Study On Fatigue Reliability Of Tower Crane Based On Crack Propagation

With the rapid development of Chinese economy and the continuous investment in infrastructure construction,the demand for engineering machinery in the field of engineering is increasing.In the meanwhile,the higher and higher full load rate of engineering machinery also puts forward higher requirements for its performance,especially the fatigue performance during its usage.The tower crane,with its large operating space,can realize the vertical and horizontal transport of materials,and it has gradually become one of the most widely used and indispensable engineering machinery in all kinds of infrastructure constructions.Therefore,it is of great significance to study the fatigue strength and the fatigue life of the tower crane under practice for it can improve the safety performance,optimize the structure design and prolong the service life of the machinery.The tower crane loads frequently during operating,and in most case the load is alternating in practical engineering application due to its own characteristics,so its structure could be faced with serious fatigue failure.The typical performance includes the connection failure caused by the stress relaxation of the joint.At the same time,due to the unavoidable small cracks in the welding structure,the fatigue crack will be easily induced by the alternating load,which will eventually cause the fatigue fracture,which could cause accidents that are difficult to redeem.Therefore,how to analyze the fatigue characteristics of the tower crane,the expansion of fatigue crack and further predict of its fatigue life have become one of the serious problems.The QTZ40 tower crane boom as the research target,focusing on the followingcontents:(1)According to its typical characteristic curve,through the establishment of the finite element model,analyzing the boundary conditions of the tower crane lifting arm,the stress analysis is carried out to calculate the stress distribution of the structure under typical working conditions.Furthermore,through the field observation,the fatigue critical location was determined from the calculation done during the operation of the variable amplitude trolley of the tower crane.(2)Taking the typical roof welding structure of the lifting arm of the tower crane into account,the analysis method of stress intensity factor is introduced to the fracture mechanics theory,the finite element model of the typical T welded joint structure is established,the fatigue crack propagation mode and characteristics are analyzed,in the meantime,the stress strength under different structural parameters is calculated.Based on the fatigue crack growth rate of welded structure,the structural parameters were optimized.(3)Based on the fatigue life estimation method of fracture mechanics,the risk position of the lifting arm of the tower crane is simplified.The stress intensity factor of the crane boom under different external loads is calculated as well,the fatigue life is predicted through the determination of the initial crack and the fracture crack length in the structure using the fracture mechanics fatigue crack propagation rate theory.Through the research towards the fatigue life of tower crane in this paper,the structure of the tower crane is constantly adjusted and optimized,and the service time of the material is raised to a reasonable length,thus the utilization rate is effectively improved,the cost of the engineering is saved and the accidents involving the tower crane are reduced.