Simulation Of Dynamic Fracture Behavior Of Engineered Cementitious Composites By PD

Ultra High Toughness Cementitious Composites(UHTCC,also known as ECC)is a new type of high-performance cement-based composite with good tensile strain hardening ability and multi-slit cracking characteristics.The dynamic failure behavior of multi-slit cracking involves a large number of discontinuous problems,and the traditional theoretical numerical analysis methods of continuous media have encountered difficulties in this problem.A multi-scale nonlocal theory based on the spatial integral equation to describe the mechanical behavior of matter——Predynamics(PD),which avoids the singularity of the traditional macro method based on the continuity assumption modeling and solves the spatial differential equation in the face of the discontinuity problem.In this paper,PD model of ECC material is established.Firstly,the PD numerical theory based on bond is briefly introduced,then two modeling methods of ECC materials are introduced: fully discrete model and semi discrete model.The fully discrete model consists of a homogeneous phase representing the matrix and a second phase containing all discrete fibers.The matrix part is modeled by solid finite element method,and the fiber part is modeled by three-dimensional embedded element method.Fully discrete modeling can truly reflect the distribution of reactants in the model,but the computational efficiency is a big short board.Moreover,for PD model theory with node spacing above 0.1mm level,it is not suitable to use fully discrete modeling for ECC.The semi discrete model is to model the matrix in two phases separately,and the other phase(fiber)acts on the matrix in the form of force.The PD semi discrete model of ECC is built by modeling the matrix and the interaction between fiber and matrix.The correction factor of bond damage is introduced to modify the point force function in PD theory,so as to describe the strain softening phenomenon of cement matrix.The simulation results are in good agreement with the existing experimental results.It is proved that PD theory is effective and accurate in dealing with the mechanical behavior of ECC material with a large number of discontinuities in the fracture behavior.Based on this model,the fracture behavior of ECC material is simulated and analyzed.The results are summarized as follows:(1)In the simulation analysis of convergence and convergence of ECC material plate,the reasonable near-field radius and grid spacing ratio are obtained.Under the consideration of calculation cost and calculation efficiency,keeping the nearfield radius =2.0 mm and grid spacing ratio =4 can meet the requirements of simulation calculation.(2)With the increase of fiber content,the sensitivity to the existing cracks is lower and the multi crack situation is more obvious when the ECC board with pre cracks is subjected to uniaxial tensile stress;with the increase of the length of pre cracks,the effect on the final crack situation of the material is not significant,and the multi crack situation appears.(3)When the ECC plate with prefabricated cracks is subjected to impact load,the bearing is damaged,resulting in cracks,and the center of the lower edge of the plate is cracked.With the decrease of the distance between the prefabricated crack and the impact load location,the more obvious the crack growth is.For the whole crack,with the increase of fiber content,the more cracks are produced,and the more inclined crack branches are produced on the shear inclined section.(4)When ECC material plate with prefabricated cracks is subjected to punching load,the single section of the prefabricated crack section will be damaged,and the incorporation of fiber will not play an obvious role,and the failure mode of the sample will not be affected by the fiber basically.(5)When the ECC plate with prefabricated cracks is wedge split tensile,with the increase of fiber content,the number of crack branches increases;in the case of the same fiber content,the influence of the depth of the prefabricated crack on the crack growth is not significant,and the crack growth speed and path are basically the same.