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Dilatancy and precursory bulging along incipient fracture zones in uniaxially compressed westerly granite

Liu, Hsi-Ping and Livanos, Alexander C. R. (1976) Dilatancy and precursory bulging along incipient fracture zones in uniaxially compressed westerly granite. Journal of Geophysical Research, 81 (20). pp. 3495-3510. ISSN 0148-0227. doi:10.1029/JB081i020p03495.

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The existence of precursory bulge along an incipient fracture zone in a uniaxially compressed Westerly granite sample has been investigated by two optical methods. The first method is the method of slit diffraction. The cross section of a cylindrical rock sample is monitored by bringing two straightedges next to the rock sample to form two slits (each slit being formed between one straightedge and one side of the rock sample) and by illuminating alternately the two slits with a collimated laser beam. The Fraunhofer diffraction pattern is recorded on film in the direction perpendicular to the straightedge and can be interpreted as the absolute value squared of the one-dimensional spatial Fourier transform of the slit under certain conditions, thereby providing a simple method of magnification of the rock surface geometry. The conditions under which the Fraunhofer diffraction pattern can be interpreted as the absolute value squared of a one-dimensional Fourier transform are related to the radius of the rock sample, width of the slit, position of the recording film plane, and nature of deformation of the rock surface and are presented in this paper. The films are digitized by a microdensitometer. The data are analyzed by digital filtering and interpolation techniques to give a strain resolution of 10^(−5). During a test with the slit diffraction method, strain inhomogeneities in terms of local bulges indicative of incipient failure zones were found to develop at ∼92% of the uniaxial compressive strength, and their propagation is traced at 2.66-s intervals until failure. Local strains in the incipient failure zones are of the order of 10^(−2) before failure takes place. Because of the large amplitude of the strain inhomogeneity prior to failure recorded by the slit diffraction method, we then tried the faster method of recording without magnification by a motion picture camera. In the second test a precursory bulge in the middle of the sample first appeared at ∼3.75 s prior to failure at a load of >99.7% of the uniaxial compressive strength. The bulge developed rapidly in successive frames until eventually a failure plane passed through this sharp bulge. The results from both tests demonstrate the formation of a concentrated weak zone as a result of the interaction and coalescence among the microcracks in the final stage of the test, which then develop into fracture zones. The bulging is the result of accentuated deformation in the weak zone because of its reduced deformation moduli. It is considered that the local bulge and orientation of the fracture zones in the first test were controlled by the stress concentration at the sample-load block interface, whereas those in the second test were controlled mainly by the inhomogeneity in the material properties within the sample. The precursor times of both tests do not fit into the empirical relationship between precursor time and fault dimension as derived from earthquakes and mine rock bursts. The precursor times of these tests are too long by 3 orders of magnitude in comparison with those given by the empirical relationship.

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Additional Information:© 1976 by the American Geophysical Union. Received September 15, 1975; revised March 25, 1976; accepted March 25, 1976. The authors wish to thank W. G. Knauss and C. D. Babcock, Jr., for providing the use of a Baldwin testing machine and R. F. Scott, Don L. Anderson, J. H. Whitcomb, and N. George for their interest and some discussions concerning this work. We also wish to thank E. Steffensen and N. 0. B. Motta for their valuable technical assistance. The motion picture was taken by a crew under the direction of J. Angier from WGBH Educational Foundation, Boston. This work was supported by the Jet Propulsion Laboratory/NASA under contract 64597 and was supported partially by the Air Force Office of Scientific Research under AFOSR 72-2234. Contribution 2673, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California.
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Air Force Office of Scientific Research (AFOSR)72-2234
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Caltech Division of Geological and Planetary Sciences2673
Issue or Number:20
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Official Citation:Liu, H.-P., and A. C. R. Livanos (1976), Dilatancy and precursory bulging along incipient fracture zones in uniaxially compressed westerly granite, J. Geophys. Res., 81(20), 3495–3510, doi:10.1029/JB081i020p03495
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:76692
Deposited By: Tony Diaz
Deposited On:19 Apr 2017 21:40
Last Modified:15 Nov 2021 17:02

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