Experimental Research And Finite Element Analysis Of Concrete Filled Steel Tubes Under Impact Load

Concrete filled steel tubes (CFT) are widely applied in civil and military constructions for its excellent properties. However, research on its impact resistant behavior is still in the immature stage. This thesis studied the impact resistant capacities of CFT by means of experiments and numerical simulation to obtain the fundamental behavior of CFT under impact load and investigate possible way to improve the behavior.The fundamentals of Split Hopkinson pressure bar (SHPB) is first introduced in this thesis. The CFT impact experiment under 3 sets of air pressure was performed using the 74mm-diameter variable cross section SHPB together with the plain concrete impact experiment under 2 sets of air pressure for comparison of the dynamic behavior. Under dynamic load the plain concrete in this experiment crushed completely while the CFT specimens can still hold their shape with little damage and thus changed the brittle failure of plain concrete to plastic failure. In this experiment the average strain rate, dynamic compression strength, ultimate strain, residual strength and the secant stiffness were obtained through processing data of the pressure bar strain recorded. The dynamic increase factor of concrete filled steel tube was obtained. A formulation to calculate the dynamic increase factor of CFT was proposed based on the existing research. Based on some assumptions the confining stress between concrete core and steel tube was calculated from the strain measured for the steel tube of CFT specimen.Due to the limit in length to diameter ratio of specimen and the loading energy in SHPB experiment the CFT and confined concrete filled tube (CCFT) high-speed impact experiment with greater length to diameter ratio was carried out using 57-diameter one stage light gas gun. The time history strain curves and the failure patterns of CFT and CCFT specimens under different velocity impact load were obtained. The effectiveness of CFRP confinement to CFT specimen was investigated. Based on the momentum theorem and other relative assumptions the dynamic increase factor was calculated to study the strain rate relativity of CFT and CCFT. The numerical analysis, as a good supplement for experiment, is conducted to simulate the light gas gun experiment using LS_DYNA in this thesis. The numerical simulation model is introduced in details, including the element selection, contact definition and related material models. The numerical results and test results are compared. Based on the simulation results, the influence of CFPR confinement and steel tube thickness variety on the impact resistant capacity of CFT is discussed.