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| Titel |
The dependence on pressure of the plastic flow of rocksalt in the temperature range 25-250Ë C: implications for the rate controlling mechanism |
| VerfasserIn |
Nawaz Muhammad, Chris Spiers, Hans De Bresser, Colin Peach |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250096427
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| Publikation (Nr.) |
 EGU/EGU2014-11934.pdf |
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| Zusammenfassung |
| Despite the large body of data that already exists, the question what microphysical
mechanisms govern plastic flow of natural rocksalt at in situ conditions has not yet been
answered to full satisfaction. In particular, the exact mechanism controlling dislocation
motion at relatively low temperature is still insufficiently understood. As a result,
uncertainties exist regarding the appropriate mechanism-based flow-law for low temperature,
hampering reliable extrapolation of lab creep data to in situ strain rates. Such extrapolation is
required for the modelling of the long term behaviour of salt for geomechanical purposes
(e.g. subsidence prognosis).
Several dislocation models have been proposed to control plastic flow of rocksalt, such as
dislocation climb, cross-slip and (impurity-controlled) glide, but none of these have been
rigorously verified. One way to test which model is appropriate is by investigating the
pressure dependence of flow of rocksalt. Dislocation glide is expected to be hardly
affected by pressure, cross slip (controlled by constriction of partial dislocations) will
become easier with increasing pressure, and dislocation climb will become more
difficult.
We performed conventional axi-symmetric compression tests on synthetic polycrystalline
salt samples with an average grain size of 300 μm. The samples were dry, in order to
eliminate the possible influence of pressure solution creep. The experiments were carried out
at temperatures in the range 25-250°C, i.e. 0.28-0.48Tm, and at pressure ranging 50-600
MPa, which is a range not previously covered for polycrystalline rocksalt. Argon gas was
used as the pressure medium.
With confining pressure increasing from 50 to 600 MPa, the rocksalt remained of the
same strength at RT, but became about 60% stronger at 125oC and about 80% stronger at
250oC at strain rate 10-6 s-1 (at 15% strain). Using a conventional (Dorn-type) power law to
describe the mechanical behaviour, stress exponents (n) were found that varied from 10.3 at
125°C to 7.1 at 250°C. The data suggest that dislocation glide may control the
behaviour of rocksalt at room temperature, but the absolute values found for n
and the decrease in n with increasing T indicate that a single dislocation climb
controlled model is not sufficient to describe the flow at 125-250°C. We evaluated the
experimental data against various models of dislocation glide, cross slip and glide, and
combinations of these, in order to come to a (composite) flow law that can be applied
in the modelling of the long term behaviour of salt at relatively low temperature. |
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