Research On Ultra-low Cycle Fatigue Fracture Of Round Steel Tube And Bolt Ball Assembly

In recent decades,with the rapid development of the national economy,steel structures have been widely used in the field of building structures.New steel material types,component types,and structural styles have emerged.Among them,space grid structure public buildings are dotted in modern cities.In addition to meeting the daily functions,these public buildings also require the function of earthquake shelters,and become the evacuation of disaster victims during disasters,emergency rescue command centers and material transshipment sites.Many scholars at home and abroad have also done a lot of experimental research on the fatigue performance of high-strength bolts,round steel tube rods and high-rise frame beam-column joints of bolt-ball joints from different angles,and accumulated a lot of data;but about the large-span spatial structure in strong earthquakes There are still few studies on the ultra-low cycle fatigue performance,which has seriously hindered thedevelopment of seismic properties of space steel structures.Therefore,it is urgent to test the ultra-low cycle fatigue performance of the bolt-and-ball joint steel pipe system.This paper is one of the contents of the National Natural Science Foundation of China: "Study on Ultra Low-cycle Fatigue Behaviors and Their Evaluation Method of Standardized Bolted Jointing Grid Structures Under Strong Earthquake"(51578358).Based on the National Natural Science Foundation of China,the research team conducted an ultra-low cycle fatigue performance test of steel-steel ball-bolted test specimens under earthquake action.This paper focuses on the fracture phenomena,energy dissipation capacity and fracture mechanism of the test specimens.The following aspects work:(1)The research team designed a set of ultra-low cycle fatigue test program for the bolt-and-ball joint steel pipe system.The specific work includes the design of the test model,and the determination of the loading system.(2)The ultra-low cycle fatigue test of 12 high-strength bolt-and-tube test specimens under axial tension-pressure reciprocating loads was completed,the test specimens were observed,the loading time nodes of the specimens were recorded,and the cracks were measured..Through the experimental phenomena and hysteresis curves,the effects of loading system and the ratio of the length of the rods on the ultra-lowcycle fatigue failure morphology of the specimens were studied.(3)The numerical simulation analysis of the high-strength bolt-and-tube test piece was carried out by using OpenSees seismic engineering simulation software.The correctness of the test results was verified by comparison test and simulation results,and the ultra-low cycle of OpenSees under the seismic action of the circular steel bar members of the truss structure.Feasibility in terms of fatigue performance.(4)The macroscopic and microscopic morphology analysis of the fractures of some specimens was carried out to judge the crack initiation and fracture mechanism of the specimens.