Stresses, strains and Mohr circles
The concept of stress, deﬁned as force per unit area, was introduced into the theory of elasticity by Cauchy in about 1822. It has become universally used as an expedient in engineering design and analysis, despite the fact that it cannot be measured directly and gives no indication of how forces are transmitted through a stressed material. Clearly the manner of transfer in a solid crystalline material, such as a metal or hard rock, is different from the point-to-point contacts in a particulate material, such as a soil. Nevertheless, in both cases it is convenient to visualize an imaginary plane within the material and calculate the stress across it by simply dividing the force across the plane by the total area of the plane.