Relaxation of Flexure‐Induced Strengthening of Ice

Increasing fetch as sea ice retreats with global warming is increasing the amplitude of ocean waves, motivating the need for a better understanding of the impact of episodic flexing on the strength of ice. Unexpectedly, recent studies showed that the flexural strength of ice increases by as much as a factor of two or more upon cyclic loading (unlike earlier results where ice had a thermo‐mechanical history that could account for the difference), possibly owing to the development of internal back stress originating from dislocation pileups. New systematic experiments reveal that the cyclically‐induced increase in flexural strength of columnar‐grained S2 freshwater and saline ice is fully relaxed upon annealing at high homologous temperatures (Th = 0.91 and 0.96). Moreover, the ice can be repeatedly strengthened to the same level by cyclic loading if allowed to anneal after each episode of strengthening. The relaxation of the original strength is attributed to the relaxation of the cyclically‐induced internal back stress. Plain Language Summary Recent studies have revealed that as the ice coverage on the Arctic Ocean decreases due to global warming, the amplitude of ocean waves increases, leading to an increase of bending stresses within the ice and to the potential for subsequent failure. However, it was recently discovered that the strength of ice increases upon flexing up and down, unlike earlier results where natural sea ice potentially had many micro‐ and macrocracks that could account for the decrease in strength. To understand the behavior of ice under repetitive loading we conducted and describe new systematic experiments in the present study. We find that the increase in flexural strength of both freshwater and saline ice is fully relaxed if the ice is allowed to anneal (to be unloaded) for up to 48 hr. In other words, the results imply that an ice cover is the weakest and, thus, most susceptible to failure after long “quiet” periods related to the absence of ocean waves. Key Points Flexural strength of freshwater and saline ice increases upon cycling by as much as a factor of two or more Cyclically‐induced increase in flexural strength of freshwater and saline ice is fully relaxed upon annealing The relaxation of the original strength is attributed to the relaxation of the cyclically‐induced internal back stress