發(fā)布時(shí)間：2018-05-23 14:11

  本文選題：應(yīng)力集中系數(shù) + 缺口試樣尺寸系數(shù)��； 參考：《中南大學(xué)》2013年博士論文

【摘要】：裝備結(jié)構(gòu)件的疲勞壽命是決定設(shè)備安全可靠服役的基本能力,因此它是裝備設(shè)計(jì)與制造中必須科學(xué)分析的重要問題�，F(xiàn)代裝備服役功能日趨強(qiáng)大,結(jié)構(gòu)日趨大型化和復(fù)雜化,對(duì)構(gòu)件的性能指標(biāo)—特別是可靠性要求更為突出,要求疲勞壽命的計(jì)算更為科學(xué)真實(shí),而疲勞壽命計(jì)算是建立在充分的試驗(yàn)基礎(chǔ)上,構(gòu)件的超大型化使構(gòu)件原型疲勞壽命試驗(yàn)十分困難,以致大型件的壽命設(shè)計(jì)成為當(dāng)今裝備設(shè)計(jì)的一大難題,本文擬在已有疲勞計(jì)算理論基礎(chǔ)上,結(jié)合大型構(gòu)件特點(diǎn)探索一種有限試驗(yàn)與計(jì)算分析結(jié)合的疲勞壽命預(yù)測(cè)方法。由于拉伸疲勞分析是其它疲勞分析的基礎(chǔ),本論文主要對(duì)拉伸作用下大型結(jié)構(gòu)件疲勞壽命的幾個(gè)主要因素的影響規(guī)律進(jìn)行研究,并在此基礎(chǔ)上形成大型結(jié)構(gòu)件疲勞壽命的預(yù)測(cè)方法。 1.針對(duì)構(gòu)件內(nèi)部缺陷隨尺寸的增大而增加,發(fā)生疲勞失效的概率也增大,特別是位于高應(yīng)力區(qū)域中(如應(yīng)力集中)的缺陷更容易萌生裂紋；以及常規(guī)的拉伸疲勞試驗(yàn)中光滑試樣尺寸效應(yīng)不顯著,用于修正大型結(jié)構(gòu)件的疲勞強(qiáng)度將產(chǎn)生較大的誤差,甚至可能得到錯(cuò)誤的結(jié)果,基于帶缺口試樣裂紋萌生和擴(kuò)展規(guī)律與大尺寸結(jié)構(gòu)因內(nèi)部缺陷而萌生裂紋并發(fā)展斷裂機(jī)制相似,本文提出缺口試樣尺寸系數(shù),用它代替光滑試樣尺寸系數(shù)進(jìn)行疲勞壽命分析。通過有限元法和TheCritical Distance(TCD)理論對(duì)幾何相似試樣進(jìn)行缺口試樣尺寸系數(shù)分析(厚度未作考慮),得到了基于梯度效應(yīng)的缺口試樣尺寸系數(shù)簡(jiǎn)易計(jì)算式。利用此式對(duì)一實(shí)例進(jìn)行尺寸系數(shù)計(jì)算,結(jié)果表明引入缺口試樣尺寸系數(shù)是合理有效的,反映了拉伸作用下的缺口試樣尺寸效應(yīng)。 2.由于疲勞失效的局部特性和裂紋常萌生于表面,當(dāng)構(gòu)件三維方向結(jié)構(gòu)變化大時(shí),本文建立了高應(yīng)力區(qū)局部結(jié)構(gòu)尺寸對(duì)疲勞強(qiáng)度影響程度的量化式子。假設(shè)缺口板試樣的壽命以裂紋形成壽命為主,通過對(duì)兩種材料(45#,Q235)的幾何相似試樣進(jìn)行拉-拉疲勞壽命測(cè)試,結(jié)果發(fā)現(xiàn)：除了應(yīng)力梯度的影響外,缺口局部高應(yīng)力方向的尺寸對(duì)疲勞壽命也產(chǎn)生一定的影響(文中稱厚度效應(yīng)),同時(shí)考慮這兩方面的影響才能更全面地反映缺口試樣的尺寸效應(yīng)。利用疲勞極限的外推法獲得了與局部特征參量L/G相關(guān)的缺口試樣疲勞強(qiáng)度經(jīng)驗(yàn)公式,并根據(jù)此式和趨勢(shì)外推法得到缺口試樣尺寸系數(shù)表達(dá)式�；谄谀M實(shí)驗(yàn)和相似性準(zhǔn)則,可建立巨型鍛壓機(jī)機(jī)架的模擬縮比模型(屬于光滑缺口試樣),利用尺寸系數(shù)表達(dá)式便可計(jì)算出機(jī)架的尺寸系數(shù)。 3.在零件的尺寸系數(shù)計(jì)算過程中,同時(shí)考慮應(yīng)力集中、尺寸和表面加工狀態(tài)的綜合影響,對(duì)大型結(jié)構(gòu)件的疲勞強(qiáng)度修正過程進(jìn)行演繹推導(dǎo),得出了新的疲勞綜合修正系數(shù)模型。通過該模型對(duì)理論應(yīng)力集中系數(shù)、表面加工系數(shù)和缺口試樣尺寸系數(shù)的敏度分析,并與未考慮組合影響的綜合修正系數(shù)表達(dá)式的敏度分析對(duì)比,結(jié)果表明了新表達(dá)式的合理性。 4.利用新的疲勞綜合修正系數(shù)對(duì)試樣疲勞壽命進(jìn)行分析,計(jì)算結(jié)果與試驗(yàn)結(jié)果基本相符,與傳統(tǒng)修正系數(shù)計(jì)算的結(jié)果對(duì)比,結(jié)果表明：用本文的疲勞綜合修正系數(shù)進(jìn)行修正具有更好的效果,更符合實(shí)驗(yàn)情況。把它應(yīng)用于大型結(jié)構(gòu)件上,基于名義應(yīng)力法可實(shí)現(xiàn)利用小試樣試驗(yàn)得到大型結(jié)構(gòu)件的疲勞壽命預(yù)測(cè),由此形成疲勞壽命預(yù)測(cè)的試驗(yàn)分析方法,為大型結(jié)構(gòu)件的疲勞壽命評(píng)估提供參考。 5.由于疲勞試驗(yàn)結(jié)果的分散性,假定疲勞強(qiáng)度和對(duì)數(shù)疲勞壽命服從正態(tài)分布,本文考慮綜合修正系數(shù)中的三個(gè)關(guān)鍵因素的分散性,利用隨機(jī)變量組合方法得到了疲勞綜合系數(shù)的分布。根據(jù)材料疲勞極限的分布,運(yùn)用綜合修正系數(shù)的分布得到零件疲勞極限的分布。在材料的中值S-N(可靠度為50%)曲線基礎(chǔ)上,作出可靠度為99.9%的大型結(jié)構(gòu)件S-N曲線,從而估算出其高可靠性壽命,使計(jì)算結(jié)果更加合理。 6.在名義應(yīng)力法基礎(chǔ)上,應(yīng)用本文的綜合修正系數(shù)修正材料的S-N曲線后實(shí)現(xiàn)了巨型鍛壓機(jī)機(jī)架的疲勞壽命預(yù)測(cè),預(yù)測(cè)結(jié)果是可以接受的,解決了其試驗(yàn)難的問題。該方法為拉伸載荷下的大型結(jié)構(gòu)件疲勞壽命評(píng)估提供了一種新的途徑。 7.大型結(jié)構(gòu)件通常是長(zhǎng)時(shí)間、高可靠服役。當(dāng)其尺寸和形狀基本確定后,運(yùn)用本文提出的疲勞壽命分析方法進(jìn)行構(gòu)件局部結(jié)構(gòu)精細(xì)設(shè)計(jì),以進(jìn)一步提高其使用壽命。本文從結(jié)構(gòu)細(xì)部設(shè)計(jì)規(guī)律方面進(jìn)行了探索,以減少局部應(yīng)力集中為目標(biāo),提出了幾種簡(jiǎn)單、經(jīng)濟(jì)的巨型鍛壓機(jī)機(jī)架結(jié)構(gòu)延壽設(shè)計(jì)方案,有限元分析結(jié)果說明了其有效性,可滿足設(shè)計(jì)壽命要求,為大型結(jié)構(gòu)件的抗疲勞設(shè)計(jì)提供參考。圖90幅,表23個(gè),參考文獻(xiàn)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 .
【學(xué)位授予單位】：中南大學(xué)
【學(xué)位級(jí)別】：博士
【學(xué)位授予年份】：2013
【分類號(hào)】：TH114

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