The traditional rock fracture feature extraction method is difficult to extract the fracture features timely and accurately, and the existing digital processing methods are not systematic. A method of quantitative identification and characterization of rock fractures based on image processing technology is proposed, and an automatic threshold segmentation algorithm is established on the basis of wavelet analysis to segment the fracture area of image. The region growth method is used to calculate the fracture area, after the image is segmented. The boundary tracking algorithm is used to extract the edge contour, and the zhang-Suen algorithm is used to calculate the crack skeleton. The momentum method is used to fit the skeleton tangent line, and the normal vector of the tangent line is calculated. The trigonometric function is transformed to calculate the crack inclination angle, and the crack width information is extracted and calculated with the DDA line drawing algorithm. The results show that the automatic threshold segmentation algorithm based on wavelet analysis is superior to the traditional Otsu algorithm in terms of segmentation effect, threshold selection and calculation speed. In the extraction and calculation of fracture characteristic values, this method not only ensures the smoothness of the skeleton, but also improves the fitting degree of the skeleton. This method is simpler and requires less calculation compared with the traditional method.

Quartz sandstone samples were tested under cyclic impact loadings by the drop weight impact test equipment to study its mechanical properties and failure process under medium strain rates. Three specimens were selected at each impact height of 0.3~0.6 m, and each specimen was subjected to 8 cycles of impact with medium strain rates of 26.33 s^-1, 29.7 s^-1, 32.03 s^-1 and 35.17 s^-1, respectively. Then, the influence of cyclic loading times on the dynamic compressive strength, elastic modulus and energy efficiency of quartz sandstone were discussed. The results show that under different medium strain rates, the dynamic compressive strength of the specimens under the impact loading of the 8th cycle is about 13 MPa, which is lower than that of the first cycle. During the process, the resistance to deformation is weakened, and the elastic modulus is significantly reduced. The dynamic compressive strength of the specimens has a positively correlation with the elastic modulus. From the perspective of energy, the dissipated energy, energy efficiency and unit volume dissipated energy of the specimen are improved after 8 cycles of impact loading. The effect is most obvious when the impact energy is 70.27 J, the dissipated energy of rock is increased by 6 J, the energy efficiency is increased by 8.8%, and the unit volume dissipated energy is increased by 50%. For fracture fractal, the fracture morphology of rock under medium strain rates includes splitting failure, edge collapse failure, block failure and crushing failure. When the strain rate increases from 26.33 s^-1 to 35.17 s^-1, the characteristic value of the average particle size of the sample fragments decreases from 24.49 mm to 21.15 mm. The fractal dimension increases from 1.07 to 1.75 linearly.