On a Mohr failure envelope diagram, the position of a stress point relative to the envelope and the normal-stress axis indicates the type of fracture that develops: extensional fractures form at very low (near-zero or negative) effective normal stress near the tensile region, hybrid (extensional-shear) fractures form at intermediate normal stress on the curved portion of the envelope, and shear fractures form at higher normal stress along the linear (Coulomb) portion of the envelope.
Progressing along the failure envelope from lowest to highest normal stress: point a (lowest normal stress, near the tensile/near-zero region) corresponds to an extensional fracture;...
Full option-by-option analysis available to enrolled students.