Many brittleness criteria have been proposed to characterise material behaviour under triaxial compression, but there is no consensus as to which criteria is the most suitable. It was shown recently that increasing σ3 can lead to contradictory intact rock behaviour within different ranges of σ3. For example, rock behaviour can be changed from class I (ductile) to class II (brittle) and then to class I again, based on the Wawersik and Fairhurst (1970) classification. Brittleness, in this case, can vary from absolute brittleness to absolute ductility. In this paper, it is argued that only two of the many existing criteria can properly describe the variation of brittleness within a wide range of confinements. These criteria rely upon energy balance and are based on sound physics principles.
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