Influence of Loading Rate on Tensile Fracture Behavior in Ring-Splitting Tests

In the ring splitting test, the tensile strength by substituting peak load into the formula may be overestimated sometimes. To verify its rationality and study the influence of loading rate on the tensile behavior in the ring test, ring tests with different loading rates (0.0005 to 0.01 mm/s) were carried out on sandstone. Three methods (digital image correlation, acoustic emission monitoring, and strain monitoring) were used to determine the crack initiation point: the non-contact strain method, the acoustic emission method, and the local strain method. The indirect tensile strength was calculated based on the crack initiation load. 3D optical scanning and scanning electron microscopy were used to obtain fracture surface characteristics. The results show that the tensile strength increases with the increasing loading rate, and measuring the tensile strength based on the cracking initiation load in the ring test is feasible. As the loading rate increases, the fracture surface becomes increasingly flat, and transgranular failure occurs. The fracture mechanism of ring specimens under different loading rates was proposed: at lower loading rates, tensile cracks propagate along weak structures and primary defects. As the loading rate increases, the tensile cracks directly penetrate the grains and propagate, increasing tensile strength.