Research On High-velocity Dynamic Mechanical Properties And Constitutive Model Of Ultra-high Strength And Ultra-high Toughness Concrete

Normal concrete is widely used in all kinds of buildings because of its advantages such as low price,easy preparation and easy access to raw materials.However,frequent natural disasters and artificial accidents usually impose high-velocity dynamic loadings on concrete structures.Because of the brittleness and easy cracking of ordinary concrete,dynamic loadings usually cause serious damage to concrete structure and pose a great threat to the safety of personnel and property inside.Therefore,High toughness and high strength concrete are gaining more and more widely used owing to its excellent performance.This paper focuses on a kind of high toughness concrete and a kind of high strength concrete,which are: Ultra High Toughness Cementitious Composites(UHTCC)with great deformation ability and Reactive Powder Concrete(RPC)with high strength.The high-speed dynamic mechanical properties and dynamic constitutive models of the above two materials were systematically studied by means of material mechanics test,structure test,theoretical analysis,numerical simulation,etc.,which provided data support and design basis for the application of the two materials in civil infrastructure and military protection engineering.The main research work and conclusions of this paper are as follows:(1)The static and dynamic mechanical properties of UHTCC and RPC were studied: static and dynamic compressive and direct tensile tests were carried out on UHTCC and RPC;High speed photography and digital image correlation(DIC)were used to observe and calculate the strain distribution of specimens during the test.Scanning electron microscopy(SEM)was used to observe the fracture failure mode of dynamic tensile specimens.The dynamic stress-strain relationship applicable to UHTCC and RPC was established.The experimental results showed that the dynamic compressive strength,tensile strength,tensile initial cracking strength and elastic modulus of UHTCC and RPC increased obviously with the increase of strain rate.The tensile strain capacity of UHTCC showed the opposite rate effect trend.The energy absorption capacity of UHTCC and RPC can be greatly improved under dynamic conditions,which is far better than that of ordinary concrete.The semi-empirical formula can well fit the relationship between the DIF of of UHTCC or RPC and the strain rate.The failure mode and morphology of the fibers were also related to the strain rate.UHTCC showed obvious strain hardening phenomenon at all strain rates,while RPC showed insignificant strain hardening phenomenon.Therefore,the proposed three-stage stress-strain relationship can better describe the stress-strain curves of the two materials at different strain rates.(2)The confining pressure effect of UHTCC and RPC was studied.Conventional triaxial compression test was conducted on UHTCC and RPC.Computed tomography(CT)was used to observe the internal damage of specimens after triaxial compression failure.Confining pressure effects of triaxial strength,axial and circumferential peak strain,maximum volume strain,elastic modulus and Poisson’s ratio of UHTCC and RPC were analyzed.The toughness index TI was used to characterize the toughness of materials at different stages.A three-stage triaxial compressive stress-strain relationship was proposed and verified.The results showed that confining pressure significantly enhanced the triaxial compressive strength and strain capacity of both UHTCC and RPC.In particular,the compressive strain capacity of UHTCC showed a secondary increase trend under the action of confining pressure,and the phenomenon of complete plasticity appeared under small normalized confining pressure.The toughness calculation results showed that both UHTCC and RPC have significant post-peak energy absorption capacity under confining pressure.CT scanning results showed that confining pressure has a positive effect on limiting crack width and volume.Williams-Warnke failure criterion can well describe the triaxial performance of UHTCC and RPC.The proposed three-stage stress-strain relationship can well predict the curves under different confining pressures.(3)The penetration resistance of UHTCC and RPC was studied.The penetration tests of UHTCC and RPC targets with different thicknesses and fiber volume ratios against 8mm projectiles at different striking velocities were carried out.The influences of thickness and fiber volume ratio on penetration results were analyzed.CT technology was used to observe the internal damage of the specimen after penetration.Based on the Forrestal’s model,the modified fitting formula of height of cratering and material penetration resistance R are proposed and verified.The experimental results showed that RPC had stronger penetration resistance than UHTCC.Increasing target thickness had a significant effect on enhancing penetration resistance.The addition of steel fiber can greatly limit the damage expansion,and the best effect was achieved when the fiber content reached 1.5% to 2%.PVA fiber had more effect on limiting surface damage.The internal damage of the target after penetration can be divided into two parts: central damage and internal crack,and each part was composed of three parts.The proposed formula of height of cratering and material penetration resistance can be combined with the model to predict the projectiles’ residual velocities when different materials resist penetration.(4)The dynamic constitutive models of UHTCC and RPC were studied and verified by numerical simulation.The parameters of K&C and HJC models of UHTCC and RPC are calibrated comprehensively.According to the disadvantages of the above models,a modified plastic model is proposed.According to the anti-penetration and anti-contact explosion test data in this paper and the literature,numerical simulation was carried out using LS-DYNA software and the calibrated model parameters and the proposed modified model were verified.The results showed that the calibrated K&C and HJC model parameters can better describe the uniaxial compression,triaxial compression and uniaxial tensile properties of UHTCC and RPC.The improved model can better reflect the strain rate effect of the tensile properties and predict the damage distribution of UHTCC and RPC targets under penetration and explosion loads by proposing reasonable strength surface transition segment and damage equations,which provided important reference for future engineering applications of two materials.