Extensive da mage to ice occ urs during ice-structure interac ti o n by microc racking, recrystalli zati o n a nd melting, Th e obj ecti ve of this wo rk was to investigate this da m a ge process unde r confin ed-stress conditi o ns beli eved to bc assoc ia ted with impact zo nes th a t occ ur durin g ice-structure inte r ac ti on, "Da m age" refers to mi c ros truc tura l mod ifi cati o n th at causes d e teriorati on o f the m ee ha nica l prope rti es, Prior exp erimental work h as shown that a sm a ll a mo unt of defo rm a ti on causes pe rm a nent da mage in ice, leading to e nh a nced creep ra tes during subsequc nt load ing, To investigate thi s soft ening, fr es hwater g ra nul a r ice was defo rmed under m od era te confineme nt (20 :'IiPa ) a t -10°C, a t two rates which brac ket duc ti le a nd brittl e beh avio r (10 2 s I a nd 10 + s I). Sa mples were deform ed to different level s of ax ial strain up to 28,8% , Thin sec tio ns we re exam i ned to assess the prog ressive cha nges in microstructure, Bo th g ra in-bound a ry a nd intra-g ra nul a r c rac king began a t strains co rrespond ing to th e peak stress (1-2% ) fo r tests at both stra in ra tes. The peak stresses we re 23.4 MPa for the tests a t 10 2 S I a nd 9.8 MPa for th e tes ts a t 10 + s I, At strains of > 1-2%, d e n se elusters of intra-g ra nul a r crac ks b ega n to develop in th e samples tes ted a t the highe r ra te, At th e lower ra te, d yna mic rec rys ta lli zati on was a ppa rently the do min a nt defo rm a ti o n m eeha nism b eyo nd the pea k stress. The a\' C rage g r a in-size decreased stro ngly during th e fi rst few per ce nt stra in a nd then m a intained a rela tivel y stabl e va lue.

During ice-structure interaction, ice will fail in a brittle manner dominated by two processes. The first corresponds to the formation of macrocracks and the consequent spalling-off of large ice pieces. The second includes an intense shear-damage process in zones, termed critical zones, where high pressures are transmitted to the structure. The shear-damage process results in microstructural changes including microcrack formation and recrystallization. A range of tests on laboratory-prepared granular ice have been conducted to determine the fundamental behaviour of ice under various stress states and stress history, particularly as it relates to changes in microstructure. The test series was designed to study three aspects: the intrinsic creep properties of intact, undamaged ice; the enhancement of creep and changes in microstructure due to damage; and the effects of different stress paths. Tests on intact ice with triaxial confining pressures and low deviatoric stresses, aimed at defining the intrinsic creep response in the absence of microcracking, showed that an accelerated creep rate occurred at relatively low deviatoric stresses. Hence, a minimum Creep rate occurred under these conditions. Recrystallization to a smaller grain-size and void formation were observed. Ice damaged uniaxially and triaxially prior to testing showed enhancement of creep under both uniaxial and triaxial loading conditions Creep rates in triaxially damaged ice were found to be non-linear with high deviatoric stresses, corresponding to a power-law dependence of creep rate. Uniaxially damaged specimens contained microcracks parallel to the stressed direction which tended to close under triaxial confinement. Damage under triaxial conditions at low confining pressures produced small recrystallized grains near zones of microcracking. At high confining pressures, a fine-grained recrystallized structure with no apparent cracking was observed uniformly across the specimen. The recrystallization process contributes significantly to the enhanced creep rates found in damaged specimens.

Tests in the field and full-scale experience with arctic structures show that the crushing of ice is accompanied by large fluctuations in load. Field experiments show that, in addition to variations of load in time, significant spatial variations across the contact surface also occur. The deformation is observed to take place in a thin layer of damaged ice, which appears near the structure or indenter surface. It is important to model the deformation and strength of ice in this zone. Various aspects of modelling are discussed in the paper, in particular, measures of damage and the relation to the deformation of ice. The relevance of various components of deformation (elastic, viscous, delayed elastic) is outlined, and two mathematical formulations for the deformation are discussed. The behaviour was investigated by a series of tests at constant strain rate as well as tests in which the strain response to stress of damaged and undamaged ice was measured. The creep rate in damaged ice is shown to be significantly enhanced, even for short-term loading. Comparisons of theory and experiment are given for constant strain-rate tests. The models have been calibrated to the experimental data described in the paper. It is a matter for future research to generalize the models to all damage levels and stress states.Des essais en nature et I'expCrience grandeur nature acquise avec les structures arctiques dimontrent que le broyage de la glace est accompagnC de fortes fluctuations de charge. Des expCriences sur le terrain montrent qu'en plus de variations de charge en fonction du temps, des variations spatiales significatives se produisent le long de la surface de contact. L'on observe que la dCformation se produit dans une couche mince de glace endommagke qui apparait prks de la structure ou de la surface de l'entaille. I1 est important de modCliser la deformation et la resistance de la glace dans cette zone. DiffCrents aspects de la modClisation sont discutCs dans cet article, et en particulier les mesures des dommages et la relation a la dCformation de la glace. La pertinence de diverses composantes de dkformation (Clastique, visqueuse, Clastique retardCe) est mise en Cvidence et deux formulations mathCmatiques pour la dCformation sont discutCes. Le comportement a Ct C CtudiC par une sCrie d'essais a vitesse de dkformation constante de mCme que des essais dans lesquels la rCaction de la dkformation a la contrainte de la glace endommagtie et intacte Ctait mesurCe. La vitesse de fluage dans la glace endommagie apparait Ctre accrue de facon significative, mCme pour un chargement a court terme.Des comparaisons entre la thCorie et llexpCrience sont donnCes pour des essais a vitesse de dkformation constante. Les modkles ont Ct C CtalonnCs avec les donnCes expCrimentales dCcrites dans l'article. La gCnCralisation des modkles a tous les niveaux de dommage et d'Ctats de contrainte devrait faire I'objet de futures recherches.

Extensive damage to ice occurs during ice structure interaction by microcracking, recrystallization and melting. The objective of this work was to investigate this damage process under confined-stress conditions believed to be associated with impact zones that occur during ice–structure interaction. “Damage” refers to microstructural modification that causes deterioration of the mechanical properties. Prior experimental work has shown that a small amount of deformation causes permanent damage in ice, leading to enhanced creep rates during subsequent loading. To investigate this softening, freshwater granular ice was deformed under moderate confinement (20 MPa) at –10°C, at two rates which bracket ductile and brittle behavior (10−2 s−1 and 10−4 s−1). Samples were deformed to different levels of axial strain up to 28.8%. Thin sections were examined to assess the progressive changes in microstructure.Both grain-boundary and intra-granular cracking began at strains corresponding to the peak stress (1–2%) for tests at both strain rates. The peak stresses were 23.4 MPa for the tests at 10−2 s−1 and 9.8 MPa for the tests at 10−4 s−1. At strains of > 1–2%, dense clusters of intra-granular cracks began to develop in the samples tested at the higher rate. At the lower rate, dynamic recrystallization was apparently the dominant deformation mechanism beyond the peak stress. The average grain-size decreased strongly during the first few per cent strain and then maintained a relatively stable value.

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