Experimental And Theoretical Study On High-Cycle Fatigue Performance Of M60 High-Strength Bolts In Grid Structures With Bolt Sphere Joints

The bolt-sphere joint grid structure is a widely used spatial structure in China,which has many advantages such as beautiful shape,reasonable force,and high degree of assembly.It is especially widely used in industrial buildings with hanging cranes.With the increase of the span of the space truss structure,the tonnage of the suspended cranes,and the frequency of operation,the load borne by the truss structure also increases accordingly.Therefore,the application of large diameter high-strength bolts with higher load-carrying capacity has become an inevitable trend,and the accompanying fatigue problems will become more prominent.At present,there is no corresponding fatigue design method in relevant domestic and foreign standards.Therefore,this paper relies on the support of the National Natural Science Foundation of China’s general project,and focuses on the experimental and theoretical research on the highcycle fatigue performance of large-diameter M60 high-strength bolts for bolt-ball node grid structures.The main research contents and results of this paper are as follows:(1)Using the Amsler fatigue testing machine,this paper conducted constant amplitude and variable amplitude fatigue tests on M60 high-strength bolts used in bolt-sphere joint grid structure.A total of 27 sets of constant amplitude fatigue test data and 8 sets of variable amplitude fatigue test data were obtained.Through regression analysis of the fatigue data,the S-N curve of M60 high-strength bolts was obtained.Based on this,a fatigue design method was established using the nominal stress amplitude Δσ as a parameter.The reference period was set at 2 million cycles,with [Δσ] = 39.3MPa.Comparison and analysis with other specifications of high-strength bolts and related design values show that as the diameter of high-strength bolts increases,the fatigue strength decreases,especially in the case of fatigue design methods specified in various national regulations that tend to be hazardous.(2)With the help of a Phenom XL scanning electron microscope,macroscopic and microscopic analysis was conducted on the typical fatigue fractures of M60 high-strength bolts,revealing the fatigue failure mechanism of M60 high-strength bolts.Stress concentration is the primary cause of fatigue crack initiation.The basic morphology characteristics of constant amplitude fatigue fractures and variable amplitude fatigue fractures were obtained.All fatigue fractures exhibited three characteristic regions,including the fatigue crack initiation zone,fatigue crack propagation zone,and instantaneous fracture zone.Constant amplitude fatigue fractures and variable amplitude fatigue fractures exhibit different characteristics as the stress amplitude changes.Under high stress amplitudes,the fatigue crack initiation zone is concentrated,and the propagation zone is relatively small with a larger instantaneous fracture zone for constant amplitude fatigue fractures.As the stress amplitude increases,the area of the propagation zone gradually decreases.In contrast,the morphology of variable amplitude fatigue fractures is much more random than that of constant amplitude fatigue fractures.The fatigue crack initiation zone and the propagation zone is dispersed,and the area of the propagation zone exhibits a decreasing trend with increasing stress amplitude.(3)Through the use of the modified Miner’s rule,the Corten-Dolan theory,fracture mechanics theory,and continuum damage mechanics theory,the fatigue life of the M60 highstrength bolt was estimated.The results indicate that the damage values calculated by the modified Miner’s rule have high variability,while those calculated by the Corten-Dolan theory are close to 1.0,with significantly reduced variability,demonstrating its applicability.The estimation results of the fracture mechanics theory and continuum damage mechanics theory are in good agreement with the experimental results,fully demonstrating their applicability.(4)Using Abaqus finite element software,a large number of numerical analyses were conducted on M60 high-strength bolts,and the stress concentration factor and fatigue notch factor values were obtained.It was revealed that severe stress concentration is the main cause of fatigue failure and the reason for low fatigue strength.By quantitatively exploring various factors such as bolt-ball diameter,thread form,thread root fillet radius,bolt head under fillet radius,bolt insertion depth,and thread root scratches,the corresponding influence rules of each stress concentration were obtained.Effective way for improving the fatigue performance of M60 large-diameter high-strength bolts were proposed.