| Résumé | Observations from in-situ video records acquired during laboratory ice-crushing experiments and medium-scale ice-indentation field tests exhibit remarkable consistency. Spalling of ice away from the contact zone produces a sawtooth pattern in the load records and the majority of the actual movement of the indentor into the ice occurs during the sharp drops in load associated with the spalls. At least half of the load is borne on relatively-intact ice (hard zones) where the interface pressure, from calculations using load data and measured hard-zone areas and from pressure sensors, is in the range 20 -70 MPa. The visual data show how spalling determines the evolution of hard-zone size and shape during the tests. A thin slurry layer of melt (~ 16%) and ice particles (~84%) produced at the hard-zone interface areas has been observed in the laboratory tests and its thickness (< 0.2 mm) determined. Similarly, significant quantities of melt/slurry produced in medium-scale indentation tests have also been documented. These observations are consistent with a process of heat generation, and consequent melt production, caused by rapid viscous flow of the thin slurry layer at the hard-zone interface. Additionally, recent analysis of data from earlier lab tests has identified a mechanism to explain how tiny ice particles from the hard-zone interface get into the slurry, to comprise the majority of its bulk. The data from in situ high-speed imaging records of ice crushing, and from records of rapid adiabatic hydraulic pressurization of ice samples in a pressure vessel, suggest that small Tyndall melt figures, produced by frequent and sharp pressure spikes during ice crushing, create a thin weakened layer at the hard-zone interface surface. The ambient flow of slurry at the interface could shear off particles from the weakened surface layer that become entrained in the slurry. The melt-production process, and the erosion/entrainment of hard-zone ice particles by the slurry layer, could account for the rapid removal of hard-zone material from the crushing interface.
The integrity of the above understandings has been demonstrated in a few cases. In one instance, the understandings, particularly with respect to ice-spalling behavior, provided a comprehensive explanation of large-scale ice-induced vibration of structures, and furthermore led to a technology (known as ‘Blade-Runners’) for mitigating the phenomenon. In a second instance, remarkable aspects of ice-crushing friction have been shown to stem from essentially the same ice-crushing physics noted above, and the slurry layer has further been shown to be highly lubricating. For example, data from recent experiments of a mock ice-skating blade have shown that crushing that occurs when the blade is sliding laterally on an ice surface, as happens when a skater applies a pushing stride to accelerate or when the skater is quickly stopping, produces regular tiny spallation events at the ice/blade interface that result in a sawtooth load pattern. Additionally, the high lubricity of the slurry layer beneath the blade during lateral sliding and also when gliding forward, where crushing on asperities and crushing due to ploughing/gouging occurs, largely accounts for the low friction force that is necessary for ice skating. |
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