【作者】 黄宁；

【导师】 钟掘； 黄明辉；

【作者基本信息】 中南大学 ， 机械工程， 2013， 博士

【摘要】 装备结构件的疲劳寿命是决定设备安全可靠服役的基本能力,因此它是装备设计与制造中必须科学分析的重要问题。现代装备服役功能日趋强大,结构日趋大型化和复杂化,对构件的性能指标—特别是可靠性要求更为突出,要求疲劳寿命的计算更为科学真实,而疲劳寿命计算是建立在充分的试验基础上,构件的超大型化使构件原型疲劳寿命试验十分困难,以致大型件的寿命设计成为当今装备设计的一大难题,本文拟在已有疲劳计算理论基础上,结合大型构件特点探索一种有限试验与计算分析结合的疲劳寿命预测方法。由于拉伸疲劳分析是其它疲劳分析的基础,本论文主要对拉伸作用下大型结构件疲劳寿命的几个主要因素的影响规律进行研究,并在此基础上形成大型结构件疲劳寿命的预测方法。1.针对构件内部缺陷随尺寸的增大而增加,发生疲劳失效的概率也增大,特别是位于高应力区域中(如应力集中)的缺陷更容易萌生裂纹；以及常规的拉伸疲劳试验中光滑试样尺寸效应不显著,用于修正大型结构件的疲劳强度将产生较大的误差,甚至可能得到错误的结果,基于带缺口试样裂纹萌生和扩展规律与大尺寸结构因内部缺陷而萌生裂纹并发展断裂机制相似,本文提出缺口试样尺寸系数,用它代替光滑试样尺寸系数进行疲劳寿命分析。通过有限元法和TheCritical Distance(TCD)理论对几何相似试样进行缺口试样尺寸系数分析(厚度未作考虑),得到了基于梯度效应的缺口试样尺寸系数简易计算式。利用此式对一实例进行尺寸系数计算,结果表明引入缺口试样尺寸系数是合理有效的,反映了拉伸作用下的缺口试样尺寸效应。2.由于疲劳失效的局部特性和裂纹常萌生于表面,当构件三维方向结构变化大时,本文建立了高应力区局部结构尺寸对疲劳强度影响程度的量化式子。假设缺口板试样的寿命以裂纹形成寿命为主,通过对两种材料(45#,Q235)的几何相似试样进行拉-拉疲劳寿命测试,结果发现：除了应力梯度的影响外,缺口局部高应力方向的尺寸对疲劳寿命也产生一定的影响(文中称厚度效应),同时考虑这两方面的影响才能更全面地反映缺口试样的尺寸效应。利用疲劳极限的外推法获得了与局部特征参量L/G相关的缺口试样疲劳强度经验公式,并根据此式和趋势外推法得到缺口试样尺寸系数表达式。基于疲劳模拟实验和相似性准则,可建立巨型锻压机机架的模拟缩比模型(属于光滑缺口试样),利用尺寸系数表达式便可计算出机架的尺寸系数。3.在零件的尺寸系数计算过程中,同时考虑应力集中、尺寸和表面加工状态的综合影响,对大型结构件的疲劳强度修正过程进行演绎推导,得出了新的疲劳综合修正系数模型。通过该模型对理论应力集中系数、表面加工系数和缺口试样尺寸系数的敏度分析,并与未考虑组合影响的综合修正系数表达式的敏度分析对比,结果表明了新表达式的合理性。4.利用新的疲劳综合修正系数对试样疲劳寿命进行分析,计算结果与试验结果基本相符,与传统修正系数计算的结果对比,结果表明：用本文的疲劳综合修正系数进行修正具有更好的效果,更符合实验情况。把它应用于大型结构件上,基于名义应力法可实现利用小试样试验得到大型结构件的疲劳寿命预测,由此形成疲劳寿命预测的试验分析方法,为大型结构件的疲劳寿命评估提供参考。5.由于疲劳试验结果的分散性,假定疲劳强度和对数疲劳寿命服从正态分布,本文考虑综合修正系数中的三个关键因素的分散性,利用随机变量组合方法得到了疲劳综合系数的分布。根据材料疲劳极限的分布,运用综合修正系数的分布得到零件疲劳极限的分布。在材料的中值S-N(可靠度为50%)曲线基础上,作出可靠度为99.9%的大型结构件S-N曲线,从而估算出其高可靠性寿命,使计算结果更加合理。6.在名义应力法基础上,应用本文的综合修正系数修正材料的S-N曲线后实现了巨型锻压机机架的疲劳寿命预测,预测结果是可以接受的,解决了其试验难的问题。该方法为拉伸载荷下的大型结构件疲劳寿命评估提供了一种新的途径。7.大型结构件通常是长时间、高可靠服役。当其尺寸和形状基本确定后,运用本文提出的疲劳寿命分析方法进行构件局部结构精细设计,以进一步提高其使用寿命。本文从结构细部设计规律方面进行了探索,以减少局部应力集中为目标,提出了几种简单、经济的巨型锻压机机架结构延寿设计方案,有限元分析结果说明了其有效性,可满足设计寿命要求,为大型结构件的抗疲劳设计提供参考。图90幅,表23个,参考文献173篇。更多还原

【Abstract】 The fatigue life of the equipment structure is to determine its basic capability of safe and reliable service, so it is an important problem for scientific analysis in design and manufacuture of equipments. Service functions of modern equipment are becoming more powerful, the structure is developing towards larger and more complex. The performance target of the components, especially reliability requirements, is more prominent, requires of the fatigue life calculation is more scientific and real. The fatigue life calculation is based on the full tests, the prototype fatigue life test of super-scale components is very difficult in experimenting, so that the life design of large-scale components becomes a major problem in today’s equipment design. Combined with characteristics of large-scale components, this article intends to explore a fatigue life prediction method of limited test and computational analysis on basis of the existing fatigue calculation theory. Because the tensile fatigue analysis is a base for other fatigue analysis, the influencing law of several major factors of large-scale components fatigue life is mainly investigated under tension load in this thesis, and the fatigue life prediction method for large-scale components is formed on this basis.1. Because defects in component increase with increasing its dimension, the probability of fatigue failure also augments. Especially, defects in high stress region easily result in initiating cracks. Due to no obvious size effect under tension in routine fatigue test, fatigue strength calculating result of large-scale component will be larger errors, sometimes wrong results may appear. Based on the similarity of fatigue crack initiation and expanding mechanism between notched-sample and defects in big dimension structure, the notched-sample size factor was presented, the smooth specimen size coefficient was replaced with it in fatigue life analysis. Finite element method (FEM) and theory of critical distance (TCD) were employed to analyze size coefficient of geometrically similar specimens (not including thickness), a simple expression of notched-sample size coefficient was obtained based on gradient effect. A size coefficient example shows that introduction of notched-sample size coefficient is reasonable and effective, the notched-sample size effect under tension is prominent.2. Based on the local characteristic of fatigue failure and surface crack, when components size in three dimension largens, it is necessary to investigate the weighed formula of local structure size in high stress region influencing on fatigue strength. It is assumed that the life of notched-plate specimen is dominated by the crack initiation life, fatigue life of geometrically similar specimens of two materials (45#, Q235) were test under tension. The results show that size in local high stress direction has a certain impact on fatigue life except for stress gradient (named thickness effect). Simultaneously considering effect of two aspects can comprehensively reflect size effect of notched-sample. An empirical formula of notched-sample fatigue strength was obtained by applying extrapolation of fatigue limit, in which a local characteristic parameter L/G represented effect of geometric size variations on fatigue performance. The notched-sample size coefficient was calculated through this formula and trend extrapolation. Based on simulated fatigue experiment and similarity rule, the simulated-scale model of large-scale rack of giant forging press machine (notched-sample) was established, its size coefficient was obtained.3. During caculating the component size coefficient, simultaneously considering three key factors:stress concentration, surface processing state and size effect, the corrected process of fatigue strength calculation of large-scale component was analysed and deduced; a new comprehensive fatigue correction factor model was established. Sensitivity analysis of model to the theoretical stress concentration coefficient, the surface processing coefficient and the notched-sample size coefficient was proceeded, and was compared with not considering combining influence of comprehensive correction factor expression, the rationality of new expression is proved.4. Fatigue life of a notched-sample was analyzed with new fatigue comprehensive correction factor, calculating results basically agree with experimental results. Compared with that of traditional correction coefficient, it shows that the new factor correcting has better effect, and is in accordance with test situation. New factor is used to large-scale component based on nominal stress method, its fatigue life prediction will be accomplished through fatigue test of small samples, which result in experiment&analysis method. It provides reference for fatigue life assessment of large-scale component.5. Due to dispersion of fatigue experimental results, this article assumes fatigue strength and logarithm fatigue life both belong to normal distribution, dispersion of three key factors of comprehensive correction factor were taken into account, fatigue comprehensive correction factor distribution was determined through random variables combination method. Fatigue limit distribution of component was obtained with comprehensive correction factor distribution based on distribution of material fatigue limit distribution. On basis of material S-N curve (50%reliability),99.9%reliability S-N curves was drawn, so the reliability life was gotten. This makes life calculation result more reasonable.6. On basis of nominal stress method, after amending S-N curve of material with new comprehensive correction factor, fatigue life prediction of rack of giant forging press machine was effectively predicted, life result can be accepted, which can solve its difficult problem in the experiment. This method is to provide a new way for large-scale component fatigue assessment under tensile load.7. Large-scale component usually require longer and more reliable service life. When its size and shape was determined, local detials structure were designed with new fatigue life prediction method, so that the service life is to be increased. Structural details design rule were explored in this article, which aimed to reduce stress concentration, a few simple, economic structure life extension design programs were presented, finite element analysis results show their effectiveness. This ensures design life requirement and provides reference for anti-fatigue design of large-scaled component.更多还原