Ideal Fracture Displacement Modes

Rock deformation experiments of rocks loaded to failure under triaxial compression demonstrate symmetrical orientation of fractures with respect to the three effective principal stresses: σ₁’ > σ₂’ > σ₃’ where compressive stress is positive. The type of fracture that will develop is dependent upon the value of minimum effective principal stress (σ₃’), the difference between the maximum and minimum effective principal stresses (σ₁’- σ₃’), and the tensile strength of the rock.

There are three ideal displacement modes of fractures based on the angle with respect to σ₁’: mode I, mode II, and mode III.

 

Mode I displacement is referred to as opening or tensile mode and is purely extensional. These fractures develop at an orientation perpendicular to σ₃’ and within the σ₁’ stress plane.

 

Mode II displacement is referred to as forward shear mode, where the fracture surfaces slide over one another in a direction perpendicular to the fracture tip.

 

Mode III displacement is referred to as transverse shear mode, where the fracture surfaces move relative to one another in a direction parallel to with the fracture tip. Both mode II and III fracture planes are oriented parallel to σ₂’ and at an angle less than 45° to σ₁’.

Source: Agust Gudmundsson (2011) Rock Fractures in Geological Processes