| Résumé | High-speed imaging (up to 30,000 images/s) has been used to observe the behavior of pyramidshaped polycrystalline freshwater ice samples during crushing at –10oC in the lab. The ice contact zone consisted of an intact high-pressure central area surrounded by pulverized spall debris and partially refrozen melt. A sawtooth pattern was evident in the load record, similar to that observed in other ice crushing and indentation studies. Spalling behavior was responsible for the sequence of load drops in the sawtooth load pattern and has been discussed before. Here we report details of system vibration associated with spalling. Essentially the spalling events serve as impulses that excite resonant vibration in the ice/apparatus system. We furthermore observed that resonant lock-in occurs when the frequency of the spalling events is in the vicinity of the dominant resonant frequencies of the ice-apparatus system. Correlation of load and actuator displacement data and visual observations of spalling and slight flexures of the thick acrylic crushing platen, through which the ice behavior was observed, enabled the identification of resonant modes and lock-in onset. Lock-in was observed at the two apparent dominant resonances in the apparatus, i.e. roughly 350 Hz and 900 Hz. These resonances appear to be associated with the pump/actuator system (350 Hz) and a flexural resonant mode of the acrylic platen (900 Hz). Spalling frequency was generally higher at the beginning of the crushing tests, when the ice was less confined and more susceptible to fracture, and decreased with increasing confinement as the tests progressed. In tests where the nominal actuator speed was 40 mm/s the higher frequency lock-in mode was briefly observed at first, and a short time later lock-in occurred extensively at the lower frequency. These observations are relevant to resonance and lock-in behavior that has been observed in ice-structure interaction at larger scales. |
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