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Abstract The combination of thinning ice, larger waves, and damage due to diurnal thermal cycling motivate the need to better understand the impact of flexing under the action of oceanic waves on the strength of thermally cracked ice. To that end, new experiments were performed on freshwater, lab‐grown ice and first‐year natural sea ice. Both materials were cracked by thermal shocking and then subsequently cyclically flexed. Initially, the thermal cracks weakened both materials. When the cracked ice of either origin was cyclically flexed under fully reversed loading, its flexural strength, initially reduced by the stress‐concentrating action of the cracks, recovered to the strength of non‐cracked, non‐flexed ice. When the cracked ice was cyclically flexed non‐reversely, its strength recovered only partially. During reversed cyclic flexing, the cracked region experienced alternately compressive and tensile stresses. We suggest compression resulted in contact of opposing crack faces followed by sintering leading to strength recovery. During non‐reversed cyclic flexing, contact and sintering were reduced and ice strength did not fully recover. The tendency for cracks to heal during cyclic flexing may lessen their threat to the structural integrity of an ice cover. 

Abstract The structural integrity of the arctic sea ice cover is under threat owing largely to the combination of thinning and larger waves. Another contributor may be thermal cracking. In concentrating stress, thermal cracks may weaken the cover. Of interest, therefore, is the strength of thermally damaged ice. To that end, new experiments were performed on sea ice and on lab‐grown saline and salt‐free ice that had been cracked by thermal shocking. As expected, the cracks weakened the materials in accord with fracture mechanics. However, within tens to hundreds of seconds of shocking, the strength recovered completely, for the ice had healed. Healing is attributed to thermally activated sintering related to surface diffusion, assisted possibly by the formation of a quasi‐liquid layer on crack faces. Whether behavior on the small scale is indicative of behavior on the large scale remains to be determined. 

The process comprises the nucleation, propagation, and interaction of cracks. Understanding their micromechanics in ice is likely to help scientists understand fracture in all kinds of materials. 

Water ice Ih exhibits brittle behavior when rapidly loaded. Under tension, it fails via crack nucleation and propagation. Compressive failure is more complicated. Under low confinement, cracks slide and interact to form a frictional (Coulombic) fault. Under high confinement, frictional sliding is suppressed and adiabatic heating through crystallographic slip leads to the formation of a plastic fault. The coefficient of static friction increases with time under load, owing to creep of asperities in contact. The coefficient of kinetic (dynamic) friction, set by the ratio of asperity shear strength to hardness, increases with velocity at lower speeds and decreases at higher speeds as contacts melt through frictional heating. Microcracks, upon reaching a critical number density (which near the ductile-to-brittle transition is nearly constant above a certain strain rate), form a pathway for percolation. Additional work is needed on the effects of porosity and crack healing. ▪ An understanding of brittle failure is essential to better predict the integrity of the Arctic and Antarctic sea ice covers and the tectonic evolution of the icy crusts of Enceladus, Europa, and other extraterrestrial satellites. ▪ Fundamental to the brittle failure of ice is the initiation and propagation of microcracks, frictional sliding across crack faces, and localization of strain through both crack interaction and adiabatic heating. 

Water ice is ordinarily fragile and breaks if extended by just <0.1 % ( 1 ). On page 187 of this issue Xu et al. ( 2 ) show that fibers of cold ice a few micrometers or less in diameter can bend without breaking into a near-circular shape tens of micrometers in radius. Upon unloading, the fibers spring back to their original shape. Such strains are near the theoretical limit of ∼15% ( 3 ), so the deformation is completely elastic. The microfibers can transmit visible light as effectively as state-of-the-art on-chip light guides ( 4 , 5 ). The authors also find that extreme bending creates a near-surface layer on the compressive side that transforms relatively quickly from ice I h (hexagonal crystal structure) to ice II (rhombohedral crystal structure). This pronounced elasticity and transparency reflect the absence of defects within the material, and the structure change implies a low barrier for the ice I h -to-II transformation. 

Abstract. New systematic experiments reveal that the flexural strength of saline S2 columnar-grained ice loaded normal to the columns can be increased upon cyclic loading by about a factor of 1.5. The experiments were conducted using reversed cyclic loading over ranges of frequencies from 0.1 to 0.6 Hz and at a temperature of −10 ∘C on saline ice of two salinities: 3.0 ± 0.9 and 5.9 ± 0.6 ‰. Acoustic emission hit rate during cycling increases with an increase in stress amplitude of cycling. Flexural strength of saline ice of 3.0 ± 0.9 ‰ salinity appears to increase linearly with increasing stress amplitude, similar to the behavior of laboratory-grown freshwater ice (Murdza et al., 2020b) and to the behavior of lake ice (Murdza et al., 2021). The flexural strength of saline ice of 5.9 ± 0.6 ‰ depends on the vertical location of the sample within the thickness of an ice puck; i.e., the strength of the upper layers, which have a lower brine content, was found to be as high as 3 times that of lower layers. The fatigue life of saline ice is erratic. Cyclic strengthening is attributed to the development of an internal back stress that opposes the applied stress and possibly originates from dislocation pileups.