Observations and Micromechanical Modeling of the Behavior of Snow/Ice Lenses Under Load in Order to Understand Avalanche Nucleation

为了了解雪崩成核，对雪/冰透镜在负载下的行为进行观察和微机械建模

• 批准号: 2227842
• 负责人: Ian Baker
• 金额: $ 57.45万
• 依托单位: Dartmouth College
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2023
• 资助国家: 美国
• 起止时间: 2023-01-01 至 2025-12-31
• 项目状态: 未结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2227842&HistoricalAwards=false
• 关键词: Observations; Micromechanical; Modeling; Behavior; Snow

The microstructural evolution of snow under a temperature gradient has been of interest for many years since this can lead to persistent weak layers, which is a possible cause of avalanches. Ice crusts can form on top or within a snowpack from a variety of meteorological conditions, including significant melt/freeze or freezing rain events. Once buried, they can persist throughout the entire winter season and act as an ideal sliding surface for dangerous slab avalanches in seasonal mountain snowpacks. These phenomena are important because the number of fatalities from avalanches in the U.S. has increased annually since the 1970s. Avalanches can also have substantial economic impacts due to road closures, the costs of rescue, and building damage. To understand avalanche nucleation, snow and layered snow-ice specimens will be deformed in a cold room while imaging with micro computed tomography. Macroscopic deformation experiments on larger samples at both different rates and different temperatures will also be conducted while imaging with a high-speed video camera. The final deformed microstructures in both cases are imaged at high resolution using a scanning electron microscope, which provides information on both the effects of crystal orientation on deformation while clearly delineating one ice crystal orientation from another. Based on the experimental observations, a multiscale computational model is being built to understand crack initiation, crack propagation, and deformation mechanisms. Three types of experiments are performed and are accompanied by micromechanical modeling. The first experiment performs dynamic loading in an X-ray micro CT of non-homogeneous snow samples in which there are differences in the layering in terms of grain size, type, and bulk density to resolve how these differences influence both the local and global deformation. Second, a X-ray micro CT is used to examine snow specimens containing artificially-created ice lenses oriented either at 90o or 45o to the loading direction. In addition to observing the microstructure as a whole and determining a wide variety of microstructural parameters that can be used to characterize the snow quantitatively, the evolution of individual ice crystals to observe bond formation and bond-breaking in detail are examined. Also, more macroscopic deformation experiments on 10 cm cubes containing an ice lens at either 45 or 90 degrees to the loading direction that are being strained at both different strain rates and different temperatures on a servo-hydraulic testing machine located in a cold room are performed. These specimens are being imaged using a high-speed video camera during loading. The observations of snow under load and subsequent high resolution SEM imaging are being used as input to build robust multiscale computational models that describe the microstructural evolution of the snowpack under load and the microscale deformation and failure mechanisms. The resulting model, which will describe the deformation as function of loading rate and temperature, and include the ductile-to-brittle transition of ice, will be transformative for our understanding of the deformation of snow and for avalanche prediction.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.

多年来，人们一直对温度梯度下雪的微观结构演变感兴趣，因为这可能导致持续的弱层，这是雪崩的可能原因。由于各种气象条件，包括重大的融化/冻结或冻雨事件，积雪的顶部或内部可能形成冰壳。一旦被掩埋，它们可以在整个冬季持续存在，在季节性的山区积雪中，它们是危险的板状雪崩的理想滑动表面。这些现象很重要，因为自20世纪70年代以来，美国雪崩造成的死亡人数每年都在增加。由于道路封闭、救援费用和建筑物损坏，雪崩还会对经济造成重大影响。为了了解雪崩的成核过程，我们将在一个寒冷的房间里对雪和层状雪冰标本进行变形，同时用微型计算机断层成像。在高速摄像机成像的同时，还将对更大的样品进行不同速率和不同温度下的宏观变形实验。在这两种情况下，最终变形的微观结构都使用扫描电子显微镜以高分辨率成像，这提供了晶体取向对变形的影响的信息，同时清楚地描绘了一种冰晶取向与另一种冰晶取向。在实验观察的基础上，建立了一个多尺度的计算模型来理解裂纹的起裂、扩展和变形机制。进行了三种类型的实验，并伴有微力学建模。第一个实验在x射线微CT上对在粒度、类型和堆积密度方面存在分层差异的非均匀雪样品进行动态加载，以解决这些差异如何影响局部和全局变形。其次，使用x射线微型CT检查含有人工制造的冰透镜的雪标本，这些冰透镜与加载方向呈90度或45度方向。除了从整体上观察微观结构，确定各种微观结构参数，这些参数可以用来定量地表征雪，我们还研究了单个冰晶的演变，以详细观察键的形成和键的断裂。同时，在冷室伺服液压试验机上，对与加载方向成45°或90°的含冰透镜的10 cm立方体进行了不同应变速率和不同温度下的宏观变形实验。在装载过程中，这些标本正在使用高速摄像机进行成像。积雪在载荷下的观测和随后的高分辨率扫描电镜成像被用作输入，以建立鲁棒的多尺度计算模型，描述积雪在载荷下的微观结构演变以及微观尺度的变形和破坏机制。由此产生的模型将变形描述为加载速率和温度的函数，并包括冰的韧性到脆性转变，这将对我们对雪的变形和雪崩预测的理解产生革命性的影响。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。