Numerical modelling of deformation and recrystallisation mechanics in ice and ice-air aggregates

Ice sheets and glaciers flow under their own weight and their flow of ice is a major contributor to both global sea-level and climate changes. The macroscopic flow of ice is affected by the properties of the microstructure, which is formed by a small aggregate of individual ice crystals. The deformation of ice is accompanied by recrystallisation, a term which describes mechanisms causing re-orientations of the crystalline lattice, the formation of new crystals or the migration of their boundaries. The ice crystal is marked by a significant viscoplastic anisotropy, which causes a distinctly higher resistance to flow, if the crystalline lattice is unfavourably oriented. With deformation, the ice grains align and develop a crystallographic-preferred orientation within the ice-aggregate, which induces a macroscopic anisotropy. A knowledge of the micro-dynamic deformation and recrystallisation mechanisms and how they affect the properties of the ice aggregate is a key to understand ice sheet dynamics. The objective of this thesis to investigate the deformation and recrystallisation mechanisms in ice and the involved changes in the microstructures of ice- and ice-air aggregates. This is done by means of two-dimensional numerical simulations using the modelling platform Elle, which optimised for modelling interacting micro-dynamic processes. The simulations couple a numerical model for viscoplastic deformation of anisotropic polycrystalline aggregates to implementations of recrystallisation mechanisms in Elle. In particular, an explicit numerical approach to consider secondary phases such as air inclusions in the numerical setup is developed, implemented and used in this thesis for the first time. Additionally, the new approach allows grain-size-reducing mechanisms, which allows the achievement of stable-state microstructures with deformation. In each scientific publication presented in the thesis, qualitative comparisons to natural polar ice accompany the numerical simulations. The results of this thesis show that the deformation and microstructures of ice are generally more heterogeneous than previously thought. Strain localisation is common in ice and related to viscoplastic anisotropy and intensified by the presence of air inclusions. Probably, strain localisation is occurring over a range of scales and has implications for the large-scale flow of ice. The thesis further demonstrates that deformation-induced recrystallisation mechanisms are common in ice and discusses their relation to strain localisation. In particular, the study points out the importance of the dissection of grains by migrating grain boundaries as an additional grain-size-reducing process in polar ice, which was not studied previously. This thesis confirms that the activation of deformation and recrystallisation mechanisms is a function of the deformation conditions such as strain rate, temperature and likely the load of impurities and dust particles. The steady-state numerical-microstructures reflect the prescribed deformation conditions, but appear largely independent from the initial microstructures. These results of this study indicate a high rate of change in crystallographic-preferred orientation and other microstructural properties. Furthermore, the thesis confirms that the development of crystallographic-preferred orientation is a function of strain rather than time or stress.Eisschilde und Gletscher fließen unter ihrem eigenen Gewicht und ihr Fluss hat einen großen Einfluss sowohl auf globale Meeresspiegel- als auch Klimaveränderungen. Das makroskopische Fließverhalten von Eis wird von den Eigenschaften der Mikrostruktur, die sich aus kleinen Gefügen von einzelnen Eiskristallen zusammensetzt, beeinflusst. Die Deformation des Eises geht mit Rekristallisation einher, ein Begriff, der Mechanismen beschreibt, die eine Neuorientierung des Kristallgitters, das Entstehen neuer Kristalle oder die Bewegung der Kristallgrenzen verursachen. Der Eiskristall ist durch eine deutliche mechanische Anisotropie gekennzeichnet, was einen deutlich höhren Widerstand gegen Deformation bedeutet, wenn das Kristallgitter ungünstig orientiert ist. Mit der Deformation richten sich Eiskristalle innhalb des Gefüges entlang einer kristallographisch bevorzugten Richtung aus, was auch makroskopisch eine Anisotropie erzeugt. Kenntnisse über die mikrodynamischen Deformations- und Rekristallisationsmechanismen und deren Einlfuss auf die Eigenschaften der Eismikrostruktur sind grundlegend für ein verbessertes Verständnis der Eisschilddynamik. Das Anliegen dieser Arbeit ist eine Untersuchung der Deformations- und Rekristallisationsmechanismen in Eis und der damit einhergehenden mikrostrukturellen Veränderung in reinem Eis und Eis mit Lufteinschlüssen. Dafür sieht die Arbeit zweidimensionale numerische Simulationen mit Hilfe der Modellierplattform Elle, die für die Modellierung von interagierenden mikrodynamischen Prozessen optimiert ist, vor. Die Simulationen koppeln ein numerisches Modell für viskoplastische Deformation unter Beachtung der Kristallanisotropie mit Implementierungen von Rekristallisations-mechanismen in Elle. Insbesondere beschäftigt sich die Arbeit erstmalig mit der Entwicklung, Implementierung und Anwednung eines numerischen Ansatzes, der weitere Phasen wie Lufteinschlüsse einbezieht. Zusätzlich erlaubt der neue Ansatz korngrößenverkleindernde Prozesse, wodurch die simulierten Mikrostrukturen mit Deformation einen Gleichgewichtszustand erreichen können. Jede wissenschaftliche Veröffentlichung in dieser Arbeit, nimmt außerdem qualitative Vergleiche von natürlichem polarem Eis mit numerischen Simulationen vor. Die Ergebnisse dieser Arbeit zeigen generell eine höhere Heterogenität in Eisdeformation und Mikrostruktur als bislang angenommen. Verformungslokalisation ist verbreitet in Eis, steht in Verbindung mit der Kristallanisotropie und wird durch Lufteinschlüsse verstärkt. Wahrscheinlich kann eine Verformungslokalisation über verschiedene Größenordnungen auftreten und hat eine Bedeutung für das großmaßstäbliche Eisfließen. Desweiteren zeigt die Arbeit, dass deformationsinduzierte Rekristallisationsmechanismen in Eis verbreitet sind und diskutiert deren Verhältnis zur Verformungslokalisation. Insbesondere zeigt die Arbeit, dass die Zerteilung von Körnern durch Korngrenzmigration ein wichtiger, jedoch bislang nicht untersuchter, korngrößenverkleindernder Prozess in polarem Eis ist. Die Arbeit bestätigt außerdem, dass die Aktivierung von Deformations- und Rekristallisationsmechanismen von Deformationsbedingungen wie Verformungsrate, Temperatur und wahrscheinlich dem Anteil von Verunreinigungen abhängt. Im Gleichgewichtszustand spiegeln die simulierten Mikrostrukturen die vorgegebenen Deformationsbedingungen wider, aber sind größtenteils unabhängig von der Ausgangsmikrostruktur. Diese Ergebnisse deuten darauf hin, dass die Rate der mikrostrukturellen Änderungen in beispielsweise kristallographisch bevorzugten Richtungen oder Korngrößen in Eis hoch ist. Außerdem bestätigt die Arbeit, dass die Entwicklung einer kristallographisch bevorzugten Richtung von der aufgebauten Verformung abhängt, nicht aber vom Zeit oder Spannungszustand

Modelling the influence of air on the deformation and recrystallisation mechanisms in polar firn and ice

Within their upper approximately thousand meters, ice sheets on Earth contain a significant amount of air and air hydrates below. In the permeable firn, this air is still exchanging with the atmosphere and is under atmospheric pressure, whereas the air bubbles are entrapped at the firn-ice transition 60 – 120 m depth. As recent research showed, the presence of air bubbles can significantly influence microdynamical processes such as grain growth and grain boundary migration (Azuma et al., 2012, Roessiger et al., 2014). Understanding the dominant deformation mechanisms has essential implications on paleo-atmosphere research and allows more realistic modelling of ice sheet dynamics. Therefore, numerical models were set up and performed focussing on the implications of the presence of bubbles on recrystallisation and the mechanical properties of ice with air inclusions. The 2D numerical microstructural modelling platform Elle was coupled to the full-field crystal plasticity code of Lebensohn (2001), which is using a Fast Fourier Transform (FFT) following the approach by Griera et al. (2013). Taking into account the mechanical anisotropy of ice, FFT calculates the viscoplastic response of polycrystalline and polyphase materials that deform by dislocation glide, predicts lattice re-orientation and using the local gradient of the strain-rate field, dislocation densities are calculated. FFT was used for the simulation of dynamic recrystallization of pure ice by Montagnat et al. (2013). Polyphase grain boundary migration driven by surface energy and internal strain energy reduction was incorporated in the code and now also enables us to model deformation of ice with air bubbles. The approach is based on the methodology of Becker et al. (2008) and Roessiger et al. (2014). During Deformation, spherical to elliptical bubble shapes are only maintained, when surface energy based recrystallisation is activated, whereas they quickly collapse at low strains in the absence of recrystallisation. The presence of bubbles leads to increased localization of stress, strain and dislocation densities, a reduction of the bulk strength of the bubbly ice is observed. Furthermore, strain-induced grain boundary migration already occuring in the uppermost levels of ice sheets (Kipfstuhl et al. 2009, Weikusat et al. 2009) is confirmed by our modelling. References Azuma, N., Miyakoshi, T., Yokoyama, S., Takata, M., 2012. Journal of Structural Geology 42, 184- 193. Becker, J.K., Bons, P.D., Jessell, M.W., 2008. Computers & Geosciences 34, 201-212. Bons, P.D., Koehn, D., Jessell, M.W. (Eds.), 2008. Microdynamic Simulation. Springer, Berlin. Kipfstuhl, S., Faria, S.H., Azuma, N., Freitag, J., Hamann, I., Kaufmann, P., Miller, H., Weiler, K., Wilhelms, F., 2009. Journal of Geophysical Research 114, B05204. Lebensohn, R.A., 2001. Acta Mater 49 (14), 2723e2737. Montagnat, M., Castelnau, O., Bons, P.D., Faria, S.H., Gagliardini, O., Gillet-Chaulet, F., Grennerat, F., Griera, A., Lebensohn, R.A., Moulinec, H., Roessiger, J., Suquet, P., 2014. Journal of Structural Geology 61, 78-108 Rößiger, J., Bons, P.D., Faria, S.H., 2014. Journal of Structural Geology 61, 123-132 Weikusat, I., Kipfstuhl, S., Faria, S.H., Azuma, N., Miyamoto, A., 2009. Journal of Glaciology 55, 461-472

Advanced material model for shear cutting of metal sheets

A finite-element simulation of the shear cutting process is used to predict thegeometry of the cutting surface. A fully-coupled Lemaitre-type model is used in the process model for the description of the material behaviour. The extended Lemaitre model considers the influence of shear and compression-dominated stress states on the propagation of damage. Tensile tests with and without notches are used for the identification of material parameters. These methods are advantageous for the analysis of different blanking processes. Since damage parameters have a strong influence on the cutting surface quality, a numerical study isconducted to analyse their influence. The results of the simulations are compared with experimental data

Nogo receptor is involved in the adhesion of dendritic cells to myelin

BACKGROUND: Nogo-66 receptor NgR1 and its structural homologue NgR2 are binding proteins for a number of myelin-associated inhibitory factors. After neuronal injury, these inhibitory factors are responsible for preventing axonal outgrowth via their interactions with NgR1 and NgR2 expressed on neurons. In vitro, cells expressing NgR1/2 are inhibited from adhering to and spreading on a myelin substrate. Neuronal injury also results in the presence of dendritic cells (DCs) in the central nervous system, where they can come into contact with myelin debris. The exact mechanisms of interaction of immune cells with CNS myelin are, however, poorly understood. METHODS: Human DCs were differentiated from peripheral blood monocytes and mouse DCs were differentiated from wild type and NgR1/NgR2 double knockout bone marrow precursors. NgR1 and NgR2 expression were determined with quantitative real time PCR and immunoblot, and adhesion of cells to myelin was quantified. RESULTS: We demonstrate that human immature myeloid DCs express NgR1 and NgR2, which are then down-regulated upon maturation. Human mature DCs also adhere to a much higher extent to a myelin substrate than immature DCs. We observe the same effect when the cells are plated on Nogo-66-His (binding peptide for NgR1), but not on control proteins. Mature DCs taken from Ngr1/2 knockout mice adhere to a much higher extent to myelin compared to wild type mouse DCs. In addition, Ngr1/2 knockout had no effect on in vitro DC differentiation or phenotype. CONCLUSIONS: These results indicate that a lack of NgR1/2 expression promotes the adhesion of DCs to myelin. This interaction could be important in neuroinflammatory disorders such as multiple sclerosis in which peripheral immune cells come into contact with myelin debris

Strain localisation and dynamic recrystallisation in the ice-air aggregate: A numerical study

We performed numerical simulations on the micro-dynamics of ice with air inclusions as a second phase. This provides first results of a numerical approach to model dynamic recrystallisation in polyphase crystalline aggregates. Our aim was to investigate the rheological effects of air inclusions and explain the onset of dynamic recrystallisation in the permeable firn. The simulations employ a full field theory crystal plasticity code coupled to codes simulating dynamic recrystallisation processes and predict time-resolved microstructure evolution in terms of lattice orientations, strain distribution, grain sizes and grain boundary network. Results show heterogeneous deformation throughout the simulations and indicate the importance of strain localisation controlled by air inclusions. This strain localisation gives rise to locally increased energies that drive dynamic recrystallisation and induce heterogeneous microstructures that are coherent with natural firn microstructures from EPICA Dronning Maud Land ice coring site in Antarctica. We conclude that although overall strains and stresses in firn are low, strain localisation associated with locally increased strain energies can explain the occurrence of dynamic recrystallisation

The Relevance of Grain Dissection for Grain Size Reduction in Polar Ice: Insights from Numerical Models and Ice Core Microstructure Analysis

The flow of ice depends on the properties of the aggregate of individual ice crystals, such as grain size or lattice orientation distributions. Therefore, an understanding of the processes controlling ice micro-dynamics is needed to ultimately develop a physically based macroscopic ice flow law. We investigated the relevance of the process of grain dissection as a grain-size-modifying process in natural ice. For that purpose, we performed numerical multi-process microstructure modelling and analysed microstructure and crystallographic orientation maps from natural deep ice-core samples from the North Greenland Eemian Ice Drilling (NEEM) project. Full crystallographic orientations measured by electron backscatter diffraction (EBSD) have been used together with c-axis orientations using an optical technique (Fabric Analyser). Grain dissection is a feature of strain-induced grain boundary migration. During grain dissection, grain boundaries bulge into a neighbouring grain in an area of high dislocation energy and merge with the opposite grain boundary. This splits the high dislocation-energy grain into two parts, effectively decreasing the local grain size. Currently, grain size reduction in ice is thought to be achieved by either the progressive transformation from dislocation walls into new high-angle grain boundaries, called subgrain rotation or polygonisation, or bulging nucleation that is assisted by subgrain rotation. Both our time-resolved numerical modelling and NEEM ice core samples show that grain dissection is a common mechanism during ice deformation and can provide an efficient process to reduce grain sizes and counter-act dynamic grain-growth in addition to polygonisation or bulging nucleation. Thus, our results show that solely strain-induced boundary migration, in absence of subgrain rotation, can reduce grain sizes in polar ice, in particular if strain energy gradients are high. We describe the microstructural characteristics that can be used to identify grain dissection in natural microstructures

The effect of dynamic recrystallisation on the rheology and microstructures of partially molten rocks

This work was founded by the joint project “Rheology of the continental crust in collision”, funded by the Procope scheme of PHC Egide in France and by the DAAD PPP scheme in Germany. M-GL acknowledges the support of the Juan de la Cierva programme of the Government of Spain’s Ministry for Science, Innovation and Universities. EGR acknowledges the support of the Beatriu de Pinós programme of the Government of Catalonia's Secretariat for Universities and Research of the Department of Economy and Knowledge (2016 BP 00208). This work benefited from discussions with Pi L. Jolivet and E. Burov within the ERC project RHEOLITH. We thank Elisabetta Mariani and Marcin Dabrowski for their helpful comments, together with the editorial guidance of Dave Healy and Bill Dunne.Peer reviewedPostprin

CD47 restricts antiviral function of alveolar macrophages during influenza virus infection

CD47 is an ubiquitously expressed surface molecule with significant impact on immune responses. However, its role for antiviral immunity is not fully understood. Here, we revealed that the expression of CD47 on immune cells seemed to disturb the antiviral immune response as CD47-deficient mice (CD47−/−) showed an augmented clearance of influenza A virus (IAV). Specifically, we have shown that enhanced viral clearance is mediated by alveolar macrophages (aMФ). Although aMФ displayed upregulation of CD47 expression during IAV infection in wildtype mice, depletion of aMФ in CD47−/− mice during IAV infection reversed the augmented viral clearance. We have also demonstrated that CD47 restricts hemoglobin (HB) expression in aMФ after IAV and severe acute respiratory syndrome coronavirus type 2 (SARS-CoV-2) infection, with HB showing antiviral properties by enhancing the IFN-β response. Our study showed a negative role for CD47 during antiviral immune responses in the lung by confining HB expression in aMФ

The importance of strain localisation in shear zones

The occurrence of various types of shear bands (C, C’, C”) in shear zones indicate that heterogeneity of strain is common in strongly deformed rocks. However, the importance of strain localisation is difficult to ascertain if suitable strain markers are lacking, which is usually the case. Numerical modelling with the finite-element method has so far not given much insight in the development of shear bands. We suggest that this is not only because the modelled strains are often not high enough, but also because this technique (that usually assumes isotropic material properties within elements) does not properly incorporate mineral deformation behaviour. We simulated high-strain, simple-shear deformation in single- and polyphase materials with a full-field theory (FFT) model coupled to the Elle modelling platform (www.elle.ws; Lebensohn 2001; Bons et al. 2008). The FFT-approach simulates visco-plastic deformation by dislocation glide, taking into account the different available slip systems and their critical resolved shear stresses in relations to the applied stresses. Griera et al. (2011; 2013) have shown that this approach is particularly well suited for strongly anisotropic minerals, such as mica and ice Ih (Llorens 2015). We modelled single- and polyphase composites of minerals with different anisotropies and strengths, roughly equivalent to minerals such as ice Ih, mica, quartz and feldspar. Single-phase polycrystalline aggregates show distinct heterogeneity of strain rate, especially in case of ice Ih, which is mechanically close to mica (see also Griera et al. 2015). Finite strain distributions are heterogeneous as well, but the patterns may differ from that of the strain rate distribution. Dynamic recrystallisation, however, usually masks any strain and strain rate localisation (Llorens 2015). In case of polyphase aggregates, equivalent to e.g. a granite, we observe extensive localisation in both syn- and antithetic shear bands. The antithetic shear bands are, however, ephemeral and best seen in movies. In the final microstructure they are very difficult to discern. We present movies that show that in all cases the distribution of both strain rate and of finite strain is much more heterogeneous than the finite microstructure tends to show. Bons, P.D., et al. (2008) Lecture Notes in Earth Sciences, 106 Griera, A. et al. (2011) Geology, 39, 275-278 Griera, A., et al. (2013) Tectonophysics, 587, 4-29 Griera, A., et al. Geotectonic Research, 97, 37-39 Lebensohn, R.A. (2001) Acta Materialia, 49, 2723-2737 Llorens, M.G. (2015) ) Numerical simulation of deformation microstructures and folds in polar ice and ductile rocks. PhD-thesis, Tübingen Univ. Llorens, M.G., et al. (in press) Journal of Glaciolog