Abstract
Understanding the fracturing mechanisms of rock on both macro- and micro-scale is important for properly designing rock engineering applications. However, there is still a lack of understanding of why macro and micro-scale fracturing mechanisms differ. In this study, acoustic emission (AE) and digital image correlation (DIC) techniques were employed to track the microcracking processes in granite specimens subjected to indirect (Brazilian) and direct tensile loadings. The moment tensor inversion of the AE waveforms and the DIC strain field data revealed that the ultimate so-called tensile macro-fracture was predominantly composed of shear microcracks in Brazilian tests and tensile microcracks in the direct tension tests. The different contributions of shear and tensile microcracks to the formation of the macro-fracture explain the difference between direct and indirect tensile strengths. Our results showed that the compressive stress in the Brazilian test due to its biaxial stress field and the grain size are the two critical factors affecting the microcracking mechanisms in the tested coarse-grained granite. Characterizing the surface of the generated macro-fractures and the results of a series of complementary tensile tests performed on fine-grained mortar specimens suggested that reducing the compressive stress and grain size decreases the contribution of shear microcracks. The results of this study can be used in rock fracture applications in granitic rocks such as hydraulic fracturing for geothermal energy extraction, where the knowledge of the cracking location and mechanisms is critical for enhancing the reservoir's productivity.
Highlights
-
So-called tensile macro-fractures are composed of both shear and tensile cracks at the microscale.
-
The ratio of shear-to-tensile microcracks depends on the grain size and stress states.
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Microcracks are predominantly shear in Brazilian and tensile in the direct tensile loading.
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Data availability
The datasets generated and/or analyzed during the current study are available from the corresponding author upon reasonable request.
Abbreviations
- \({\sigma }_{Bt}\) :
-
Brazilian tensile strength
- \({\sigma }_{Dt}\) :
-
Direct tensile strength
- \({F}_{p}\) :
-
Applied external load at failure
- D:
-
Brazilian disc diameter
- T:
-
Brazilian disc thickness
- A:
-
Nominal cross-sectional area of prismatic specimens
- A0 :
-
Average focal amplitude of acoustic emissions
- Ai :
-
Maximum signal amplitude received by the ith sensor
- ri :
-
Signal source distance to the ith sensor
- n:
-
Number of sensors receiving the same AE signal
- \({\varepsilon }^{e}\) :
-
Elastic strain component
- \({\varepsilon }^{p}\) :
-
Plastic strain component
- \({\varepsilon }^{t}\) :
-
Total strain
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Acknowledgements
The first two authors thank the Natural Sciences and Engineering Research Council (NSERC) [Grant no. RGPIN-2020-07109] of Canada for funding this research program and the Fonds the recherche du Québec – Nature et technologies (FRQNT) for financing the research infrastructure. The authors would also like to acknowledge Mr. Danick Charbonneau and Mr. Jean-Christophe Lacasse, technicians at the Rock Mechanics Laboratory of the University of Sherbrooke, for their valuable cooperation. The authors are also grateful for the support by Correlated Solutions, Inc., for kindly providing us with a free license for DIC software. Thanks should also go to Mr. Alex Loignon for his support in setting up the DIC system.
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Shams, G., Rivard, P. & Moradian, O. Micro-scale Fracturing Mechanisms in Rocks During Tensile Failure. Rock Mech Rock Eng 56, 4019–4041 (2023). https://doi.org/10.1007/s00603-023-03275-6
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DOI: https://doi.org/10.1007/s00603-023-03275-6