True Mode $$\mathrm {III}$$ Fracturing of Rocks: An Axially Double-Edge Notched Brazilian Disk Test
Bahador Bahrami, Morteza Nejati, M.R. Ayatollahi, Thomas Driesner
Abstract
Abstract A new test, referred to as axially double-edge notched Brazilian disk (ANBD), is proposed to measure true mode $$\mathrm {III}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>III</mml:mi> </mml:math> fracture toughness ( $$K_{\mathrm {IIIc}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>K</mml:mi> <mml:mi>IIIc</mml:mi> </mml:msub> </mml:math> ) 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 $$\mathrm {III}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>III</mml:mi> </mml:math> 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 $$\mathrm {III}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>III</mml:mi> </mml:math> fracturing. The influences of the test parameters of ligament length and loading angle on $$K_{\mathrm {IIIc}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>K</mml:mi> <mml:mi>IIIc</mml:mi> </mml:msub> </mml:math> are also investigated. A comparison study shows that $$K_{\mathrm {IIIc}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>K</mml:mi> <mml:mi>IIIc</mml:mi> </mml:msub> </mml:math> values are similar to $$K_{\mathrm {IIc}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>K</mml:mi> <mml:mi>IIc</mml:mi> </mml:msub> </mml:math> but almost 2.5 times greater than $$K_{\mathrm {Ic}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>K</mml:mi> <mml:mi>Ic</mml:mi> </mml:msub> </mml:math> . This demonstrates that the true mode $$\mathrm {III}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>III</mml:mi> </mml:math> test offers a similar shear-based fracturing mechanism to the true mode $$\mathrm {II}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>II</mml:mi> </mml:math> , which is significantly more energy-consuming than the tension-based mode $$\mathrm {I}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>I</mml:mi> </mml:math> failure type.