces, which is stored in the form of elastic potential energy.

For a wire of length (L0) stretched by a distance \left( x \right) the restoring elastic force is

F=(Stress)(Area)=y\left[ \frac{x}{{{L}_{0}}} \right]A

Work required for increasing an element length

dW=Fdx=\frac{{{y}_{A}}}{{{L}_{0}}}x\,dx

Total work required in stretching the wire is

W=\int\limits_{0}^{\Delta \ell }{Fdx=\frac{{{y}_{A}}}{{{L}_{0}}}}\int\limits_{0}^{\Delta \ell }{x\,dx}

=\frac{{{y}_{A}}}{{{L}_{0}}}\left[ \frac{{{x}^{2}}}{2} \right]_{0}^{\Delta \ell } =\frac{{{y}_{A}}{{\left( \Delta \ell \right)}^{2}}}{2{{L}_{0}}}

(c) Analogy of rod as a spring

From definition of young’s modulus

y=\frac{Stress}{Strain}=\frac{FL}{A\,\Delta L}

F=\frac{{{y}_{A}}\Delta L}{L}

This expression is analogy of spring force

F=kx

k=\frac{yA}{L}=constant

### Bulk Modulus (B)

Within elastic limit the ratio of the volume stress and the volume strain is called bulk modulus of elasticity.

B=\frac{Volume\,stress}{Volume\,strain}=\frac{\frac{F}{A}}{-\frac{\Delta V}{V}}=\frac{\Delta P}{\frac{-\Delta V}{V}}

### Rigidity Modulus

Within elastic limit, the ratio of shearing stress (or) tangential stress and shearing strain (or) tangential strain is called modulus of rigidity.

\eta =\frac{Shearing\,stress}{Shearing\,strain}=\frac{\frac{{{F}_{tangential}}}{A}}{\phi }

\phi -angle of shear.

### Poisson’s Ratio

Within the elastic limit, the ratio of lateral strain (or) transverse train and longitudinal strain is called Poisson’s ratio. In case of a circular bar of material, the change in the diameter of the circular bar material to its diameter due to deformation in the longitudinal direction.

Poisson’s\,ratio(\sigma )=\frac{lateral\,strain}{longitudinal\,strain}=\frac{\beta }{\alpha }