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

雪在温度梯度下的微观结构演变多年来一直受到关注，因为这可能导致持续的薄弱层，这可能是雪崩的原因。冰壳可以在积雪的顶部或内部形成，原因是各种气象条件，包括显著的融化/冻结或冻雨事件。一旦被掩埋，它们可以在整个冬季持续存在，并作为季节性山区积雪中危险的板状雪崩的理想滑动面。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。雪崩还可能由于道路封闭、救援费用和建筑物损坏而产生重大经济影响。为了了解雪崩成核，雪和分层的冰雪标本将在冷室中变形，同时用微型计算机断层扫描成像。还将在不同速率和不同温度下对较大样品进行宏观变形实验，同时使用高速摄像机进行成像。在这两种情况下，最终变形的微观结构在高分辨率下使用扫描电子显微镜成像，它提供了信息的晶体取向对变形的影响，同时清楚地描绘一个冰晶取向从另一个。基于实验观察，一个多尺度的计算模型正在建立，以了解裂纹萌生，裂纹扩展和变形机制。三种类型的实验进行，并伴随着微观力学建模。第一个实验中进行动态加载的X射线微CT的非均匀的雪样品，其中有不同的粒度，类型和体积密度方面的分层，以解决这些差异如何影响本地和全球的变形。其次，X射线显微CT用于检查雪标本含有人工创建的冰透镜，定向在90 °或45 °的加载方向。除了观察作为一个整体的微观结构，并确定各种各样的微观结构参数，可用于定量表征雪，个别冰晶的演变，以观察债券的形成和债券断裂详细检查。此外，在位于冷室中的伺服液压试验机上，对与加载方向成45度或90度的含有冰透镜的10 cm立方体进行以不同应变速率和不同温度应变的更宏观的变形实验。这些样品在加载过程中使用高速摄像机进行成像。在负载和随后的高分辨率SEM成像下的雪的观察被用作输入，以建立强大的多尺度计算模型，描述在负载和微观变形和故障机制下的积雪的微观结构演变。由此产生的模型将描述变形作为加载速率和温度的函数，并包括冰的韧脆转变，这将是我们对雪变形的理解和雪崩预测的变革。该奖项反映了NSF的法定使命，并通过使用基金会的智力价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.