In this study, a simple approach is proposed for the formulation of a rock failure criterion based on octahedral shear stress
theory, while maintaining and modifying some of the given restrictions.
The motivation for the combined compression-and-shear plate-impact experiments stems from the fact that in classical metal plasticity based on dislocation mechanisms, the volumetric and shear behaviors are typically considered uncoupled: the classical plasticity theory addresses shear only, and the von Mises criterion leads to a convenient unifying constitutive description that is independent of the deformation rates in terms of the octahedral shear stress
. Even in the elegantly developed theory of linear viscoelasticity (1-3), the volumetric and shear behaviors are considered to be uncoupled.
The process is driven by the mean stress [bar.[sigma]] and the octahedral shear stress
[tau], to be defined below.
Where [[tau].sub.oct] is the octahedral shear stress
Sweeney and Ward  showed that in PVC above the glass transition temperature, the different modes of uniaxial, planar, and equibiaxial tensile deformation are comparable provided the results are expressed in terms of the octahedral shear stress
The octahedral shear stress
, [[tau].sub.oct], and hydrostatic stress, [[sigma].sub.h], are defined as:
Conversion from axial stress to octahedral shear stress
could also be made easily, but since data is plotted here as principal stresses such a conversion was not made.
According to their analysis the maximum octahedral shear stress
and principal stress are 141% and 35.9% higher around the crack-tip void than around the crack-tip rubber particle with the maximum hydrostatic tension unchanged.