Direct loading of cylindrical WST specimens for Mode I rock fracture energy evaluation: a proof-of-concept study

Abstract

This study presents a proof-of-concept assessment of an alternative loading configuration for evaluating Mode I rock fracture energy using cylindrical Wedge Splitting Test (WST) specimens subjected to direct tensile opening at the specimen groove. The system uses a simple shear-to-tensile (ST) fixture installed inside a conventional direct shear apparatus, which converts horizontal shear displacement into tensile opening displacement. Four Corvio sandstone specimens were tested, supported by Digital Image Correlation (DIC) observations and numerical simulations. The experiments produced nearly symmetrical fracture patterns, consistent load-displacement responses, and stable post-peak softening behavior. DIC observations showed crack initiation at the notch tip followed by vertical propagation through the ligament, supporting a Mode I-dominated fracture process. The stress intensity factor analysis yielded \(\:{K}_{I}=0.090\) MPa·m 1/2 and a mode mixity ratio of \(\:\left|{K}_{II}\right|/{K}_{I}=6.51\times\:{10}^{-4}\) , indicating negligible Mode II contribution. The DEM simulation showed that broken bonds remained localized along the intended fracture path, with no damage observed away from the main crack trajectory. The fracture energy ranged from 50.6 to 59.6 N/m and averaged 55.8 N/m, with a coefficient of variation of 7.7%. Overall, the results demonstrate the feasibility of the proposed direct-loading configuration for the tested sandstone. However, broader validation requires a larger specimen series, different rock types, direct CMOD measurement, and direct comparison with conventional WST testing.