Flexural Fatigue of Saline Ice

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The sea ice cover on polar oceans is cyclically bent owing to the action of ocean waves, rather like the back and forth bending of a paper clip. Just as a paper clip will eventually break with repeated bending, so too may the sea ice cover. This type of breaking is termed fatigue failure, a mode of failure that occurs without warning. Fatigue failure of the sea ice cover and its attendant breakup into many small pieces can imperil on-ice navigation by humans and animals, can increase the rate at which the ice cover melts, and can reduce the protective barrier from ocean waves afforded Arctic coastal villages and their indigenous people. In other words, as sea ice retreats and oceanic fetch increases, the propensity for fatigue failure and its consequences is expected to increase owing to an increase in ocean wave amplitude and, hence, in ice stress. This project will investigate the fatigue failure of ice under bending. Its intellectual merit resides in describing, and then understanding, a destructive phenomenon about which little is known. More broadly, the project provides insight and, ultimately, predictive models for the stability of Antarctic ice shelves and the integrity of the lithosphere of icy satellites that possess putative oceans, such as Jupiter’s Europa and Saturn’s Enceladus. This project builds upon a preliminary study in the laboratory which showed that when subjected to flexing under four-point loading at the rate of one cycle every ten minutes (0.1 Hz), plates of both saline ice and freshwater ice at -10 degrees C exhibit erratic behavior, failing in some tests after as few as two to three cycles and in others after two thousand cycles. While fatigue life of materials in general is known to be somewhat variable, the magnitude of the variability that we noted seems unusually large. More surprising still is the observation that the flexural strength of ice that survived many cycles actually increases, by as much as a factor of two. This cyclic strengthening is opposite to fatigue weakening that has been reported in the literature from in-situ studies of flexed beams of Antarctic sea ice. We hypothesize that the difference between in-situ and laboratory behavior may reflect the role on fatigue life of thermal-mechanical history, particularly of stress-induced micro-cracks produced in natural settings. The goal of this project is to test this hypothesis in a systematic manner by introducing into test specimens different populations of micro-cracks and then exploring their role, cognizance being taken of possible annealing/blunting effects with time. Variables will include micro-crack density, salinity (0-7 ppt), temperature (-25 to-3 degrees C), flexing frequency (0.01 to 1.0 Hz), stress (0.1 to 1 MPa) and hold time between cracking and flexing (1 to 48 hrs).This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.