Abstract
A new test, referred to as axially double-edge notched Brazilian disk (ANBD), is proposed to measure true mode mathrm {III} fracture toughness (K_{mathrm {IIIc}}) of rock materials. The term true denotes a shear-induced fracturing via self-planar crack extension as opposed to a twisted tension-based one commonly observed in many mode mathrm {III} experiments of rocks. The ANBD test follows a straightforward procedure thanks to its simple core-based geometry and diametrical compression loading setup. Finite element analyses are employed to evaluate the stress intensity variations along the crack front and to calculate the point-wise stress intensity factors (SIFs) for different geometry and loading configurations. The results of ANBD tests conducted on granitic samples demonstrate the good performance of this test in yielding true mode mathrm {III} fracturing. The influences of the test parameters of ligament length and loading angle on K_{mathrm {IIIc}} are also investigated. A comparison study shows that K_{mathrm {IIIc}} values are similar to K_{mathrm {IIc}} but almost 2.5 times greater than K_{mathrm {Ic}}. This demonstrates that the true mode mathrm {III} test offers a similar shear-based fracturing mechanism to the true mode mathrm {II}, which is significantly more energy-consuming than the tension-based mode mathrm {I} failure type.
Highlights
Rock fracturing is an important topic in many geo-science and geo-engineering applications such as CO2 sequestration, wastewater injection, geothermal energy, tunnelling, mining, etc
Rock cores were taken from the Bedretto Underground Laboratory for Geoenergies (BULG) of ETH Zurich, that is situated in the Bedretto tunnel, a 5.2 km long access gallery to the Furka railway tunnel in southern Switzerland (Gischig et al 2020)
The first one is the pulverised surface of the fracture, which was identified among important features of shear fracture surfaces by Bobet and Einstein (1998)
Summary
Rock fracturing is an important topic in many geo-science and geo-engineering applications such as CO2 sequestration, wastewater injection, geothermal energy, tunnelling, mining, etc. Fractures in rock masses are often subjected to the combinations of mechanical and hydraulic loadings, and as a result, they may evolve in complex three-dimensional trajectories. Different tensile or shear loadings is of great importance in rock mechanics and engineering. The singular stress field near the crack front is presented in the form of the superposition of three loading modes I, II, III. The singular stress field within xy plane is characterised by Eq (1a), where point-wise mode I and mode II stress intensity factors, KI and KII , determine the intensity of in-plane stresses. The out-of-plane shear stresses are characterised by Eq (1b), with KIII being the point-wise mode III stress intensity factor (Sun and Jin 2012)
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