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3.1 j. Strain partitioning in polymineralic rocks (I.C. Stretton, in collaboration with S.J. Covey-Crump, P.F. Schofield/London, U.K. and K.S. Knight/Didcot, U.K.)

The deformation of polymineralic rocks is sufficiently complex to have received relatively little experimental attention. Yet an understanding of the deformation behavior of such materials is widely recognised as being of fundamental importance if we are to obtain an accurate understanding of the deformation of the Earth's lithosphere. Consequently, we are presently participating in a collaborative project studying the mechanical behavior of polymineralic aggregates with a view to developing generally applicable, mechanical constitutive equations for such materials that are founded upon a sound micromechanical basis. The application of time-of-flight neutron diffraction potentially provides a technique for addressing a long standing (and hitherto largely unsolved) problem in this important subject area, namely how to determine the controls on the stress and strain partitioning between the component phases during deformation. The penetrating nature of neutrons means that neutron diffraction is an ideal technique for measuring the strain partitioning between two phases in a polycrystalline aggregate. At low strains where a large component of the deformation is elastic, neutron diffraction may be the only currently available technique to provide such information.

We have performed a series of ambient temperature, uniaxial deformation experiments on calcite-halite two phase aggregates in-situ within the PEARL beam line at ISIS. Samples with varying halite:calcite ratios were made from powders that had been intimately mixed and then cold-pressed to 200 MPa. The grain size of both phases is approximately 50 µm. The choice of calcite and halite reflects the need for geological materials that (a) have well characterized mechanical properties, (b) have a large strength contrast, (c) are readily distinguishable by neutron diffraction, (d) have one phase (halite) that is ductile at room temperature and pressure, and (e) for which samples of controlled grain size may be readily fabricated.

Experiments were performed on samples with 80% and 50% halite and the single phase end member samples to investigate the volume fraction dependence of stress/strain partitioning. The maximum load applied to the sample had previously been determined in experiments to ensure that constant load could be maintained and that the specimen would not fail catastrophically. Measurements were made at a minimum of 4 different loads, approximately 5, 10, 15, 20 MPa. The resolution of the elastic strain measurements (50 microstrain) is several orders of magnitude less than the strains that experienced by the calcite and halite.

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