Flexural Fatigue of Saline Ice

极地海洋上的海冰由于海浪的作用而周期性弯曲，就像回形针的来回弯曲一样。就像回形针最终会因反复弯曲而断裂一样，海冰覆盖也可能会断裂。这种类型的断裂被称为疲劳失效，这是一种在没有警告的情况下发生的失效模式。海冰的疲劳破坏和随之而来的分裂成许多小块可能危及人类和动物的冰上航行，可能会增加冰盖融化的速度，并可能减少北极沿海村庄及其土著人民对海浪的保护屏障。换句话说，随着海冰的退缩和海洋取水量的增加，预计疲劳破坏的倾向及其后果将由于海浪幅度的增加而增加，从而导致冰应力的增加。本项目将研究冰在弯曲作用下的疲劳破坏。它的智力价值在于描述并理解一种破坏性的现象，人们对此知之甚少。更广泛地说，该项目为南极冰架的稳定性和冰卫星岩石圈的完整性提供了洞察力，并最终提供了预测模型，这些卫星可能拥有木星的欧罗巴和土星的土卫二。这个项目建立在实验室的初步研究基础上，该研究表明，当以每十分钟一个循环(0.1赫兹)的速度在四点加载下弯曲时，盐冰板和淡水冰板在零下10摄氏度时都表现出不稳定的行为，在一些测试中仅在两到三次循环后失败，在另一些测试中在2000次循环后失败。虽然材料的疲劳寿命在某种程度上是可变的，但我们注意到的变异性的大小似乎异常大。更令人惊讶的是，观察到在多次循环中幸存下来的冰的弯曲强度实际上增加了两倍。这种循环强化与疲劳减弱是相反的，在文献中已经报道了对南极海冰弯曲梁的现场研究。我们假设，现场行为和实验室行为之间的差异可能反映了热机械历史对疲劳寿命的作用，特别是在自然环境中产生的应力诱导的微裂纹。本项目的目标是以系统的方式检验这一假设，方法是将不同群体的微裂纹引入测试样品，然后探索它们的作用，并意识到可能的退火/钝化效应随时间的变化。变量将包括微裂缝密度、盐度(0-7ppt)、温度(-25至-3摄氏度)、弯曲频率(0.01至1.0赫兹)、应力(0.1至1兆帕)和破裂与弯曲之间的保持时间(1至48小时)。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

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.