The mechanical stress state in the subsurface is on a grand scale the resultant of forces linked to the solar system: the attraction of the sun and the moon and the centrifugal forces related to Earth’s movements. The geometry of a stress state determines the deformation that rocks undergo. For years, geology has used deformation to determine present and past tectonic stresses. This analysis is done on various scales: by observing fold geometry and mapping faults in the field, and by laboratory study the deformation of the elements that make up the rocks. Many rocks belong to a transition category called elasto-plastic, often involving an elastic section at the center of the function, surrounded by two plastic extremities. A triaxial test that has caused a sample to rupture and its corresponding Mohr circle describe the maximum strength of the sample under the particular conditions of the test stresses.