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.

温度梯度下的雪的微观结构演变已经引起了很多年的关注，因为这可能导致持续的弱层，这是雪崩的可能原因。冰壳可以在各种气象条件下的顶部或积雪中形成，包括明显的熔体/冷冻或冻结雨活动。一旦被埋葬，它们就可以在整个冬季都坚持下去，并充当季节性山区积雪中危险平板雪崩的理想滑动表面。这些现象很重要，因为自1970年代以来，美国雪崩的死亡人数每年增加。由于封路，救援成本和建筑损失，雪崩也会产生重大的经济影响。为了了解雪崩成核，在使用微型计算机断层扫描成像时，在冷房间中会变形雪和分层的雪冰样标本。在使用高速摄像机进行成像时，还将进行较大样品的宏观变形实验。在两种情况下，最终变形的显微结构均使用扫描电子显微镜以高分辨率成像，该显微镜提供了有关晶体方向对变形的影响的信息，同时清楚地从另一个界定了一个冰晶方向。基于实验观察结果，正在构建一个多尺度计算模型，以了解裂纹启动，裂纹繁殖和变形机制。进行三种类型的实验，并伴随微力模型。第一个实验在非均匀积雪样本的X射线微CT中执行动态载荷，在晶粒尺寸，类型和散装密度方面存在差异，以解决这些差异如何影响局部和全局变形。其次，X射线微CT用于检查包含以90O或45o为导向的人为创建的冰镜头的雪样样本，直到载荷方向。除了观察整个微观结构并确定可用于定量表征雪的各种微观结构参数外，还检查了单个冰晶体以详细观察键形成键和破坏键的键。同样，在10厘米立方体上进行了更大的宏观变形实验，该实验在45或90度的载荷方向上均以不同的应变速率紧张，并且在寒冷室内均处于不同的应变速率和不同的温度下，该方向都在寒冷室内进行。 。这些标本在加载过程中使用高速摄像机进行成像。负载下的雪和随后的高分辨率SEM成像的观察被用作构建强大的多尺度计算模型的输入，这些模型描述了载荷下的雪堆的微观结构演变以及微观的变形和故障机制。最终的模型将变形描述为加载速率和温度的函数，并包括冰的延性过渡，将是我们对雪的变形和雪崩预测的理解。使命，并被认为是通过基金会的知识分子优点和更广泛影响的审查标准通过评估值得支持的。