Evaluation of ring test with reference to deformation rate and specimen geometry in assessing the tensile behaviors of granite

Abstract

Ring test for indirect tensile strength measurements utilizes disc specimens with a hole at the center of the disc. Such specimens are found to limit the stresses at the specimen-platen contact and transfer the tensile stress to the upper- and lower-hole boundaries in ring specimens. Researchers observed that the tensile strength of a ring specimen tends to decrease as the ratio of the hole-radius to the disc-radius (ρ) increases. However, there has been no research on how the tensile strength derived from ring specimens varies when loaded under different quasi-static strain rates or deformation rates. The strain analysis in case of ring tests also does not seem to have gained attention. In this study, ring specimens of Malanjkhand granite (India) with varying ρ (0.13, 0.17, 0.21 and 0.25) were quasi-statically loaded at 0.5 mm/min, 1.5 mm/min and 5.5 mm/min corresponding to strain rates of 1.75 × 10⁻⁴ s⁻¹, 5.26 × 10⁻⁴ and 1.93 × 10⁻³ s⁻¹, respectively. The combined effect of the deformation rate and specimen geometry (ρ) on the indirect tensile strength and deformation behavior was investigated. The Ring Tensile Strength (RTS) is found to be higher than the Brazilian Tensile Strength (BTS). RTS shows dependency on both the geometry of the ring specimen as well as the deformation rate. The tensile Tangent Deformation Modulus (D_v) and the ratio of the horizontal strain to vertical strain, coined as Ring Strain Ratio (RSR), were estimated in this study from the tensile stress and vertical-horizontal strain data. A numerical finite element analysis was also performed in Abaqus to observe the stress distribution in both ring and Brazilian discs, where the results were found to be broadly conformable.