【作者】 刘斌；

【导师】 张开林；

【作者基本信息】 西南交通大学 ， 交通运输工程（专业学位）， 2017， 硕士

【摘要】 疲劳失效是焊接结构最主要的破坏形式之一,铁路机车转向架上的焊接构件在实际工作中往往承受多轴载荷的作用,因此发展多轴疲劳理论,建立可应用于工程实际的多轴疲劳评估方法具有重要的实际意义。相对于单轴疲劳而言,多轴疲劳无论在力学分析方面还是物理机制方面都更为复杂,目前仍是国际疲劳研究领域的难点和热点问题。目前国内外普遍采用传统的名义应力法和结构应力法(热点应力法)。名义应力法忽略了结构焊缝细节的影响,存在名义应力参量难以确定,与标准中典型焊接接头无法匹配的困难。结构应力法考虑了焊缝细节尺寸,但焊趾和焊根缺口处具有强烈的网格敏感性,容易导致局部应力奇异,因此更多的情况是采用公式外推法估算焊缝热点处的应力状态,即便如此,该方法也存在焊根处无法进行疲劳分析的困难。为了克服传统抗疲劳设计方法的缺陷,本文引入更为准确可靠的缺口应力法进行焊接接头的疲劳寿命研究,缺口应力法可将不同的焊接接头试验数据统一在同一疲劳寿命曲线坐标系下。鉴于当前焊接接头疲劳评估曲线只适用于单轴加载情况,本文基于缺口应力法和多轴准则对焊接接头进行了多轴疲劳分析。文章选取了工程运用中几种常见的焊接接头,通过联合焊接接头的多轴疲劳试验名义应力数据和有限元法所计算的缺口应力集中系数,可以获得焊接接头的各向缺口应力;根据VonMises、IIW和EESH三种多轴疲劳准则可分别计算出其等效缺口应力;然后采用最小二乘法分别拟合出不同的适用于多轴疲劳评估的缺口寿命S-N曲线,最后将新的多轴缺口寿命曲线应用于转向架构架主要焊缝的疲劳寿命预测。与IIW标准推荐的单轴曲线相比,这些多轴缺口疲劳曲线具有更低的倾斜度和更高的疲劳等级。由于焊接接头缺口应力集中系数能否准确确定将对缺口应力的计算结果造成直接的影响,因此在第四章中讨论了焊缝参数对缺口应力的影响。通过分析可知,焊喉深度的减小对于等效缺口应力的影响非常有限,而焊趾角度的减小效果更为明显;砂轮打磨对减小等效缺口应力也非常明显,相对于原焊缝设计参数,焊趾在6mm的磨削半径下,缺口应力至少可以降低20%。更多还原

【Abstract】 Fatigue failure is one of the most important failure modes in welded structures,the welding components on the bogie of railway locomotives often bear the effect of multi-axle loads.Therefore,it is of great practical significance to develop the multi-axial fatigue theories and establish the multi-axial fatigue assessment methods which can be applied to engineering practice.Compared with uniaxial fatigue,multi-axial fatigue is more complicated in mechanics analysis and physical mechanism,it is still a difficult and hot issue in the field of international fatigue research at present.At present,the traditional nominal stress method and structural stress method(hot spot stress method)are widely used at home and abroad.The nominal stress method ignores the influence of structural weld details,therefore,it is difficult to determine the nominal stress parameters,and it is difficult to match the typical welding joints.The structural stress method takes into account the size of weld details,however,there is a strong mesh sensitivity at the weld toe and the weld root notch,which can easily lead to the local stress singularites,therefore,in many cases,the extrapolation method is used to estimate the stress state at the weld hot spot.Even so,the method also has difficulties in fatigue analysis at the root of weld.In order to overcome the defects of traditional anti fatigue design method,in this paper,the fatigue life of welded joints is studied by a more accurate and reliable notch stress method,so that the experimental data of different welding joints can be unified in the same fatigue life curve coordinate system by notch stress method.In view of that the fatigue assessment of welded joints is only applicable to uniaxial loading,in this paper,the multi-axial fatigue analysis of welded joints is carried out based on the notch stress method and the multi-axial criterion.In this paper,several common welded joints used in engineering are selected,and the notch stress concentration factor is calculated by using the nominal stress data and the finite element method,so that the notch stress of welded joints can be obtained;According to Von Mises、ⅡW and EESH these three multi-axial fatigue criteria,the S-N curve can be calculated;Then,we use the least square method to fit the different fatigue life curves;In the end,the new multi axle notch life curve is used to predict the fatigue life of the main frame of bogie frame.Compared with the uniaxial curve recommended by IIW standard,these multi-axial notch fatigue curves have lower inclination and higher fatigue rating.On account of that whether the stress concentration factor of welded joint can be determined accurately will have a direct impact on the calculation results of notch stress,therefore,we discussed the influence of welding parameters on notch stress in the fourth chapter.Through analysis,the effect of the decrease of the throat depth on the equivalent notch stress is very limited,and the effect of reducing the weld toe angle is more obvious;Grinding wheel grinding is very obvious to reduce the equivalent notch stress,compared with the original design parameters,when the weld toe is in the grinding radius of 6mm,the notch stress can be reduced by at least 20%.更多还原