Investigation On The Dynamic Mechanical Properties And Anti-Bullet Mechanism Of Concrete Under Triaxial Static Loading

Concrete is not only the most important engineering protection material,but also widely used in shipbuilding,machinery industry,marine development,Geothermal Engineering and other projects.Because it is often impacted by earthquake,wave,explosion and other external loads,it is in a complex stress state.Therefore,in the field of protection engineering,the focus is still on the dynamic mechanical properties and ballistic performance of concrete under complex stress state.Due to the difficulties in equipment manufacture and experimental principle,relevant experimental equipment and experimental data are very rare.On the basis of the conventional Hopkinson experimental technology,this paper overcomes the defects of the current hydraulic lateral experimental device,designs and develops a dynamic impact loading system which can realize the true triaxial static load.Therefore,the generation and propagation process of stress wave in dynamic shock compression process under triaxial stress state are analyzed,and reliable experimental principle,signal measurement method and data processing method are established.Then the dynamic mechanical properties of concrete under complex stress state are systematically studied.At the same time,an experimental platform of concrete penetration under true triaxial static load is developed by using a bullet fired from a hole in a horizontal bar,and the ballistic resistance characteristics of concrete under complex stress are explored.The two sets of systems constitute an experimental platform,which can be used for independent experiments to reveal the dynamic strength and ballistic resistance characteristics of concrete materials;and it can also carry out combined experiments to reveal the ballistic performance of concrete after impact or the dynamic strength characteristics of concrete after penetration.Based on this idea,the main work of this paper is as follows:In this paper,firstly,the dynamic compression experimental device under true triaxial static load is introduced in detail,and the functions of various structural components are described.The generation and propagation process of stress wave in the dynamic compression process are described,and the signal measurement methods and corresponding data processing methods are introduced.Secondly,the experimental system is calibrated,and the influence of true triaxial static load on the waveform of different parts of the square rod and the unloading waveform on the incident bar are discussed.Then the influence of lateral inertia effect on uniaxial strength is evaluated.Finally,the influence of transverse shear stress on the overall stress balance is analyzed by numerical calculation.The results show that the value of transverse shear stress is far less than the incident wave amplitude,which does not affect the overall stress balance and can be ignored.Based on the dynamic compression experimental device under true triaxial static load,the strength change and deformation mode of concrete samples under different stress states and velocities are studied.With the increase of lateral restraint,the failure mode of concrete gradually changes from brittle failure mode to plastic failure mode.With the increase of impact velocity,the strength of x-axis increases,the values of lateral engineering stress and engineering strain measured are lower,and also increase with the increase of x-axis impact velocity.By analyzing the experimental data of concrete under true triaxial dynamic loading,it is found that the dynamic stress-strain relationship,hydrostatic pressure volume strain relationship and equivalent stress-strain relationship all show obvious strain rate effect and load path correlation.When the D-P strength criterion is used to describe the dynamic strength,it is found that the material parameters(a0 and k0)and the strength parameters(c and ?)show obvious strain rate effect.With the increase of strain rate,the material parameter a0 decreases,while the parameter k0 increases;the friction angle(?)increases,while the cohesion(c)decreases.In order to study the anti-bullet characteristics of concrete under complex stress state,a real triaxial penetration experimental device is developed in this paper.The experimental principle,signal measurement method and corresponding data processing method of the device are introduced in detail.Based on the experimental device,the anti-bullet resistance of concrete target under different penetration velocity and different lateral limit states,namely axial resistance,reaming resistance and lateral friction stress are studied.The results show that with the increase of penetration velocity,the axial resistance and crater depth increase,and the dynamic reaming resistance and lateral friction stress also increase.At the same penetration velocity,with the increase of lateral pressure,the axial resistance and reaming resistance increase gradually,which is more conducive to the generation of lateral friction stress.In order to quantitatively analyze the shape and size of the crater,a 3D optical digital microscope was prepared to observe the crater after the penetration experiment,and the crater depth and diameter were statistically analyzed.Based on the empirical formula and semi empirical formula,the crater depth of concrete target is compared and analyzed,and it is found that there is a certain deviation from the experimental results;and the classical NDRC formula is improved,and it is found that the strength of concrete increases with the increase of lateral restraint.Finally,based on the dimensional analysis,a dimensionless formula of penetration depth is proposed,and the evolution process of the effect of inertia,target strength and compression characteristics with the change of initial stress state is studied by combining with numerical simulation.Based on ABAQUS software,the strength and anti-bullet resistance characteristics of concrete under complex dynamic load conditions are simulated and calculated.Under the condition of true triaxial static load,when the specimen is impacted first and then penetrated,the axial resistance and reaming resistance of the bullet increase and the lateral friction stress decreases due to the compaction of sample after the first impact load.When the specimen is penetrated first and then impacted,its strength is lower than that under the simple dynamic compression,and there is a certain anisotropy.When the sample is simultaneously penetrated and compressed,due to the coupling effect,the dynamic strength changes and the lateral strength decreases obviously.The research in this part provides some guidance for the following combinatorial experiments.