Principal Stresses and Strains: It has been observed that at any point in a strained material, there are three planes, mutually perpendicular to each other, which carry direct stresses only, and no shear stresses.

A little consideration will show that out of these three direct stresses, one will be maximum, the other minimum and the third an intermediate between the two. These particular planes, which have no shear stress, are known as principal planes. The magnitude of direct stress, across a principal plane, is known as principal stress.

According to Newton’s law of viscosity, the shear stress on a layer of a fluid is directly proportional to the rate of shear strain. Ideal fluid is a fluid with zero viscosity. Real fluids possess viscosity.