| As a large lifting equipment running on the upper part of the nuclear island,the polar cranes for nuclear power plants is one of the eleven main equipments of nuclear power plants,and its security is highly concerned.As a key component of supporting carriage and load-bearing in polar cranes for nuclear power plants,the main beam is mainly subjected to alternating load during service,and its main form of failure is fatigue fracture.The breakage of the main beam will cause the entire polar cranes for nuclear power plants to collapse,damage the reactor assembly and result in incalculable consequences.In order to ensure the safe and reliable operation of polar cranes for nuclear power plants,it is of great significance to carry out research on fatigue life and reliability of the main beam of polar cranes for nuclear power plants.In this paper,the main beam of polar cranes for third generation nuclear power plants is studied.First,the failure mode and the research method of fatigue life of the main beam of polar cranes for nuclear power plants are investigated and determined.Then,on the basis of establishing the finite element model of the main beam of polar cranes for nuclear power plants,the finite element analysis is carried out in combination with the particularity of the main beam structure.According to the results of finite element analysis and Paris formula of crack propagation,the fatigue life of the main beam is studied,and the results of the fatigue life of the main beam are verified by the equivalent test.Finally,in view of the problem of lack of fatigue life data of the main beam,the fatigue life simulation experiment is carried out by Monte Carlo simulation,and the reliability of the fatigue life of the main beam is studied based on the simulation results.The main research of this paper is as follows:(1)According to the actual size of the main beam of the third generation polar cranes for nuclear power plants,a finite element model of the main beam is established on the basis of appropriate simplified structure.Four typical load conditions are determined based on the characteristics of polar cranes for nuclear power plants.Finite element stress-strain analysis is performed under four load conditions.Based on the results of finite element analysis,the predicted location of fatigue life of the main beam is determined.(2)Based on the analysis results of dangerous locations,the initial cracks are analyzed and judged.Since the materials of two dangerous locations are different(one on the parent material and one on the weld),the difference in mechanical properties is considered when determining the initial cracks in these two parts;According to the influence of the initial crack on the inside and the joint of the weld,the initial crack size of the two dangerous parts is determined.(3)Consider the fatigue life prediction of the main beam in the stage of crack formation.In the fatigue life prediction of the main beam of a general bridge crane,we usually acquiesce in that there are weld defects in welds,and the life of the crack formation stage is neglected.In contrast,the life of the crack formation stage of the beams of polar cranes for nuclear power plants cannot be ignored after the defect detection is carried out according to JB/T12214-2015.Therefore,the crack formation stage is taken into account when the fatigue life prediction of the main beam of polar cranes for nuclear power plants is carried out using the idea of the equivalent initial crack.(4)Getting the constant material value of Q345 steel in the Paris formula through the test data of Q345 steel compact tension specimen.The fatigue life prediction of the main beam is completed by combining the finite element stress analysis results of the main beam and Paris formula.The designed equivalent test verifies the rationality to use the welds of mid-span of the main beam as the fatigue life prediction position and the influence of the constraint mode of the end of the main beam on the fatigue life of the main beam ends. |
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