Research On Fatigue Life Of Overhead Traveling Crane Girder

Main girder and other components of overhead traveling crane are typical welded structures, the primary failure type of which is fatigue crack. Since there are weld defects in welds, using traditional strength theory based on no defect and no crack to design, crane's security in rating load can't be insured. Crack initiation and formation life is short; the fatigue life mostly decides on crack's stable propagation period. It is of great meaning for preventing crack accident and guiding overhead traveling crane's design production . test and management to study propagation mechanism of crane's welded structures and estimate surplus fatigue life of crane in service.Aiming at frequent crack accident of overhead traveling crane, this paper took a 50tx33m overhead traveling crane of a cold rolling mill plant for subject investigated to analyze stress of it's bridge structure with the research method combining FEM with field testing. Base on it, "shell-to-solid submodel" method was used to finely analyze high stress and severe stressconcentration region-welds in mid-span of the main girder, and stress distribution of it isobtained. The analysis results shows welds in mid-span, suffered from highest stress and severe stress concentration, are primary crack initiation sites and weakness influencing the increase of carrying capacity of overhead traveling crane.According to dangerous site's load spectrum established by the combination of field test and finite element analysis, structure fatigue reliability design are made by combining modified Miner theorem and Corten-Dolan nonlinear damage accumulation theory and modified p—S—N curve. The analysis results shows that the effect of load sequence isn't prominent for structures suffering from low stress and no peak load. The results accords with the designed life of the crane by and large, showing that calculation results meet required precision for engineering, and that life fatigue model is practicable.Crack propagation life of welded structures in variable amplitude load is studied by combining equivalent stress method and Miner linear damage accumulation theory. Calculation shows: if there are initial cracks in welds of mid-span of the main girder, propagation speed of cracks accelerates sharply and the main girder will fail soon when it reaches a critical value. For structural fatigue cracks in case of no peak load in the working course, equivalent stress method and Miner linear damage accumulation theory, taking no count of overload effect variable amplitude load sequence and interrelationship, can be adopted to calculate crack propagation life, the calculation process of which is relatively simple and the calculation results meet required engineering precision.One method used in the paper, combining modified Miner theorem and Corten-Dolandamage theory to make fatigue reliability design, applicable for not only fatigue damage and life analysis of overhead traveling crane but also that of general metal structure; another method used in the paper, combining finite element analysis and fracture mechanics analysis, provide a practicable engineering method for fatigue life estimation of structure with initial defects.