Abstract：Based on CT test of a certain sandstone under uniaxial compression，the safety zone，the damaging zone and the fracture zone of the sandstone are defined. Then，the divisional criteria by CT number which is used to acquire the binary images of the divisional zones from the CT images at each loading stage are discussed and determined through experiment. On the basis of the divisional criteria，the above three zones of each scanning section at each loading stage are obtained. Furthermore，taking the 5th scanning section for example，its divisional results at each loading stage are given. Then，utilizing the image to measure the technique of mathematical morphology is adopted to analyze the CT number information on these zones in each scanning section individually，which is picked over and calculated through image disposal technology. Moreover，the changing law of CT number on these zones of sandstone changing with the loading process and the procreant conditions of the damaging zone and the fracture zone are presented. Finally，according to the statistical results of CT number on these three kinds of zone obtained at each loading stage，a statistical damage variable based on CT number is defined；and the equation of damage evolution pertinent to this variable is established too. Based on all these data，a constitutive model which can reflect the divisional damage-fracture of sandstone is constructed.
