The use of notch and short crack approaches to fretting fatigue threshold prediction: Theory and experimental validation

In this paper the similarities between fretted and notched components in terms of stress gradient and the consequent “size effect” are discussed. Critical distance and short crack arrest approaches for the prediction of fretting fatigue thresholds are then presented and the predictions are compared with experimental results. Two geometry and alloy combinations are considered in order to validate the prediction by the means of experimental results. In particular, both Hertzian fretting tests performed on Al4%Cu alloys and experiments carried out employing ‘flat and rounded’ contact pads, made of Ti6Al4V alloy, are used for the comparison. It is shown that both criteria provide good predictive capabilities. However, the short crack arrest method is less empirical and can be adapted to a wider range of applications (e.g. surface treated components).     

The effect of rubber contact on the fretting fatigue strength of railway wheel tire

The cause of the ICE train derailment, which occurred in 1998 at Eschede, was fatigue failure originating on the inside of the wheel tire. Rubber-sprung resilient wheels were used for the trailer cars. The wheel tire is mounted on the wheel disc. Thirty-four rubber pads were arranged between the wheel disc and the wheel tire. It was postulated that fretting fatigue between the rubber block and the inner side of the tire might have an influence on the initiation of the incipient crack. In order to clarify the influence of the rubber contact on the fatigue strength of the tire, fretting fatigue experiments under rubber contact conditions were performed. During the fundamental fretting fatigue test using bridge pads and small size carbon steel specimens, no typical fretting damage such as fretting wear and minute cracks were observed due to contact of the rubber. Stress conditions of the rubber-sprung wheel under vertical and lateral wheel loads were evaluated by a three-dimensional elastic stress analysis. Since the rubber is a super-elastic material, the Mooney-Rivlin model was used in the FEM calculation. It was found that the wheel tire is subjected to a cyclic stress during one revolution of the wheel and the maximum stress occurred at the center of the inner surface of the tire where the fatigue crack initiated. Fatigue strength of the wheel tire was determined by the rotating bending fatigue testing of specimens taken from the tire. It was found that the tire with an 862 mm diameter at a wheel load of 80 kN had a safety factor more than 3.5 from a fatigue limit diagram with a failure probability of 0.01. To confirm the fretting damage under the rubber contact and the result of the fatigue strength evaluation, fatigue tests of a full size wheel were made. After 20 million cycles at the wheel load of 280 kN, which was just below the endurance limit estimated by the endurance limit diagram, no fretting damage and no fatigue cracks were observed. The wheel was, however, fractured at 1.56 million cycles under the maximum load of 308 kN, which was just above the endurance limit. The estimation of the safety factor of 3.5 estimated from the endurance diagram was confirmed by the full size fatigue testing. It was concluded that there was no effect of fretting due to the rubber contact on the fatigue strength of the rubber-sprung single-ring railway wheel.     

切应力状态下疲劳短裂纹的试验研究

通过扭转疲劳试验和塑料复型膜技术研究了中碳钢试样的疲劳短裂纹特性。疲劳短裂纹发生在长度方向与最大切应力相近的片状铁素体中，并在初期迅速扩展。但当其达到晶界时，裂纹扩展速度降低，甚至停止增长，成为不扩展裂纹。只有当应力强度因子满足ＬＥＦＭ条件时，裂纹才会克服晶界障碍继续扩展，最后形成主导裂纹导致试样破坏。疲劳断裂包含有全部三种断裂模式。     

Propagation in fretting fatigue from a surface defect

This paper analyses the growth of cracks in fretting fatigue from an initial flaw at the surface. Different crack growth laws are used in order to take into consideration the particular behaviour of short cracks. This methodology is applied to estimate life in various fretting fatigue tests with spherical contact characterized by two different geometries. The material used in the experiments is Al7075. The two geometries present significant differences in the evolution of the stresses, crack growth, etc. which are discussed. The approaches used to model short crack growth give different results, some of them being in good agreement with the experiments.     

机车电机小齿轮轴油槽结构优化的有限元分析

牵引电机小齿轮轴的失效是其油槽边缘处的旋转弯曲微动疲劳所致。采用ABAQUS有限元软件,模拟倒圆曲率半径对小齿轮轴的疲劳寿命的影响,并利用SWT多轴疲劳准则对结构优化的结果进行了分析。有限元分析结果与失效分析的结果有很好的一致性;SWT参数可很好地预测微动疲劳裂纹萌生位置;适当增加小齿轮轴油槽的倒圆曲率半径,可以提高小齿轮轴的微动疲劳寿命。     

Fretting fatigue in hydrogen gas

To clarify the effect of hydrogen gas on fretting fatigue strength of the materials, which supposed to be used for hydrogen utilization machines, fretting fatigue tests were conducted in hydrogen gas. It is important to take fretting fatigue into account in strength design, because many fatigue failure accidents have occurred at joints or contact parts between components. As a part of the experiments, an austenitic stainless steel was focused in this paper. The material was SUS 304. Fretting fatigue strength in hydrogen gas decreased compared with that in air. Tangential force coefficient increased in the reverse order of fretting fatigue strength. Therefore, one of the reasons of the decrease of fretting fatigue strength was that tangential force was different depending on the environment. Absorption of hydrogen occurred during fretting in hydrogen gas was detected. The absorption could be considered as one of the causes of the decrease of fretting fatigue strength, since fretting fatigue life of pre-charged specimen was decreased and also the crack propagation threshold of short fatigue crack was reduced by hydrogen charge.     

Friction behavior of quenched and tempered steel in partial and gross slip conditions in fretting point contact

Tangential traction caused by friction in contacting surfaces is a major factor in fretting fatigue that increases stress levels and leads to a reduction in fatigue life. Friction in fretting contact was studied in partial, mixed and gross slip conditions on quenched and tempered steel. Measurements were made with sphere-on-plane contact geometry for polished and ground surfaces. Friction was evaluated from on-line energy ratio and, after the tests, from wear marks. A maximum friction coefficient of over 1.0 was measured at mixed slip zone with polished surfaces, whereas ground surfaces promote lower values in similar operating conditions. The friction coefficient dependence on load cycles and loading frequency is also presented and briefly discussed. The friction data and understanding thus gained is to be used for evaluation of crack initiation with the numerical fretting fatigue model.     

Quantitative analysis of fretting fatigue degradation in 7075-T6 aluminum alloy

Fretting fatigue failures are commonly observed in the aviation industry. The objective of this study was to understand the fretting fatigue mechanism by characterization of fretting fatigue degradation to gain insight into the process of crack formation from pits in 7075-T6 aluminum alloy. This paper focuses on the quantitative analysis of fretting fatigue degradation in terms of pit depths and dominant crack formation. For 60 percent of the specimens, the dominant crack nucleated from a pit other than the maximum-depth pit observed on the fracture surface.     

Modeling of geometry effects in fretting fatigue

The stress field that results from two bodies in contact is an important aspect that governs the fretting fatigue behavior of materials. Applied loads as well as contact geometries influence the contact stresses. The profile of an indenter and the boundary conditions provide sufficient information from which the surface tractions and the corresponding subsurface stresses have been calculated in a semi-infinite halfspace using singular integral equations. In this investigation, a numerical subroutine was developed to calculate the surface tractions and the corresponding surface and subsurface stresses of an arbitrary finite thickness infinite plate subjected to loading through a random indenter. The results from the detailed stress analysis of the contact region are required by both an initiation and fracture mechanics approach. While initiation criteria involving stress gradient fields, such as sharp notches and edges of contact in fretting fatigue, are not well established or agreed upon, stress intensity factor calculations using tools such as weight functions are more reliable. The stress intensity analysis, which is used to determine whether an initiated crack will continue to grow if it is above the threshold, depends on many variables in the stress analysis such as pad and specimen geometry, loading configuration and friction coefficient. The contact stress analysis has been used to determine equivalent stress parameters that are related to the initiation of a crack. Similarly the numerical subroutine for the contact stresses is used in conjunction with the stress intensity analysis to determine the influence of the geometry, loading configuration and friction coefficient on the stress intensity factor. Results from high-cycle fretting fatigue experiments are used to determine the threshold stress intensity factor for a given configuration. The combination of the numerical and experimental analysis is then used to develop a tool for high-cycle fretting fatigue based on a threshold approach involving a go–no go criterion.     

Fretting fatigue in dovetail blade roots: Experiment and analysis

A biaxial fatigue experiment is described which is capable of simulating the loading experienced by a dovetail blade root in an aircraft gas turbine. A comprehensive stress analysis of the experimental configuration has been undertaken and a semi-analytical approach has been developed to provide accurate estimates of surface tractions and subsurface stress fields. The forces (normal and shear) and moment for the semi-analytical model are calculated from the global finite element analysis of the dovetail in which coarse elements were used. The bulk stress and the bending stress in the disk and blade were evaluated by calculating the membrane and bending stresses across the thickness of the disk and blade. Short crack arrest methods have been applied to the results of the stress analysis to predict the fatigue performance of the blade specimens. The results show a good agreement with the experimental observations.     

New results for the fretting-induced stress concentration on Hertzian and flat rounded contacts

Recent work on fretting fatigue has emphasized the role of stress concentration on fretting damage, while previous work had concentrated on empirical parameters to assess influence of fretting on fatigue life. In particular, analogies with fatigue in the presence of a crack or a notch have been noticed, suggesting that the stress field induced by frictional contact per se may explain the reduction of fatigue life due to fretting.In the paper, new analytical and numerical solutions are produced for the stress concentration induced in typical fretting contacts involving the Hertzian geometry or the flat punch with rounded corners in view of application to the dovetail joints. Normal and tangential load (in the Cattaneo–Mindlin sense) is considered with “moderate” or “large” bulk stresses.     

Finite element model of the contact between a vibrating conductor and a suspension clamp

Aeolian vibrations may lead to failure of the overhead conductors of electrical transmission lines. Damages are caused by fretting fatigue at the attachment position of pieces of hardware. This phenomenon depends much on contact mechanics. The contact between a wire and a suspension clamp, a critical location, was modelled using the finite element method. Results from strain measurements on vibrating conductors served as input. The numerical results gave estimates of stresses and slip amplitudes. We can use these results to compute crack initiation criteria. The Ruiz and Chen criterion was chosen here and results compared well with experimental data.     

16MnR钢在不同应力比下的疲劳裂纹扩展的试验研究及模拟

采用3.8 mm厚带有圆形缺口的CT试样,研究了16MnR钢在不同应力比的恒幅循环载荷作用下的疲劳裂纹扩展。开发了一种基于疲劳损伤的方法来模拟疲劳裂纹扩展速率。将16MnR钢的循环塑性本构模型通过用户材料子程序UMAT嵌入到ABAQUS中。把有限元计算得到的疲劳裂纹尖端附近区域的弹塑性循环应力应变结果,代入到疲劳损伤模型中,得到每个加载循环在裂尖各点产生的疲劳损伤值。通过疲劳损伤准则,导出疲劳裂纹稳定扩展速率的计算公式。疲劳裂纹扩展试验验证了模拟结果。实验结果和模拟结果都表明,该试样厚度下,应力比对裂纹扩展速率几乎没有影响。     

Life estimation of Ti-6Al-4V specimens subjected to fretting fatigue and effect of surface treatments

Suitability of different multi-axial parameters in predicting fretting fatigue life of Ti-6Al-4V specimens has been investigated. Ameliorating effect of surface treatments on fretting fatigue has been studied. In simple uni-axial/multi-axial fatigue tests, nucleation as well as propagation of cracks occur under the influence of identical stresses. Hence nucleation accounts for most of the total life. Fretting fatigue crack nucleation occurs due to very large contact stresses, effect of which is felt only close to the surface (due to steep gradients). Propagation mostly occurs due to lower stresses in the bulk of the material (negligible influence of contact tractions) and forms a significant portion of total life. Total life has to be taken as sum of initiation life calculated from different multi-axial fatigue parameters and propagation life from conventional fracture mechanics approach. Steep stress gradients necessitate the adoption of a statistics based approach to predict the crack initiation life, based on an assumed distribution of flaws. The quality of comparison between predicted and experimentally observed failure lives provides confidence in the notion that conventional fatigue life prediction tools can be used to assess fretting fatigue failure. Effect of surface treatments like shot-peening with or without additional surface coatings on total life of the specimen and on friction coefficient has been studied.     

微动疲劳寿命的估算方法研究

着重分析了零构件由于微动磨损而造成的疲劳失效机制 ,说明了在这种微动疲劳模式下疲劳寿命的组成情况 ,用门槛值应力公式估算了当磨蚀坑根部萌生扩展性裂纹时蚀坑的临界深度尺寸 ,并分析了微动裂纹尖端的应力强度因子 ,得出了计算微动裂纹萌生尺寸的表达式 ,最后用上述方法计算了螺纹联接件的微动磨损寿命与裂纹萌生尺寸 ,用局部应力应变法计算了微动裂纹的萌生寿命 ,所得到的估测寿命与试验值相符 ,由此可见 ,该微动疲劳寿命的估测方法是合理的、有效的     

Al-Li 8090 和 Ti-6Al-4V 的 20 kHz 微动疲劳研究

用超声疲劳试验技术研究了Ａｌ－Ｌｉ８０９０铝锂合金和Ｔｉ－６Ａｌ－４Ｖ钛合金在２０ｋＨｚ时的微动损伤现象。试验结果表明，在极高频率下，也有微动损伤发生，并可引发疲劳裂纹的萌生和扩展，导致微动疲劳破坏。     

基于疲劳短裂纹行为的疲劳寿命估算方法

在疲劳短裂纹形成和扩展行为基础上，提出了一种疲劳寿命估算方法。计算结果表明，该方法具有满意的预测精度。     

一种基于连续损伤力学的低周疲劳寿命预测模型

基于连续介质基本守恒定律和连续损伤力学,可将材料疲劳损伤造成的有效承载面积减小表示为平均应变的函数,在此基础上,按微裂纹阶段和疲劳裂纹阶段对材料低周疲劳的损伤演化进行了分析,并建立了一种低周疲劳寿命预测模型。对316L钢光滑试样进行420℃环境下应力控制的低周疲劳试验,采用上述方法进行损伤描述和寿命预测。结果表明微裂纹阶段是材料低周疲劳寿命消耗的主要阶段,采用各寿命段采样数据获得的寿命预测结果与试验结果较 符合。     

Fretting fatigue behavior of SUS304 stainless steel under pressurized hot water

Fretting fatigue behavior of the sensitized SUS304 stainless steel under a pressurized hot water at 7.3 MPa and 288 °C was investigated. The tests were carried out under a contact pressure of 100 MPa and a frequency of 20 Hz. From the experimental result, combined effect of pressurized hot water and localized high tangential stress due to fretting resulted in nucleation of intergranular crack along the outer edge of contact region at lower stress amplitudes, while a fretting fatigue crack was nucleated at the highest tangential force point independently from these intergranular cracks at higher stress amplitudes. No intergranular crack nucleation was observed for fretting fatigue at the same temperature in air. The higher stress ratio reduced the fatigue strength, where the crack tip was exposed more in corrosive environment due to the high mean stress compared to the lower stress ratio.