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.