The Effect of Loading Conditions and Specimen Size on the Tensile Strength of Ring-Shaped Specimens

Abstract

This study introduces a custom-built direct tension testing apparatus designed for ring-shaped rock specimens. The apparatus applies a uniform axial load to the inner surface of the ring and is compatible with standard geotechnical testing equipment. Using this device, 176 granite specimens sourced from Horseshoe Bend, Idaho, were tested under three configurations: Brazilian, indirect ring tension, and direct ring tension. Only specimens exhibiting valid diametrical splitting failure modes (126 out of 176) were included in the final analysis. Direct tension tests yielded the lowest tensile strengths, followed by Brazilian tests, with indirect tension tests producing the highest values. While scatter existed within all methods, tensile strength generally decreased with increasing specimen volume, indicating a potential size effect. Notably, smaller specimens tend to produce less variable results, which may suggest the influence of a grain size effect, where large grains in larger specimens act as internal defects, lowering strength and affecting failure behavior. Shape effects were assessed using the relative hole radius (inner radius divided by outer radius). For both indirect and direct tension tests, tensile strength increased with increasing relative hole radius, although the trend was more prominent in indirect tests. Normalized failure load (P*) decreased with increasing relative hole radius in indirect tests but remained relatively constant in direct tests, deviating from published trends. No clear correlation was observed between failure mode and relative hole radius. This research confirms that specimen geometry and internal heterogeneity significantly influence tensile strength in rock. The custom apparatus provides a reliable platform for direct tension testing and lays the groundwork for future investigations into the effects of grain structure, improved specimen preparation methods, and standardized tensile strength protocols for brittle geomaterials.