What is the difference between neutral axis and principal axis?
The Neutral axis is the axis passing through the section of a member undergoing pure bending where the stress is zero. Basically it is the point where the member goes from tension to compression. The principal axis can also be related the members in pure bending, but also applies to material under many different types of stress. Essentially it is the orientation of a material under which the normal stresses are maximum and shear stresses are zero. Instead of looking at the material as a whole you are looking at a differential peice of the material. You usually use mohrs circles to find this orientation.
