Study On Macroscopic Waveform Response And Mesoscopic Numerical Calculation Of Concrete Under Impact Loading

With the intensification of global regional conflicts,the threat of terrorist attacks and the frequent occurrence of accidental explosions,the research on explosion and impact resistance of building structures has gradually become the focus.Concrete is one of the most widely used building materials.To grasp its dynamic mechanical properties is the basis of studying the nonlinear strong dynamic load response behavior of building structures.However,as a typical heterogeneous three-phase composite material,the existing homogenization research method based on macro continuum mechanics is no longer applicable.Therefore,it is necessary to understand the dynamic mechanical response characteristics and damage mechanism of concrete from the perspective of mesomechanics.In this paper,the dynamic mechanical properties of concrete materials are studied by macroscopic impact test and mesoscopic numerical simulation.The"double peak"phenomenon of reflected wave is observed in the large-diameter SHPB impact compression test,and the response characteristics are accurately reproduced by the simulation of 3D mesoscopic model generated by image processing technology and the new dynamic elastoplastic damage constitutive model（IUD）.Considering the transverse dispersion effect of large diameter bars,the numerical tests of Hopkinson bundle bar（HBB）system on concrete specimen were carried out.The difference of local failure reveal the formation mechanism of reflected wave"double peak".Subsequently,the response waveform and damage characteristics of concrete under complex stress states and stress history conditions were studied in depth through multi-axial dynamic and static coupling loading numerical test.The simulation results further validate the"double peak"mechanism of reflected wave proposed in this paper.The main research contents and conclusions include:（1）Impact compression and splitting tests based on large diameter SHPB system.The dynamic compression and splitting experiments of concrete were carried out by using（37）100 mm SHPB system.Combined with high-speed camera technology and digital image correlation method,the macroscopic response behavior of concrete was studied,and its waveform response characteristics and deformation and failure process were analyzed.It is found that in the large-diameter SHPB experiment of concrete,the reflected waveform shows the characteristic of"double peak",and the peak value of the second peak is affected by the damage degree of the specimen.With the increase of impact pressure,the residual fragments of the specimen gradually transform from long strips with significant tensile characteristics to fragments with small size differences in all directions.（2）Establishment of three-dimensional meso-numerical model of concrete and verification of IUD constitutive parameters.Aiming at the deficiency of the representation of aggregate shape characteristics in the current concrete meso-model building methods,a 3D concrete meso-model generation method based on the fractal characteristics of aggregate was proposed to reflect the real shape of aggregate.Combined with the IUD constitutive model,the deformation and failure process of concrete in the large-diameter SHPB experiment is reproduced,which verifies the reliability of the meso-concrete model,and compares the simulation results of the homogeneous model,which reflects the superiority of the meso-concrete model.（3）The numerical test of concrete in HBB system.Considering the wave dispersion problem in large diameter SHPB system,the meso-dynamic mechanical behavior of concrete in HBB system is studied.The single bar with small square section can keep the shape of waveform well and reduce the dispersion effect.Under the loading of HBB system,the different positions of single bar will cause different reflected wave responses due to the difference of corresponding specimen boundary conditions.Preferential destruction of the part adjacent to the free surface causes the reflected wave to double peak,while in the central part,more stress waves are transmitted to the transmission bar due to the passive confining pressure effect.Based on the above phenomenon,combined with previous SHPB test results of rock and concrete with different dimensions,it is considered that the phenomenon of reflected wave double peak in large-diameter SHPB concrete test is the result of the interaction of principal stress sensitivity and specimen size.（4）Dynamic mechanical properties of concrete under complex stress states and stress history.The mesoscopic calculation model of true triaxial prestress loading was established,and the dynamic responses of concrete specimens under the conditions of uniaxial,biaxial and triaxial prestress were numerically studied.And the waveform response characteristics and deformation and failure process of concrete are analyzed.The dynamic strength,waveform characteristics and failure modes of concrete all show constraint dependence.Under uniaxial loading,the dynamic strength decreases with the increase of axial prestressσ₁.Under biaxial loading,similar to the static strength,when the maximum principal stressσ₁remains unchanged,the dynamic strength of the specimen increases first and then decreases with the increase of the intermediate principal stress.Under the triaxial passive confining pressure,the reflected wave shows the characteristic of"double peak"and the position of the trough is delayed backwards.The specimen is affected by the passive confining pressure,which increases the yield strength of the material and delays the damage evolution process of the material.With the increase of hydrostatic pressure,the"double peak"characteristic of reflected wave disappears.Under uniaxial loading,axial cracks and circumferential cracks are the main failure modes.Under biaxial loading,delamination fractures occur along the direction of dynamic loading,and shear oblique cracks dominate the side.However,under the action of triaxial compression,the damage of specimen is slight,and shear fracture occurs with the increase of incident wave amplitude.Figure[116]Table[8]Reference[154]...