基于多轴疲劳准则的齿轮点蚀寿命预测

综合齿轮动力学和弹性流体动力润滑理论,建立基于临界平面法多轴疲劳寿命预测模型.首先根据齿轮啮合特性获取齿面接触的时变参数,采用平均滤波方法模拟齿面磨合后的粗糙状态,并将齿面粗糙形貌带入油膜厚度计算,并基于量纲化差分方法建立齿轮的热弹流动力润滑模型;随后,通过润滑界面压力和摩擦力的分布计算近表面应力状态,确定接触近表面任意平面的应力与应变幅;最终,采用临界平面方法计算Smith-Watson-Topper(SWT)参数和最易萌生裂纹的平面,最终确定齿轮疲劳点蚀寿命,并试验验证模型有效性.结果表明：粗糙表面造成压力、油膜厚度和温度等波动较大,最大应力集中分布在表面,疲劳点蚀的微裂纹首先在表面萌生;齿轮疲劳点蚀数值模型可有效预测不同润滑条件下的疲劳点蚀寿命.     

斜齿轮弹流润滑下的接触疲劳寿命计算

经典齿轮接触疲劳强度理论是基于光滑表面赫兹干接触理论,而实际齿面具有粗糙度,且啮合轮齿多数处于混合润滑状态.本文基于齿轮润滑接触分析建立了渐开线斜齿轮的接触疲劳寿命计算模型.模型由齿轮润滑接触分析模型和基于次表面应力分布的疲劳寿命模型组成.首先将斜齿圆柱齿轮一对齿的瞬时啮合等效为两反向圆锥的接触问题,建立了齿轮的有限长弹流润滑计算模型,考虑了齿轮啮合周期内瞬时载荷、接触线长、卷吸速度等因素的影响,基于统一雷诺方程方法求得啮合齿对间的润滑压力和油膜厚度分布;在此基础上,计算轮齿接触区次表面的米歇斯应力分布,根据Zaretsky接触疲劳寿命计算模型,对齿轮组的接触疲劳寿命进行模拟预测.针对不同工况参数下接触疲劳寿命计算表明:润滑油黏度、轮齿表面粗糙度等因素对齿面接触疲劳寿命均有显著的影响.     

渐开线直齿轮弹流润滑条件下的多轴疲劳寿命预估

本文中基于弹流润滑分析和次表面应力建立了渐开线直齿轮多轴疲劳寿命计算模型.相对于传统的单轴疲劳模型,考虑了齿轮固定点的应力历史和材料属性对疲劳寿命的影响,并可以得到齿轮在完整啮合过程中的寿命分布.首先建立齿轮的有限长弹流计算模型,得到齿轮啮合过程中的油膜压力和油膜厚度,再根据油膜压力计算出次表面的应力分布;通过分析齿轮计算区域随啮合过程移动的关系,得到固定点的应力历史,再根据基于应力历史的多轴疲劳寿命模型对齿轮的完整啮合过程进行寿命预估.计算分析了不同粗糙度幅值对轮齿各点寿命大小和分布的影响.研究表明:齿面粗糙度对疲劳寿命的影响显著,随着粗糙度幅值的增大,表层下最大应力向齿面移动,导致低疲劳寿命区向齿面发展且逐步扩展到整个单齿啮合区;而表面粗糙度降低到一定程度则对疲劳寿命的影响变得不明显.     

混合润滑下短齿啮合对行星齿轮接触疲劳的影响

少齿差行星齿轮为避免齿顶干涉,通常会减小齿高,这可能会导致齿面实际接触宽度小于理论赫兹接触宽度,降低齿面接触强度.鉴于此,为研究少齿差行星传动短齿制对齿轮接触疲劳的影响,综合考虑了轮齿接触宽度、楔形间隙、齿宽有限长和齿面粗糙度等因素,建立少齿差行星齿轮短齿啮合的混合润滑统一方程,求解出啮合齿对间的压力分布、摩擦系数和轮齿接触区次表面应力分布,根据Zaretsky接触疲劳寿命计算模型,对不同工况下不同啮合位置的轮齿接触疲劳寿命进行预测.结果表明:接触宽度在少齿差行星齿轮的疲劳寿命预测中不容忽视,短齿啮合模型下的楔形间隙对啮入和啮出过程的疲劳寿命有不同影响.     

缺口件多轴疲劳寿命预估新方法

局部应力应变法(LSSM)以危险点处的局部应力应变历程进行寿命评价,忽略了非比例附加强化及缺口附近应力梯度对疲劳损伤的影响.论文通过分析裂纹萌生面(临界面)上剪应力和正应力分布状态,提出一种计算缺口件多轴疲劳损伤影响区的数学计算方法,建立考虑法向正/剪应力梯度与非比例附加强化效应的寿命预估模型.首先,基于能量法和临界面理论,借助坐标变换原理将应变能密度最大的材料平面定义为临界面,建立确定裂纹萌生方向的计算方法.其次,以临界面上特定路径为积分方向,将缺口件三维问题转化为线性问题,简化计算过程.综合考虑峰值应力和等效应力场强对多轴疲劳寿命的贡献度,建立表征缺口件多轴疲劳损伤演化过程的损伤参量.最后,通过TC4合金和Q345钢多轴疲劳试验验证论文模型的可行性和精确度,并与LSSM法、FS模型、SWT模型进行对比分析.     

一种经验型疲劳小裂纹扩展模型

金属材料疲劳寿命由裂纹萌生和裂纹扩展寿命两部分组成，其中对于萌生寿命中的小裂纹分析是精确描述裂纹萌生寿命的关键．而小裂纹在扩展过程中由于尺寸相对较小，导致传统线弹性断裂力学预测方法失效，需要对其进行改进，考虑裂纹尖端塑性区引起的残余压应力对小裂纹扩展速度的影响．本文针对此问题进行了初步分析，通过对塑性区引起的残余应力的量化，结合小裂纹门槛值特性，提出了一种经验型修正的小裂纹扩展模型，用于定量预测裂纹的萌生寿命．使用铝合金6082-T6缺口试样进行了疲劳实验，并与理论结果进行了对比，验证了所提模型的有效性．     

渗碳淬硬层疲劳裂纹闭合模型

本文在Newman裂纹闭合模型的基础上,以裂纹面附加残留位移为主要线索,综合考虑了渗层组织、强度、初始残留应力分布及残余奥氏体转变对裂纹闭合行为的影响,提出了一个渗碳硬层疲劳裂纹闭合计算模型。文中利用该模型对疲劳裂纹扩张过程中卸载后裂纹面接触应力的分布、渗层裂纹闭合效应进行了成功地计算;借助于实测的渗碳层da/dn—ΔK_(eff)关系,较好地预测了渗层裂纹扩张速率的变化规律;并在定量分析残余奥氏体转变、初始残留应力分布对闭合行为的影响方面进行了有益的尝试。     

基于多因素的焊接结构疲劳裂纹扩展速率分析

以Donahue等提出的疲劳裂纹扩展速率计算模型为基础,通过引入形状系数、张开比和残余应力等参数,建立了适用于焊接结构的疲劳裂纹扩展速率计算模型,分析了多种因素对焊接结构疲劳裂纹扩展速率的影响规律。结果表明,焊板厚度和焊缝余高的变化均会对焊接结构疲劳裂纹的扩展速率产生影响,在对焊接结构表面形状进行设计时应保有一定的焊缝余高;有效应力比的增大会降低焊接结构疲劳裂纹的扩展速率,且裂纹深度的变化不会改变有效应力比对焊接结构疲劳裂纹扩展速率的影响;残余应力的增大会提高焊接结构疲劳裂纹的扩展速率,且残余应力对疲劳裂纹扩展速率的促进作用随着裂纹深度的增加而增大,在对焊接结构的疲劳性能进行设计时须考虑残余应力对结构性能的影响。     

高强度钢的超高周疲劳裂纹扩展模型研究

当疲劳寿命在106或107周时,Wöhler S-N 曲线被看作渐近于水平轴,107的疲劳强度被看成是疲劳极限。现代应用要求延长零件的工作寿命,实际齿轮部件应用超过107循环的疲劳失效。本文应用压电超声疲劳试验机对经过热处理和渗碳处理后的低铬合金钢材料进行研究,采用红外摄像仪观测试件表面的温度场随疲劳裂纹萌生和扩展的过程。试验条件是室温,应力比为0.1（R=0.1）,频率为20KHz。通过对表面渗碳处理后试件的断口分析,探讨表面渗碳处理、微观结构和与杂质有关的断裂机理,根据Paris公式建立超高周疲劳裂纹扩展模型。对裂纹扩展过程中裂纹尖端的塑性区的分析结果,结合传热学原理,建立热耗散模型,有限元方法的数值解结果较好地符合红外摄像仪的观测的试验结果。     

谐振载荷作用下工程结构振动疲劳寿命预估的损伤力学-有限元法

建立了预估谐振载荷作用下结构振动疲劳寿命的损伤力学-有限元方法。首先根据损伤热力学原理,构建了损伤演化方程,建立光滑试件在恒幅应变交变载荷作用下寿命预估方法;进一步由损伤力学守恒积分原理,得到恒幅重复载荷作用下应力与寿命的关系式;然后根据标准件疲劳试验结果,拟合得到损伤演化方程中的材质参数;最后利用APDL语言编程对ANSYS软件进行了二次开发,借助ANSYS软件对谐振载荷作用下结构振动疲劳裂纹萌生寿命进行预估。作为算例,本文利用该方法预估了LC9CgS1铝合金梁谐振载荷作用下疲劳裂纹萌生寿命。     

齿向修形对滤波减速器润滑性能的影响分析

综合考虑了滤波减速器齿向修形参数、真实齿面粗糙度和瞬态效应等因素,建立了轮齿混合润滑数学模型,数值计算了不同修形参数值对应不同啮合点的最大压力和中心膜厚,分析了齿面粗糙度和转速对润滑性能的影响.结果表明:修形参数r和Ry均存在一个优化范围,使得轮齿表面最大油膜压力显著降低,边缘效应弱化,而中心膜厚则随着r和Ry的增大而逐渐增大;未修形轮齿边缘油膜压力受粗糙度的影响而急剧增大,边缘效应更加显著,修形后轮齿的边缘效应得到了明显改善,因此,轮齿修形也因粗糙表面的存在而显得更加重要;随着转速逐渐降低,轮齿表面的平均油膜厚度逐渐变小,接触比逐渐增大,轮齿表面由弹流润滑逐渐转为混合润滑,最后演变为边界润滑.     

Cracking simulation of a tubesheet under different loadings

For shell-and-tube heat exchangers, tubesheet cracking is a major failure form. Owing to complicated structures, loadings and environments, mechanisms for the crack nucleation and propagation often puzzle engineers and as a result, it is hard to take effective measures to prevent this kind of failure from happening again. In this paper, three dimensional finite element models were established to investigate a real tubesheet cracking with the emphasis on the driving forces for the crack propagation from a fracture mechanics point of view. Three different loadings, namely residual expansion stress, crack face pressure and transverse pressure, and three crack growth patterns were considered. In order to obtain the residual stresses, the hydraulic expanding process of tube-to-tubesheet joint was simulated. Residual contact pressures between the tube and tubesheet and the induced residual stress distributions in the tubesheet were computed. The possibility for crack propagation in the tubesheet under the action of the different loadings was investigated in terms of the strain energy density factor. Results show that surface crack propagation may be driven by all the three loadings especially the transverse pressure. But when surface cracks come into the interior of the tubesheet along the thickness, as acted along the whole tubesheet thickness, the residual expansion stress would play key roles in crack propagation.     

Dynamic characteristic analysis of spur gear system considering tooth contact state caused by shaft misalignment

The effect of gear contact state change due to shaft misalignment on meshing stiffness is usually neglected in the traditional stiffness calculation model with misalignment error, the further influence mechanism of shaft misalignment on gear dynamic characteristics is also unclear. To address these shortcomings, a new mesh stiffness calculation model with misaligned gear considering the effects of tooth contact state is proposed by combining the improved loaded tooth contact analysis (LTCA) model. Then the effects of tooth contact state changes aroused by shaft misalignment on the meshing stiffness excitation are investigated. Moreover, a dynamic model of the misaligned gear system with 8 degrees of freedom (DOF) is established, and based on which the dynamic characteristics of the gear system are investigated and verified by experiment. The study results indicate that the proposed model can be used to evaluate the stiffness excitation and dynamic characteristics of the misaligned gear system with the tooth contact state taken into consideration. This study can provide a theoretical method for evaluating and identifying shaft misalignment errors.

     

Elastic-viscoplastic field at mixed-mode interface crack-tip under compression and shear

For a compression-shear mixed mode interface crack, it is difficult to solve the stress and strain fields considering the material viscosity, the crack-tip singularity, the frictional effect, and the mixed loading level. In this paper, a mechanical model of the dynamic propagation interface crack for the compression-shear mixed mode is proposed using an elastic-viscoplastic constitutive model. The governing equations of propagation crack interface at the crack-tip are given. The numerical analysis is performed for the interface crack of the compression-shear mixed mode by introducing a displacement function and some boundary conditions. The distributed regularities of stress field of the interface crack-tip are discussed with several special parameters. The final results show that the viscosity effect and the frictional contact effect on the crack surface and the mixed-load parameter are important factors in studying the mixed mode interface crack- tip fields. These fields are controlled by the viscosity coefficient, the Mach number, and the singularity exponent.     

渐开线斜齿轮非稳态弹流润滑数值模拟研究

建立了渐开线斜齿轮啮合的弹流润滑计算模型,将斜齿圆柱齿轮啮合的齿面接触等效为有限长线接触的弹流润滑问题.考虑斜齿轮啮合的实际因素,将斜齿轮啮合过程中的等效曲率半径和齿面载荷的变化反映到弹流润滑计算模型中,应用统一Reynolds方程方法求得轮齿在1个完整啮合周期内的瞬时弹流润滑数值解.结果表明：斜齿轮啮合线上各点处的膜厚、压力均有较大不同,各接触点处的油膜厚度受综合曲率半径的影响较大;斜齿轮传动非稳态效应相对较弱;小齿轮齿根附近和节点位置处润滑状态较差;适当增大压力角可以改善齿轮的润滑.     

基于载荷分担理论的渐开线斜齿轮热混合弹流润滑分析

沿接触线把斜齿轮分成许多小薄片,每一薄片看成具有当量角速度的直齿轮,根据欧拉方程得到任一接触点处的曲率半径和表面速度.然后基于载荷分担、弹流润滑和粗糙线接触理论,建立了考虑表面粗糙度的斜齿轮传动混合热弹流润滑模型.研究了斜齿轮传动稳态载荷分布下牛顿流体和Carreau流体时的润滑特性.结果表明:牛顿流体和Carreau非牛顿流体模型下,中心油膜厚度、油膜承载比例、油膜温升随时间和接触线的变化规律相同.牛顿流体下的摩擦系数较工程实际偏大.Carreau非牛顿流体模型下摩擦系数和工程实际相符,其随接触线啮合位置的变化规律与油膜厚度正好相反.     

Study on safety characteristics of the spur gear pair considering time-varying backlash in the established multi-level safety domains

Nonlinear Dynamics - Multi-state meshing characteristics induced by backlash and contact ratio in the gear pair are an unsolved puzzle about its relationship to safety characteristics of the gear...     

Effect of plastic zone on the fatigue crack propagation behavior between two fatigue cracks

In this study, the fatigue crack propagation behavior in the stress interaction field between two different fatigue cracks is studied by experiment and finite element analysis. In the experiment, the offset distance between two cracks and the applied stress are varied to create different stress interaction fields. The size of the plastic zone area is used to examine the crack propagation path and rate. Three types of crack propagation in the interaction field were found by experiment, and the crack propagation behavior of two cracks was significantly changed as different stresses were applied. The size of the plastic zone obtained by finite element analysis can be used to explain crack propagation behavior qualitatively.