Micro XCT Image Based Three-dimensional Meso-scale Fracture Simulations Of UHPFRC

In recent years,ultra-high-performance fiber-reinforced concrete(UHPFRC)is increasingly used in high-rise and long-span constructions due to its high strength,high toughness and good durability.The excellent mechanical properties of UHPFRC primarily originate from the reinforcing effect of the addition of fibres.The research on the effect of fibres on the fracture properties of UHPFRC is of great significance for the further optimisation of UHPFRC.In this thesis,the mircro X-ray computed tomography(μXCT)and finite element simulation were used to study the dynamic failure fracture behaviour of ordinary concrete and the static fracture performance of UHPFRC at the mesoscopic level.This thesis firstly proposed a systematic framework of converting concrete/UHPFRC μXCT images to meso-scale finite element models including image processing,image segmentation and image based meshing.For μXCT image processing and segmentation,this paper adopted three segmentation methods,namely the direct thresholding method,the regional thresholding method,and the deep-learning based segmentation method.These three methods are suitable for images with different dimensions,material phases and qualities,and they were compared by segmenting a μXCT image of a UHPFRC beam.For the fibre phase in UHPFRC,a skeleton optimisation and smoothing algorithm was proposed in this thesis,which can effectively separate the adhering fibre phase in the image and obtain independent and regular fibre skeleton profiles.After image segmentation and skeleton optimisation,this thesis introduced image-based structured and unstructured mesh generation methods.Secondly,three-point bending tests were carried out on two groups of UHPFRC beams with a size of 40×40×160 mm.The first group was continuously loaded until fail,and then μXCT scanning was performed;the second group was unloaded after 5 load steps for μXCT scanning,and then loaded and scanned again,so as to obtain 3D μXCT images of UHPFRC specimens during the entire loading process.Through the analysis of μXCT images,the influence of fibres and pores on the fracture mechanism of UHPFRC,such as crack propagation along the fibre-matrix interface,fibre crack bridging effect,fibre debonding,matrix spalling,crack initiation position and crack propagation direction,were discussed.In addition,μXCT image based finite element simulations of UHPFRC under three-point bending were carried out.Fibres and pores were explicitly included in the model,and the fibre-matrix interface was modelled indirectly using equivalent fibre constitutive relationships.The load-displacement curves and the crack morphology obtained from the simulations were in good agreement with the experimental results.Thirdly,this thesis proposed a new three-dimensional cohesive element for simulating the fibre-matrix interface.Combined with the improved kinematic sliding type multi-point constraint method(MPC),the proposed method can effectively model fibre-matrix interface behaviour such as fibre constraints in the normal direction,cohesive bond-debonding,and frictional sliding.For three different types of fibres,three corresponding cohesive and frictional coupled interface constitutive models are proposed in this work.This modeling strategy along with the three constitutive models were firstly verified by modelling single fibre pull-out tests with different pullout angles.The simulated results were in good agreement were with the experimental results in terms of load-slip curves and fracture modes.Furthermore,this method was used to simulate the fibre reinforced concrete under uniaxial tensile and three-point bending.The simulated results were in good agreement with experimental data.Fourthly,this thesis proposed a mesh non-conforming cohesive element insertion method for modeling large amount of fibres in SFRC and UHPFRC.In this method,fibre nodes are independent to matrix mesh while fibres and fibre-matrix interface were explicitly modelled thereby avoiding excessively dense matrix mesh.The mesh non-conforming method was firstly validated by modeling a cylindrical SFRC specimen under direct tensile.The simulated load-displacement were well compared to the curve obtained using mesh conforming method and experimental data,indicating that the mesh non-conforming method is feasible and efficacious.For a further application,the mesh non-conforming method was used in a μXCT image based UHPFRC model with more than 10000 fibre elements.This model was simulated under uniaxial tension and compression.Good agreement was found between the modelling results and the test data in terms of load-displacement curve and fracture patterns.Finally,in order to simulate the fracture behaviour of concrete under extremely high strain rate loads such as impact and explosion,this thesis adopted a continuum-discrete coupled node splitting method.93 2D concrete models and a 3D concrete cubic model were simulated under 6 strain rates in the range of 10-5/s-1000/s.The simulated stress-strain curves and fracture patterns are very different under different strain rates.The predicted compressive dynamic increase factor(CDIF)-strain rate curve is well within the range of experimental data and very close to empirical curves.To further study the mechanisms causing concrete strain rate effect,quantitative analysis of three factors,namely the end friction,the concrete mesostructure and the inertia effect,on CDIF were carried out through Monte Carlo simulations of the 93 2D models.In addition,this thesis adopted the arbitrary Lagrangian Eulerian method(ALE)and the smooth particle hydrodynamics(SPH),align with the μXCT image based mesoscale concrete model,to simulate the fracture behaviour of concrete under blast load and fluid-solid interaction respectively.The simulation results show the discrete fracture behaviour of concrete under such extreme loads.