Stress is nothing but the force applied to the cross-sectional area and strain is nothing but change in dimensions of given material by original dimensions, after the load is applied. Think of a long bar, with one end fixed to a wall, and you’re pulling on the other end. Stress is how hard you’re pulling. The strain is how far the bar has stretched.
Let us understand stress-strain in brief-
When any body is subjected to external loading, then elements from the body will try to oppose that force, that internal resistance offered by elements from the body is known as stress.
When force is applied to any deformable body which behaves as a continuum i.e. its mass is continuously distributed through the body, internal forces manifest inside the body. Quantitatively, stress is the average internal force acting upon a unit area of a surface within the body. Its SI unit is Pascal (N/m^2).
In the context of three-dimensional bodies, there are two types of stresses, shear stress and normal stress. The former occurs as a reaction to body forces and the latter as a reaction to surface forces. Body forces act on the entire volume of the body e.g. gravitational force and surface forces act across a particular internal or external surface element of the body.
Normal stress and Shear stress
Often in real life, most mechanical bodies experience both types of stresses simultaneously.
It is the ratio of change in dimension of the body due to external loading to original dimension of the body before loading.
When force is applied to a body similar to the one described above, its configuration (set of positions of all particles of the body) tends to change. This is called deformation. It often involves a change in shape, but the definition of deformation includes rigid body motions which do not involve shape change. The term strain refers to the relative deformation of a body. In two dimensions, it would refer to the relative change in length, which is (new length/old length). It is dimensionless.