The Mechanisms of Deformation of Pure and Debris-Laden Ice

OPP-0117371学院这是由寒冷地区工程与环境实验室和达特茅斯学院共同提出的建议。尽管在理解冰川流动机制方面取得了很大进展，但在理解应力与由此产生的流动速度之间的关系以及低浓度碎片或颗粒包裹体对冰流动速度的影响方面仍然存在不确定性。有迹象表明，在低应力水平下，并不普遍观察到幂定律行为(n=3到4)。实验室实验表明，在低应力下，通常需要n1和线性应力依赖关系，才能从冰盖模型的计算中产生合理的结果。然而，流动的应力依赖性的根本原因尚不清楚，目前还没有基于物理的过程的定量模型。事实上，有证据表明，含有少量碎片的冰实际上比纯冰或富含沉积物的冰变形得更快。鉴于碎屑层对整个冰盖运动的重要性，人们非常有兴趣了解这种影响的原因并建立适当的本构关系。最近的迹象表明，流量的应力依赖性和沉积物的影响都可以用基于位错的本构模型来处理。由于无法量化有意义大小的样品中的位错过程，基于位错的模型在冰上的应用一直受到阻碍。然而，已经开发出一种克服这一障碍的方法，现在可以根据样品的物理性质来确定样品的有效位错密度(单位体积材料的位错线长度)。此外，循环加载和蠕变实验的结合提供了一种跟踪位错密度随试件的应力/应变路径和热历史的函数的方法。这些实验将被用来建立颗粒淡水冰和含有碎屑的颗粒冰的位错密度-应力/应变-温度关系。将关注碎屑浓度和热历史对内生(变形前)位错密度的影响以及因变形而演化的位错密度。这种方法已经成功地应用于具有各种微结构的冰，但最广泛地应用于海冰，并发展了一个基于位错的定量模型。这项研究将在实验室制备的颗粒淡水冰的情况下验证该模型的关键方面，将其扩展到低沉积物浓度的冰的情况，并通过使用静态和变形试件上的同步辐射x射线形貌术直接观察位错-粒子相互作用。随后的工作将检查实地岩心以进行核实。

AbstractOPP-0117371ColeThis is a collaborative proposal by the Cold Regions Engineering and Environmental Laboratory and Dartmouth College. Despite considerable progress in understanding the mechanisms of glacier flow, uncertainties still remain in understanding the relationship between stress and the resulting flow rate, and the effect of low concentrations of debris or particulate inclusions on the flow rate of ice. There are indications that power law behavior (n =3 to 4) is not universally observed at low stress levels. Laboratory experiments indicate that at low stresses, n1, and a linear stress dependence is often required to produce sensible results from calculations in ice sheet models. However, the underlying causes for the stress dependency of flow are not known and a physically based quantitative model of the process is not currently available. In fact, there is evidence that ice with small quantities of debris actually deforms faster than either pure ice or sediment-rich ice. Given the importance of the debris-laden layers to overall ice sheet movement, there is great interest in understanding the reason for this effect and developing a suitable constitutive relationship. Recent indications show that both the stress dependence of the flow rate and the influence of sediments can be addressed with a dislocation-based constitutive model. The application of dislocation-based models to ice has been hampered by an inability to quantify dislocation processes in specimens of a meaningful size. However, an approach has been developed that overcomes this obstacle, and a specimen's effective dislocation density (length of dislocation lines per unit volume of material)can now be determined as a function of its physical properties. Moreover, a combination of cyclic loading and creep experiments provides a way to track the dislocation density as a function of the specimen's stress/strain path and thermal history. Such experiments will be used to establish the dislocation density-stress/strain-temperature relationships for granular freshwater ice and debris-laden granular ice. Attention will be paid to the influence of debris concentration and thermal history on the grown-in (pre-deformation)dislocation density and on the dislocation density that evolves as a result of deformation. This approach has been successfully applied to ice having a variety of microstructures, but most extensively to sea ice, and a quantitative dislocation-based model has been developed. This study will verify critical aspects of the model for the case of laboratory-prepared granular freshwater ice, extend it to the case of ice with low sediment concentrations, and provide direct observations of the dislocation-particle interactions through the use of synchrotron x-ray topography on static and deforming specimens. Subsequent work will examine field cores for verification.