Scanning electron microscope analysis of fretting fatigue damage

Scanning electron microscope examinations of surfaces damaged by fretting fatigue were performed to establish variations in microstructural and environmental effects on the fretting fatigue process of ferrite/pearlite and martensite microstructures of a $\text{.40}\text{.50}$ carbon steel. The observations suggest differences in fretting mechanisms in laboratory air and “vacuum” conditions as well as differences due to surface hardness.     

Radial fretting fatigue damage of surface coatings

Radial fretting tests with a 52100 steel ball-on-flat contact have been carried out under different normal loads. TiN, MoS₂ and TiN+MoS₂ coatings on a 1045 steel flat were examined. The normal loads amplitude used were 200, 400 and 800 N at speeds of 12 and 1.2 mm/min. Dynamic analysis in combination with microscopic examinations by SEM and EDX have been performed. It was observed that the vertical stiffness increased with the increase of loading speed and number of cycles. The metallographic examinations showed that little damage was observed for the MoS₂ coating, which exhibited excellent radial fretting fatigue resistance. For the TiN coating, micro-cracks appeared at the lower load while delamination occurred at the higher load. For the TiN+MoS₂ composite coating, the vertical stiffness increased but accompanied by some micro-cracks. As a result of the study, the radial fretting test is proposed as one possible new method to evaluate coating life.     

Correlation between fretting and plain fatigue using fatigue damage gradient

Fretting fatigue is correlated with plain fatigue in order to develop a method to estimate fretting fatigue life from plain fatigue data. Fretting fatigue experiments as well as plain ones were conducted to obtain fatigue life data at various conditions. Finite element analyses were conducted to evaluate the Smith-Watson-Topper (SWT) fatigue damage parameter around crack initiation location. It is revealed that the SWT in fretting fatigue decays exponentially away from the surface. Moreover, a correlation function exists that relates the gradient of normalized SWT at the surface to the maximum SWT ratio of plain fatigue to fretting fatigue at the same life. It is demonstrated that equivalent SWT for fretting fatigue, which is determined from the correlation function, can be compared directly with plain fatigue data for estimation of fretting fatigue life.     

车轴轮座微动损伤对车轴疲劳寿命的影响

使用有限元软件ANSYS计算轮座损伤对车轴疲劳寿命的影响.首先计算车轴的稳态受力,这时主要考虑两种栽荷:过盈应力和额定载重应力,关键是考虑两种载荷的复合作用效应.采用外推方法获得车轴材料的S-N曲线,然后将其导入有限元程序进行计算.计算结果表明:有微动损伤的车轴疲劳寿命比无微动损伤的疲劳寿命明显减少.     

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.     

Subsurface damage development during fretting fatigue of high strength steel

The results of fretting fatigue experiments performed on two high-strength structural steels, PH 13-8 Mo stainless steel and quenched and tempered 4340 steel, are evaluated. Observations regarding the subsurface deformation and cracking behavior of the steels are compared and contrasted. It was found that the fretting stresses influenced early crack growth to a greater depth in PH 13-8 Mo stainless steel than in 4340 steel. In addition, experiments on PH 13-8 Mo led to the development of a region below the fretting scar that underwent a microstructural transformation, while experiments on 4340 steel did not. Likely reasons for this discrepancy are suggested. Differences in the formation of oxide layers and the occurrence of adhesion between the two materials are also discussed.     

Mean stress effects in fretting fatigue life estimation method using fatigue damage gradient correction factor

In our previous study, we developed a fretting fatigue life estimation method that considers stress gradient effect [Journal of Mechanical Science and Technology 28 (2014) 2153–2159]. In this method, fatigue damage value at the cracking location is corrected with the factor that is a function of fatigue damage gradient, and the corrected value is treated as the fatigue damage value in plain fatigue for life estimation. In the present study, we examined the effect of mean stress on fatigue damage gradient correction function, because the reliability of the developed method was only verified at a stress ratio (R) of ?1 in previous studies. Fretting fatigue experiments were conducted to obtain the fatigue life data of three different fretting pad shapes with R values ranging from ?1.0 to 0.3. Finite element analyses were then conducted to evaluate the fatigue damage parameter in the cracking region. The results revealed that fretting fatigue life decreases at increased stress ratio. Furthermore, the fatigue damage gradient correction function was unaffected by the stress ratio, although it is affected by plastic deformation at the cracking location. Thus, a correction function for the occurrence of plastic deformation and another for the absence of plastic deformation are necessary. The developed method was demonstrated to predict the fretting fatigue life at various levels of stress ratio with the use of plain fatigue data.     

Effect of surface treatments on fretting fatigue damage of biomedical titanium alloys

Fretting fatigue is an adhesive wear damage caused by tangential micromotion under normal force at contact areas. It is observed along the contact points of hip implants and bone plates. Surface-modified biomedical titanium alloys offer better resistance against fretting damage. PVD TiN coatings and plasma nitriding have proved effective in minimizing friction and delaying the failure of materials. In the present study, attempt has been made to explain the fretting fatigue failure mechanism sequence of PVD TiN-coated and plasma-nitrided Ti–6Al–4V and Ti–6Al–4V couple through friction measurement and microscopic examination.     

Mechanics of fretting fatigue tests

A popular fretting fatigue test, in which oscillating tension is applied in phase with the fretting force, is analysed. The configuration is a generalization of the well-known Mindlin contact problem, and it is shown that the addition of bulk tension has a substantial effect on the stick-slip geometry and the shear traction at the interface. The largest tension induced, which is thought to be responsible for the initiation of fatigue cracks, is also slightly increased.     

Finite element analysis of shot-peening effect on fretting fatigue parameters

Shot peening is widely used to improve the fretting fatigue strength of critical surfaces. Fretting fatigue occurs in contacting parts that are subjected to fluctuating loads and sliding movements at the same time. This paper presents a sequential finite element simulation to investigate the shot peening effects on normal stress, shear stress, bulk stress and slip amplitude, which are considered to be the controlling parameters of fretting damage. The results demonstrated that among the modifications related to shot peening, compressive residual stress has a dominant effect on the fretting parameters.     

Metallographic analysis of fretting fatigue damage in Ti-6Al-4V MA and 7075-T6 aluminum

Fretting fatigue tests were conducted utilizing axial fatigue load application at various maximum fatigue stress levels and normal pressures. Materials investigated were titanium Ti-6A1-4V mill annealed (MA) and aluminum 7075-T6. Subsequent to testing, the specimens were sectioned and metallographically examined to investigate the relationship between fretting damage, fretting induced cracking and reduction of specimen fatigue life.At all maximum fatigue stress levels investigated a given amount of fretting damage was required before any fatigue life reduction occurred. Presumably, the damage leads to the development of cracks in the fretted areas. Metallographie studies of the fretted areas have revealed that multiple cracks form and are propagated by fatigue. Some evidence was found to indicate that fretting debris is forced into the microcracks as they develop, thus explaining, in part, the significant reduction in life caused by the fretting.     

Effects of environment on fretting fatigue

Fretting fatigue tests of a carbon steel and an aluminum alloy were carried out in various environments and the effects of oxygen and water vapor were investigated by tangential force measurements, the initiation and propagation of cracks and hardness and structural changes of the damaged surface layer. With carbon steel the effect of water vapor is negligible but oxygen has a deleterious effect on the initiation and propagation of fretting fatigue cracks. However, with an aluminum alloy the effect of oxygen is small but water vapor accelerates the initiation and propagation of cracks. Environmental effects are more dominant than the stress conditions with an aluminum alloy; material softening and structural change of the surface layer occur.     

Crack propagation behaviour in fretting fatigue

Fretting fatigue and normal, or unfretting, fatigue tests of a stainless steel SUS304L and an aluminium alloy A2024-T3 were carried out to investigate the effects of the contact pressure and the stress ratio on the crack propagation behaviour. The crack propagation behaviour was represented by the crack propagation rate da/dNversus the crack length a or the stress intensity factors ΔK_eff and K_max In fretting fatigue, crack propagation was divided into two stages, namely S_I and S_II. The value of da/dN in the S_I stage was very high, even under a stress intensity factor less than the threshold for normal fatigue, and decreased gradually with crack growth because of crack closure and the decreasing fretting effect. The decrease in da/dN was marked in the case of high contact pressure and low stress ratio such as when R = −0.33, where R denotes the minimum stress divided by the maximum stress. During fretting fatigue crack closure occurred at an oblique short crack in the early stages of crack propagation in both the SUS304L steel and the A2024-T3 alloy; it also occurred at the oblique cracked surface of the shear lips formed in the A2024-T3 alloy during crack growth. However, in the S_II stage, which followed the S_I stage, da/dN increased with crack growth as for normal fatigue.     

Evaluation of methods for reducing fretting fatigue damage in 2024-t3 aluminum lap joints

Fatigue strength of aluminum lap joints subjected to fretting can vary widely depending on the type of treatments applied to the faying surfaces. Many materials normally selected for their lubricity or good wear properties cannot be used in a bolted joint because of their interference with the load transfer requirements of the joint. Thus the best methods found in this evaluation in order of their effectiveness were bonded and shotpeened, bonded alone, shotpeened alone, and bonded steel wear pads. These techniques increased the fatigue strength at 10⁷ cycles of an untreated joint from 12 k.s.i. to a maximum of 23 k.s.i.     

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.     

Initiation and propagation of fretting fatigue cracks

Fretting fatigue tests of a carbon steel were carried out. Fatigue cracks were measured by means of electrical resistance and observed with a scanning electron microscope. The mechanism of fretting fatigue failure is discussed from the experimental results. Small fatigue cracks are initiated early in life and some grow to be propagating cracks. Cracks grow to a given depth by tangential stress combined with repeated stress and then propagate with repeated stress alone, causing a knee point in the propagation curve. Fretting fatigue damage is saturated in the first 20–25 % of life which coincides with the knee point. The condition of non-propagating cracks is also known.     

Fundamental investigations of electrical conductor fretting fatigue

Fretting is known to be the main factor leading to conductor individual wire breaks under aeolian vibration in the vicinity of a clamp. In this paper, previous studies on overhead electrical conductor bending fatigue are summarized. Results obtained with several conductor types and clamps are compared. A general fretting analysis as well as testing procedure are suggested. Influence of the main mechanical parameters on the occurrence of several types of degradation processes is discussed.