Mohr's circle

Summary

Mohr's circle is a graphical picture of how the normal and shear stress on a plane through a point change as the plane is rotated. In two dimensions, the pairs (normal stress, shear stress) for all orientations lie on one circle. Its centre is the mean normal stress and its radius is the maximum shear stress. The circle meets the horizontal axis at the principal stresses, so it gives them, and the maximum shear stress, at a glance.