Experimental Study On Sea Ice-water Thermodynamic Process And Characteristics Of Sea Ice Uniaxial Compressive Strength

The mechanical properties and failure mechanism of sea ice are key issue in structure design for offshore structures and vessels in ice-covered regions.Under driven forces from wind and current,rafted ice and ice ridges can be formed from level ice.Compared with level ice,these ice features can multiple ice thickness rapidly and have different mechanical properties.For the rafted ice and ice ridges,the sea ice strength is dependent on not only the level ice but also the ice crystal structure under the influence of thermodynamic process.Considering both thermal process and failure process comprehensively,it would be benefit to estimate the ice load in engineering applications.In the thesis,we studied the level ice,ice ridge and rafted ice from both heat transfer process and mechanical properties.In the thermodynamic process study of sea ice,submerging tests were carried out to analyze the transient heat transfer between ice and water.The variation of ice temperature and ice thickness growth were measured while the ice crystal structure was also observed at microscale.In the mechanical properties study of sea ice,field measurements were performed on the level ice and artificial rafted ice.Based on uniaxial compression tests,the differences on compressive strength and failure pattern were discussed considering the characteristics of consolidation layer.In this study,the main contents are listed as following.Firstly,the ice temperature development and phase change characteristics were studied with submerging tests and Finite Difference Method.During the transient heat transfer process,the temperature difference leads to a strong heat transfer at the water-ice interface.Based on the physical experiment and numerical simulation results,a dimension analysis was carried out.It shows that the dimensionless ice temperature is dominated only by the Fourier number,while the dimensionless ice growth can be expressed by the combination of Stefan number,initial temperature and initial thickness.Based on the non-dimensional expressions,the ice temperature and ice thickness can be predicted with various initial conditions.Due to the brine activities in sea ice,the thermodynamic properties of sea ice and seawater are dependent on the temperature and salinity,and affect the characteristics of ice temperature and thickness during the thermodynamic process.For saline ice,the high salinity lead to higher rate of ice growth and slowed down the warming up process.Secondly,the ice growth process is a competition between phase change and salt rejection based on the observation of ice crystal structure at microscale.Under slow freezing rate and low water salinity,the rejection process is effective and lead to small grains and smooth ice-water interface.For the high freezing rate in high salinity water,we can find the dense brine channels and a super cooled layer in ice samples,which inhibits the solidification process.Under this condition,small size grains and a needle shape interface were preferred.The experimental results showed that the grain size has positive correlation with the difference between initial ice temperature and freezing point,water salinity,and has negative correlation with ice salinity,Therefore,the ice-water interface geometry and grain structure have close relationship with the ice growth rate and the water salinity.Thirdly,the brittle-ductile transition and anisotropic properties of level ice were performed with field uniaxial compression tests in the Bohai Sea.The tests were carried out under various strain rates in both vertical and horizontal directions,while the ice deformation,load and failure patterns were recorded.Under low strain rates,the ice failure is dominated by creep,where the ice sample deformation is carried by grain sliding and dislocation lubricated by brine.Under high strain rates,the brittle behavior takes over due to the creep deformation is insufficient to relax the stress concentration at crack tips.The condition for crack development is the trigger for brittle-ductile transition.The natural sea ice normally owns columnar structure at microscale.Thus the loading direction has obvious influences on failure patterns and compressive strength.The compressive strength of sea ice in vertical direction is obviously higher than that in horizontal direction.In this work,the compressive strength of sea ice in vertical direction is three times higher than that of in horizontal direction.Besides that,the critical strain rate for the ductile-brittle transition under horizontal loading is ten times higher than that under vertical loading.Moreover,compression tests were performed with artificial rafted ice,which was produced without damaging ice surface condition.The rafted ice includes three layers:upper level ice,underwater level ice and freeze bonds.Under low strain rates,the rafted ice failed in a similar manner to level ice since the freeze bonds had very little influence on creep deformation.Under high strain rates,the rafted ice fractured in brittle mode and the granular grains in the freeze bond weakened the sea ice samples.Therefore,the overall strength of rafted ice is weaker than that of level ice in brittle failure model.The experimental results also denote that the rafted ice had a low strain rate at ductile-brittle transition that that of level ice.Finally,the main contribution of the thesis is concluded and possibilities of further work are suggested.