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| Titel |
Micromechanics of brittle creep and implications for the strength of the upper crust |
| VerfasserIn |
N. Brantut, P. Baud, M. J. Heap, P. G. Meredith |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250068553
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| Zusammenfassung |
| In the upper crust, the chemical influence of pore water or other aqueous solutions promotes
time dependent brittle deformation through sub-critical crack growth. Sub-critical crack
growth allows rocks to deform and fail at stresses far below their short-term failure
strength, and even at constant applied stress (“brittle creep”). Here we present a new
micromechanical model describing time dependent brittle creep of water-saturated rocks
under triaxial stress conditions. Macroscopic brittle creep is modelled on the basis of
microcrack extension under compressive stresses due to sub-critical crack growth. The
incremental strains due to the growth of cracks in compression are derived from the
sliding wing crack model of Ashby and Sammis (1990). Crack length evolution is
computed from Charles’ power law description of stress corrosion crack growth. The
macroscopic strains and strain rates computed from the model are non-linear and compare
well with experimental results obtained on granite, low porosity sandstone and
basalt samples. Primary creep (decelerating strain rate) corresponds to decelerating
crack growth, due to an initial decrease in stress intensity factor with increasing
crack length in compression. Tertiary creep (accelerating strain rate as failure is
approached) corresponds to an increase in crack growth rate due to crack interactions.
Secondary creep, with apparently constant strain rate, arises as merely an inflexion
between these two end-member phases. The strain rate at the inflexion point can
be estimated analytically as a function of model parameters, effective confining
pressure and temperature conditions, which provides an approximate creep law for the
process. The creep law is used to infer the long term differential stress as a function of
depth in the upper crust for tectonic loading rates: sub-critical cracking induces an
offset of the rock strength, which is equivalent to a decrease in cohesion. For porous
rocks, the competition between sub-critical crack growth and pressure solution has
also been investigated, and brittle creep appears to be the dominant mechanism
only at high (> 10-10Â s-1) strain rates and shallow (< 1Â km) depth in the crust. |
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