Meso-topological Evolution Mechanism Of Granular Materials During Loading And Unloading Based On Complex Network

The mechanical characteristics of geotechnical particle materials are closely related to engineering safety,and are very important for geotechnical engineering and geological disasters.They are composed of a large number of discrete particle solids and pores,and there are interactions of multi-scale discrete microstructures inside them.At present,the constitutive model of geotechnical granular materials still has shortcomings in describing complex physical and mechanical properties,which can be analyzed in depth from the microscopic scale based on topology theory.In this paper,the discrete element triaxial loading and unloading test is used to study the evolution of macroscopic and microscopic structures of granular materials under complex loading and unloading paths,and to explain the irreversible mechanical behavior of granular materials during loading and unloading from the perspective of complex networks.The main research contents and innovations are as follows :(1)The stress-strain response characteristics of granular materials with different compactness at the macro level were obtained by discrete element triaxial loading test,and the loading tests of samples with various compactness were carried out under multiple sets of confining pressures.During the loading process,the deviatoric stress of the dense sample rises to the peak and then undergoes strain softening.The volumetric strain changes from shear contraction to dilatancy.The deviatoric stress of the loose sample exhibits strain hardening and continuous shear contraction,which are in line with the universal law.(2)The loading and unloading test was carried out with the data obtained from the triaxial loading test.The dense and loose particle systems with confining pressure of100 k Pa were used as the test objects.Four loading and unloading paths were used.The samples with different compactness will produce ’ hysteresis loops ’ after undergoing the loading and unloading process.The size of the hysteresis loop has a certain influence on the macroscopic stress-strain response of the granular material.During the loading and unloading process,the average coordination number and average clustering coefficient of the dense sample decreased during the loading period,and increased during the unloading period,while the loose sample showed the opposite law.(3)The complex network(the analysis tool of particle system)is used to analyze the fine microscopic characteristic parameters,and the threshold(the ratio of the contact force between particles to the average contact force)is introduced to divide the strong and weak contact system of the particle system.The fabric characteristics of the strong contact force system and the transmission law of the contact force are studied.It is found that there is not much correlation between the loading and unloading history and the probability density distribution of the contact force.When the number of particles in the maximum cluster reaches the minimum value,the relative length will also drop sharply.At the same time,with the increase of the threshold value,the peak point of the radius and diameter of the dense sample network reaches the peak value at the threshold value [1.5,2],and the threshold value of the loose sample [2,2.5].After reaching the peak value,it decreases rapidly to the minimum value,and the force transmission process will not be completed inside the particle system.The variation of the equivalent anisotropy coefficient of the strong contact system with the threshold is close to the variation trend curve of the deviatoric stress.The equivalent anisotropy coefficients corresponding to the same deviatoric stress and different loading and unloading state points are different.The fabric change may be one of the reasons for irreversible deformation.Figure 53 Table 1 Reference 111...