Mesoscale Hydraulic Fracture Simulation Of Concrete Based On The Pore Pressure Cohesive Element

As a widely used material in hydraulic engineering construction,concrete has low tensile strength and is susceptible to fracture,and the development of micro-cracks can be exacerbated by high fluid pressure in hydraulic structures,which has a significant impact on the safety and normal use of the structures.As a multiphase composite material,the meso-structures of concrete(mortar,aggregate,pores,etc.)affect its macroscopic mechanical properties.The mesoscale research on the seepage-fracture mechanism of concrete is of great significance for the evaluation of structural safety and protection of engineering.In this thesis,mesoscale simulations were conducted to study the whole process of damage and fracture in concrete,so as to improve the understanding of the complicated fracture and hydro-fracturing behavior of concrete.In this thesis,an innovative numerical approach coupling the scaled boundary finite element method(SBFEM),the continuous damage phase-field model(PF-CZM)and the discrete cohesive crack model was first developed to simulate complicated mesoscale fracture in concrete.In this approach,each aggregate was simulated by a single semi-analytical SBFEM based polygon or polyhedron without internal nodal discretization,which can significantly reduce the number of degrees of freedom(Do Fs).The damage and fracture in the mortar was simulated by the PF-CZM,and the aggregate-mortar interfaces are modelled by zero-thickness cohesive interfacial elements(CIEs),this new approach thus takes full advantages of the flexibility and semi-analytical accuracy of SBFEM,the mesh independence of PF-CZM,and the ease-of-use of CIEs.Two-dimensional(2D)and three-dimensional(3D)mesoscale simulations of the mode-I and mixed-mode fracture tests of concrete specimens were performed.The load-displacement curves and crack morphology obtained from the simulations were in good agreement with the experimental results.The developed approach can effectively simulate the complicated fracture process in concrete members,revealing the effects of meso-structures on crack initiation and propagation.Secondly,this thesis proposed mesoscale numerical approaches for hydro-fracturing simulations of concrete based on 2D and 3D pore pressure cohesive elements(COH2D6P and COH3D6 P in ABAQUS).The random aggregate generation and packing algorithm and the XCT image-based direct convertion algorithm were used to establish the concrete mesoscale finite element(FE)meshes.The 2D mesoscale simulations of hydro-fracturing tests of concrete shown that the peak pressure and crack path were in good agreement with the experimental results.Furthermore,the effects of fluid viscosity,natural fracture,pores and confining pressure on the hydro-fracturing behavior of concrete were studied.The results shown that it was more difficult to crack the concrete specimens using more viscous injecting fluids,and pores may lead to a significant reduction in the hydro-fracture resistance.The natural fracture and confining pressure could change the hydro-fractuing direction and final pattern of cracks.The developed 3D mesoscale models can effectively simulate the whole hydro-cracking process in concrete,and reveal the effects of random aggregates and weak interfaces on the hydro-cracking surfaces.The simulated results were in good agreement with the crack surfaces obtained from concrete tests.Thirdly,this thesis developed a FEM-SBFEM coupled method for 2D and 3D mesoscale hydro-fracturing simulations of concrete specimens using the elastic SBFEM based polygons or polyhedrons to model aggregates in FE models.This method simplifies the mesh generation process and can effectively reduce the number of Do Fs for models with high aggregate content.The hydro-fracturing tests of concrete specimens were modelled.The simulated crack paths and peak fluid pressure were in good agreement with the pure FE models and the experimental results,and the application scope of SBFEM was expanded.Fourthly,Weibull Random fields(RF)of tensile strength were used to indirectly characterize the mesoscopic heterogeneity in concrete,and a mesoscale numerical approach was proposed to simulate hydro-fracturing of concrete.This approach avoids the processes of packing random aggregates and complicated mesh generation.The hydro-cracking tests of concrete were simulated firstly based on the 2D random field hydro-fracturing models.The results shown that the peak fluid pressure was hardly dependent on the mesh size,and the simulated results were in good agreement with the random aggregate models.Moreover,2D Monte Carlo simulations(MSC)were carried out,and the results shown that the strength variance has significant effects on the evolution of fluid pressure.As the variance decreases,the distribution law of the peak pressure was more consistent with the Gausssian distribution.The hydro-cracking surfaces obtained from the 3D models were compared well with the random aggregate models and the experimental data,which verified the effectiveness of the proposed method for simulating 3D complicated hydro-fracturing.Finally,this thesis developed an explicit numerical scheme-based user-defined pore pressure cohesive element(VUEL)subroutine in ABAQUS,to improve computational efficiency and investigate the dynamic hydro-fracturing behavior of concrete.The 2D and 3D mesoscale dynamic hydro-fracturing models of concrete were developed by inserting VUEL in 2D and 3D FE meshes,respectively.The homogenous examples and hydro-fracturing tests were simulated firstly.The results shown that the characteristics of dynamic hydro-cracking could be effectively captured by this VUEL,and parallel computing by multiple CPU cores could significantly reduce modelling times.Furthermore,3D mesoscale models with different injection rates and fracture parameters(strength and fracture energy)were simulated,and their effects on the whole process of hydrao-cracking,fluid pressure evolution and crack patterns were studied.