The force applied per unit area in mechanics is understood as stress.
σ=FA
The ratio of internal force F that is produced when a substance is deformed, to the area A where force is applied is referred to as stress.
Strain can be referred to as the ratio of the amount of deformation that the body experiences in the direction of force applied to the initial sizes of the body. The relation of deformation in terms of the length of the solid is shown below:
ε=δlL
where,
As strain defines the relative change in shape and it's a dimensionless quantity.
The material's stress-strain curve delineates the connection between stress and strain for materials. In other words, a stress-strain curve is a graphical representation that shows the reaction of a material when a load is applied.