In geological vocabulary, the term "brittle" is used to describe deformation behaviour in which the rock loses continuity (Van der Pluijm & Marshak 2004), or as the permanent change that occurs in a solid material due to the growth of fractures and/or due to sliding on fractures after the rock stresses exceed some critical value (Davis & Reynolds 1996). Brittle structures can be identified at many scales. Within crystal grains, microfractures can initiate either at the grain boundary where atomic disorder is higher, creating nucleation sites, or within the grain along cleavage or initiated by inclusions (pores or impurities). Grain boundaries themselves can exhibit brittle separation, dilation and shear displacement. At the micro-scale, such fractures or cracks can propagate across or between multiple grains and can be extensional, sheared or a combination. Coalescence of brittle microstructures can lead to macroscopic fractures (a general term) including cracks and ultimately joints in extension or shear fractures of different scales. Infilling of such features over time with vein minerals restores continuity in an engineering sense but does not change the geological designation of these "brittle" structures. Ultimately larger scale discontinuities can result from the interconnection of joints, bedding weakness planes (Perras 2014) and shear fractures resulting in shear zones or larger scale extensional partings. The orientation of these macro structures is controlled by primary anisotropy, induced fabric (Ghazvinian 2015) and/or by paleo- and neo-tectonic stresses.
The geological designation of brittle is related only to a disruption at any scale to the physical continuity of the solid material at any point in the deformation process. In true ductile or plastic deformation, there is at no time a physical break in the solid continuum at any scale (atomic bonding is transferred or replaced but never broken). This is very different from the definition of "brittle, plastic and ductile" used in typical engineering applications. In a very basic engineering usage, brittle behaviour refers to a significant loss of strength after elastic limits are exceeded. Plastic or ductile behaviour refers to a sustained level of strength, after elastic deformation limits and after yield begins, for a sustained set of boundary conditions including confinement. The terms strain-weakening or -hardening are appended to indicate a measured loss or gain of strength, respectively. This designation still makes a distinction between moderate weakening and significantly brittle behaviour. This phenomenological description, however, does not address the micromechanics behind the yield behaviour and does not capture the processes in which progressive accumulation of small-scale brittle damage-processes or ductile creep that ultimately lead to delayed macroscopic brittle failure.