| Grid structure is widely used because of its convenient construction,beautiful shape and other characteristics.It not only needs to meet the use function of stadium,airport,conference room and other daily activities,but also plays the role of refuge in the disaster earthquake.It can be seen from the major earthquakes in history that the failure of grid structure under the action of catastrophic earthquake shows obvious characteristics of ultra-low cycle fatigue failure.As the key part connecting the upper grid and the lower support,the anchor bolt of the grid support node often breaks due to the combined action of tension,shear and other complex stresses under strong earthquake,which leads to the failure of the whole structure.In this paper,rubber plate bearing is taken as the research object to study its ultra-low cycle fatigue performance and failure mechanism,which is of great significance for improving building safety and building a resilient city in the future.Based on the National Natural Science Foundation Project "Research on ultra low cycle fatigue performance and evaluation method of bolt ball grid structure under strong earthquake"(51578358),the following work is carried out:(1)According to the specification,the joint and test loading device of M27 anchor bolt rubber plate bearing were designed together with the research group;(2)The ultra-low cycle fatigue tests of four rubber plate bearings were carried out by using bidirectional loading method,and the failure process of the specimens was recorded.The ultimate bearing capacity,hysteretic behavior,energy dissipation capacity,horizontal stiffness and other fatigue properties of rubber plate bearing are studied;(3)The fracture morphology of anchor bolt was studied from macro and micro perspectives,and the fracture mechanism of anchor bolt was analyzed;(4)ABAQUS was used to establish a finite element model for numerical analysis,and the failure process and stress change of anchor bolt in damage accumulation were analyzed.The main conclusions are as follows:(1)The failure process of the specimen is basically the same,which can be summarized into the rubber staggered movement,support slip,bottom plate lifting and bolt bending,etc.Finally,the failure of rubber plate bearing joint is caused by the fracture of anchor bolt.The life of anchor bolt is less than 100 times in the range of 20-87 times,which belongs to ultralow cycle fatigue fracture.(2)Under the influence of slip,the hysteresis curve of the specimen has obvious "pinching" phenomenon,and the fullness degree is poor.When the vertical pressure increases from 100 k N to 200 k N,the plastic development degree of anchor bolt is higher and the life of specimen is lower.The ultimate load,initial stiffness and average energy consumption are increased.(3)The statistics of necking rate of anchor bolt show that the plastic deformation of anchor bolt after fracture is small,and the necking position is within 0-90 mm from the root of anchor bolt,which is caused by tensile force and extrusion of hole wall of steel floor anchor bolt.(4)The fracture is an obvious bi-directional bending fatigue fracture,which is composed of two fatigue sources,two propagation zones and one instantaneous fracture zone.The cracking sequence,crack propagation rate and plastic development degree of each anchor bolt are different.According to the microscopic results,the fracture of anchor bolt shows both ductile fracture and brittle fracture characteristics,and the ultimate fracture causes are different when the anchor bolt position is different;(5)The results of numerical simulation and test are basically consistent.There is obvious stress concentration at the root of anchor bolt,and the axial stress is far greater than the tangential stress,which is about 2.4 times of the latter.It is the weakest place of the whole rubber plate bearing joint.Under the action of metal damage initiation criterion and evolution law,the root element of anchor bolt withdraws from work in turn until the anchor bolt breaks. |
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