Macro-and Microscopic Study Of Granular System Based On X-ray Imaging And FDEM Numerical Modelling

Granular materials are ubiquitous in nature,engineering construction and industry,which are composed of a large number of discrete particles and show as a multi-body dissipative system.Especially for the rockfill materials,as the application carrier of complex rockfill-dam engineering construction,they are composed of lumps,gravel and sand and have complex irregular shape,which shows more complex physical and mechanical properties and are closely related to hydropower engineering construction and safe operation.The micromechanical statistical characteristics on the particle scale level determine many unique macroscopic mechanical properties of particle materials.Therefore,it is necessary to study on the microscopic structure and dynamics properties of granular materials.So as to fully understand the relationship between the macroscopic and microscopic mechanical properties of granular materials.The study of the complex mechanical properties of three-dimensional particle systems from the particle scale has put forward higher requirements for the experimental testing and numerical simulation methods,especially for the geo-granular materials with irregular shapes.Therefore,accurate measurement of the dynamic properties and the effect of the multi-scale morphology characteristics on the macro-and microscopic mechanical characteristics of the particle system need to be further studied.In this paper,a virtual test platform is established by combining the advantages of X-ray micro-computed tomography(micro-CT)and finite-discrete element(FDEM)numerical modelling method,aim to investigate the dynamic properties and force transfer characteristics of shear granular system with complex particle shapes.Complex multi-scale morphology virtual particle systems are developed based on the spherical harmonic(SH)analysis,as well as a series of numerical shear tests are carried out by FDEM.We study the effect of particle multi-scale morphologies on the mechanics of granular materials,such as the macroscopic strength,microscopic plastic deformation and microscopic fabric of the particle system.And the difference and connection between the influences of different scale morphology on the mechanical properties are discussed.The main contents are:In combination with micro-CT technology,the in-situ triaxial shear test of the Ottawa sand sample are conducted,and a set of CT image processing algorithm are developed to accurately obtain the coordinates and orientation of each particle.SH analysis are used to reconstruct and characterize the multi-scale morphological features of particles,and the particle tracking and matching algorithm based the SH rotation invariants is mainly developed to accurately match the same particle under the large strain interval,so as to ensure the accuracy of analyzing the particle kinematics from the experimental test.Based on CT image data,the particle contact retrieval algorithm is developed,which successfully identified the particle contact position and quantified contact information,and develop the particle contact tracking algorithm to accurately obtain the particle contact evolution process of the shear particle system.It provides technical support for exploring the particle interaction and force transfer path of shear granular system under experimental testing.A numerical sample generation method for real particle shape are proposed,which realizes the explicit description and reconstruction of the multi-scale morphological characteristics of real particles,and ensures the consistency of the statistical characteristics of microstructure and the particle size distribution of test samples.Without calibrating the numerical model parameters of FDEM,FDEM simulation quantitatively reproduces the tested macroscopic mechanical response,microscopic dynamic characteristics and contact evolution characteristics,which reflects the importance of considering the multi-scale morphological characteristics of particles and the microscopic structure characteristics of particle system in the numerical simulation.The local plastic behavior of the shear granular system is discussed.The local plastic deformation of particles is characterized by structural defects and exhibits highly dynamic spatial heterogeneity.The particle clusters with high plastic deformation are proved to the basic plastic carrier,resulting in irreversible macroscopic plastic deformation of the granular system.The contact force transfer characteristics are study based on FDEM simulation.The spatial heterogeneity of granular dynamics leads to the evolution of the contact force network,and the collapse of the force chain is consistent with the formation of the shear band,which is the product of the larger structural pores in the shear band induced by the local plastic deformation of particles.Finally,the proposed tool,combining experimental X-ray CT and numerical FDEM,sheds new light on bridging length scales from particle size to granular system,making accurately understanding physical mechanism of granular system failure simple and flexible.Based on SH analysis,the virtual particles with complex shapes are generated by taking the SH rotation invariant and the SH-based fractal dimension as the shape control index.Quantitative comparison of the multi-scale morphological characteristics of virtual Ottawa sand particles and its parent particles verifies the reliability and effectiveness of the virtual particle generation method based on SH analysis.Through statistical analysis of the morphological characteristics of virtual particles generated by different SH invariants and fractal dimension,it is found that as the values of SH invariants and fractal dimension increase,the generated virtual particles transit from particles with smooth surface to coarse particles with multi-edges and uneven surface.The influence of the two control indexes on the overall morphological characteristics of the particles shows the opposite evolutionary law.As the value of the SH invariant increases,the generated virtual particles shape transit from round to long and narrow flat,on the contrary,the particle shapes controlled by the fractal dimension transit from long and narrow flat to round.For a numerical sample,the same SH invariant and fractal dimension are specified for all particle,respectively.The generated virtual particles are almost with unique sphericity,convexity and roundness,which provides a new approach for a series of quantitative studies on the influence of particle morphology on the behaviors of particle materials.Triaxial shear tests are carried out on eight groups of virtual particle systems with different shapes to investigate the effects of particle shapes on the macroscopic mechanical response,microscopic dynamic characteristics and microstructure anisotropy of shear granular systems.Compared with the spherical particle system,the long,narrow,flat and rough angular particles generally improve the macroscopic shear strength of the granular system,forming a structure with higher shear strength and dilatancy.By comparing the effects of particle multi-scale morphology on the macroscopic shear strength,it is found that the overall morphology(extension length and flatness)plays a leading role on the macroscopic shear strength,and the more narrow and flat particles can significantly improve the macroscopic shear strength of the granular system.On the aspect of microscopic dynamics,compared with the spherical particle system,the complex particle shape is more likely to drive the local plastic behavior of particles,showing higher plastic deformation events.Influence of local morphological characteristics(sharp edges and rough surface)on local plastic behavior plays a leading role,more irregular edges and rough particles are more likely to form the spatial distribution of relative scattered clusters and more clusters with larger size,rendering the significant dynamics heterogeneity and showing the decentralized local failure mode.As the influence on the transmission characteristics,compared with the spherical particle system,the complex particle shape system produces a very large contact force chain,which significantly improves the fabric anisotropy degree.The overall morphological characteristics play a leading role in the influence on the fabric,and the system with long and narrow flat particles significantly improves the micro fabric anisotropy and the shear strength at the macro level.The quantitative relationship between particle stress and particle non-affine motion shows that particle plastic rearrangement is closely related to particle stress fluctuation.The evolution for the peak friction angle,the cluster structure and the fabric anisotropy parameter with the shape factors exhibit the consistent linear relationships,providing a new understanding of the macroscopic and microscopic relationship of the influence of particle shapes on the mechanical properties of the granular system.