Meso-scale Fracture Modelling Of Concrete Based On XCT Images And Scaled Boundary Finite Element Method

Concrete is widely used in civil engineering constructions.It is highly important to study the multi-scale properties of concrete material in order to optimise and assess the material and structural performance.At the meso-scale,concrete is a multi-phasic composite material consisting of aggregates,mortar matrix and pores.Since these phases are random in distribution and geometric features,the nonlinear and macroscopic behaviour of concrete is dependent on the heterogeneity of its meso-structures.Thus it is necessary to conduct meso-scale modelling of concrete to investigate the relationship between the meso-phases and the macroscopic behaviour,and to accurately understand the complex damage and fracture mechanisms of concrete materials.The following five tasks have been accomplished in this study:Firstly,three-dimensional(3D)meso-scale finite element models of concrete based on in-situ X-ray Computed omography(XCT)images are developed and validated.The continuum damaged plasticity model is used to simulate the complicated damage and fracture behaviour.Excellent agreement is found between the modeling results and the XCT test data in terms of the load-displacement curves and fracture patterns.3D uniaxial compression and tension tests are also simulated,and it is found that the distribution of pores have profound influences on the strength and crack patterns.The image-based 3D models are proved very promising in elucidating the fundamental mechanisms of complicated crack initiation and propagation behavior.Secondly,the dynamic damage and fracture behavior of concrete under compression with different strain rates is statisticaly investigated by Monte Carlo simulations of realistic meso-scale models based on the XCT images,using the concrete damaged plasticity model.It is found that both the strain rate and the meso-scale heterogeneity have significant effects on the dynamic behaviour of concrete at macro-scale.The predicted compressive dynamic increase factor-strain rate curve is in good agreement with existing experimental data and empirical curves.It is found that the realistic XCT-image based meso-models are very promising in effectively elucidating the complicated and fundamental dynamic failure mechanisms of concrete.As the strain rate increases,more and more macro-cracks appear,forming a complicated network,and this network under high strain rates tends to connect all the pores within the whole specimen.Additionally,the dynamic compressive behavior under high strain rates is more sensitive to the pore fraction,suggesting that minimizing the internal defects should be more emphasized for dynamic loadings.Thirdly,a highly efficient FE-SBFE coupled method in the meso-scale fracture modelling of concrete is developed.The scaled boundary finite element method(SBFEM)is implemented in ABAQUS using a user-defined element(UEL)subroutine for the first time.In this way,the advantages of SBFEM,such as the semi-analytical solutions and the flexibility in domain discretisation,are combined with powerful nonlinear solvers and pre/post-processing functionalities available in ABAQUS,to solve complicated problems with higher accuracy and fewer degrees of freedom than the traditional FEM.Each aggregate is modelled by one SBFE polygon or a few,and only the boundary is discretised by nodes.This leads to considerable reductions in degrees of freedom.The mortar matrix is modelled by the traditional FEM and discrete cracks are modelled by pre-inserted cohesive elements inside the mortar and on the mortar-aggregate interfaces.The crack surfaces and the load-displacement curves are found in good agreement with experiments.Fourthly,an efficient numerical homogenization approach soley based on the SBFEM is developed for meso-scale concrete samples with randomly generated and packed aggregates and pores.A simple algorithm is devised to discretise samples into meshes consisting of semi-analytical SBFE polygons only.As each aggregate is modelled by one SBFE polygon and only polygonal boundaries are discretised into nodes,the degrees of freedom of a model is significantly reduced compared with conventional finite element models.The volumetrically averaged stress inside a SBFE polygon is semi-analytically integrated,leading to high accuracy in the homogenised elastic properties.The effects of model size and porosity are statistically studied by extensive Monte Carlo simulations.A size effect law taking porosity into account is proposed to predict effective elastic moduli which are in good agreement with experimental data.Finally,random polyhedral aggregates with complex shapes are automatically generated in the finite element software ABAQUS using a Python script.In this way,the powerful pre-processing module of ABAQUS is applied to the meso-scale modelling of concrete.Using a highly efficient C++ program,discrete cohesive elements with zero-thickness are pre-inserted on the polyhedral aggregate-mortar interfaces and within the mortar matrix,to realistically simulate 3D crack initiation and propagation in ABAQUS.Parametric studies are carried out to investigate the influence of cohesive fracture properties on the load-carrying capacity and cracking process of concrete as well as the crack width.It is found that the crack patterns are highly dependent on the strength-ratio and fracture energy-ratio of mortar and interface.The meso-structural features of aggregates also have significant effects on the complicated 3D cracking process.