| Ultra-High Performance Concrete(UHPC),as a new type of cement-based composite material,has outstanding mechanical properties,excellent durability and good working performance.In recent years,the application of UHPC in roads,bridges and marine structures has become more and more extensive.The UHPC structure will be subjected to bending loads during the service process,so that the internal defects and micro-cracks will develop into macro-cracks,and finally the structure will be fractured and damaged.value.In this paper,the combined method of experiment and numerical simulation is used to study the bending fracture behavior of UHPC,and analyze the influence of related factors on the bending fracture of UHPC.Peridynamics(PD)is a non-local theory.The constitutive equations in the form of spatial integrals cleverly avoid the external failure criteria of traditional methods when dealing with crack initiation and propagation problems.Damage localization and crack propagation are natural occurring.In this paper,based on Ordinary State-Based Peridynamics,the interaction between the fiber and the matrix is regarded as the external force of the matrix,and a semidiscrete model of UHPC is established.In the calculation of the fiber force,the buffering effect of the inclined fibers and the spalling effect of the matrix are considered,and the modified expression of the fiber force is deduced.The effectiveness of the semi-discrete model in this paper is preliminarily demonstrated by simulating the tensile test of the UHPC double-notch specimen and comparing it with the test results.The three-point bending test and cube compressive strength test of UHPC were carried out.At the same time,the numerical image correlation method is applied to the experimental study of UHPC three-point bending fracture,and the bending behavior and crack propagation process of UHPC are analyzed.It is found that for the deflection softened UHPC,the peak load can be used as the initial crack load of UHPC,and the change of crack opening displacement is very small in the process of crack generation through the specimen.A semi-discrete model of UHPC three-point bending specimen is established,and the three-point bending test of UHPC is simulated.The simulation results are in good agreement with the experimental results.On this basis,the effects of fiber distribution,fiber volume fraction,fiber length and seam height ratio on the bending-fracture behavior of UHPC were investigated using a semi-discrete model.The results show that the cracks of the UHPC threepoint bending specimens initiate at the prefabricated notch and gradually expand to the midspan loading point.During the entire crack propagation process,the cracks initiate and expand spontaneously,and there is no preset crack propagation path.The advantages of PD method in simulating crack initiation and propagation are demonstrated.The addition of fibers changed the crack propagation path of UHPC beams.The longer the fiber length and the higher the fiber volume fraction,the more obvious the inhibition effect on the crack propagation of UHPC beams.Aiming at the mixed failure problem of UHPC,by changing the initial crack position,a semi-discrete model of UHPC beam without fiber and with fiber was established,and the effect of fiber on UHPC hybrid crack propagation was studied.The results show that in the mixed failure mode,the bridging effect of fibers significantly changes the crack propagation path of the UHPC matrix.The research results show that the semi-discrete model in this paper can reasonably simulate the fracture process of UHPC under bending loads,and can more accurately analyze the crack propagation and fracture properties of UHPC caused by changes in related parameters.The process has high research value. |
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