Hysteresis and dichotomous mechanics in cyclic crushing failure of confined freshwater columnar ice

Cyclic crushing experiments with a haversine velocity waveform were performed on passively confined, freshwater columnar ice specimens for a variety of velocities and frequencies. The aim of the experiments was to study the ice deformation and failure behavior in crushing when loaded at a predefined displacement pattern closely resembling the frequency lock-in regime of ice-induced vibrations. The focus of the experiments was on the development of load and ice deformation behavior at the grain and ice specimen scales during each cycle. To this end, the deformation and failure of the ice were observed with crossed-polarized light to highlight the microstructure in-situ during cyclic crushing. It was shown that there are dichotomous mechanical behaviors of the damaged and confined ice during a single crushing cycle: brittle at high velocity and non-brittle at low velocity. At low velocity, ice fracture was interrupted and stress relaxation occurred until the predefined velocity began increasing in the cycle. The stress relaxation in the load was accompanied by stress-optic effects in the ice. It was found that a load peak-velocity hysteresis developed in each crushing cycle: peak loads following the non-brittle behavior were temporarily higher than the peak loads of the brittle behavior. The temporary load peak enhancement tended to increase with increasing duration of stress relaxation, i.e. the peak enhancement tended to increase with decreasing velocity and frequency. Negligible peak enhancement and stress relaxation duration were observed for the highest frequency and mean velocity tested of 2 Hz and 10 mm s⁻¹, respectively. For tests with a minimum velocity of 1 mm s⁻¹, no stress relaxation was observed in the load measurement. Preliminary results from deviating from the haversine velocity waveform by increasing the minimum velocity showed that the stress relaxation duration decreases, but the non-brittle peak load does not decrease. It is speculated that ice anelastic ice behavior could account for the rapid stress relaxation at low velocity. It is unclear what causes the hysteresis, although it is speculated that dynamic strain aging might play a role. The change in ice behavior during the experiments demonstrates a mechanism which develops rapidly and might therefore incite the development of the frequency lock-in regime of ice-induced vibrations of vertically-sided structures.