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

Program: Civil Engineering MS

Committee Chair: Nick Hudyma

Committee: Nick Hudyma, Yang Lu, Bhaskar Chittoori

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.

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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.