Mohr's circle [SEP] named after
Mohr's circle
Mohr's circle, named after Christian Otto Mohr, is a two-dimensional graphical representation of the transformation law for the Cauchy stress tensor.
Mohr's circle is often used in 
calculations relating to mechanical engineering for materials' strength, geotechnical engineering for strength of soils and, structural engineering for strength of built structures respectively. Examples of these are to be found in many soil mechanics textbooks such as "Smith's Soil Mechanics" by Smith and "Statics: Engineering Mechanics" by Bedford and Fowler
among many many other resources. It is also used for calculating stresses in many planes by reducing them to vertical and horizontal components. These are called principal planes in which principal stresses are calculated; Mohr's circle can also be used to find the principal planes and the principal stresses in a graphical representation. 
After performing a stress analysis on a material body assumed as a continuum, the components of the Cauchy stress tensor at a particular material point are known with respect to a coordinate system. The Mohr circle is then used