Universal criteria for rock brittleness estimation under triaxial compression

Brittleness is one of the most important mechanical properties of rock; however, the concept of brittleness in rock mechanics is yet to be precisely defined. Many brittleness criteria have been proposed to characterise material behaviour under compression, but there is no consensus as to which criteria is the most suitable and reliable.This paper considers brittleness at compression as the rock capability to self-sustaining macroscopic failure in the post-peak region due to elastic energy accumulated within the loaded material. The applicability of various criteria for assessing rock brittleness from this point of view is analysed. It is shown that only two of many existing criteria can describe properly the intrinsic material brittleness within the whole range of brittleness variation from the absolute brittleness to ductility. These criteria rely upon post-peak energy balance and are based on sound physics principles. Unlike other existing criteria they allow for the representation of two classes of rock behaviour (Class I to Class II) in the form of continuous, monotonic and unambiguous scale of brittleness. The effect of confining pressure on rock brittleness is analysed where rock behaviour can be changed from Class I to Class II and then to Class I again.

► Various criteria for assessing rock brittleness at triaxial compression are analyzed.
► Only two criteria characterise properly brittleness from absolute value to ductility.
► These criteria rely upon post-peak energy balance.