大型结构件的疲劳寿命预测方法研究

发布时间：2018-05-23 14:11

本文选题：应力集中系数 + 缺口试样尺寸系数　；参考：《中南大学》2013年博士论文

【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。
[Abstract]:The fatigue life of the equipment structural members is the basic ability to determine the safe and reliable service of the equipment , so it is an important problem that must be scientifically analyzed in the equipment design and manufacture . The fatigue life calculation is more scientific and true , and the fatigue life calculation is more scientific and true , and the fatigue life calculation is based on the existing fatigue calculation theory .

1 . In view of the increase of the internal defects of the component with the increase of the size , the probability of fatigue failure is also increased , especially in the high - stress region ( such as stress concentration ) , the defect is more prone to crack initiation ;
Based on the finite element method and the Critical Distance ( TCD ) theory , a simple calculation formula of the notch sample size coefficient based on the gradient effect is obtained . The results show that the notch sample size coefficient is reasonable and effective , which reflects the notch sample size effect under the tensile action .

2 . Due to the local characteristics of fatigue failure and the crack propagation on the surface , the effect of local structure size on fatigue strength is established . It is found that the fatigue life of the notch specimen is mainly influenced by the stress gradient . The fatigue life of the notch specimen is also influenced by the external extrapolation method . Based on the fatigue simulation experiment and the similarity criterion , the model of the simulation shrinkage ratio of the frame of the huge forging press can be established .

3 . In the process of calculating the dimension coefficient of the parts , taking into account the comprehensive influence of stress concentration , dimension and surface machining state , the fatigue strength correction process of large structural members is deduced , and a new model of fatigue comprehensive correction coefficient is obtained .

4 . The fatigue life of the specimen is analyzed by using the new fatigue comprehensive correction factor . The results are in agreement with the experimental results . The results show that the fatigue life prediction of large structural members can be obtained by using the method of nominal stress based on the nominal stress method , and the fatigue life prediction of large structural members can be obtained by using the nominal stress method . The fatigue life prediction method is used to provide a reference for the fatigue life assessment of large structural members .

5 . Owing to the dispersion of fatigue test results , it is assumed that the fatigue strength and the logarithmic fatigue life are subject to the normal distribution , and the distribution of the fatigue comprehensive coefficient is obtained by using the random variable combination method . Based on the distribution of the fatigue limit of the material , the distribution of the fatigue limit of the parts is obtained by using the distribution of the comprehensive correction coefficient . The S - N curve of the large structural member with a reliability of 99.9 % is obtained based on the curve of the median S - N ( reliability of 50 % ) of the material , so that the reliability life of the material is estimated , and the calculation result is more reasonable .

6 . On the basis of the nominal stress method , the fatigue life prediction of the frame of the giant forging press is realized by applying the S - N curve of the composite correction coefficient correction material , and the prediction result is acceptable , and the problem of difficult test is solved . The method provides a new approach for the fatigue life assessment of large structural members under tensile load .

7 . Large structural members usually serve for a long period of time and are highly reliable . When the size and shape are basically determined , the fatigue life analysis method proposed in this paper is designed to improve its service life .
【学位授予单位】：中南大学
【学位级别】：博士
【学位授予年份】：2013
【分类号】：TH114

【参考文献】