Abstract: The critical fracture characteristics of rock, such as the length of process zone and crack mouth opening displacement(COD), are the fundamentals to solve the fracture problems of rock by using fracture mechanics. Traditional measure methods, such as strain gauge and LVDT, etc., cannot obtain the full-field displacement of rock specimen. In that case, it's difficult to achieve the above mentioned critical fracture characteristic of rock. Digital image correlation (DIC) method is a optical measure method which can obtain the accurate full-field displacement of the specimen by comparing the original image and deformed image. A series of three-point bending tests is performed on the semi-circular rock specimens(relatively hard marble and relatively soft yellow sandstone), and the DIC method is used to measure the full-field displacements of the specimens. The obtained displacements are further analyzed, and the critical fracture characteristics of the specimens including the length of process zone and the crack mouth opening displacement are revealed. It is shown that the length of process zone in marble is obviously shorter than that in yellow sandstone, while the values of COD of yellow sandstone are higher than the ones of the marble with same crack length, which demonstrates that the soft yellow sandstone is prone to be affected by boundary effect. Those researches can help us to further understand the fracture process of rock ,and to solve rock engineering problem using fracture mechanics.

Key words: rock, critical fracture characteristic, length of process zone, crack mouth opening displacement, digital image correlation