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