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.
