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Nonlinear Processes in Geophysics An interactive open-access journal of the European Geosciences Union
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Volume 24, issue 3 | Copyright

Special issue: Waves in media with pre-existing or emerging inhomogeneities...

Nonlin. Processes Geophys., 24, 543-551, 2017
https://doi.org/10.5194/npg-24-543-2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 05 Sep 2017

Research article | 05 Sep 2017

Extracting real-crack properties from non-linear elastic behaviour of rocks: abundance of cracks with dominating normal compliance and rocks with negative Poisson ratios

Vladimir Y. Zaitsev1, Andrey V. Radostin1, Elena Pasternak2, and Arcady Dyskin3 Vladimir Y. Zaitsev et al.
  • 1Institute of Applied Physics, Russian Academy of Sciences, Nizhny Novgorod, 603950, Russia
  • 2School of Mechanical and Chemical Engineering, The University of Western Australia, Perth, 6009, Australia
  • 3School of Civil, Environmental and Mining Engineering, The University of Western Australia, Perth, 6009, Australia

Abstract. Results of examination of experimental data on non-linear elasticity of rocks using experimentally determined pressure dependences of P- and S-wave velocities from various literature sources are presented. Overall, over 90 rock samples are considered. Interpretation of the data is performed using an effective-medium description in which cracks are considered as compliant defects with explicitly introduced shear and normal compliances without specifying a particular crack model with an a priori given ratio of the compliances. Comparison with the experimental data indicated abundance (∼ 80%) of cracks with the normal-to-shear compliance ratios that significantly exceed the values typical of conventionally used crack models (such as penny-shaped cuts or thin ellipsoidal cracks). Correspondingly, rocks with such cracks demonstrate a strongly decreased Poisson ratio including a significant (∼45%) portion of rocks exhibiting negative Poisson ratios at lower pressures, for which the concentration of not yet closed cracks is maximal. The obtained results indicate the necessity for further development of crack models to account for the revealed numerous examples of cracks with strong domination of normal compliance. Discovering such a significant number of naturally auxetic rocks is in contrast to the conventional viewpoint that occurrence of a negative Poisson ratio is an exotic fact that is mostly discussed for artificial structures.

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A new method of analysing pressure wave dependences is presented and tested against the published experimental data. Upon the results of examination of more than 90 rock samples, it was found that a significant portion of rocks (~ 45 %) exhibit negative Poisson ratios at lower pressures. Such a significant number of naturally auxetic rocks suggests that the occurrence of negative Poisson ratios is not as exotic as assumed previously.
A new method of analysing pressure wave dependences is presented and tested against the...
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