We have discussed various topics in the category of thermal engineering, now we are going to start our discussion on another category of mechanical engineering here i.e. strength of material. First we will see basic terms and principles in strength of material.

Today we will see here the basic concept of stress and strain with the help of this post.

As we know that a body will have large number of molecules or we can say that body consist molecules or small-small particles. Molecular forces act between these molecules of the body. If we will apply an external force over the body then there will be some changes or deformations in the body.

But molecular forces which will be acting between the molecules of the body will resist the deformation in the body due to the external force and this is termed as internal resistance developed by the body in order to resist the deformation in the body due to the external force.

Resistance by which material of the body resists the changes or deformations in the boy will be termed as strength of material.

### Stress

When an external force will be applied on a body, there will be deformation or changes in the size and shape of the body. But molecular forces which will be acting between the molecules of the body will resist the deformation in the body due to the external force and this is termed as internal resistance developed by the body in order to resist the deformation in the body due to the external force.

This internal resistance force per unit area will be termed as stress.

We must note it here that one loaded member will remain in equilibrium condition when applied force or load and internal resistance developed by the body against the deformation are same.
Let us see here one uniform bar and axial force P is acting over the bar as shown in figure. If we consider one section x-x, we can also see here the internal resistance force developed by the body against the deformation. Now, we will see the mathematical expression of stress.

#### Stress = Internal resistance force/area of cross section

Let us see now the unit of stress, stress unit will be based on the unit of external force applied and unit of area. If we consider the S.I unit system, unit of external force will be Newton i.e. N and unit of area will be m2 and therefore unit of stress will be N/m2.

Let us see here the various types of stress
1. Tensile stress
2. Compressive stress
3. Shear stress

We will study and analyze each type of stress in detail in our next post.

### Strain

As we have already discussed that when an external force will be applied on a body, there will be deformation or changes in the size and shape of the body. Strain is basically defined as the ratio of change of dimension of the body to the original dimension of the body. There will not be any unit for strain and hence it will be considered as dimensionless term.

Let us see here the following figure, axial force is acting over the bar and hence there is change in length of the bar as shown in figure. Strain will be determined here as the ratio of change in length of the bar to the original length of the bar and we have displayed same in following figure.
Let us see here the various types of Strain
1. Tensile strain
2. Compressive strain
3. Shear strain
4. Volumetric strain

We will study and analyze each type of strain in detail in our next post.

### Reference:

Strength of material, By R. K. Bansal
Strength of material, By S. Timoshekno

We will see another important topic i.e. types of stress and strain in the category of strength of material.