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  • 1
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    Dynamic recrystallisation of ice aggregates during co-axial viscoplastic deformation: a numerical approach (2016)
    INT GLACIOL SOC
    In:  EPIC3Journal of Glaciology, INT GLACIOL SOC, 62(232), pp. 359-377, ISSN: 0022-1430
    Details
    Publication Date: 2016-08-01
    Description: Results of numerical simulations of co-axial deformation of pure ice up to high-strain, combining full-field modelling with recrystallisation are presented. Grain size and lattice preferred orientation analysis and comparisons between simulations at different strain-rates show how recrystallisation has a major effect on the microstructure, developing larger and equi-dimensional grains, but a relatively minor effect on the development of a preferred orientation of c-axes. Although c-axis distributions do not vary much, recrystallisation appears to have a distinct effect on the relative activities of slip systems, activating the pyramidal slip system and affecting the distribution of a-axes. The simulations reveal that the survival probability of individual grains is strongly related to the initial grain size, but only weakly dependent on hard or soft orientations with respect to the flow field. Dynamic recrystallisation reduces initial hardening, which is followed by a steady state characteristic of pure-shear deformation.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , isiRev
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  • 2
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    Full-field predictions of ice dynamic recrystallisation under simple shear conditions (2016)
    ELSEVIER SCIENCE BV
    In:  EPIC3Earth and Planetary Science Letters, ELSEVIER SCIENCE BV, 450, pp. 233-242, ISSN: 0012-821X
    Details
    Publication Date: 2019-07-17
    Description: Understanding the flow of ice on the microstructural scale is essential for improving our knowledge of large-scale ice dynamics, and thus our ability to predict future changes of ice sheets. Polar ice behaves anisotropically during flow, which can lead to strain localisation. In order to study how dynamic recrystallisation affects to strain localisation in deep levels of polar ice sheets, we present a series of numerical simulations of ice polycrystals deformed under simple-shear conditions. The models explicitly simulate the evolution of microstructures using a full-field approach, based on the coupling of a viscoplastic deformation code (VPFFT) with dynamic recrystallisation codes. The simulations provide new insights into the distribution of stress, strain rate and lattice orientation fields with progressive strain, up to a shear strain of three. Our simulations show how the recrystallisation processes have a strong influence on the resulting microstructure (grain size and shape), while the development of lattice preferred orientations (LPO) appears to be less affected. Activation of non-basal slip systems is enhanced by recrystallisation and induces a strain hardening behaviour up to the onset of strain localisation and strain weakening behaviour. Simulations demonstrate that the strong intrinsic anisotropy of ice crystals is transferred to the polycrystalline scale and results in the development of strain localisation bands than can be masked by grain boundary migration. Therefore, the finite-strain history is non-directly reflected by the final microstructure. Masked strain localisation can be recognised in ice cores, such as the EDML, from the presence of stepped boundaries, microshear and grains with zig-zag geometries.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , isiRev
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  • 3
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    Competition between grain growth and grain-size reduction in polar ice (2018)
    Details
    Publication Date: 2021-03-29
    Description: Static (or ‘normal’) grain growth, i.e. grain boundary migration driven solely by grain boundary energy, is considered to be an important process in polar ice. Many ice-core studies report a continual increase in average grain size with depth in the upper hundreds of metres of ice sheets, while at deeper levels grain size appears to reach a steady state as a consequence of a balance between grain growth and grain-size reduction by dynamic recrystallization. The growth factor k in the normal grain growth law is important for any process where grain growth plays a role, and it is normally assumed to be a temperature-dependent material property. Here we show, using numerical simulations with the program Elle, that the factor k also incorporates the effect of the microstructure on grain growth. For example, a change in grain-size distribution from normal to log-normal in a thin section is found to correspond to an increase in k by a factor of 3.5.
    Keywords: grain; growth; grain size; reduction; polar; ice ; 551
    Language: English
    Type: article , publishedVersion
    Format: 7
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    CITATION GEO-LEO
  • 4
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    Dynamic recrystallisation of ice aggregates during co-axial viscoplastic deformation: a numerical approach (2016)
    Cambridge University Press
    In:  EPIC3Journal of Glaciology, Cambridge University Press, 62, pp. 359-377, ISSN: 0022-1430
    Details
    Publication Date: 2019-12-02
    Description: Results of numerical simulations of co-axial deformation of pure ice up to high-strain, combining full-field modelling with recrystallisation are presented. Grain size and lattice preferred orientation analysis and comparisons between simulations at different strain-rates show how recrystallisation has a major effect on the microstructure, developing larger and equi-dimensional grains, but a relatively minor effect on the development of a preferred orientation of c-axes. Although c-axis distributions do not vary much, recrystallisation appears to have a distinct effect on the relative activities of slip systems, activating the pyramidal slip system and affecting the distribution of a-axes. The simulations reveal that the survival probability of individual grains is strongly related to the initial grain size, but only weakly dependent on hard or soft orientations with respect to the flow field. Dynamic recrystallisation reduces initial hardening, which is followed by a steady state characteristic of pure-shear deformation.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , isiRev
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  • 5
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    FFT simulation of dynamic recrystallization in polar ice (2014)
    EGU
    In:  EPIC3EGU General Assembly, Vienna, 2014-04-27-2014-05-02EGU General Assembly, EGU
    Details
    Publication Date: 2014-05-12
    Description: Research on ice flow is a key to understand how climate changes affect polar ice. Numerical modelling provides a better insight into the mechanics of ice from the microstructure to the ice sheet scale. The mechanics of polar ice are very sensitive to temperature changes mainly due to active recrystallization processes, as the material is very close to its melting point. We present numerical simulations that predict the microstructural evolution of ice polycrystals during deformation and dynamic recrystallization at large strain, using a full-field approach. The crystal plasticity code (Lebensohn, 2001) is used to calculate the response of a polycrystalline aggregate that deforms by dislocation glide, applying a Fast Fourier Transform (FFT). The coupling between FFT and the ELLE microstructural evolution platform allows us to include recrystallization in the aggregate, which is simulated by means of two main processes: (1) recovery and subgrain rotation, which locally reduces the crystal misorientation, and (2) grain boundary migration, which is driven by grain boundary curvature and intra-grain strain energies. This contribution presents a comparison of numerical simulations under pure and simple shear conditions up to high strain at different strain rates. The results show a strong effect of the recrystallization on the final microstructure. Dynamic recrystallization masks the strain rate and finite strain heterogeneity resulting from the strong slip anisotropy of ice. However, this strong effect does not significantly modify the single-maximum pattern of c-axes that are distributed at a low angle to the shortening direction in both pure and simple shear. In both cases, recrystallization produces larger and more equidimensional grains, with smooth boundaries. References: R. A. Lebensohn. N-site modelling of a 3D viscoplastic polycrystal using Fast Fourier Transform. 2001. Acta Materialia 49, pp 2723-2737.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , notRev
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  • 6
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    Computational analysis of dynamic recrystallization of ice aggregates during viscoplastic deformation (2015)
    Copernicus
    In:  EPIC3EGU General Assembly 2015, Vienna, 2015-04-13-2015-04-17Vienna, Copernicus
    Details
    Publication Date: 2015-05-11
    Description: Ice is a common mineral at the Earth’s surface. How much ice is stored in the Greenland and Antarctic ice sheets depends on its mechanical properties. Therefore properties of ice directly impact on human society through its role in controlling sea level. The bulk behaviour of large ice masses is the result of the behaviour of the ensemble of individual ice grains. This is strongly influenced by the viscoplastic anisotropy of these grains and their lattice orientation. Numerical modelling provides a better insight into the mechanics of ice from the micro to the ice sheet scale. We present numerical simulations that predict the microstructural evolution of an aggregate of pure ice grains at different strain rates. We simulate co-axial deformation and dynamic recrystallization up to large strain using a full-field approach. The crystal plasticity code (Lebensohn et al., 2009) is used to calculate the response of a polycrystalline aggregate that deforms by purely dislocation glide, applying a Fast Fourier Transform (FFT). This code is coupled with the ELLE microstructural modelling platform to include intracrystalline recovery, as well as grain boundary migration driven by the reduction of surface and strain energies. The results show a strong effect of recrystallization on the final microstructure, producing larger and more equiaxed grains, with smooth boundaries. This effect does not significantly modify the single-maximum pattern of c-axes that are distributed at a low angle to the shortening direction. However, in experiments with significant recrystallization the a-axes rotate towards the elongation axis at the same time as the c-axes rotate towards the compression axis. If slip systems on prismatic and/or pyramidal planes are active, it is thought that a-axes gradually concentrate with depth (Miyamoto, 2005). The bulk activity of the slip systems is different depending on the relative activity of deformation versus recrystallization: the non-basal slip systems are more active at high strain in experiments with dynamic recrystallization compared to those experiments with low recrystallization activity.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , notRev
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  • 7
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    Shear localisation in anisotropic, non-linear viscous materials that develop a CPO: A numerical study (2019)
    Elsevier
    In:  EPIC3Journal of Structural Geology, Elsevier, 124, pp. 81-90, ISSN: 01918141
    Details
    Publication Date: 2020-01-02
    Description: Localisation of ductile deformation in rocks is commonly found at all scales from crustal shear zones down to grain scale shear bands. Of the various mechanisms for localisation, mechanical anisotropy has received relatively little attention, especially in numerical modelling. Mechanical anisotropy can be due to dislocation creep of minerals (e.g. ice or mica) and/or layering in rocks (e.g. bedding, cleavage). We simulated simple-shear deformation of a locally anisotropic, single-phase power-law rheology material up to shear strain of five. Localisation of shear rate in narrow shear bands occurs, depending on the magnitude of anisotropy and the stress exponent. At high anisotropy values, strain-rate frequency distributions become approximately log-normal with heavy, exponential tails. Localisation due to anisotropy is scale-independent and thus provides a single mechanism for a self-organised hierarchy of shear bands and zones from mm-to km-scales. The numerical simulations are compared with the natural example of the Northern Shear Belt at Cap de Creus, NE Spain.
    Repository Name: EPIC Alfred Wegener Institut
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  • 8
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    Strain localization and porphyroclast rotation (2011)
    Geological Society of America (GSA)
    Details
    Publication Date: 2011-03-01
    Description: It has been debated for decades whether rigid inclusions, such as porphyroclasts and porphyroblasts, do or do not rotate in a softer matrix during deformation. Experiments and numerical simulations with viscous matrix rheologies show ongoing rotation of circular inclusions, whereas using Mohr-Coulomb plasticity results in nonrotation. Because the rocks in which inclusions are found normally undergo deformation by dislocation creep, we applied a full-field crystal plasticity approach to investigate the rotation behavior of rigid circular inclusions. We show that the inclusion's rotation strongly depends on the anisotropy of the matrix minerals. Strongly anisotropic minerals will develop shear bands that reduce the rotation of inclusions. Inhibition of rotation can only occur after a significant amount of strain. Our results may help to explain why geologic rigid objects often show evidence for rotation, but not necessarily in accordance with the viscous theory that is usually applied to these systems.
    Print ISSN: 0091-7613
    Electronic ISSN: 1943-2682
    Topics: Geosciences
    Published by Geological Society of America (GSA)
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  • 9
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    Dynamic recrystallisation of ice aggregates during co-axial viscoplastic deformation: a numerical approach (2016)
    Cambridge University Press
    Details
    Publication Date: 2016-03-18
    Description: Results of numerical simulations of co-axial deformation of pure ice up to high-strain, combining full-field modelling with recrystallisation are presented. Grain size and lattice preferred orientation analysis and comparisons between simulations at different strain-rates show how recrystallisation has a major effect on the microstructure, developing larger and equi-dimensional grains, but a relatively minor effect on the development of a preferred orientation of c-axes. Although c-axis distributions do not vary much, recrystallisation appears to have a distinct effect on the relative activities of slip systems, activating the pyramidal slip system and affecting the distribution of a-axes. The simulations reveal that the survival probability of individual grains is strongly related to the initial grain size, but only weakly dependent on hard or soft orientations with respect to the flow field. Dynamic recrystallisation reduces initial hardening, which is followed by a steady state characteristic of pure-shear deformation.
    Print ISSN: 0022-1430
    Electronic ISSN: 1727-5652
    Topics: Geography , Geosciences
    Published by Cambridge University Press on behalf of International Glaciological Society.
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  • 10
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    Intracontinental Orogeny Enhanced by Far-Field Extension and Local Weak Crust (2018)
    American Geophysical Union
    Details
    Publication Date: 2018-12-01
    Print ISSN: 0278-7407
    Electronic ISSN: 1944-9194
    Topics: Geosciences
    Published by American Geophysical Union
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