Direct Mode I Fracture Toughness Testing Using Short Rod Specimens: A New Loading Fixture

Reliable determination of Mode I fracture toughness is essential for evaluating crack initiation and propagation in rock engineering applications such as hydraulic fracturing, underground energy storage, geothermal energy extraction, and CO₂ sequestration. The Short Rod (SR) test is the only ISRM-suggested direct method for determining Mode I rock fracture toughness, but its wider application is limited by a relatively complex loading system. This study proposes a simple shear-to-tensile (ST) fixture for conducting SR tests using a conventional direct shear apparatus. The CNC-cut steel fixture converts the applied shear load into direct tensile loading on the specimen. Tests on gypsum produced symmetric fracture paths and consistent load-displacement responses, with a mean KIC of 0.287 ± 0.008 MPa√m. SCB tests on the same material yielded 0.288 ± 0.020 MPa√m, showing close agreement between the two methods. DEM simulation further captured fracture initiation at the chevron-notch tip and progressive crack propagation through the specimen ligament. The results demonstrate the feasibility of the proposed configuration as a proof of concept for direct SR testing, with future studies aimed at extending its validation to stronger rock materials.