The plastic deformation of brittle solids

Many normally brittle solids can undergo considerable plastic deformation when, in addition to an axial stress, they are subjected to a hydrostatic pressure. If the brittle strength of the material is Og and the specimen has a hydrostatic pressure p applied to it, the tensile stress needed to produce brittle fracture is Og + p. This tensile stress gives rise to a shear stress resolved in the slip direction in the slip plane and if this resolved shear stress reaches a value corresponding to the axial yield stress Oy before it reaches Og 4- p the material will deform plastically without brittle fracture taking place. 

This change in behaviour under a confining pressure accounts for the macroscopic plastic deformation of many materials normally considered brittle. Many rocks, for example, deform plastically under a sufficiently high confining pressure. 

---