Molecular Dynamics Study Of Micro Structure And Mechanical Properties Of S? Methane Hydrate

Recently,the natural gas hydrate had been found in the universe.Also,the hydrate can be the phase change material and water for space exploration.In addition,hydrate has the advantages of widely distributed,large scale reserved,high calorific value,etc.It has drawn more and more attentions from the scholars of the world.Before the exploitation and application of hydrates,it is of great theoretical significance and practical value to investigate the basic mechanical properties of hydrates,the deformation and destruction mechanisms,and their interaction mechanisms with sediments.However,due to the extremely harsh environment of hydrate exists,it is difficult to accurately test its mechanical properties.And the micro-mechanisms of deformation and failure of hydrate are still unclear.The using of conventional testing methods and theoretical analysis methods to study the mechanical properties of hydrates and the deformation of micro-mechanism have some limitations.In this paper,the micro structure and mechanical properties of sI methane hydrate were studied by molecular dynamics simulation.Through the tensile or compressive deformation of the sI methane hydrate structure systems,the mechanical properties of the hydrates and the micro-mechanism during deformation are revealed.The work also provides the reference data and theoretical basis for the technical application and engineering exploitation of hydrates.Firstly,an unit cell of complete perfect sI methane hydrate was constructed based on the reference data.And the model size effect on the mechanical properties and microstructure of the hydrate was investigated.Through the tensile and compressive deformation of methane hydrate systems with different sizes,the microcosmic configuration changes and the stress-strain curves were analyzed.It was found that the methane hydrate has brittle material properties.Also,the maximum stress of the system has a scale effect in a certain scale.Then,the effect of defects on the hydrate microscopic configuration and mechanical properties was studied.By deleting certain particles of the intact hydrate structure,the hydrate structures with defects are obtained.After the structures with defects are stabilized and relaxed,the tensile deformation test is conducted to study the evolution of microscopic configuration.And it is found that the system owns the mechanical properties of the brittle material.Statistical analysis of stress-strain curves for different systems revealed that the mechanical properties of the system were reduced when the total number of defects was 9.02%.At this time,the maximum stress value was lower than the maximum stress value of the complete structure,and the brittle fracture strain was lower.The mechanical performance of other systems with defect ratios lower than 9.02% are better than that of the perfect structure.After analyzing the ordered parameters of different structures,it was found that the F4 order parameters can effectively determine the microstructural fracture strain interval.While the F3 order parameter can indicate the fracture position of the model in the tensile direction.Finally,the microstructure and mechanical properties of the hydrate/water/ice mixed system were studied.The hydrates of the hydrate/water/ice mixed system were stretched and deformed.The changes of the microscopic configuration were studied.And it is found that the mixed system remained brittle.It is fractured and the maximum stress value is lower than that of the intact hydrate system.The strain at which the hydrate/water/ice mixed system breaks is smaller than that of the hydrate system.F3 and F4 ordering parameters can determine the fracture location of the hydrate/water mixture,as well as the interfacial changes in the microscopic hydrate/water/ice mixed system.