Cyclic loading of freshwater ice produced in the laboratory and sea ice collected in the Beaufort Sea with thermal microcracks, 2022-2023

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. All experiments were conducted in the Ice Research Laboratory, Thayer School of Engineering, Dartmouth College. Freshwater ice was produced in the laboratory, while sea ice was collected in the Beaufort Sea. 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

海冰变薄、海浪增大以及日热循环所造成的破坏，使得我们亟需更深入了解海浪作用下的冰体弯曲对热致开裂冰（thermally-cracked ice）强度的影响。为此，我们针对实验室培育的淡水冰与第一年自然海冰开展了全新实验。所有实验均在达特茅斯学院（Dartmouth College）塞耶工程学院（Thayer School of Engineering）的冰体研究实验室（Ice Research Laboratory）完成。其中淡水冰由实验室制备，海冰则采自波弗特海（Beaufort Sea）。两类冰样均通过热冲击产生开裂，随后进行循环弯曲测试。初始阶段，热开裂均削弱了两种冰样的强度。当两种来源的开裂冰体承受全反向加载（fully reversed loading）循环弯曲时，其原本因裂纹应力集中（stress-concentrating）效应而降低的抗弯强度（flexural strength），会恢复至未开裂、未弯曲冰体的强度水平。而当开裂冰体承受非反向循环弯曲时，其强度仅能实现部分恢复。在反向循环弯曲过程中，开裂区域会交替承受压应力与拉应力。我们推测，压应力会使裂纹相对的面发生接触并产生烧结（sintering），进而实现强度恢复。而非反向循环弯曲过程中，裂纹面的接触与烧结程度较低，冰体强度无法完全恢复。循环弯曲过程中裂纹愈合的趋势，或可降低其对冰盖结构完整性的威胁。