Principle stresses and strains

There are generally two types of stresses, which we deal with in the structural mechanics or structural analysis of the Civil engineering. One is normal stress and another is shear stress. An element of a structure can be loaded with different combinations of the loading. There can be normal loading or shear loading on a face or the combination of both. These combination of loads results in the induction of the maximum stresses along some planes. There will be one particular plane of the element where the shear stress will be equal to zero but the normal stress will be maximum.
Principle Stress: 
This is the normal stress on a particular plane of a structural element subjected to external loads, where its value is maximum, but the value of the shear stress is zero.
Strain produced due to the principle stress is known as the principle strain.