15th International Conference on Port and Ocean Engineering Under Arctic Conditions, 23-27 August 1999, Espoo, Finland
Observations from in-situ video records acquired during laboratory ice crushing experiments and medium scale indentation tests at Hobson's Choice Ice Island exhibit remarkable consistency. Spalling of ice away from the contact zone produces a sawtooth pattern in the load records and nearly all of the actual movement of the indentor into the ice occurs during the sharp drops in load associated with the spalls. The majority of the load is borne on relatively intact ice (hard spots) where the pressure, from calculations using load data and measured hard spot areas and from pressure sensors, is in the range 40 - 70 MPa. The visual data show how spalling determines the evolution of hard spot size and shape during the tests. Melt produced in the hard spot areas has been observed in the laboratory tests and its thickness measured. Significant quantities of melt produced in Hobson's Choice tests have also been documented. All of these observations are consistent with a process of heat generation, and consequent melt production, caused by viscous flow of a thin layer of liquid in the high pressure zones. This process accounts for the bulk of the energy dissipation in the experiments and explains how an indentor can rapidly move forward on hard spots consisting of relatively undamaged ice.