Gray and texture features of strain field for fractured sandstone during failure process

In order to explore the deformation and fracturing mechanism and precursory characteristics of fractured rock mass,the flaws with different inclinations were prefabricated in the red sandstone plate specimens by using the high-pressure water-jet cutting technology.The uniaxial compression experiments were conducted on the fractured sandstone specimens.The digital image correlation method was applied for a non-contact measurement of strain field during loading.Six gray feature parameters including mean,standard deviation,third-order moment,smoothness,consistency and entropy and four texture feature parameters including contrast,correlation,angular second moment and entropy were used to quantify the gray scale and texture evolution of strain field during the deformation and fracturing process of fractured sandstone specimens.The results show that the grayscale of strain field changes from low gray level to high gray level during the loading process.The shape of gray histogram changes from “pyknic” type to “lanky” type,accompanied by an increase in the peak frequency.The gray/texture feature parameter of strain field-time curve of the fracture sandstone specimen can be divided into the fluctuation stage,steady stage and mutation stage.The abrupt changes in the gray/texture feature parameters of strain fields are related to the crack initiation and propagation,and the corresponding variation trend can be grouped into the growth type,attenuation type and little change type.The absolute values of Pearson’s correlation coefficients between the gray feature parameters and texture feature parameters are mostly greater than 0.5,indicating that there exists a significant or high correlation between the two types of feature parameters.The first abrupt change in the gray/texture feature parameters is associated with the initiation of tensile cracks,and appears when the specimen is close to failure.It can be regarded as a precursory point and can be used as an early warning signal of imminent instability of fractured rock mass.The ratio of precursor stress to peak stress is in the range of 0.83-0.97,and the ratio of precursor time to final failure time is in the range of 0.85-0.95.Compared with intact sandstone specimen,the gray/texture feature parameters have a better identification ability of failure precursor for fractured sandstone specimens.This is because the existence of pre-flaw makes the discontinuity of strain field more pronounced.Finally,the recommended monitoring gray/texture feature parameters are proposed for intact rock and fractured rock mass,respectively.