Mohr diagram showing a Coulomb failure envelope based on a set of experiments with increasing differential stress. The circles represent differential stress states at the instant of shear failure. The envelope is represented by two straight lines, on which the dots represent failure planes.(a) A brittle failure envelope as depicted on a Mohr diagram. Within the envelope (shaded area), stress states are stable, but outside the envelope, stress states are unstable. (b) A stress state that is stable, because the Mohr circle, which passes through values for ?1 and ?3 and defines the stress state, falls entirely inside the envelope. (c) A stress state at the instant of failure. The Mohr circle touches the envelope. (d) A stress state that is impossible.A representative composite failure envelope on a Mohr diagram. The different parts of the envelope are labeledSketches of the fracture geometries that form during failure. Note that the geometry depends on the part of the failure envelope that represents failure conditions, because the slope of the envelope is not constant
Pic courtesy: Earth Structure – An introduction to structural geology and tectonics
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