Abstract: Strength thresholds of crack initiation ( ) and crack damage ( ) of hard rock are of great significance not only for understanding the progressive failure process and brittle failure mechanism but also for establishing the crack initiation strength criterion and long-term strength criterion. In view of this, four methods to calculate the crack initiation and damage strength, including crack volumetric strain model, acoustic emission method, lateral strain and volumetric strain observation method, moving point regression technique, are summarized systematically; and marble and granite in uniaxial stress state are taken as examples to show problems in determining and by each method. Results show that and can be obtained conveniently and precisely through crack volumetric strain model which has a clear physical meaning; progressive failure and damage process of rock can be comprehended qualitatively or semi-quantitatively by analysis of acoustic emission hit rate; but and can not be acquired accurately. Curve of volumetric strain stiffness is strongly influenced by data intervals and characteristics of different stages during loading are not obvious when moving point regression technique is used, which makes the determination of difficult. The result could contribute to understanding the importance and calculating methods of crack initiation and damage strength thresholds and provide a useful reference to choose appropriate calculating method.

Key words: crack initiation strength, crack damage strength, acoustic emission, volumetric strain stiffness, calculating method