Seligman awardee 2013
Paul Duval has been at the forefront of international research to understand the physics of ice flow for 40 years. Many would say that he is still the leading researcher in microscale ice physics. He has made outstanding contributions to our understanding of ice flow with regard to the effects of impurities, including water, the impact of crystal size variations, and the rate-controlling processes of the deformation of ice. He characterized ice creep and unveiled the micromechanisms underlying the process. In addition, he championed modern modelling of the depth evolution of crystallographic texture and of the attendant development of plasticly anisotropic viscoplastic flow within both the Greenland and the Antarctic ice covers. Computational glaciologists continue to incorporate these effects into refined interpretations of ice core data and GCM models, much to the benefit of future climate predictions.
Early in his career, Paul published three papers that stand out as fundamental building blocks in glaciology. ‘Fluage et recrystallisation dynamique de la glace polycristalline’ (Duval (1972) C. R. Acad. Sci. (Paris), 275, 337–339) made clear for the first time that dynamic recrystallization contributes significantly to the acceleration of creep following the secondary stage. ‘Anelastic behavior of polycrystalline ice’ (Duval (1978), J. Glaciol., 21(85), 621–628) showed that, when load is removed, anelastic or recoverable strain accounts for a significant fraction of primary creep, an observation he explained in terms of dislocation back-slip. ‘Rate-controlling processes in the creep of polycrystalline ice’ (Duval and others (1983) J. Phys. Chem., 87, 4066–4074) established not only that plastic strain of relatively warm material (–10˚C) compressed under intermediate-to-high deviatoric stress (0.1–10 MPa) results principally from basal slip, but also that the rate of deformation is governed by a non-basal process. Most importantly, this paper showed that creepstrengthening (i.e. the reduction by a factor of 100 or more of the deformation rate upon transition from primary to secondary creep) is caused by the development of long-range interactions among the stress field of dislocations. This result laid the foundation for the understanding of the a later discovery that dislocations in ice move cooperatively and intermittently rather than individually and continuously, in the form of ‘avalanches' or microbursts (Weiss and Grasso (1997) J. Phys. Chem. B, 101, 6113–6117).
As Paul continued to explore the nature of creep, including, for instance, the study of creep-induced grain growth in polar ice (Montagnat and Duval (2000) Earth Planet. Sci. Lett., 183, 179-186), he also initiated a novel study on the modelling of inelastic deformation within glaciers and polar ice sheets. He realized early on that in such work it is critical, particularly in relation to the dating of ice cores, to take into account the plastic anisotropy that develops through the evolution of crystallographic texture: otherwise, predictions of ice age can be in error by thousands of years. Therefore, he incorporated not only an understanding of the physics of deformation but also new developments in materials mechanics on viscoplastic self-consistency (Hutchinson (1976) Proc. R. Soc. London Ser. A, 348, 101–127; Hutchinson (1977) Metal. Trans. A, 8, 1465–1469). This approach led to several excellent papers. For instance, ‘Modelling fabric development along the GRIP ice core, central Greenland’ (Castelnau, Duval and others (1996) Ann. Glaciol., 23, 194–201), ‘Viscoplastic modeling of texture development in polycrystalline ice with a self consistent approach’ (Castelnau, Duval and others (1996) J. Geophys. Res., 101, 13851–13878) and ‘Modelling viscoplastic behavior of anisotropic polycrystalline ice’ (Castelnau and others (1997) Acta Mater., 45, 4823–4834) all showed very good agreement with observation, thereby establishing the power of the combined physics–mechanics approach. A later paper, ‘Elastoviscoplastic micromechanical modelling of the transient creep of ice’ (Castelnau and others (2008) J. Geophys. Res., 113, B11203), accounted (in terms of the rheology of single crystals) for the permanent creep rate of several highly anisotropic samples harvested from Greenland.
More recent developments in which Paul is playing an important role include the study of dislocation ‘avalanches’. The observation (Weiss and Grasso (1997) J. Phys. Chem. B, 101, 6113–6117) that dislocations in ice move cooperatively and intermittently rather than individually and continuously is now attributed to the kind of long-range interactions among dislocations that Paul described in his 1983 paper (Duval and others (1983) J. Phys. Chem., 87, 4066–4074). Among others, notably Weiss and others (e.g. Weiss and others (2007) Phys. Rev. B, 76, 224110), Paul and his collaborators (e.g. Montagnat and others (2006) Phil. Mag., 86, 4259; Chevy and others (2010) Acta Mater., 58, 1837–1849) have added to this evolving story by showing that the dislocation arrangement in deformed ice exhibits invariance of spatial scale and is therefore fractal in character. Another development, published in ‘On the role of long range internal stresses on grain nucleation during discontinuous recrystallization’ (Duval and others (2012) Mater. Sci. Eng. A, 546, 207–211) is the idea that long-range dislocation interactions can eliminate the need for a critical nucleus, leading to the possibility of spontaneous nucleation not only in ice but also in other plasticly anisotropic materials such as zirconium. These developments in dislocation avalanches and barrier-free nucleation will inspire fundamental research on the plasticity of ice and other materials for years to come.
Paul’s book, co-authored with Erland Shulson, Creep and Fracture of Ice (Cambridge University Press, 2009) is the first reasonably complete account of the physical mechanisms underlying the inelastic deformation of ice on scales small and large. Throughout the book, mechanisms that govern the behaviour of ice are related to mechanisms that control the behaviour of other materials, in support of the a view that Paul has held for many years (expressed directly in ‘Creep and plasticity of glacier ice: a materials science perspective’; Duval and others (2010) J. Glaciol., 56(200), 1059–1068) that ice is a model material.
Paul is a humble person, with little to be humble about. He has served his profession well, with integrity and vigour and with clarity of thought. He has always been a keen teacher and communicator of his knowledge, keen to discuss research projects with others. He has inspired many students, several of whom are now recognized as excellent scientists in their own right. In November 2011, an international symposium (linked with the new ESF MicroDICE project) was held at Grenoble in his honour. That this symposium was instigated and organized by his current and past students demonstrates their admiration for his contribution to their own successes, and is a credit to him and to this work over the past 40 years.
Paul Duval has made a superb contribution to glaciology over the years, and the Awards Committee feel that the award of the Seligman Crystal is a fitting acknowledgement of his achievements.