The Mechanisms of Deformation of Pure and Debris-Laden Ice

纯冰和碎屑冰的变形机制

• 批准号: 0117371
• 负责人: David Cole
• 金额: $ 45万
• 依托单位: Department of Army Cold Regions Research & Engineering Lab
• 依托单位国家: 美国
• 项目类别: Interagency Agreement
• 财政年份: 2001
• 资助国家: 美国
• 起止时间: 2001-10-01 至 2005-09-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0117371&HistoricalAwards=false
• 关键词: Mechanisms; Deformation; Pure; Debris; Laden

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.

这是一个由寒冷地区工程和环境实验室和达特茅斯学院合作提出的建议。 尽管在了解冰川流动的机制方面取得了相当大的进展，但在了解应力与由此产生的流速之间的关系以及低浓度碎片或颗粒夹杂物对冰流速的影响方面仍然存在不确定性。有迹象表明，幂律行为（n =3至4）在低应力水平下并不普遍。实验室实验表明，在低应力下，n1和线性应力依赖性通常需要从冰盖模型的计算中产生合理的结果。然而，压力依赖性的流动的根本原因是未知的，目前还没有一个物理基础的定量模型的过程。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。考虑到碎屑层对整个冰盖运动的重要性，人们对理解这种影响的原因和建立合适的本构关系非常感兴趣。最近的迹象表明，这两个应力依赖性的流量和沉积物的影响，可以解决与基于位错的本构模型。基于位错的模型在冰中的应用一直受到阻碍，因为无法量化有意义尺寸的试样中的位错过程。然而，已经开发了一种方法，克服了这一障碍，和一个试样的有效位错密度（位错线的长度每单位体积的材料），现在可以确定其物理性能的函数。 此外，循环加载和蠕变实验的组合提供了一种方法来跟踪作为试样的应力/应变路径和热历史的函数的位错密度。这些实验将被用来建立位错密度-应力/应变-温度关系的粒状淡水冰和碎片负载的粒状冰。将注意到碎片浓度和热历史的影响上生长的（变形前）位错密度和位错密度的演变作为变形的结果。这种方法已成功地应用于冰具有各种微观结构，但最广泛的海冰，并已开发出一个定量的位错为基础的模型。本研究将验证实验室制备的颗粒状淡水冰的情况下，模型的关键方面，将其扩展到低沉积物浓度的冰的情况下，并通过使用同步加速器X-射线形貌静态和变形标本的位错粒子相互作用提供直接观察。随后的工作将检查现场岩心进行验证。