Research On Ultra-low Cycle Fatigue Performance Of Φ100 Bolt Ball And Φ60×3.5 Round Steel Pipe Joint Connection Specimen

With the development of the times,spatial grid structures have been widely used in large buildings such as stadiums,hangars,and stations.Such buildings can not only meet the needs of people’s daily life,but also provide people with emergency shelters after the earthquake.Earthquake damage studies have shown that some spatial grid structures will undergo relatively large plastic deformation and ultra-low cycle fatigue failure under strong earthquakes,and the fracture at the joint between the bolt ball and the round steel pipe is one of the main failure modes.As an important part of the grid structure,the bolted ball joints may cause the grid structure to collapse and cause the grid structure building to fail to function as an emergency shelter.Therefore,studying the failure mechanism and ultra-low cycle fatigue performance of bolted ball joints under axial reciprocating loading has great reference significance for the design of large-span spatial grid structures for earthquake prevention and disaster reduction and the construction of resilient cities in the future.The research content of this paper is mainly based on the National Natural Science Foundation of China "Research on Ultra-low Cycle Fatigue Performance and Evaluation Method of Bolted Ball Mesh Structure under Strong Earthquakes"(51578358).According to the existing test conditions,the participant of research group designed an ultra-low cycle fatigue loading scheme and a loading device.Four loading systems were used to perform large displacement axial reciprocating loading tests on 12 sets of Φ100 bolt balls and Φ60×3.5 round steel pipe joints.Through experimental phenomena and data analysis,the deformation characteristics,failure mechanism,hysteretic energy consumption,stiffness degradation,cumulative damage and other ultra-low cycle fatigue properties of the specimen under different loading systems are obtained.At the same time,using ABAQUS finite element software to simulate and analyze the test process,the research results obtained are as follows:(1)For different loading systems,the failure modes of the test pieces connected by the bolt ball and the round steel pipe joint are basically the same.The failure occurred at the middle node connection of the test piece.The test piece experienced bending instability and highstrength bolt cracks at the middle bolt ball joint.The process of initiation,crack propagation until fracture occurs;The necking of the damaged bolt is obvious,all the bolt threads are worn seriously,and the sleeve has obvious extrusion deformation,while the bolt ball and the round steel pipe did not undergo obvious plastic deformation during the test;The average life of the specimen under the four loading systems is 34,6,18,and 20 times,and the bolt fracture bread contains the crack origin area,crack propagation area and instantaneous failure area,which has obvious ultra-low cycle fatigue failure characteristics.(2)The main energy-consuming parts of the test piece are high-strength bolts,sealing plates and sleeves.Because the volume of the three plastic deformations is small,the hysteresis curve is shown as pinching type,not full,asymmetry,and energy consumption capacity is poor.(3)The trend of the skeleton curve of the specimens under different loading systems is basically the same,and the tension zone and the compression zone basically overlap,but the fracture points are different due to the damage accumulation process.(4)The loading system has a significant impact on the damage and degradation of the specimen,and the magnitude of the tension and compression amplitude has a greater impact on the damage,stiffness degradation and bearing capacity degradation of the specimen.The increase of the compression amplitude will increase the bending deformation of the highstrength bolt,accelerate the damage of the specimen,the stiffness and bearing capacity are significantly degraded,and the fatigue life will be reduced;The increase of the tensile amplitude will deepen the degree of plasticity of the specimen,accelerate the accumulation of damage of the specimen,cause the deterioration of the stiffness of the specimen to accelerate,and reduce the fatigue life.(5)Using ABAQUS finite element analysis software to conduct a preliminary simulation analysis on the loading process of the test piece,the simulation results are basically consistent with the test trend.