Collaborative Research: Experimental and Theoretical Study of Crack Dynamics in Sea Ice

将对控制冰的动态断裂的物理过程进行实验研究。这项研究的动机是需要了解淡水冰和盐冰裂缝速度之间的显著差异（一到三个数量级）。除了对冰物理具有重要意义外，该研究对涉及冰结构相互作用和破冰作业的工程问题也至关重要。几个研究小组最近发现，在盐水冰中裂缝扩展的最大速度比在淡水冰中要低一到三个数量级。在含盐冰中，较低的裂缝速度可能会显著增加冰力（由于冰破碎率较低）。在巨大冰块的动力学（冰川的运动、海冰的漂移）中也应考虑到它们。最近达特茅斯学院冰研究实验室的实验结果清楚地表明，盐水冰中未冻盐水的液体包裹体对裂纹扩展有很强的延缓作用。然而，这种发育迟缓的物理机制尚不清楚。数据及其初步分析指出了裂纹减速的几种可能机制：裂纹与包裹体之间的弹性相互作用；孔隙和裂纹网络中液体扩散运动的能量耗散声波在液体包裹体中的衰减；水在孔隙中运动的惯性效应；在裂纹尖端后留下的液体“斑块”的负毛细压力。这些机制将在精心设计的实验中进行检验，同时通过理论分析进行检验。在达特茅斯（冰研究实验室）进行的实验将包括测量冰样品的裂缝速度和动态断裂韧性，这些冰样品具有不同大小和形状的人工孔隙，这些孔隙中充满了不同密度、粘度和表面张力的液体。一项平行的理论研究，将在塔夫茨大学进行，将包括对上述物理机制的彻底分析。塔夫茨大学的理论工作和达特茅斯大学的实验项目计划密切合作。理论观点将在实验中进行检验，旨在确定几种微观机制的相对重要性。然后，获取的数据将馈送并修正理论模型。由于裂缝与孔隙流体动态相互作用问题的困难，以及可能的微观机制的多样性，实验与理论相结合的方法是必要的。一旦完成，这项研究将解释并定量模拟海冰中非常缓慢的裂缝速度。这将有重要的工程应用。海水冰对动态断裂的高阻力和非常慢的裂缝速度（与淡水冰相比）决定了冰在破碎和破碎过程中的动力（冰-结构相互作用；破冰操作及其由于非常慢的裂缝速度而受到的限制等）。这些结果也将与除冰以外的流体饱和或含有细液滴的材料的动态断裂有关。这些例子是岩石，也可能是混凝土。另一个可能的应用是含有石油或煤油夹杂物的岩土材料的动态断裂。

Experimental studies of the physical processes that govern the dynamic fracture of ice will be conducted. The study is motivated by the need to understand the striking difference between velocities of cracks in freshwater ice and in saline ice (one-to-three orders of magnitude). Besides its importance for the ice physics, the study is essential for engineering problems involving ice-structure interactions and ice-breaking operations. Several groups of researchers recently discovered that maximal speeds of fracture propagation in the saline ice are one-to-three orders of magnitude lower than the ones in the freshwater ice. Low crack speeds in the saline ice may significantly increase the ice forces (due to low rates of ice fragmentation). They should also be taken into account in dynamics of very large masses of ice (motion of glaciers, drift of sea ice). The experimental results recently obtained at Ice Research Laboratory of Dartmouth College clearly indicate that the liquid inclusions of unfrozen saline water in saline ice strongly retard crack propagation. Yet, the physical mechanisms of this retardation are not understood. The data and its preliminary analysis point to several possible mechanisms of crack deceleration: elastic interactions between cracks and liquid inclusions; dissipation of energy due to diffusive motion of liquid in the network of pores and cracks; sonic wave attenuation in liquid inclusions; inertia effects due to movements of water in pores; negative capillary pressure of liquid "patches' left behind the crack tip. These mechanisms will be examined in carefully designed experiments and, in parallel, by a theoretical analysis. The experiments to be conducted at Dartmouth ( Ice Research Laboratory), will include measurements of crack velocity in and the dynamic fracture toughness of ice samples with artificial pores of various sizes and shapes that are filled with liquids of different density, viscosity and surface tension. A parallel theoretical investigation , to be done at Tufts University, will include thorough analyses of the physical mechanisms listed above. Close collaboration between the theoretical work at Tufts and the experimental program at Dartmouth is planned. Theoretical ideas will be tested in experiments designed to identify the relative importance of several micromechanisms. The acquired data will then feed and correct the theoretical modeling. The combined experimental-theoretical approach is necessary due to the difficulty of the problem of dynamic interaction between cracks and fluids in pores, and due to the multiplicity of possible micro mechanisms. When accomplished, the study will explain and quantitatively model very slow crack speeds in sea ice. This will have important engineering applications. High resistance of sea water ice to dynamic fracture and very slow crack speeds (as compared to the ones in fresh water ice) determine dynamic forces in ice during its crushing and fragmentation (ice-structure interactions; ice breaking operations and their limitations due to very slow crack speed, etc.). The results will also be relevant for the dynamic fracture of materials other than ice that are either fluid-saturated or contain fine liquid droplets. The examples are rocks and, possibly, concrete. Another possible application is the dynamic fracture of geomaterials containing inclusions of oil or kerosene.