Fatigue Life Analysis On Box Girder Of Crane Based On Equivalent Structural Method

Crane is the most common and important special equipment and mainlycomposed of welded box structure, so it is obviously the failure of weldedcomponents, to a large extent, determines the failure of the structure. Because of allkinds of uncertainty factors in the process of welding, welded components inevitablyexists some welding defects (porosity, slag inclusion, lack of fusion, etc.) as well asstress concentration. So the fatigue failure often occurs mainly at the welded position.Due to welding node stress singularity, when we use the traditional finiteelement method (FEM) to get results of the stress at the welded position, the finiteelement mesh size and type are quite sensitive. As a result, it is difficult to accuratelycalculate the stress corresponding to engineering practice, and to effectively predictthe fatigue life of structures. In order to effectively predict the fatigue performance ofthe crane, the background and the home and abroad development situation of fatiguelife analysis are reviewed and summarized. Meanwhile, by comparing with theproblems in the current existing evaluation methods, the thesis determines the methodof this topic research: Mesh-sensitive structure stress method and the master S-Ncurve approach (Equivalent structural stress method) predict the fatigue life of weldedcomponents. Secondly, for the sake of effectively take the advantages of equivalentstructural stress method, the paper focus on its theory, and combine with existingknowledge, the important theoretical formula of this method is derived. Thenaccording to the crane fatigue fracture form, this paper took with weight bearing50tons, span of31.5meters overhead crane as research object to analyze staticperformances of box girder with FEM. The results show the static strength andstiffness satisfies the design requirements. The effectiveness of simplified mechanicalmodel, modeling method and the result of the analysis is verified with comparingstatic performance with test results. According to static performances the mostdangerous place of girder can be located and taken as a sub-modeling including ofweld details, and then we carries on the static analysis. The mathematical model of fatigue master S-N curve was summarized; the mesh-insensitive and accuracy of lifeprediction of equivalent stress method has been proved combining with examples.Finally, the equivalent structural stress was calculated by transforming structuralstress of the dangerous location, and fatigue life of the metal structure has beenpredicted, the results show that the main welds have good anti-fatigue performancesand satisfy the design requirements.Finally, some imperfections are analyzed and summarized during researchingprocess in this paper, and the same time some questions are put forward to be solvedin the later research.