The relative displacements of the various parts of an elastic body under the action of an external force produce an internal force tending to restore the body to its original configuration. Such internal force, generated in the body by the external force per unit area of the body is known as stress.

According to Newton’s third law, if the system in the body is in equilibrium, the internal force is equal in magnitude but opposite in direction to the applied force. In this case, the externally applied force per unit area of the body in a state of equilibrium measures the stress.

The kind of deformation on a body depends on the way forces are applied. There are applied.

There are four types of deformations, three of which are shown in Figure 7.

In this section, we will focus on tensile deformation and in the next section, shear deformations may be considered.