Experimental And Theoretical Study On Variable Amplitude Fatigue Performance Of M20 Torsional-Shear High Strength Bolt

As one of the main forms of steel structure connection,high strength bolt connection is widely used in the field of prefabricated buildings because of its convenient disassembly and reliable connection.From the perspective of structural resistance,the loads on the structure in the whole life cycle are all alternating loads except the self-weight.Under the reciprocating action of these alternating loads,the fatigue performance of joints is crucial to the safety of the structure,and the fatigue failure of high-strength bolts at joints cannot be ignored.In terms of high-strength bolted joints,the current research results in China and abroad focus on construction measures and joint stiffness,but there are few studies on the fatigue performance of high-strength bolts.In this paper,the M20 torsion-shear type high strength bolt commonly used in prefabricated steel structure buildings is taken as the research object,and its fatigue performance is discussed in depth through fatigue test,numerical simulation and theoretical analysis.The main research contents and conclusions are as follows.1.Based on the beam-column end-plate connection in prefabricated steel structure,the Ttype connector was designed.The amplitude fatigue test of ’ low-high ’ loading mode under full preload and the amplitude fatigue test of ’ low-high ’ mode,’ high-low ’ mode,’ high-low-high ’mode and ’ low-high-low ’ mode under no preload were completed respectively.A total of 25 groups of fatigue failure data were obtained.The S-N curve of variable amplitude fatigue was fitted by equivalent stress amplitude method,and the design method of variable amplitude fatigue was established by allowable stress amplitude and hot spot stress amplitude.The results show that the pre-tightening force can greatly improve the fatigue performance of high strength bolts.For M20 high strength bolts,the fatigue life under full pre-tightening force is 3.7 times of that without pre-tightening force.Compared with the constant amplitude fatigue life,it is found that the fatigue life of the variable amplitude fatigue is greater than that of the constant amplitude fatigue under the same stress amplitude.It is proved that the low stress amplitude will play a certain exercise effect on the fatigue performance of the material,so as to improve the fatigue life of high strength bolts.2.For M20 torsion-shear high strength bolts in T-joints,the effects of different preloads(0kN,50 kN,100 kN,155 kN)on the mechanical properties of M20 torsion-shear high strength bolts were discussed by using ABAQUS software.The theoretical stress concentration coefficient of M20 torsion-shear high strength bolts was 3.55,and the effective stress concentration coefficient was 3.44,which provided parameter support for the establishment of fatigue design method.3.Through the macroscopic and microscopic analysis of fatigue fracture,the typical characteristics of variable amplitude fatigue fracture of high strength bolts under full pretightening force and no pre-tightening force are discussed,and the causes of abnormal fatigue life and excessive discreteness are explained from the microscopic point of view.It is found that the shorter the fatigue life of bolts,the more crack sources,the smaller the crack propagation area and the larger the instantaneous fracture area.From the microscopic point of view,the crack width and spacing in the propagation area are small,the morphology of the instantaneous fracture area is complex,the number of dimples is small,and the shape is more irregular.4.The variable amplitude fatigue life obtained from the test is estimated by three cumulative damage laws,respectively.The results show that the modified Miner rule and Corten-Dolan theory can be used to estimate the fatigue life well,and the accuracy and discreteness are better than those of the Miner rule.5.The variable amplitude fatigue life is estimated by the fracture mechanics method,and the estimated value is in good agreement with the experimental value.The estimated fatigue life under high stress amplitude is larger,and the estimated fatigue life under low stress amplitude is more accurate.In addition,the assumption of initial crack size and the selection of parameters such as material fracture toughness will have a great influence on the results of fatigue life estimation.This study provides a reference for the variable amplitude fatigue design of tensile connection of high strength bolts,and also provides a basis for the fatigue calculation of high strength bolts with preload in the specification.