【作者】 罗明军；

【导师】 张华；

【作者基本信息】 南昌大学 ， 机械设计及理论， 2014， 博士

【摘要】 工程上，受到模态频率相近的动态载荷的作用，结构表面产生的应力水平虽然低于静态疲劳极限值，但是仍可能由于疲劳损伤的累积而产生疲劳破坏，而且结构的实际疲劳寿命远低于依据传统普通疲劳计算而得的预测结果。至今为止，这仍是传统的疲劳寿命分析理论无法清楚阐述的难题之一。本文介绍了振动疲劳与传统普通疲劳之间的基本概念，提出了振动疲劳与传统的普通疲劳的主要区别在于激励频率是否涵盖了结构的工作模态频率。通过对某后扭力梁结构在服役状态下容易出现疲劳破坏的分析，提出了一种解决汽车底盘结构振动疲劳的办法，即运用多体动力学、有限元法和应变工作模态试验与分析相结合的手段，从载荷识别、工作模态分析、动应力数值估算与试验验证、激励频率对结构动应力影响分析等方面进行研究。在此基础上，阐述振动疲劳分析的若干关键技术，本文完成的主要工作如下：1、创建了后扭力梁结构的有限元模型，分析获得了结构固有模态频率，并应用模态试验验证了结构有限元模型的准确性。根据后扭力梁结构实际工作状态下的装配关系，确定了后扭力梁结构的3种可能的边界条件，并运用随机子空间法识别得到了不同边界条件下后扭力梁的工作模态参数，研究表明结构在服役状态下其边界条件较为复杂。2、为解决大规模应变测试中可能面临的数据采集通道不足的问题，采用二向应变片代替应变花测试结构表面的应变信号，并推导了薄壁结构件的主应力计算公式。为验证推导的主应力公式的正确性，设计和加工了U型薄壁结构试件，开展应变测试。研究表明基于二向应变片计算的薄壁件主应力能够反映结构的真实应力水平，为后续工程上的应用提供参考。将研究结果应用于后扭力梁结构应力分析，通过实车道路试验验证了该推导方法的正确性和精度。3、考虑后扭力梁结构轴头处橡胶衬管内的衬垫非线性特性，应用ADAMS和Matlab/Simulink模块建立了后扭力梁结构刚柔耦合多体动力学模型，基于样车道路试验采集的悬架弹簧的位移信号，通过迭代仿真获得了后扭力梁左、右轴头载荷。为了验证其准确性，基于整车台架试验对仿真结果进行了验证。试验表明联合ADAMS和Matlab/Simulink对后扭力梁结构左、右轴头载荷迭代仿真具有较高的准确性，误差控制在10%。4、采用准静态、直接积分和模态叠加法分析后扭力梁结构的动应力，重点研究了关键部位横梁的动应力变化。通过分析各方法对振动疲劳动应力计算精度，对比了三种方法的优缺点和适用性。最后，通过整车台架应变试验对数值分析结果进行验证。试验表明准静态法难以直观表达结构的实际动应力，而模态叠加和直接积分法能较准确描述结构动应力。5、考虑了结构尺寸和加工工艺等对材料疲劳特性曲线S-N的影响，经修正获得了部件的S-N曲线。基于准静态、直接积分和模态叠加法在时域范畴内对后扭力梁结构在不同路段的疲劳寿命进行数值仿真分析，找到对应路况下结构疲劳寿命薄弱区域，并通过台架应变试验对结果进行了验证。试验表明这三种方法均能精确定位结构疲劳薄弱区域，但准静态法不能准确预测结构振动疲劳寿命，而直接积分和模态叠加法可较为准确预测结构振动疲劳寿命。6、对后扭力梁结构的轴头载荷进行频谱分析，获得了轴头载荷的频域范围和能量分布信息。在频域内运用Dirlik方法预测结构疲劳寿命，获得了结构在全路段、共振三路段、石块路段工况下的疲劳寿命。为研究加载频率对结构动应力和疲劳寿命的影响，提出了采用放大因子来评价共振频率激励对结构动应力的贡献的研究方法。结果表明，等能量的共振频率激励使得结构产生的疲劳损伤约为非共振激励的4倍。更多还原

【Abstract】 When structure is suffering from dynamic load which is close to the naturalfrequency, stress level is lower than the static fatigue stress limit in the project. But itmay produce crack. In other words, the actual life of the structure is lower thanprediction results on the basis of the static fatigue analysis method. It is one ofproblems difficulty resolved by the traditional theory of fatigue life analysis so far.This paper has introduced the basic concept between vibration fatigue and thetraditional ordinary fatigue, and pointed out the primary difference whether theexcitation frequency covers work modal frequency between vibration fatigue and thetraditional ordinary fatigue. Aiming at vibration fatigue damage problem on reartorsion beam when a certain type of vehicle with rear torsion beam suspension is inservice condition, a method used to predict vibration fatigue life of complex structurewith rigid-flexible coupled was proposed in the paper. Based on vibration fatigueanalysis of a rear torsion beam which was a typical chassis component in a passengervehicle, key techniques for vibration fatigue analysis were described by usingmulti-body dynamics, finite element method and the strain operational modal test andanalysis, including load identification, operational modal analysis, numericalprediction method of dynamic stress and the influence analysis of dynamic stressfrom excitation frequency.1、The finite model of rear torsion beam was established and its free modalfrequencies were acquired. The three possible boundary conditions were studiedaccording to assembly relationship of rear torsion beam in working status. Theoperational modal parameters of rear torsion beam were identified by using thestochastic subspace method based on the strain modal test platform. Studies haveshown that the boundary condition of structure in the state of serving is morecomplicated.2、In order to solve the problem of insufficient data acquisition channel in largescale testing, the strain signals on structure surface were tested by two-direction straingauge instead of three-direction strain gauge. The principal stress calculation formula of thin-walled structure was conducted based on the strain gauge. The U-shapedthin-walled structure specimen was designed and processed. The numerical principalstress was verified by surface strain testing system of the thin-walled structure. Thestudy has shown the principal stress of thin-walled structure calculated based on thestrain gauge can reflect the real stress level, and provide reference for subsequentengineering application. Then the research result was applied into stress analysis ofrear torsion beam, and the correctness and precision of the method was validatedthrough the real vehicle road test.3、In the dynamic analysis of rear torsion beam, the nonlinear characteristics ofrubber lining tube in spindle head were considered. Based on displacement signal ofthe suspension spring on the real vehicle road test and multi-body dynamic model ofthe torsion beam with rigid-flexible coupled via the ADAMS of dynamic simulationsoftware, the load of left and right spindle noses of rear torsion beam were simulatedvia iterate. In order to verify the accuracy of the method, the load of spindle nose wasverified based on the test bench. Experiment has shown that the simulation resultshad high accuracy and the error was in10%.4、The quasi-static method, the direct-integral method and modal-superpositionmethod used to analyze the dynamic stress of structure were presented. And thedynamic stress of the key parts of transom was paid much attention. In the analysis ofdynamic stress, the relation between load frequency and the operational modalfrequency of structure was especially studied. The applicability of the method ofvibration fatigue analysis was analyzed. Finally the calculation results ware verifiedon the bench test of whole vehicle. Tests has showed that the quasi-static method isdifficult to directly express the actual dynamic stress of structure, but the modalsuperposition method and direct integration method can accurately describe thestructure dynamic stress.5、 Considering with the influence of material fatigue characteristic fromstructure size and processing technology, the S-N curve of part was obtained byamendment. The fatigue life analysis of rear torsion beam in time domain method ondifferent road was calculated by using the quasi-static method, the direct-integralmethod and modal-iteration method. And analysis results were verified by bench strain test. Test has showed that the quasi-static method can not accurately predictvibration fatigue life of structures, but the direct integral method and modalsuperposition method can accurately predict vibration fatigue life of structure.6、The frequency range and energy distribution information of load spectrum ofspindle nose was given through spectrum analysis of load of spindle nose. In order toget the frequency response characteristics of structure, a method used to impel the teststructure on spindle nose by way of sine sweep method in indoor test bench ispresented. Based on method of Dirlik, the fatigue life of rear torsion beam waspredicted by using fatigue frequency domain analysis method. The fatigue life of reartorsion beam was analyzed in the whole road, resonance-three section and therock-road. In order to study the influence of the dynamic stress and fatigue life fromload frequency, the zoom factor is presented, which is used to the contribution of thedynamic stress from resonance excitation frequency. The results have shown that thefatigue damage of structure from resonance excitation is about four times from thesame energy non-resonant incentive.更多还原