A Theoretical Study of Residual Stress Effects on Fatigue Life Prediction

Recently, the trend has been toward the use of the full subsurface stress field in rolling element bearing fatigue life prediction (stress field-based life models). By using the stress field-based bearing life models, more accurate assessments of such things as fitting practice and thermal treatments on the bearing performance are achieved. However, one aspect missing in most models has been the consideration of the changing residual stress during operation of the bearing. This study was conducted to investigate the time dependent residual stress on contact fatigue life predictions.

This study concluded that the changes in residual stress during operation were most likely a fatigue reaction of the material to the pre-fatigue residual stress and cyclic contact stress fields. The materials fatigue response changes the instantaneous values of the material constants in most stress field-based life equations, thus making them in-calculable. As such, the pre-fatigue residual stress field should be used in the stress field-based models.     

A Comparative Study of Residual Stress Distribution Induced by Hard Machining Versus Grinding

This study investigates the residual stress distribution induced by hard machining and grinding and compares its impact on fatigue parameters. The residual stress distribution below hard turned and ground surfaces is investigated after a thermally damaged layer is removed. Fatigue parameters are computed based on the residual stress distribution to compare the impact of the residual stress distribution on the fatigue performance. Rolling contact fatigue tests are then performed to substantiate the computations. The effect of residual stresses on crack initiation depth is shown to be significant for the ground specimen. The maximum shear stress at crack initiation depth of the hard turned specimen is smaller than that of the ground specimen. Due to a significant increase in crack initiation life, the predicted rolling contact fatigue life of the hard turned specimen is longer than that of the ground specimen. The overall average in the ratios of predicted life to experimental life for the hard turned specimen is closer to 1 than that for the ground specimen. The results demonstrate that the hard turned specimen shows better rolling contact fatigue performance and better accuracy in the fatigue life prediction.     

氮化硅陶瓷球滚动接触疲劳寿命模型

针对球与圆柱接触模型中的陶瓷球,利用WEIBULL断裂统计方法导出球疲劳失效概率与寿命之间的关系方程,在相关额定寿命与最大接触应力的数值解基础上,基于最大主拉应力,构建氮化硅陶瓷球的滚动接触疲劳寿命与接触应力的数学模型.经与不同接触应力水平下的滚动接触疲劳寿命试验结果验证,表明该拉应力一寿命模型的正确性,从而验证了氮化硅陶瓷球的滚动接触疲劳失效源于最大主拉应力,而非基于最大切应力的设想.通过与L.P切应力一寿命模型预测结果的比较,表明拉应力一寿命模型适合于陶瓷球的接触疲劳寿命预测.     

Numerical Investigation on the Effects of Machining-Induced White Layer during Rolling Contact

It is well known that a thin phase-transformed white layer can be formed on component surfaces produced by hard machining. However, it is not clear as to how the white layer affects component performance, for example, in rolling contact fatigue. This study aims to determine the effects of white layer and associated residual stress on rolling contact stresses and strains. It is nearly impossible for an experimental study to identify the effects of white layer alone on rolling contact. Furthermore, small-scale contact stresses and strains (less than 30 μm) of the phase-transformed region are difficult to measure using the current experimental techniques. Therefore, a finite element analysis simulation model of rolling contact incorporating machining-induced surface integrity has been developed in this study. Three cases were investigated to decouple the effects of surface integrity factors: surface with white layer only, surface with residual stress only, and surface with white layer and residual stress. The simulation results show that distinct material properties of the white layer significantly influence the magnitudes and distributions of near-surface stresses and strains instead of those in the subsurface. Furthermore, it can be inferred that the white layer would affect near-surface fatigue damage instead of subsurface fatigue damage. The simulated near-surface fatigue damage mechanisms have been substantiated by the fatigue test data.     

Critical stresses in rolling contact fatigue

An analysis of the main hypotheses concerning the critical stresses in rolling contact fatigue is presented. It is considered that none of these hypotheses correlates adequately with all the experimental aspects of the phenomenon of rolling contact fatigue. A new hypothesis is proposed; this hypothesis states that the equivalent stress is the critical stress in rolling contact fatigue. By considering the influence of the residual stresses it is shown how the optimum fatigue life in rolling contact can be achieved.     

FE-simulation of the effects of machining-induced residual stress profile on rolling contact of hard machined components

Compared with grinding, hard turning may induce a relatively deep compressive residual stress. However, the interactions between the residual stress profile and applied load and their effects on rolling contact stresses and strains are poorly understood, and are difficult to measure using the current experimental techniques due to the small-scale of the phenomena. A new 2-D finite element simulation model of bearing rolling contact has been developed, for the first time, to incorporate the machining-induced residual stress profile instead of only surface residual stresses. Three cases using the simulation model were assessed: (a) measured residual stress by hard turning, (b) measured residual stress by grinding, and (c) free of residual stress. It was found that distinct residual stress patterns hardly affect neither the magnitudes nor the locations of peak stresses and strains below the surface. However, they have a significant influence on surface deformations. The slope and depth of a compressive residual stress profile are key factors for rolling contact fatigue damage, which was substantiated by the available experimental data. Equivalent plastic strain could be a parameter to characterize the relative fatigue damage. The magnitudes of machining-induced residual stress are reduced in rolling contact. The predicted residual stress pattern and magnitude agree with the test data in general. In addition, rolling contact is more sensitive to normal load and residual stress pattern than tangential load.     

Rolling/sliding contact fatigue life prediction of sintered and hardened steels

Among the several competing mass production methods, powder metallurgy (PM) techniques give much contribution as a means of saving material and energy. It also delivers the material with good fatigue properties that are good substitute for the conventional materials. Application of PM parts is getting wider in many areas such as automotive parts, agricultural equipment, business machine, electrical and electronics, hardware and industrial applications, etc. Many structural machine components made of PM route like gears and bearings experiences rolling–sliding contact fatigue conditions (RSCF). Contact fatigue is a type of failure commonly found in the surface of bearings, gears etc., and this type of failures decide the useful life of these structural parts. Presence of friction between the bodies normally encountered in service, many influence the contact fatigue behavior. The use of new materials and in particular sintered materials requires a thorough understanding of contact fatigue behavior for better product design and development. RSCF on sintered and hardened of 7 gm/cc density (FLC 4608-110HT, Fe–Ni PM steel) were carried out in a specially designed test rig for different contact parameters such as contact stress and slide to roll ratio. Twin-disc type rolling/sliding experimental set-up has been developed in laboratory to simulate the sliding and rolling between two rollers. Contact stress distribution and RSCF life prediction models suitable for sintered and hardened steel were also developed. The predicted life was compared with the experimental RSCF life for sintered and hardened steel.     

A Model for Rolling Bearing Life with Surface and Subsurface Survival—Tribological Effects

Until now the estimation of rolling bearing life has been based on engineering models that consider an equivalent stress, originated beneath the contact surface, that is applied to the stressed volume of the rolling contact. Through the years, fatigue surface–originated failures, resulting from reduced lubrication or contamination, have been incorporated into the estimation of the bearing life by applying a penalty to the overall equivalent stress of the rolling contact. Due to this simplification, the accounting of some specific failure modes originated directly at the surface of the rolling contact can be challenging. In the present article, this issue is addressed by developing a general approach for rolling contact life in which the surface-originated damage is explicitly formulated into the basic fatigue equations of the rolling contact. This is achieved by introducing a function to describe surface-originated failures and coupling it with the traditional subsurface-originated fatigue risk of the rolling contact. The article presents the fundamental theory of the new model and its general behavior. The ability of the present general method to provide an account for the surface–subsurface competing fatigue mechanisms taking place in rolling bearings is discussed with reference to endurance testing data.     

滚动轴承疲劳寿命的影响因素

对影响滚动轴承疲劳寿命的各种因素进行了综述，包括各种疲劳诱导应力(最大切应力、最大动态切应力、Von Mises应力和八面切应力)，切向力、残余应力和环向应力疲劳极限应力、表面粗糙度、弹流润滑、表面处理、润滑油中污染颗粒、温度、速度、不失效寿命和钢材纯净度(氧含量)的影响，为轴承寿命的准确预测提供参考。     

Explicit finite element modeling of subsurface initiated spalling in rolling contacts

An explicit finite element model was developed to investigate crack initiation and spall formation in machine elements subject to rolling contact fatigue. The modeling approach utilizes continuum damage mechanics to capture the initiation and propagation of fatigue damage that leads to the formation of a surface spall. The material microstructure is modeled via a randomly generated Voronoi tessellation. The material parameters for the model were obtained independently from torsional fatigue life data for 52100 bearing steel. The life scatter (Weibull slope) and the spall geometry obtained from the model correlate well with experimental results available in the open literature.     

Rolling contact fatigue of case-hardened chromium steel

The rolling contact fatigue of case-hardened steel surfaces in lubricated heavily loaded contact was studied. Three different case-hardening treatments were tested with a ratio of slide to roll of ?5%. Other ratios of slide to roll were used to determine the effect of tangential traction on the rolling contact fatigue endurance. When the actual contact width was measured after testing, the scatter of the fatigue results was reduced. The depth of the maximum plastic strain was determined by measuring the hardness before and after testing and was found to correspond to the occurrence of the lowest ratio of the critical shear stress to the amplitude of the load-induced orthogonal shear stress. The role of residual stresses in rolling contact fatigue is discussed. It was found that a more detailed knowledge of lubricant behaviour in heavily loaded contacts is needed to reveal the true distribution of tangential traction on the contact surface. This affects the angle of the plane and the value of the maximum amplitude of the shear stress beneath the contact zone.     

Ring crack propagation in silicon nitride under rolling contact

Silicon nitride has been found to have the optimum combination of properties which are suitable for rolling element bearing applications to withstand high loads, severe environments, and high speeds. Surface ring cracks are difficult to detect but are found on the surface of silicon nitride balls. These ring crack defects decrease the rolling contact fatigue life considerably. This paper presents an experimental study and numerical analysis of ring crack propagation in rolling contact. The contribution of this study is to provide understanding of ring crack propagation behaviour and life prediction in rolling contact. Rolling contact tests are performed on the silicon nitride/steel elements. Silicon nitride ball surfaces are examined before testing using a dye-penetrant technique and optical microscopy. The surfaces are examined using optical microscopy and scanning electron microscopy during testing and after failure. The numerical calculations are based on a 3D model of ring crack growth. The rolling contact loading is simulated by a repeated Hertzian surface load with normal pressure and tangential traction. Fracture mechanics analysis is utilised to determine the stress intensity along the crack front and the stress intensity factors are analysed using a 3D boundary element model. Life predictions from the present calculations are in line with the experimental observations.     

基于两参数Weibull分布的概率疲劳S-N曲线模型

发展了基于两参数Weibull分布的概率疲劳S-N曲线模型及构建方法,用于描述随机滚动接触疲劳应力幅-寿命关系。除考虑数据分散性的统计分布规律外,模型还考虑样本数量相关的寿命置信度预测效应, 引入最小正态寿命的置信下限评价。Weibull分布的尺度参数和形状参数,寿命置信度评估参数,以及概率S-N曲线的指数,全部由试验数据来确定。G20CrNi2Mo轴承钢的滚动接触疲劳试验数据的分析证实所提出方法的可应用性。并且证实该滚动接触疲劳寿命的分布规律,宜采用包括Weibull分布在内的有偏统计模型来描述。     

A Mathematical Model of Spalling Fatigue Failure in Rolling Contact

Variables affecting the fatigue life of a rolling contact are identified. A mathematical model of sub-surface and surface crack propagation is presented. The life to failure of volume elements in the vicinity of a defect (defect life) is formulated. A term “severity” of a microdefect has been defined. The model is characterized by the inclusion of bulk material parameters, defect characteristics and parameters of geometry, stress, lubrication and surface topography. A statistical expression for the life of an entire rolling body is based on the defect life formula. The new model comprises current standard bearing life prediction formulas as a special case.     

Numerical modeling of sub-surface initiated spalling in rolling contacts

A 3D finite element model was developed to investigate the influence of microstructure topology on the stochastic nature of rolling contact fatigue. Grains of the material microstructure are modeled with random Voronoi tessellations. Continuum damage mechanics and mesh partitioning are implemented to capture the initiation and propagation phases of fatigue damage that lead to spalling. Simulated fatigue spalling is shown to progress similarly to experimental observations of rolling contact fatigue. The fatigue lives obtained with the model exhibit scatter on par with empirical measures and are fit well by 2 and 3-parameter Weibull distributions.     

Changes in the stress field in the elastohydrodynamic contact zone due to some operating factors and their possible role in the rolling contact fatigue of cylindrical surfaces

A theoretical analysis has been carried out of changes in the stress field in the elastohydrodynamic contact zone of cylindrical surfaces due to operating variables. Their possible role in rolling contact fatigue has been assessed by accelerated rolling contact fatigue tests. The results show that changes of the elastohydrodynamic pressure distribution in the contact zone associated with increase of the viscosity-velocity parameter induce considerable changes in the stress field in the contact zone. The poor correlation of rolling contact fatigue life with material effects according to elastohydrodynamic theory and the considerable changes in rolling contact fatigue life due to lubrication effects suggests that the explanation lies outside elastohydrodynamic theory and possibly in the theory of asperity lubrication.     

焊接残余应力对桥壳疲劳寿命的影响研究

桥壳作为驱动桥的核心零部件,其疲劳寿命对驱动桥乃至整车安全性有决定性的影响,对于制造过程中使用焊接工艺的桥壳,焊接残余应力的影响不容忽略。以某商用车驱动桥桥壳为研究对象,在获得其焊接残余应力分布的基础上,分析焊接残余应力对桥壳在静态载荷和动态循环载荷工况下应力应变响应的影响。使用应变-寿命分析方法对桥壳在弯曲疲劳试验工况下的寿命进行预测,并与台架试验结果进行对比,结果表明考虑焊接残余应力时,疲劳寿命次数和破坏位置的预测结果与试验结果吻合较好,验证桥壳疲劳寿命预测模型的准确性。与不考虑焊接残余应力的模型相比,焊接残余应力导致桥壳疲劳寿命次数降低,且失效位置不同,说明了疲劳寿命预测时考虑焊接残余应力的必要性。本文方法可推广应用于含有焊接残余应力的结构疲劳寿命预测,为结构优化设计提供指导。     

考虑残余应力的螺旋锥齿轮接触疲劳裂纹萌生-扩展寿命计算方法研究

考虑渗碳、磨齿、喷丸等工艺产生的齿面残余应力,建立齿面接触应力与残余应力的复合应力场,提出一种螺旋锥齿轮接触疲劳裂纹萌生-扩展寿命计算方法。构建齿轮有限元接触分析模型,计算多轴交变接触应力场。考虑空间螺旋曲面残余应力分布的复杂性,将变曲率齿面离散为网状节点;测量各节点表面与次表面的残余应力,建立齿面残余应力场。基于Dang Van多轴疲劳准则,构建齿面裂纹萌生模型;计及残余应力与裂纹闭合效应,构建齿面裂纹扩展模型。计算复合应力场下齿轮接触疲劳寿命,研究残余应力对齿面裂纹萌生-扩展寿命的影响规律。结果发现：复杂齿面空间变曲率会影响喷丸等工艺产生的残余应力分布,中心区域的残余压应力高出齿面边缘区域约20%;复合应力场下齿面裂纹萌生位置与寿命主要取决于接触应力,残余应力会改变齿面节点平均应力进而影响疲劳寿命;齿面裂纹扩展寿命约占全寿命的10%,表征齿轮接触疲劳快速失效至迅速断裂。上述研究对于高性能齿轮传动的长寿命、高可靠性设计具有一定的参考价值。     

Probabilistic fatigue life prediction methodology for notched components based on simple smooth fatigue tests

The methodology of probabilistic fatigue life prediction for notched components based on smooth specimens is presented. Weakestlink theory incorporating Walker strain model has been utilized in this approach. The effects of stress ratio and stress gradient have been considered. Weibull distribution and median rank estimator are used to describe fatigue statistics. Fatigue tests under different stress ratios were conducted on smooth and notched specimens of titanium alloy TC11. The proposed procedures were checked against the test data of TC11 notched specimens. Prediction results of 50 % survival rate are all within a factor of two scatter band of the test results.     

Particle Damage in Hertzian Contacts and Life Ratings of Rolling Bearings

Particle denting, and contamination marks found on bearing raceways, can induce stress concentrations and facilitate surface initiated fatigue. The lubricant film developed at the dent and related local surface stresses are also significant to the crack initiation mechanism. In this article, a new methodology is presented to link the micro-EHL film and related local stresses to the fatigue life of rolling bearings. The applied methodology is based on Fourier analysis of the harmonic components of the surface microgeometry to predict stresses and induced lubricant film. The application of this method to actual bearing surfaces is discussed and analyzed in relation to some existing microcontact EHL solutions. A global evaluation of the use of the method to rolling bearings dynamic load ratings is also carried out. A comparison between experimentally obtained rolling bearing life and lives predicted using the present theory indicate the global ability of the model to describe the effect of the lubrication quality on the life expectancy of rolling contacts. From this analysis, an assessment of some typical equations used in rolling bearing dynamic ratings is carried out. It is found that the degree of lubrication of the rolling contact and the cleanliness conditions of the oil are indeed significant to the prediction of the life expectancy of the bearing.