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The Physics of Ice Crushing Associated with Indentation and Impact

Presented by: 
Robert Gagnon National Research Council of Canada
Tuesday 5th December 2017 - 11:30 to 12:30
INI Seminar Room 1
Ice crushing occurs in many contexts such as ice interaction with bridges, piers, ship hulls, offshore structures, rock beds under glaciers and ice-on-ice sliding/crushing interaction within glaciers and extraterrestrial ice masses (on Saturn’s moon Enceladus). In the cases of skate blades, sled runners and curling stones local crushing on ice asperities and/or small-scale ice unevenness, and due to gouging/plowing, occurs. In-situ imaging records from small and medium scale ice-crushing experiments show that repetitive spallation of ice from a relatively-intact hard zone in the central contact region produces a sawtooth load pattern, and most of the actual ice indentation occurs during the associated sharp drops in load. At least half of the load is borne on the hard zone, where the interface pressure is ~ 20-70 MPa. The rest of the load is borne on surrounding shattered spall debris, where the pressure is ~ 0-10 MPa. Spalling influences the evolution of hard-zone size and shape during the tests. The hard zones are regions where a thin squeeze-film slurry layer of pressurized melt and ice particles is present between the ice and the contacting surface. The viscous flow of the layer generates heat that accounts for the rapid-melting component of the removal of ice from the hard zones during ice crushing. A similar process occurs at ice-on-ice contact of fragments in the surrounding crushed-ice matrix as it extrudes away from the high-pressure zones. The melting accounts for the bulk of the energy dissipation and partly explains how an indentor can rapidly move forward on hard zones. The slurry layer thickness in small-scale lab tests is ~ 0.02 - 0.17 mm, where its liquid fraction is about 16%. The layer acts as a self-generating squeeze film that is powered by the energy supplied by the loading system. When ice crushing includes a sliding component the layer’s flow characteristics and high lubricity lead to very low friction coefficients, even on rough surfaces.
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University of Cambridge Research Councils UK
    Clay Mathematics Institute London Mathematical Society NM Rothschild and Sons