| Résumé | High-speed imaging (15,000 images/s) has been used to observe the behavior of truncated- pyramid freshwater ice samples crushing against an initially-flat freshwater ice ‘platen’ at –10oC at a rate of 20 mm/s. For most tests a polycrystalline sample was crushed against a monocrystalline platen. Two tests involved the cases where both the pyramid sample and platen were monocrystalline and where they were both polycrystalline. The ice behavior was viewed through the ice platen. Remarkably the behavior of the ice in all cases resembled that of prior tests where ice pyramids were crushed against rigid steel and acrylic platens and where observations included spalling, high-pressure zones and low-pressure zones. Spalling rates for the polycrystalline sample tests were significantly influenced by the resonant frequency of the ice-apparatus system, ~ 350 Hz, and the load records showed several extended segments of lock-in at that frequency. This implies that the spalling rates were generally in the vicinity of the resonant frequency of the system and this lead, at times, to the rates adjusting to match the system resonant frequency. While small elastic oscillations of the ice-apparatus system were evident in the sawtooth load record for the monocrystalline ice sample test these did not result in lock-in since the spalling rate was much lower than the polycrystalline case and much more stress built up in the system between spalling events, that is, the load sawteeth had much higher amplitudes and longer periods. The difference in behavior of the monocrystalline and polycrystalline ice can be attributed to the presence of grain boundaries and the random c-axis orientations of the grains, where both promote spalling fractures. Adhesive effects, due to rapid refreezing of liquid produced during crushing, were also observed in the load records at the ends of the tests where load went negative briefly as the crushed ice pyramid was pulled away from the ice platen. |
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