Study On Mesoscopic Model And Mechanism Of Concrete Hydraulic Fracture Based On Particle Flow Code

It is inevitable that the concrete structure has initial cracks,and when high water pressure is applied,the initial cracks may further expand to form macroscopic cracks,resulting in hydraulic fracturing failure of the structure.The hydraulic fracturing problem of concrete structure has become one of the focal points in the field of hydraulic engineering.Especially in recent years,a number of high concrete dams(200m and above)were built successively in China,and the hydraulic fracturing problem of these dams is more prominent,so it is necessary to study this problem in depth.Based on the particle flow code method,a fluid-solid coupling model of concrete hydraulic fracturing on meso-scale is established in this paper.The model can reflect the change process of the model from a mesoscopic perspective,and has a significant advantage of analyzing the failure mechanism.The main work and conclusions of this thesis are as follows:(1)The sensitivity of mesoscopic parameters of the particle flow code model with parallelbond was analyzed by biaxial compression test.The results show that the particle size ratio and particle stiffness have little effect on the macroscopic mechanical properties of the model.The friction coefficient has a great influence on the initial elastic modulus,peak strength and strain of the model.The strength of the parallel-bond has a significant influence on the peak strength and the strain at peak stress.The stiffness of the parallel-bond mainly affects the initial elastic modulus of the model.The parallel-bond stiffness ratio has a significant effect on the initial elastic modulus,peak strength and stress-strain softening stage of the model.The initial elastic modulus and peak strength of the model are significantly improved by increasing the coefficient of parallel-bond radius.The results of this study provides a basis for calibrating the mesoscopic parameters of the concrete model.(2)The method of generating irregular-shaped aggregates was improved,and three groups of different parallel-bond parameter values were selected for the interior of aggregates,the interior of mortar and the interface between aggregates and the mortar,respectively,then a three-phase mesoscopic model of concrete was established.The parameter values of C25 concrete were calibrated by adjusting the parameter values through several uniaxial compressive strength tests.The test result showed that the three-phase mesoscopic model can simulate the mechanical properties of C25 concrete better and can be used to carry out hydraulic fracturing test.(3)The original fluid-solid coupling model of particle flow code was improved.An independent linked list was re-created for pipes,which were divided into three types: pipes in aggregates,mortar and interface zone,which reflected the heterogeneity of concrete.The fluidsolid coupling effect was more comprehensively considered by calculating and updating the volumes of each domain.The improved fluid-solid coupling model was used to simulate the seepage process in concrete,and the results were compared with those in the literature to verify that the model can better reflect the seepage law in concrete,and can be further used in the numerical test of hydraulic fracturing of concrete.(4)According to the concrete hydraulic fracturing model and boundary conditions of laboratory tests,hydraulic fracturing tests on three-phase mesoscopic concrete model were carried out.The tests results show that under different axial pressures,critical fracturing pressures increase with the increasing value of axial pressure,and the relationship between them is linear;when the pre-cracks in models are perpendicular to axial pressures,the cracks extend in the direction of the initial crack,and the mesomechanism of hydraulic fracturing failure in the model is normal tensile failure.On this basis,the hydraulic fracture tests were performed on the models with mixed mode?-? pre-cracks.The main reason for the hydraulic fracturing of mixed mode?-? cracks is still the normal tensile failure of the parallel-bond between particles from the mesoscopic perspective.Under the same axial pressure,the critical fracturing pressure is the lowest when the crack angle ? is 0°;when the fracture angle ? is between 0° and 60°,the critical fracturing pressure increases with increasing the crack angle;when the crack angle ? is equal to 90°,the critical fracturing pressure is slightly lower than that of 60°crack angle ?.The particle flow code model adopted in this paper can reveal the mechanical mechanism of hydraulic fracturing failure from the mesoscopic perspective,so it can be further used to analyze more complicated situations and provide a new way for the study of concrete hydraulic fracturing.