Compressive Failure Of Ice Research Articles

The Dynamic Failure Behaviour of High-Pressure Zones during Medium-Scale Ice Indentation Tests

Results from medium-scale ice-crushing dynamic tests are presented in this paper based on a series of indentation experiments on confined ice samples using spherical indenters to simulate high-pressure zones (hpzs) with areas on the order of 103–104 mm2. The effects of ice temperature, interaction speed, indenter size and structural compliance on failure behaviour and associated structural dynamics have been studied. Observed failure behaviour consisted of a combination of continuous crushing extrusion and intermittent spalling, both of which were highly dependent on test conditions. Overall, the effects of the studied conditions on ice failure behaviour and associated interaction dynamics were found to be similar to the results reported from previous small-scale experiments, suggesting scale independence of the mechanisms that dominate ice failure behaviour. In general, warmer ice and smaller contact areas are associated with continuous extrusion with intermittent spalling, resulting in smoother peak pressures, while colder ice and larger contact areas tend to result in fracture-dominated behaviour with sharp peaks and substantial load drops. Ice temperature was also found to significantly influence interaction dynamics, with colder ice showing larger amplitude and longer duration dynamic activity, and higher peak pressures. Interaction speed was observed to primarily affect dynamic aspects of ice–structure interactions, with faster tests leading to higher failure frequencies. Similarly, structural compliance was found to mainly impact failure frequency, as well as the extent of load drops, with compliant structures tending to produce more significant load drops following failure. Overall, these experiments have helped enhance our understanding of compressive ice failure and contribute to improved models for dynamic ice–structure interactions.

Experimental investigation of oscillation of loads in ice high-pressure zones, part 1: Single indentor system

The paper commences with a review of ice-induced vibrations in full-scale structures in the field, in medium-scale tests, and particularly, in small-scale indentation tests. The mechanics of compressive ice failure is reviewed, with emphasis on the processes leading to the formation of a layer of microstructurally modified material under the indenter, as well as fracture. Previous tests in the present series using small-scale indentors are reviewed, and a recently conducted series of tests is introduced. This series involved confined ice specimens of cylindrical shape 154mm in diameter moving at various constant displacement rates against a 20-mm diameter indentor attached to an elastic-compliant structure. The displacement rates (ice speeds) were chosen so as to induce “sawtooth” vibrations. Two beam structures, one stiff and the other flexible, were used. In general, the desired sawtooth pattern was achieved, particularly with the more flexible system, but also with the stiffer one at higher ice speeds. Some results involving “lock-in” were obtained with the stiffer setup. Measurements were made of the distribution of pressure at the ice–indentor interface using pressure-sensitive film, and thin sections were taken from the indented samples. The thin sections in all cases showed the presence of a microstructurally modified layer of ice adjacent to the indentor. Pressure distributions showed that regular vibrations were associated mainly with activity in the layer, whereas the larger spall fractures generally caused irregular load patterns with significant loss of contact area under the indentor. The regular failure can be analysed as a self-sustained oscillation, which is common among dissipative systems. The system in this case comprises the moving ice field and the compliant structure with which the ice interacts, resulting in autonomous vibrations of the sawtooth variety. Regularity was interrupted by the larger spalling fractures.

Low-speed friction and brittle compressive failure of ice: fundamental processes in ice mechanics

Frictional sliding plays a fundamental role in the mechanics of ice, on scales small and large. Whether across opposing faces of microcracks within laboratory specimens, across strike-slip like features within the sea ice cover on the Arctic Ocean, or across faults within the crust of icy satellites of the Outer Solar System, sliding of ice upon itself is central to the process of brittle compressive failure. In that regard, ice is similar to rock and to other crystalline materials. This review addresses the role of sliding in brittle compressive failure and then focuses on the nature of friction per se. Both static friction and low-velocity (<0.1ms-1) kinetic friction are described at temperatures from near the equilibrium melting point to as low as -175°C. The corresponding coefficients of friction are discussed in terms of the interaction and deformation of asperities that protrude from opposing surfaces. The coefficients appear to be governed largely by asperity creep at lower velocities and by creep plus localised melting at higher velocities. At still higher velocities (beyond the scope of this review), friction of warm ice is governed by hydrodynamic effects. The review closes with comments on the compressive strength of ice, on the deformation of the arctic sea ice cover and on the tectonic evolution of Saturn's Enceladus.

Probabilistic fracture mechanics analysis of spalling during edge indentation in ice

The design of marine structures for ice environments requires ice load estimates, which are greatly influenced by fracture and associated scale effects. During edge indentation of natural ice, fracture processes are highly influenced by variability associated with flaw structure and contact conditions. A probabilistic framework is the most appropriate for modeling such interactions. In the present analysis, a probabilistic fracture mechanics model has been developed to model localized fracture events for ice specimens with ice edge taper angles ranging from 0° to 45°. Crack instability criteria have been formulated for compressive and tensile states of stress. This approach varies from traditional probabilistic failure models which assume that zones of compressive stress have zero failure probability and include only tensile zones in the calculation of failure probability. For compressive edge loading, experimental results indicate that local edge fractures (spalls) are most likely to emanate from regions of the ice where cracks would be subjected to axial compression and lateral tension. Compressive contact loads required to propagate fractures from zones of bi-axial tension or bi-axial compression were found to be considerably higher and are much less probable. Estimates of the mean pressure required to trigger a fracture event as a function of ice thickness were generated for thicknesses in the range of 0.2–2.0m. These results demonstrate that observed scale effects in ice pressure data can be explained by probabilistic aspects of fracture. Simulations of fracture probabilities as a function of loading eccentricity confirmed that extreme loads of interest in engineering design are most likely to correspond to high pressure zones located near the center of thickness of the ice sheet. Results indicate that fractures occur more readily for flat ice edges and higher pressures are required to propagate fractures for edges with more highly tapered edges. This result provides a theoretical basis to explore the interplay between fracture and crushing, both of which occur during compressive ice failure: fractures create tapered edges that localize contact in manner that supports crushing; crushing, over several cycles, flattens the ice edge in a manner that in turn promotes fracture. While the application presented here is focused on ice mechanics, the analysis methodology could readily be applied to other brittle materials.

Damage and Fracture during Contact between a Spherical Indenter and Ice: Experimental Results and Finite Element Simulations

Effective modeling of ice material behavior requires treatment of both damage and fracture. In this paper, a discussion of local ice damage and fracture processes observed during laboratory-scale indentation experiments conducted on unconfined polycrystalline ice using a spherical indenter is provided. Particular emphasis is placed on the interplay between fracture and extrusion processes corresponding to the failure of ice under a single zone of high contact pressure. Simulations have been conducted using a continuum damage mechanics user-material routine in conjunction with element removal techniques to simulate pressure-softening and discrete fracture processes, respectively. Simulation results provide good agreement with test data and provide important insights into interplay between damage and fracture mechanisms associated with compressive ice failure. Finite element modeling was found to work well for modeling pressure softening effects and for replicating the effects of discrete fracture events. For interactions involving multiple failure events, further work is needed to develop models that account for random aspects of fracture associated with flaw structure, contact geometry and the geometry of individual spalls.

Do loading path and specimen thickness affect the brittle compressive failure of ice?

AbstractCompressive experiments were performed on square (160 mm × 160 mm) prismatic specimens of columnar-grained, S2 freshwater ice, biaxially loaded across the columns at −10°C. The work focused on brittle behavior, achieved by deforming the specimens at an applied strain rate of 4.5 ± 1.2 × 10 3s 1 in the direction of shortening. The results show that the specimen thickness (25–150 mm) has no detectable effect on the terminal failure strength of the ice. Likewise, the strength of the ice when loaded under proportional loading, where the minor stress varies during the test, was similar to that when loaded under a constant minor stress, implying that terminal failure depends only on the stress state and not on the path taken.

Brittle compressive failure of ice: proportional straining vs proportional loading

AbstractProportional straining experiments have been performed on columnar-grained S2 freshwater ice biaxially compressed across the columns at –10°C at a strain rate of (4.5 ± 1.5) × 10–3 s–1. The results are compared with those obtained earlier (Iliescu and Schulson, 2004) from the same kind of material deformed to terminal failure under the same conditions, but through proportional loading. The exercise shows that the biaxial strength is practically independent of the path taken, at least under low confinement where Coulombic shear faulting limits terminal failure. First-year sea ice is expected to exhibit the same behavior.

Brittle compressive failure of ice: monotonic versus cyclic loading

Biaxial compression experiments (at −10 °C at strain rates between 3−510 −3 s −1 have confirmed that the terminal strength of fresh-water S2 ice brought to failure under moderate confinement (( σ 22/ σ 11)≅0.07) in about 10 rapid compressive cycles of increasingly larger loads is greater by a factor of 1.5 than the strength of the same material loaded monotonically. The strengthening effect becomes discernible when the ice is brought to failure in approximately 4–5 cycles, and then saturates after roughly 10 cycles even though inelastic strain at failure continues to increase. Damage accumulates more slowly under cyclic loading, but eventually leads to the development of the same kind of macroscopic shear fault that accompanies terminal failure under monotonic loading, provided that the loading and unloading strain rates are relatively high (3−510 −3 s −1 ) and equal. Upon lowering the unloading rate by one order of magnitude, ductile behavior is induced and shear faulting is suppressed. The behavior is explained in terms of the relaxation of internal stress.

High pressure zone formation during compressive ice failure

An understanding of the mechanics and physics of the formation of the high pressure zones that form during ice–structure interactions is sought. The influences of time, temperature and scale on the formation of these high pressure zones are explored in this paper. Line-like and localized high pressure contact zones are modeled via elastic-brittle hollow cylinder and hollow sphere idealizations, respectively. For both simultaneous and non-simultaneous contact, the critical lengths of stable cracking that may occur prior to flaking and flexural failure are strongly linked to the current level of specific pressure parameters for both line-like and localized high pressure zones. The stability aspects of the in-plane cracking, and the link between the maximum possible crack lengths and the relative magnitudes of the local and far-field pressures help explain the transitions observed within the realms of ductile, intermittent, and brittle crushing.

The Brittle Failure of Ice under Compression

This paper reviews progress during the past decade in understanding the brittle compressive failure of ice. Evidence is presented for the frictional crack sliding-wing mechanism and for the role of localized fragmentation. The ductile-to-brittle transition is explained in terms of the suppression of crack growth and is modeled in terms of Ashby−Hallam wing crack mechanics and Riedel−Rice crack-tip creep. The brittle compressive strength is related to ice−ice friction. Modeling and experiment suggest that the transition strain rate of cracked ice scales as (crack size)-1.5 and that its brittle strength scales as (crack size)-0.5, at least for small specimens. Possible size effects are noted.

Wing crack models of the brittle compressive failure of ice

This paper presents possible models for brittle compressive failure of ice based on the formation and growth of wing cracks. The limitations of each model are discussed, along with the implications of the model's respective functionality. Those results which are available in the literature are discussed in the light of the models. It is shown that none of the existing models describes the available data completely. Recent attempts to describe brittle failure in ice have concentrated on the Wing Crack model put forward by Ashby and Hallam, proposing that failure arises from the stable propagation of such wing cracks. However, at least two other possibilities must be considered. One is that wing cracks may indeed cause failure, but by an unstable propagation mechanism. The second is that rather than wing cracks causing the failure, the initiation of new cracks into the aggregate may give rise to a catastrophic reduction in stress. Given the failure of any of the models to provide good fits to the available data under the three test conditions, and their incorrect functionality for at least one of those test conditions, it is concluded that a model of the brittle compressive failure of ice that is based on wing cracks is inherently incorrect. New models or descriptions of the failure process must be found.

Micromechanical prediction of the compressive failure of ice: numerical simulations

Abstract The brittle compressive failure of polycrystalline ice is studied in this paper with a micromechanical model developed by Wu and Niu [1995, Mech. Mater. 20, 9–32]. Using physically definable parameters, the failure stresses and the effective compliances are predicted for several microstructures generated from a graph model. It is shown that (i) the uniaxial and biaxial compressive strengths are strongly dependent on the intrinsic heterogeneities with a scatter between 25–80% of the mean values, (ii) neither the critical crack density nor a critical damage measure based on the effective compliance concept are suitable phenomenological failure parameters, (iii) crack-crack interactions play a major role in statistical compressive fracture although the critical crack densities satisfy a weak interaction assumption, (iv) a change in failure mode can be indicated by relative changes in the effective compliance components, (v) the effective compliance is strongly anisotropic under compression and its scatter due to microstructural variations is less than that of the peak stress, (vi) the failure strains decrease with increase in the grain size, but increase with increase: in the friction coefficient, and (vii) the volumetric strains at failure are generally small and may have a grain size dependence.

Micromechanical prediction of the compressive failure of ice: model development

A micromechanica model of the brittle compressive fracture of polycrystalline ice is presented in this paper. The model is appropriate for describing ice behavior in the ductile-to-brittle transition domain between −10 and −50°C. It is based on a statistical description of the ice microstructure, which contains crystals of random sizes and orientations and a random distribution of grain boundary crack precursors. The analysis takes into account microstructural stresses originating from the elastic anisotropy of the constituent crystals. A Coulombic friction model describes the frictional sliding of the precursors, which nucleate into cracks if a fracture criterion is satisfied. Crack-crack interactions, taking into account their arbitrary orientations, locations and sizes, are considered in the analysis, and brittle failure is predicted on the basis of a mode I fracture criterion. The interacting model is used to study the dependence of the failure stresses on the ice microstructure, temperature, rate of loading, mean grain size, and lateral confinement. Furthermore, microstructurally dependent expressions are developed for the elastic compliance of cracked ice, which is used to predict the deformation behavior prior to failure. The micromechanical predictions are also analyzed with the aim of providing insights for the formulation of phenomenological damage and failure theories.

The growth of wing cracks and the brittle compressive failure of ice

High-speed photography has revealed the macroscopically stable growth of wing cracks in columnar, fresh-water ice compressed uniaxially within the brittle regime. The growth is influenced by the interaction of the cracks with free surfaces and is in excellent agreement with the Ashby-Hallam model [ Acta metall. 34, 497 (1986)].