Research On Ultra-low Cycle Fatigue Performance Of Tube-ball Composite Specimens Under Constant Amplitude Loading

With the rapid development of urban construction,bolted spherical grid structure has been widely used to construct large-scale public infrastructure such as gymnasium,high-speed railway station building,convention and exhibition center.In addition to meeting the regular daily use functions,such buildings often shoulder the unique mission of providing emergency shelters for the people in response to emergencies,especially earthquakes.The existing seismic damage investigation results show that although the bolted spherical grid structure has good seismic performance,there are still ultra-low cycle fatigue failure phenomena of bending,depression,opening,and fracture in the rod element after several large plastic strains if it exceeds its seismic fortification level,which seriously threatens the safety and function of the building.Therefore,it is of great significance to study the ultra-low cycle fatigue performance of rod elements under axial cyclic loading and explore the optimization method of seismic design of bolted spherical grid structures to improve the seismic resilience of such buildings.This study is part of the National Natural Science Foundation of China Project "Research on Ultra-low Cycle Fatigue Properties and Evaluation Methods of Bolted Sphere Grid Structures under Strong Earthquakes"(51578358).Based on the existing experimental conditions in the laboratory,the ultra-low cycle fatigue test of six tube-ball composite specimens of the same specification under cyclic reciprocating loading with large axial displacement were carried out by three constant amplitude loading regimes.The failure process of each specimen was observed,and the seismic performance indicators such as hysteresis curve,skeleton curve,bearing capacity degradation,and cumulative energy consumption were studied;then,under the premise of considering the ductility damage and initial geometric defects of steel,Abaqus/Explicit is used to simulate the finite element analysis of each specimen,in order to provide reference for the seismic resilience design of bolted spherical grid structure.The main research contents and results are as follows:(1)Under the same loading regime,the test results of two identical specimens are basically coincident,indicating the repeatability of the test process and the stability of the loading device.Under each constant amplitude loading regime,the deformation characteristics of the tube-ball composite specimen during the test are basically similar,and the failure is mainly concentrated in the plastic hinge region in the middle of the steel tube,which has experienced five stages: bending,depression,crack initiation,opening,and fracture.By comparing the typical deformation characteristics and the corresponding cycles of each specimen,it can be found that the bending and depression of the specimen begin with compression,and the crack initiation and fracture occur when tension occurs.With the increase of the compression amplitude,the number of cycles corresponding to the occurrence of depression,crack initiation,opening and fracture of the specimen decreased significantly.(2)The hysteresis curves of each tube-ball composite specimen obtained from the test are all Z-shaped,and there is an obvious pinch phenomenon,indicating that the energy dissipation capacity of the tube-ball composite specimen is poor;the main characteristics of the skeleton curve are as follows: under compression,the bearing capacity of the specimen has experienced three stages: linear growth,gradual degradation and sharp attenuation,and with the increase of the compression amplitude,the peak load of the skeleton curve and its corresponding displacement are reduced,indicating that the ultimate compressive bearing capacity and deformation capacity of the specimens against reciprocating load are reduced.When subjected to tension,the bearing capacity of each tube-ball composite specimen decreases sharply after reaching the peak value in the linear growth stage,and the peak load decreases with the increase of the compression amplitude,mainly due to the damage accumulation degree of the tube-ball composite specimen in the compression process increases with the increase of the compression amplitude.(3)The energy dissipation of tube-ball composite specimens is mainly carried out by the material damage in the plastic hinge region of the middle of the steel tube.Since the length range of the plastic hinge is relatively limited,most materials in the plastic hinge region enter the plastic state successively after the formation of the plastic hinge,and the cross section of the steel tube also undergoes different degrees of elliptical deformation,while other parts outside the plastic hinge region are still in the elastic state.As the loading continues,the plastic hinge adapts to the rotation and deformation of the rod by increasing its plastic strain,and further dissipates the input energy.The bolts and sleeves of the bolted spherical joints at both ends of the specimen also dissipate the energy through deformation and friction,but the total amount is relatively small.(4)Considering the ductility damage and initial geometric defects of steel,the finite element simulation analysis of the ultra-low cycle fatigue test of tube-ball composite specimens under constant amplitude loading was carried out by Abaqus / Explicit,and the finite element modeling method was checked based on the test results.On this basis,the influence of different steel tube slenderness ratio,bolt diameter and steel tube wall thickness on the ultra-low cycle fatigue performance of tube-ball composite specimens was studied by parametric analysis.The results show that the steel tube slenderness ratio is the main factor affecting the ultra-low cycle fatigue life and energy dissipation capacity of the specimen.Under the same loading regime,the ultimate tensile and compressive bearing capacity and deformation capacity of the specimen increase with the increase of the steel tube wall thickness.The diameter of high-strength bolts used in bolted spherical joints at both ends of the rod has the greatest influence on the energy dissipation capacity of the specimen,which indicates that the bending stiffness of the connecting bolts will directly affect the deformation performance of the rod.Therefore,only by selecting the bolt diameter matching the rod specification can the energy dissipation capacity of the steel tube be fully exerted.