Experimental Study On Dynamic Mechanical Properties Of Concrete Filled Steel Tube Under Impact Loading

With the increase of vehicle colliding with piers,ship colliding with piers,terrorist bombings and accidental explosions,the possibility of bridges and other engineering structures subjected to impact and blast loads is increasing,more and more scholars focus on dynamic mechanical properties of engineering materials under impulsive loading.The existing researches show that the concrete filled steel tube(CFST)has good mechanical properties under quasi-static loading,but the research on its dynamic characteristics is relatively few,this paper uses the combined method of experimental research,numerical analysis and theoretical analysis on the dynamic mechanical properties of CFST,the main work is as follows:(1)The high speed impact tests of CFST were conducted with a diameter of 75mm Split Hopkinson Pressure Bar(SHPB)experimental apparatus,study the influence of the different impact velocities and different specimen parameters on the strain rate effect and related mechanical properties of CFST.(2)A detailed comparison K&C,CSC and HJC model in LS-DYNA was done;establish a numerical model of concrete SHPB tests to analysis the failure process of concrete,stress-strain curves and deformation characteristics in SHPB impact test.(3)Set up a numerical model of concrete filled steel tube SHPB tests to simulate the high speed impact of concrete filled steel tube,the influence of the steel ratio,steel strength,concrete strength,slenderness ratio,hollow ratio on the dynamic mechanical performance of CFST were researched.(4)The dynamic stress-strain relationship of concrete,concrete confined with steel tube and concrete filled steel tube are studied respectively.The main conclusions are as follows:(1)The SHPB experimental study shows that:the critical strain rate and the sensitivity of strain rate of CFST is lower than concrete,but has good plastic deformation capacity,energy dissipation capacity and impact resistance;within strain rate range in this paper,the dynamic binding of concrete steel tube increases with the strain rate increasing;the steel ratio is the most important factor of dynamic magnification factor,followed by concrete strength;the dynamic amplification coefficient calculation formula of CFST presented in this article can reflect the dynamic enhancement of dynamic compressive strength of CFST under impact loading.(2)K&C,HJC and CSC models can reflect the strain rate effect and strain softening properties of concrete under impact loading.The failure mode of K&C and CSC model is the core collapse,and the HJC model shows the phenomenon of nuclear retention.On the whole,the K&C model is most suitable for the dynamic response analysis of concrete under impact loading.(3)The numerical simulation study of concrete filled steel tube SHPB tests shows that:the thickness of steel tube,steel strength,concrete strength,slenderness ratio and hollow ratio can influence the dynamic characteristics of CFST,which the thickness of steel tube,steel strength and hollow ratio has great influence on the restraint stress of steel tube.(4)The dynamic stress-strain relationship of unconstrained concrete,concrete confined with steel tube and concrete filled steel tube were presented in this article,which are agreement with the experimental results or numerical simulation results and can reflect the basic mechanical behavior of concrete and steel tube under impact loading.It shows that the dynamic stress-strain relationship established in this paper can provide a reference for engineering design and analysis.