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Variations of the Mechanical Properties of a Sandstone Due to Deformation... |
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Variations of the Mechanical Properties of a Sandstone Due to Deformation and Confi ning Pressure,in Relation to Microstructures Authors: J. Gustkiewicz, J.F. Gamond, E. Carrio-Schaffhauser
The infl uence of confi ning pressures up to 380 MPa and axial strains up to 20% were studied on mean
porosity (10%) Tumlin sandstone, before and beyond rupture, through complementary compressibility tests
and single and cyclic loading triaxial tests, associated with microstructural, mineralogical and porosimetrical
observations. Concavity of the curves stemming from compressibility tests, singular points observed on
curves resulting from triaxial test data, and stabilization of Young’s modulus identify a particular confi ning
pressure range about 150 MPa. The similarity of the confi ning pressure range stemming from the three
types of observations suggests it is linked to a particular stage, the closure of planar cracks in the rock.
However, the microstructural, mineralogical and porosimetrical observations show that the only planar
cracks in Tumlin sandstones are the microcavities between the illite platelets fi lling the linear grain boundaries.
It is proposed that these microcavities, and not planar cracks randomly distributed in the material,
are the ones that close when the confi ning pressure of about 150 MPa is achieved. This has a signifi cant
infl uence on the innermost deformation mechanisms during strain hardening before rupture.
Observation of the macroscopic shear zones shows that the confi ning pressure at the brittle-ductile
transition is about 300 MPa. It is proposed that the stabilization of angle γ between conjugate shear surfaces
for increasing axial deformation can be considered a good indicator of the rock entering the fully ductile
state within the range 350-400 MPa confi ning pressure. For confi ning pressures below 350 MPa, the increase
of confi ning pressure increases the value of angle γ at the onset of the conjugate shear surfaces.
The infl uence of axial strain on the macroscopic features is particularly noticeable beyond 200-250
MPa. When axial strain is higher than 15-20%, the number of shear bands, and therefore the width of
the fracture zone, increases. Beyond these values, stabilization of the elastic component of strain and
continuous increase of the permanent component suggest continuous damage of the material. |
2009