【作者】 杨先勇；

【导师】 刘安中； 李友荣；

【作者基本信息】 武汉科技大学 ， 机械设计及理论， 2005， 硕士

【摘要】 桥式起重机主梁等构件是典型的焊接结构,其主要失效形式是疲劳断裂。由于在焊缝部位存在焊接缺陷,采用传统的建立在构件无缺陷、无裂纹基础上的强度理论进行设计计算时,往往不能确保起重机在额定载荷下使用的安全性。裂纹的萌生和形成寿命很短,疲劳寿命主要取决于疲劳裂纹的稳定扩展阶段。因此,应用断裂力学方法研究起重机焊接结构的疲劳裂纹扩展机理,并估算在役起重机的剩余疲劳寿命,对预防疲劳断裂事故的发生,指导起重机的设计、制造、检验和管理具有重要意义。 针对桥式起重机常出现的断裂事故,本文以某冷轧带钢厂现役50t×33m桥式起重机为研究对象,采用有限元法结合现场实测的研究方法,对桥架结构进行了应力分析。在此基础上对应力高且有严重应力集中的危险部位——主梁跨中焊缝采用“板到体子模型”技术,对其进行细化分析,得到跨中焊缝的应力分布。计算表明:主梁跨中焊缝应力最大,且存在严重应力集中,是桥式起重机主要裂纹起源,是影响桥式起重机承载能力的薄弱环节。 根据现场试验和有限元计算编制的各危险部位的载荷谱,采用修正Miner法则结合Corten-Dolan非线性损伤理论和修正后的p—S—N曲线对结构进行疲劳可靠性设计,计算表明:对于应力水平低、没有显著尖峰载荷的结构,加载顺序的影响不大。计算结果与起重机设计寿命基本相符,表明计算结果满足工程上精度要求,该寿命模型是实用和可行的。 采用等效应力法和Miner线性累积损伤法则相结合的方法对焊接结构在变幅载荷下裂纹扩展寿命进行了研究。计算发现:如果主梁跨中焊缝出现初始裂纹,当裂纹扩展速率达到某一临界值时,裂纹的扩展速率明显加快,主梁很快失效;对于工作过程中没有显著的尖峰载荷的结构疲劳裂纹,进行裂纹扩展寿命计算时,可以采用忽略裂纹扩展过载效应和不同幅值载荷循环之间顺序与相互影响的均方根等效应力法与Miner累积损伤法,计算过程相对简单且计算结果满足工程上精度要求。 本文所采用的修正Miner法则结合Corten-Dolan损伤理论进行疲劳可靠性设计的方法不仅适用于桥式起重机的疲劳损伤与寿命分析,也适用于一般的金属结构疲劳损伤与寿命分析;本文采用有限元分析与断裂力学分析相结合的方法为含初始缺陷结构的寿命预测提供了一种工程上可行的方法。更多还原

【Abstract】 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.更多还原