Discrete Multiscale Modeling Of Concrete Material With Application To Penetration

As an important infrastructure material,concrete material has been widely used in civil engineering and military defense area.To understand the dynamic response and damge mode of concrete structure under projectile impact and blast loading is significant in the designing earth penetration weapon as well as construction of concrete shelter structure.Consequently,the topic about projectile penetration in concrete medium is one of most interesting research area in recent years.In this paper,a discrete framwork titiled Lattice Discrete Particle Model is extended to simulate thick concrete penetration problem considering thermal effect and dynamic response of rebar concrete and fiber reinforced concrete under impact and blast.The proposed model succsfully predict the depth of penetrtion,projectile residual velocity after perforation,projectile deceleration and crack propogation and distribution.This paper includes the following issues:Based on exsisting LDPM concrete constitutive model,this paper developed a new model couping the mechanical response with the thermal damage to simulate the tran-site problems like impact.The mesostructure of conrete is constructed by aggregate size information and mixture proportion,which is followed by potential facets genera-tion of LDPM element.Constitutive relationships are built on these facets including:elastic response,fracture response for tension and tension-shear coupling,pore collapse and material compaction,friction behavior.The macroscopic response of pressure de-pendent.yield criterion,softing,load history effect,pore collapse and triaxial response under confinement.This paper comes up the idea.of thermal damage according to the uniaxial and triaxial compression response decay under high temperature.By analyzing one dimension heat transfer equation,assumption of adiabatics is adopted herein for the constitutive model application in the penetration like background.A series straight fiber pullout tests are then conducted to show the fiber/rebar sliding mechanism in concrete matrix.A fiber/rebar reinforced model is formulated by modifying Gauss integration point distribution and defining a fiber/rebar-matrix interaction model.After parameter calibration,the proposed model is validated through a serious hooked fiber pullout tests.The dyanmic constitutive model is derived by considering the rate effect is due to the cohensive fracture.The crack velocity related cohension due to heat accumulation is expressed by Maxwell-Boltzmann formula.With the mesoscale effective strain,rate dependent LDPM constitutive model is formulated with no shear-compression coupling.The model validation is performed by simulating dynamic Hopkinson bar tests,notched concrte plate tension tests and concrete ball impact fragmentation test.Phenomenon like strength improvement,crack propagation and fragmentation under different situations are well captured.For the application of thick concrete target penetration,LDPM is used for the sim-ulation of C40 and 23 MPa strength concrete targets.High strength projectile body is assumed as rigid even the striking velocity is between 833-1402m/s.The numerical re-sults suggest that the thermal degradation becomes more and more important as striking velocity increases.And with the thermal effect considered,the model is able to predict the experimental results with a better accuracy.Calibrating 23 MPa strength concrete with hydrostatic and triaxial compression tests simulation,LDPM simulation results capture the deceleration during penetration process and match the measured penetration depth.Given the Forrestal resistance function,the target static resistance is independent from the projectile nose shape.With the Bazant's size effect law and split respose size effect from LDPM simulation,a modified Drucker-Prager Cap model is proposed to describe the concrete constitutive law.A dynamic spherical cavity expansion analysis is devel-oped with projectile diameter effect incoporated.The solution of governing equations is then used for penetration depth calculation of 7.62 and 76.2mm diameter projectile pen-etration showing a better prediction than BRL and NDRC.Constant projcetile velocity normal penetration in thick concrete simulation also show a projectile diameter effect:30mm diameter get 100 MPa higher restance than that of 60mm diameter case.Test of a composte target of ultra high performance fiber reinforced concrete and armour steel is subjected to projectile normal impact is performed to get the peneration resistant capbility.Combining LDPM and HJC FEM,the simulation is validated with respect to the composite target residual penetration depth.Boundary condition effect is evaluated through LDPM extensive simulations,the laterl confinement and the target lamination significantly changes the residual penetraion depth in steel.The fiber content doesn't affect the penetration resistance,while the damge mode and crack distribution is significantly different due to the arrest posed by fibers reinforced.With LDPM,this paper simulates rebar concrete slab perforation and panel response under blast loading.The numerical results match the 40 MPa and 145 MPa strength con-crete plate damage mode and crater diameter due to different weight TNT contact blast effect.Then,perforation LDPM simulation of 48 MPa and 140 MPa rebar concrete plate is performed to predict the projectile residual velocity.The higher the striking velocity the more accuracy the LDPM predictions are.Both experiment and simulation show no significant effect of the rebar effect on the residual velocity for 48 MPa strength concrete target perforation.However,the LDPM modelling suggests that the project impact lo-cation affects the perforation response to some extent.For the cases with projectile not striking the reinforcement,the residual velocities of different striking velocities projectiles is almost the same as plain concrete perforation cases.On the other hand,the plain con-crete target is spilited into pieces while the rebar concrete damage is localized due to the rebar effect.The back target effect on the perforation response is shown to be dependent on the target thickness and independent on the penetration velocity.