Coupling effect of specimen size and stress rate on the splitting failure of limestone: tensile strength, crack extension, and fragment size

Limestone as one of the primary raw materials for concrete, which is mechanically crushed into different sizes according to the grading requirements of aggregates, or is ground into powder to produce cement. Particle size and loading rate are the two main factors influencing the breaking effect and power consumption of the crusher. Under quasi-static loading conditions, rock strength decreases with increasing particle size, whereas under dynamic loading conditions, rock strength increases with loading stress/strain rate. In order to investigate these two opposite effects, in this study, limestone was precisely machined to produce Brazilian disc specimens of various diameters with a fixed length to diameter ratio of 0.5. Static and dynamic indirect tensile strength tests were conducted. The quasi-static tensile strength followed the Bažant size effect law. Additionally, the variations of dynamic tensile strengths tested by the split Hopkinson pressure bar with stress rate were described by the Asadi model. The findings indicate that as the diameter of the Brazilian disc increases, the dynamic tensile strength increase becomes more significant. High-speed cameras captured the progressive failure process of the Brazilian disc specimens under approximate stress rates, which revealed a noticeable delay in the initiation of main tensile cracks and secondary shear cracks in larger diameter specimens compared to smaller diameter ones. Furthermore, by collecting fragments and fitting particle size distribution, it was found that the dimensionless median size of large diameter specimens decreases significantly faster than that of small diameter specimens as the stress rate increased. Based on the microstructural characteristics of rocks and the forming process of crack initiation and propagation at the macroscopic level, macroscopic viscosity during crack kinematic processes is considered as a critical factor contributing to the dynamic size effect. Additionally, there a critical loading rate for the transformation of the Brazilian splitting failure mechanism, which is beneficial for improving the mechanically crushing efficiency of limestone under unit energy consumption.