Study On Impact Resistance Of CFRP Strenthened Concretefilled Steel Tube Members Under Lateral Impact

Due to the advantages of convenient construction and excellent mechanical properties,concrete-filled steel tubular（CFST）structures have been widely used in buildings,bridges and other structures.Impact loads may be expected on CFST members from cars,ships,airplanes or explosive attacks during the long-term service period.The CFST members,as key structural components,may cause the failure of the structure or the progressive collapse under accidental loadings,which will furtherly threaten people’s lives and property.In this paper,the typical CFST members commonly used in high-speed railway station buildings in China are taken as the research object.Experimental investigation and finite element simulation are carried out to understand the dynamic responses of bare and CFRP externally wrapped CFST members under train derailment impact and to explore the influence of reasonable wrapping method of CFRP laminates on members’ impact resistance.The main research contents are as follows:（1）A non-mid-span impact test for bare and CFRP strengthened CFST members was conducted to obtain the impact test phenomenon,impact force-time history curve and displacement-time history curve of the specimen.It is shown from the results that both specimens exhibit bending failures under lateral impact loadings and roughly share the same changing trend in impact force and displacement time history curves.The CFRP wrapped member is noticed to have larger values of impact force platform and displacement,with its impact curve appearing to decline earlier,which shows that CFRP external wrapping has an adverse effect on the impact resistance of the member.（2）The process of establishing the finite element numerical model of bare and CFRP wrapped CFST specimens using LS-DYNA finite element software is presented,including material model selection,element types,meshing,boundary conditions and contact definitions,etc.,and the rationality and accuracy of finite element model are validated by comparing the predicted results with the experimental data.（3）The influence of CFRP parameters such as wrapping direction,wrapping length and wrapping layers on the dynamic response under lateral impact is investigated through finite element simulation.The results show that: the impact resistance of longitudinal wrapped memeber is the best,the impact force platform value and displacement peak value can be increased by 20.9% and reduced by 28.8% respectively compared with CFST specimens,while the effect of circumferential wrapping is related to the impact height: when the impact height is low,the impact resistance of wrapped component can be improved,but with the increase of impact height,the impact resistance of wrapped component will be weakened;With the extension of CFRP wrapping range,the impact resistance of the wrapped specimen is improved,and the best wrapping effect can be achieved when the wrapping range is 900mm;When the specimen is wrapped with two CFRP layers in hoop-longitudinal direction,the impact force platform value of the wrapped specimen is 11.05% higher than that of the bare one and the peak displacement is reduced by 25.71%;In the case of double-layer wrapping,generally speaking,it is better to set the small-angle fiber layup as the surface layer,while for three-layer wrapping,the influence of CFRP wrapping sequence on the impact resistance of the specimen has not formed a uniform law;For the multi-layer longitudinal-wrapped specimen,the thicker the CFRP layers,the better the impact resistance of the specimen,while the thickening of CFRP generally hurts the lateral bending stiffness of the specimen if wrapped in hoop direction with multiple CFRP layers.（4）The influence of typical CFRP reinforcement forms（longitudinal layers,hoop layers,and a combination of longitudinal and hoop layers）on the lateral impact resistance of CFST members is analyzed to reveal the influence laws on the bending moment time history curve at the impact cross-section,the bending moment and strain distribution along the specimen and the energy variation of each part of the specimen during the impact process.The results show that the plastic strain accumulation rate of the steel tube at the impact position and the local area of the near span bearing is accelerated and the damage degree is intensified when the hoop layers are adopted,while longitudinal layers or a combination of longitudinal and hoop layers can effectively restrain the local deformation of the steel tube and delay the local strain rate;The longitudinal and hoop-longitudinal CFRP wrapping can increase the dynamic plastic limit bending moment of each section of the specimen and shorten the plastic development time,while the hoop wrapping will aggravate the plastic development and accelerate the accumulation of local damage;A lower total internal energy level reveals that the energy consumption due to the overall deformation of the specimen is reduced for hoop wrapping,while the total internal energy level of the specimens wrapped in only longitudinal direction or a combination of hoop and longitudinal direction is slightly improved,which shows the overall energy consumption capacity is increased.