Molecular Dynamics Simulation Of The Mechanical Properties Of Polycrystalline Ice

Ice Ih,which widely exists in the natural world,has many unique properties with profound influence on the natural environment and human life.Natural ice is polycrystalline on the condition of common temperature and pressure.The study of the mechanical properties of polycrystalline ice is a part of the foundations of glaciology,polar engineering and earth science.When observing a bulk of polycrystalline ice at the microscopic scale,its grain boundary premelts at a common temperature,and the dislocation overcomes the barrier presented by proton disorder to glide.It is still unclear how these phenomena can influence the mechanical property of ice.The molecular dynamic simulation in this study provides the detailed mechanical behaviour of the grain boundary and dislocation in ice under stress at the microscopic scale.The relationship between the mechanical property of ice at different scales is discussed.By simulating the tensile process of polycrystalline ice at different temperatures and grain sizes,it is found that polycrystalline ice appears obviously amorphous at the pre-melting grain boundaries when the temperature is close to the melting point,while the grain size affects the mechanical properties of polycrystalline ice by affecting the proportion of amorphous molecules.However,at lower temperatures,the polycrystalline ice breaks through brittle failure,and the grain size basically does not affect the stress concentration process and has little effect on the mechanical properties of polycrystalline ice due to the lack of grain boundary slip.In addition,by simulating and comparing the deformation process of polycrystalline ice with and without salt ions under the same structure,it is found that salt ions significantly weaken the strength of polycrystalline ice.At high temperatures,salt ions weaken the strength of polycrystalline ice by affecting the proportion of amorphous molecules.On the other hand,salt ions reduce the strength of polycrystalline ice by reducing the crack nucleation stress at grain boundaries at low temperatures.The simulation of the failure process of two types of polycrystalline ice is important for further understanding the failure of ice as well as other materials in the pre-melting state.The main mechanism of plastic deformation of ice is dislocation movement,yet the microscopic process of dislocation movement in ice is still unknown.The simulation of basal and non-basal plane dislocations in this paper shows that dislocations depend on their slip planes,and the corresponding microscopic mechanism to overcome the barrier caused by proton disorder is also different.The basal dislocation of crystalline state can overcome the potential barrier with the help of the Bjerrum defect,and it also can emit such Bjerrum defect based on the reconstructed defect.The non-basal plane dislocations are pre-melting and do not need to overcome the proton disorder barrier.The simulation of the dislocation movement process in ice explains the difference in the velocity of the two types of dislocations,which is helpful to further understand the plastic deformation process of ice.