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Hybrid fracture and the transition from extension fracture to shear fracture

Abstract

Fracture is a fundamental mechanism of material failure. Two basic types of brittle fractures are commonly observed in rock deformation experiments—extension (opening mode) fractures and shear fractures1,2. For nearly half a century it has been hypothesized that extension and shear fractures represent end-members of a continuous spectrum of brittle fracture types3,4,5,6. However, observations of transitional fractures that display both opening and shear modes (hybrids) in naturally deformed rock have often remained ambiguous, and a clear demonstration of hybrid fracture formation has not been provided by experiments4. Here we present the results of triaxial extension experiments on Carrara marble that show a continuous transition from extension fracture to shear fracture with an increase in compressive stress. Hybrid fractures form under mixed tensile and compressive stress states at acute angles to the maximum principal compressive stress. Fracture angles are greater than those observed for extension fractures and less than those observed for shear fractures. Fracture surfaces also display a progressive change from an extension to shear fracture morphology.

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Acknowledgements

We thank M. Finn, A. K. Kronenberg and J. S. Chester for discussions and suggestions, J. N. Magouirk and E. C. Powell for assistance in the laboratory, and J. S. Chester for reviews of the manuscript. This work was partially supported by US NSF.

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Correspondence to Frederick M. Chester.

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Further reading

Figure 1: A representation using the Mohr diagram of the hypothesis that the brittle failure envelope and the transition from extension fracture to shear fracture is continuous.
Figure 2: Photographs of the undeformed and deformed notch-cut dog-bone samples of Carrara marble.
Figure 3: Fracture angle and fracture strength as a function of confining pressure.

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