Study On Ultra-low Cycle Fatigue Properties Of The Joints Connected By The Bolt-ball Of Ф120 And M24 High-strength Bolts

Bolt ball grid structure is widely used in crowded public buildings such as stadiums and stations.In addition to meeting daily functions,such buildings also need to function as earthquake refuges after earthquakes.Its safety is closely related to the safety of people’s lives and properties.The seismic damage investigation and research results show that the bolt-ball joint is the weak link in bolted ball grid structure,which is prone to ultra low cycle fatigue failure under strong earthquakes,and the earthquake damage is characterized by pulling off,breaking,bending and breaking of high-strength bolts.Therefore,the study of ultra-low cycle fatigue performance of bolted ball joints under strong earthquakes has become an important issue in the field of disaster prevention and mitigation and the construction of resilient cities.The research content of this paper is mainly based on the National Natural Science Foundation of China project: Study on Ultra-low Cycle Fatigue Performance and Evaluation Method of Bolted Ball Grid Structure under Strong Earthquake(51578358).Research on the bolted ball joints in the bolted ball grid structure which is more severe in earthquakes,designed the Ф 120 bolt ball,M24 high strength bolts and Ф 88.5 x 4 round steel tube specimens of bolt ball node connections,a test apparatus that can realize axial reciprocal loading is adopted,large displacement reciprocating tension and compression loading tests under three loading systems were carried out on the specimens.The failure process,failure mechanism,hysteretic performance,skeleton curve,energy dissipation capacity,stiffness and bearing capacity degradation and other seismic performance of bolt-ball joint connection were studied,and the test process was simulated and analyzed by finite element simulation method.Combining the test and simulation results,the following research results are obtained:(1)In the ultra-low cycle fatigue tests of the specimens with nine bolt-ball joints in three groups,the fracture failure all occurred at the central bolt-ball joints,where the high-strength bolts went through a process of tensile,bending,cracking,crack propagation and instantaneous fracture,the fracture position is at the boundary between the high strength bolt with thread and without thread,and the fatigue life are less than 15 times;The neck shrinkage of the broken high strength bolt is obvious and the thread buckle is seriously worn.The macroscopic section of the high-strength bolt has the crack origin zone,the crack propagation zone and the instantaneous fracture zone,which shows the typical characteristics of ultra-low cycle fatigue failure.In the process of failure,the sleeve is constantly extruded,resulting in inhomogeneous plastic extrusion deformation.(2)Under the action of large displacement axial cyclic load,the hysteretic curve pinching phenomenon of bolt-ball joint specimens is serious,and the energy dissipation capacity is poor;The skeleton curves of all specimens under different loading regimes show obvious tension and compression asymmetry due to compression instability,because of the different tensile amplitude,the stress process of the specimen is different.(3)Under different loading regimes,the degradation law of tensile bearing capacity and stiffness of specimens is consistent,showing a "stepped" degradation in the early stage of loading,and a linear degradation in the late stage with the progress of loading;The bearing capacity and stiffness of specimens under compression drop sharply due to instability during the first loading,and then slowly degenerate to zero with the increase of loading times.The damage accumulation rate of the specimen is related to the loading system.With the increase of the tensile loading amplitude,the damage accumulation rate is accelerated,which shows that the energy dissipation accumulation rate,the bearing capacity and the stiffness degradation rate are accelerated.(4)Based on the energy dissipation,the damage model formula of bolt-ball joint specimens is proposed,and the relationship between the damage degree and the failure characteristics of the specimens is established.(5)The ABAQUS software is used to conduct a preliminary simulation analysis on the test process of the bolted ball joint connection test piece,the simulation results are basically consistent with the test trend.