Research On Dynamic Instability Of Z-section Cold-formed Steel Members Under Axial Impact

Cold-formed steel members have the advantages of light weight and high strength,and are widely used in building structures,even in the field of military engineering,which may be subjected to impact loads.However,there are few studies on the dynamic instability of common cold-formed steel members（C-section,Z-section,etc.）under axial impact at home and abroad,which brings inconvenience to the design and construction of steel structure engineering that may encounter dynamic impact.So the design and specifications of cold-formed steel structures are limited to static conditions.In order to promote the development of cold-formed steel structure to the dynamic field,it is necessary to research the instability process and influencing factors of cold-formed steel members under dynamic impact.In this paper,cold-formed thin-walled steel members with yield strength of 345MPa and550MPa are studied.Firstly,The mechanical properties of experimental steel in the strain rate of10^-410² is obtained by MTS810 universal material testing machine and HTM-5020 high-speed stretching machine.And considering the strain rate effect of the material,the Johnson-Cook strength model of the material was fitted.The SU5000 scanning electron microscopy（SEM）was used to analysis the sample after fracture.Then the finite element software（CUFSM）is used to design the dimension of the first deformation distortion under static axial compression.The axial compression test of the component is carried out by WAW-2000 universal material testing machine.The ultimate bearing capacity and residual deformation of the component is obtained.Finally,the drop hammer impact test was carried out on the Z-type cold-formed thin-walled steel members on the large drop hammer impact test platform.The instability deformation of the components during the impact process and the speed change process of the hammerhead were analyzed.The commercial finite element analysis software ABAQUS was used to combine the experiments.The material constitutive model of steel establishes a finite element model that can reflect the impact process of the drop weight test.Considering the strain rate effect,geometric nonlinearity and initial defects of the specimen,the impact time history curve and the speed time history curve of the hammer head are obtained.The displacement time history curve,the velocity time history curve and the propagation process of the stress wave in the member of the top web,the flange edge and the curling edge of the member can be obtained by analyzing the residual deformation of the sample and the time history curves obtained by the simulation.Conclusion:Z-shaped cold-formed thin-walled steel members are accompanied by dynamic buckling during dynamic impact,and with the increase of impact force,the members will yield deformation on the basis of dynamic buckling,while the propagation of stress waves and the dynamics of the members The buckling phase is coupled such that a dynamic buckling step occurs.During the impact process,it is also found that some sizes of component crimping and flanges are prone to instability and separate from the hammerhead.It is necessary to optimize the design of the dimensions and,if necessary,add corresponding constraints.