Fretting fatigue in 2XXX series aerospace aluminium alloys

This research investigated the effects of microstructural characteristics on the fretting response in 2XXX series aerospace aluminium alloys. Fretting fatigue tests were conducted to determine the influence of slip character, alloy purity, grain structure and yield strength on fretting crack nucleation and growth. Crack length measurements and micrographs of the specimens indicated there was no significant difference in the fretting response of these alloys based on their microstructural characteristics. Results also showed that fretting caused cracks to nucleate in the first 1–5% of total life which resulted in much shorter fatigue lives. Additionally, fretting normalized the nucleation time in all alloys, eliminating the differences in intrinsic fatigue nucleation resistance. This resulted in the alloys with the highest stress-life (S–N) fatigue properties exhibiting a greater reduction in fatigue strength under fretting conditions. The total fretting fatigue life appeared to be primarily determined by the fatigue crack propagation resistance of the alloys.     

Fretting fatigue crack initiation and propagation behavior of Inconel 690 alloy

Several studies have been conducted on the fretting fatigue limit characteristics of Inconel alloy tube material used in steam generators of nuclear power plants. Nevertheless, additional research on fretting fatigue crack initiation and propagation behavior is necessary in order to evaluate its fretting fatigue life more accurately. In this study, crack growth tests of fretting fatigue are conducted, and the characteristics of fatigue crack initiation and propagation are analyzed on Inconel 690 alloy. Also, plain fatigue crack growth tests are performed on the same material, and the results are compared with those of fretting fatigue crack growth tests. From both of the plain and fretting fatigue crack growth test results, the ΔK-da/dN diagrams are obtained and the crack growth rates are compared. It is found that the crack growth rate for fretting fatigue tests is faster than that for plain fatigue tests under a certain value of DK. However, over this value of DK, the crack growth rate for fretting fatigue tests becomes slower than that for plain fatigue tests due to debris which is produced by fretting and trapped in the propagated cracks. Finally, the fracture surfaces examined by an optical microscope, and the initiation angles of the oblique cracks are determined under various applied stresses. Also, the microstructure of the fracture surfaces is observed by a Scanning electron microscopy (SEM).     

Characterisation of fretting fatigue damage using synchrotron X-ray micro-tomography

The fretting fatigue damage of mechanical joints is studied experimentally by considering the following scenario: first, the crack nucleation in the joint is investigated via fretting tests. Second, the propagation of the fretting cracks under cyclic uniaxial tensile loading is carried out and analysed. Synchrotron radiation X-ray micro-tomography is used to image for the very first time the three-dimensional shape of the initial fretting cracks as well as that of the propagating cracks at different stages of the fatigue life, thanks to a dedicated set-up. This technique brings new experimental data on the influence of the local microstructure on fretting fatigue damage, initiation and growth.     

Observations on fretting damage transition to cracking; state of the art and preliminary observations

Interrupted fretting fatigue experiments were performed on 7075-T6 aluminum alloy and fretting damage characterized by confocal and scanning electron microscopy. Strain, induced by specimen fatigue, produces a small amplitude oscillatory motion between the fatigue specimen and the fretting pad. A fretting fatigue damage threshold exists in this material. Hundred percent fretting fatigue was defined as the average total cycles to fracture based on specimens 1–5 (both axial and normal forces were applied). Specimens had fretting damage induced at 100%, 80%, 60%, 40%, and 20% of the maximum fretting fatigue cycles to fracture. A positive correlation was not found between the depth of fretting damage and crack formation, but there appeared to be a stronger relationship between the fretting damaged surface areas, proximity of pits and crack nucleation sites.     

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.     

Numerical Estimation of Fretting Fatigue Lifetime Using Damage and Fracture Mechanics

Fretting fatigue is a complex tribological phenomenon that can cause premature failure of connected components that have small relative oscillatory movement. The fraction of fretting fatigue lifetime spent in crack initiation and in crack propagation depends on many factors, e.g., contact stresses, amount of slip, frequency, environmental conditions, etc., and varies from one application to another. Therefore, both crack initiation and propagation phases are important in analysing fretting fatigue. In this investigation, a numerical approach is used to predict these two portions and estimate fretting fatigue failure lifetime under a conformal contact configuration. For this purpose, an uncoupled damage evolution law based on principles of continuum damage mechanics is developed for modelling crack initiation. The extended finite element method approach is used for calculating crack propagation lifetimes. The estimated results are validated with previously reported experimental data and compared with other available methods in the literature.     

On the application of a micromechanical small fatigue crack growth model to predict fretting fatigue life in AA7075-T6 under spherical contact

This work presents a method for assessing the fretting fatigue life by estimating the fatigue crack growth rate from the regime of microcracks to the final failure, which is achieved using a two-threshold small fatigue crack growth model. The propagation thresholds are associated with the interaction of the "monotonic plastic zone" and the "cyclic plastic zone" with the microstructure of the material. The predicted fatigue life and the estimated non-propagating cracks agree very well with the experimental fretting fatigue tests with spherical contact in 7075-T6 aluminium alloy.     

Prediction of fretting fatigue behavior under elastic-plastic conditions

Fretting fatigue generally leads to the degradation of the fatigue strength of a material due to cyclic micro-slip between two contacting materials. Fretting fatigue is regarded as an important issue in designing aerospace structures. While many studies have evaluated fretting fatigue behavior under elastic deformation conditions, few have focused on fretting fatigue behavior under elastic-plastic deformation conditions, especially the crack orientation and fatigue life prediction for Ti-6Al-4V. The primary goal of this study was to characterize the fretting fatigue crack initiation behavior in the presence of plasticity. Experimental tests were performed using pad configurations involving elastic-plastic deformations. To calculate stress distributions under elastic-plastic fretting fatigue conditions, FEA was also performed. Several parametric approaches were used to predict fretting fatigue life along with stress distribution resulting from FEA. However, those parameters using surface stresses were unable to establish an equivalence between elastic fretting fatigue data and elastic-plastic fretting fatigue data. Based on this observation, the critical distance methods, which are commonly used in notch analysis, were applied to the fretting fatigue problem. In conclusion, the effective strain range method when used in conjunction with the SMSSR parameter showed a good correlation of data points between the pad configurations involving elastic and elastic plastic deformations.     

Progression of fretting fatigue damage in Ti–6Al–4V

An investigation was conducted to explore the nature of fretting fatigue damage in the stages prior to crack formation. In the unique experimental apparatus employed in this study, where total slip never occurs, several locations on each test specimen exist where cracks can develop due to local contact conditions. Under the test conditions used, not all of the sites had cracks upon test completion. This study evaluated the condition of non-cracked sites on several fretted specimens in an effort to identify differences between these and sites where small cracks were observed.A single test condition of 620 MPa average applied static clamping stress and 250 MPa applied axial fatigue stress for R=0.5 was selected, which corresponds to a fretting fatigue life of 10⁷ cycles based on prior work. For specimens tested to 10⁶ cycles, or 10% of life, several destructive and non-destructive characterization methods were chosen: scanning electron microscopy (SEM), residual stress measurement and transmission electron microscopy (TEM). Each site at which crack nucleation could be expected was inspected in the SEM and was then characterized using surface X-ray diffraction to quantify the residual stresses field near that location. Then TEM foils were cut from one area on a specimen with tiny cracks and dislocation densities were observed. A novel technique was used which permitted TEM samples to be obtained from regions in close proximity on the original specimen.Comparisons were made between as-received (AR) and stress-relief annealed (SRA) specimens, on which the stress-relief was applied prior to fretting fatigue testing. SEM inspection was useful for qualitative analysis of wear debris and identification of cracks as small as 20 μm, but was unable to provide quantitative data on the level of fretting fatigue damage beyond crack size. Although differences were noted in the residual stresses for the SRA versus the AR specimens, no residual stress peaks were noted in the edge of contact regions where cracks would eventually develop. TEM observations in the vicinity of the crack nucleation region showed that the dislocation structure decayed rapidly into the specimen thickness. The cause of the dislocations was attributed to plastic deformation caused by the clamping stresses.     

Fretting fatigue crack initiation lifetime predictor tool: Using damage mechanics approach

Fretting fatigue is a combination of two complex mechanical phenomena. Fretting appears between components that are subjected to small relative oscillatory motions. Once these connected components undergo cyclic fatigue load, fretting fatigue occurs. In general, fretting fatigue failure process can be divided into two main portions, namely crack initiation and crack propagation. Fretting fatigue crack initiation characteristics are very difficult to detect because damages such as micro-cracks are always hidden between two contact surfaces.In this paper Continuum Damage Mechanics (CDM) approach in conjunction with Finite Element Analyses (FEA) is used to find a predictor tool for fretting fatigue crack initiation lifetime. For this purpose an uncoupled damage evolution law is developed to model fretting fatigue crack initiation lifetime at various fretting condition such as contact geometry, axial stress, normal load and tangential load. The predicted results are validated with published experimental data from literature.     

An experimental study on bending fretting fatigue characteristics of 316L austenitic stainless steel

Bending fretting fatigue tests of 316L austenitic stainless steel plates against 52100 steel cylinders have been carried out under same normal load and varied bending loads. Tests of plain bending fatigue were carried out as a control group. The S-N curves of the bending fatigue were made. The results indicated that there was an obvious drop of life under the condition of bending fretting fatigue due to higher local contact stress. A dislocation model of micro-crack nucleation mechanism, as a manner of zig-zag mode, was created to explain the nucleation of fretting fatigue cracks.     

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.     

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.     

The effects of laser peening and shot peening on fretting fatigue in Ti-6Al-4V coupons

This paper describes testing of Ti-6Al-4V coupons in fretting fatigue and compares the effects of mechanical surface treatments on performance. Fretting fatigue tests were performed using a proving ring for fretting load, bridge-type fretting pads, and applied tension-tension cyclic fatigue stress. As-machined (AM), shot peened (SP), and laser peened (LP) coupons were evaluated, and data generated to compare residual stress, surface condition, lifetime, and fractographic detail encountered for each. Near-surface residual stress in SP and LP coupons was similar. The layer of compressive residual stress was far deeper in LP coupons than in SP coupons and, consequently, subsurface tensile residual stress was significantly greater in LP coupons than in SP coupons. SP coupons exhibited a rough surface and had the greatest volume of fretting-induced wear. LP coupons exhibited a wavy surface and had a small volume of wear localized at wave peaks. SP coupons had the greatest fretting fatigue lifetime, with significant improvement over AM coupons. Lifetimes of LP coupons were similar to those for SP coupons at high fatigue stress, but fell between AM and SP coupons at lower fatigue stress. Fractographic evaluation showed that fractures of AM samples were preceded by initiation of fretting-induced cracks, transition of a lead fretting crack to mode-I fatigue crack growth, and crack growth to failure. SP and LP samples exhibited behavior similar to AM samples at high fatigue stress, but in coupons tested at low stress the lead crack initiated subsurface, near the measured depth of maximum tensile residual stress, despite the presence of fretting-induced cracks. The level of fatigue stress above which lead cracks were initiated by fretting was higher for LP than for SP, and was predicted with good accuracy using an analysis based on linear elastic fracture mechanics, the fatigue crack growth threshold stress intensity factor range, and superposition of measured residual stress and applied fatigue stress.     

Finite Element Analysis of Localized Plasticity in Al 2024-T3 Subjected to Fretting Fatigue

Fretting fatigue is a combination of two complex mechanical phenomena, namely, fretting and fatigue. Fretting appears between components that are subjected to small relative oscillatory motion. Once these components undergo cyclic fatigue load at the same time, fretting fatigue occurs. Fretting fatigue is an important issue in aerospace structural design. Many studies have investigated fretting fatigue behavior; however, the majority have assumed elastic deformation and very few have considered the effect of plasticity. The main goal of this study is to monitor the effect of different fretting fatigue primary variables on localized plasticity in an aluminum alloy (Al 2024-T3) test specimen. In order to extract the stress distribution at the contact interface under elasto-plastic conditions, a modified finite element contact model was used. The contact model was verified through comparison with an elastic analytical solution. Then, a bilinear elasto-plastic isotropic hardening model with a von Mises yield surface was implemented to simulate the material behavior of the aluminum alloy. The effect of different fretting fatigue primary variables, such as axial stress, contact geometry, and coefficient of friction, on localized plasticity was investigated. Finally, the relationship between the location of maximum localized plasticity and Ruiz fretting damage parameter with the crack initiation site is discussed.     

钢丝微动疲劳裂纹萌生寿命预测研究*

钢丝微动疲劳过程中,钢丝裂纹萌生特性直接影响其裂纹扩展特性,进而制约钢丝微动疲劳寿命,因此开展钢丝微动疲劳裂纹萌生寿命预测研究具有重要意义。基于有限元法、摩擦学理论和断裂力学理论,运用Smith-Watson-Topper（SWT）多轴疲劳寿命准则建立考虑磨损的钢丝微动疲劳裂纹萌生寿命预测模型,基于多种不同的钢丝疲劳参数估算方法对钢丝的微动疲劳裂纹萌生寿命进行了预测,并探究接触载荷、疲劳载荷、交叉角度及钢丝直径等微动疲劳参数对钢丝微动疲劳裂纹萌生寿命的影响规律。结果表明：基于中值法的预测结果最接近实际值;在微动疲劳过程中,钢丝微动疲劳裂纹萌生寿命主要与接触载荷和疲劳载荷相关。通过引入微动损伤参数建立简化的适用于钢丝绳的钢丝微动疲劳裂纹萌生寿命预测模型,通过与考虑磨损的预测模型计算结果进行对比验证了该模型的准确性。     

A theoretical model for the prediction of initial growth angles and sites of fretting fatigue cracks

The crack initiation mechanisms under fretting fatigue conditions are investigated. Fretting fatigue tests have been conducted on an aluminium alloy. Two major crack initiation mechanisms are experimentally observed. They are theoretically identified by employing a simple dislocation dipole model and taking into consideration the reverse sliding along dislocation layers and the influence of the distance between these layers. It is shown that cracks may initiate either by an extrusion-intrusion mechanism or a fatigue tensile process. Two parameters respectively linked to each of these initiation mechanisms are proposed to predict crack location. The theoretical crack initial growth directions and locations are shown to correlate very well with the experimental ones. With regards to these initiation processes, a spall detachment mechanism is proposed considering the interaction of microcracks.     

Palliatives in fretting: A dynamical approach

Fretting damages are connected to numerous aspects like friction, wear, contact mechanics, fatigue and material sciences. Its quantification also requests to consider the loading history as well as the sliding condition. Based on a “fretting sliding” approach, and considering fretting wear test conditions, various palliative solutions have been investigated. Shot peening treatment, introducing compressive residual stresses, appears pertinent against crack propagation but ineffective against crack nucleation due to the activation of surface relaxation phenomena. Hard thin coatings present stable residual stresses independently of the sliding conditions. However, they only delay the crack nucleation process, when the coating is worn through, cracking phenomena are activated. To quantify the coating endurance against wear, an energy density approach has been developed. The stability of this approach has been confirmed regarding the contact size effect and illustrated through the analysis of synergic interaction between soft thick coating and solid lubricant.