Evolution law of strain field and precursor identification of flawed sandstone based on Digital Image Correlation method

Previous studies on the application of Digital Image Correlation(DIC) method in rock mechanics have mainly focused on obtaining the contours of deformation fields, whereas little attention has been paid to perform quantitative analysis on these deformation data by combining some indicators. The digital image correlation method was applied for real-time and non-contact monitoring deformation of flawed sandstone specimens during the compressive loading process. The differentiation characteristic of strain field was quantitatively described by using the variance. The differentiation rate of strain field was quantitatively described by using the finite difference derivative of the variance variation curve. The evolution laws of strain field and precursor characteristics were analyzed. The results show that the deformation and fracturing process in flawed sandstone specimens can be divided into four stages, namely, compaction, elastic deformation, steady crack propagation, rapid crack propagation and failure. The crack initiation and propagation behavior during the loading process is associated with the generation and development of strain localization zones in the strain field, resulting in the variations of variance and differentiation rate of strain field. The evolution process of variance of strain field shows an obvious periodic characteristic. The variance of strain field-axial strain curve can be divided into three stages, namely, steady differentiation, accelerated differentiation and accelerated acceleration differentiation. When the tensile crack appears, the first peak occurs in the differential rate of the strain field-axial strain curve, which could be regarded as a precursor before the failure of fractured rock mass. The ratio of precursor stress to peak stress is in the ranges of 0.80～0.96. The research results have theoretical significance for the failure prediction of engineering rock mass.