Understanding the static rate dependence of early fracture behavior of cemented paste backfill using digital image correlation and acoustic emission techniques

•The static rate dependence of the early fracture evolution of the cemented paste backfill (CPB) was investigated.•A method to calculate the fractal dimension of acoustic emission RA is proposed.•The synergetic law of acoustic emission b-value and RA fractal dimension was obtained.•The strain field, displacement field, and dilatancy characteristics of the CPB are analyzed and discussed. In mining engineering practice, cemented paste backfill (CPB) are subjected to loads at different static rates, so it is crucial to clarify the mechanical evolution mechanism of CPBs at different static rates. This work aims to understand the early fracture evolution mechanism of the CPB at static rates. First, the CPB with different static rates were monitored in real-time using acoustic emission (AE) and digital image correlation (DIC) techniques, and the effects of static rates on the CPB compressive strength as well as failure characteristics were analyzed; then, the intrinsic relationship between AE rise time/amplitude (RA) and average frequency (AF) was used to classify the crack types, and AE b-values and RA fractal dimensions were discussed for the crack evolution of the scale; finally, the strain field, displacement field, and dilation characteristics of the CPB were investigated. The results show that the increase in static rate increases the compressive strength of the CPB increase first and then decreases, and there is a critical loading rate effect. The final failure form of the specimen is transformed from tensile failure to tensile-shear composite failure with the increase of static rate. In addition, the growth of the loading rate will inhibit the formation of tensile cracks and promote the development and expansion of shear cracks. The trend of the AE b-value and the RA fractal dimension have a synergistic feature, which can be used as a sudden drop before and after the peak intensity as the failure precursor information of the CPB. The increased static rate will make the CPB reach the dilatancy state in advance, and the changing pattern of dilatancy stress synergizes with the peak strength. The study results provide a theoretical basis for evaluating the structural safety of the CPB.