Investigation of microstructure effect on fretting fatigue crack initiation using crystal plasticity

The onset of fretting fatigue is characterized by material microstructural changes in which the extent of the damage is comparable to grain size, and hence, the microstructure characteristics could have a significant effect on fatigue crack initiation. In this paper, a three‐dimensional finite element crystal plasticity framework is presented for simulation of the fretting fatigue. Controlled Poisson Voronoi tessellation (CPVT) method is employed to generate the polycrystalline region. In the CPVT method, regularity parameter controls the shape of grains. In this study, the impact of grain size and regularity parameter on crack initiation life and initiation site has been investigated. Cumulative plastic slip was used as a parameter of microstructure‐sensitive fatigue indicator. This parameter could effectively predict the location of crack initiation and its life. The results show that regularity parameter has a significant effect on the location of crack initiation. Furthermore, the effect of grain size on the fretting fatigue life of 316L stainless steel was investigated experimentally through testing different specimens with different grain sizes, to validate the simulation results.     

A multiaxial fretting fatigue test for spline coupling contact

A novel fretting fatigue experimental methodology is presented for mimicking the salient fretting variables for arbitrary axial locations within a complex spline coupling geometry, under combined torque, axial loading and rotating bending moment. The approach permits the simulation, in a simplified test arrangement, of the superimposed multiaxial fretting conditions between the spline teeth. This is achieved via the combination of a low frequency in-plane cyclic normal clamping load and a higher frequency out-of-plane cyclic fatigue stress. The latter is known from spline fatigue tests to play a critical role, along with torque and axial loads, in fretting fatigue cracking of splines.     

The fretting fatigue limit based on local stress at the contact edge

It is well known that fretting fatigue is affected by various factors such as contact pressure, relative slip, contact length, specimen size, loading type and so on. In this study, the reason why these factors affect fretting fatigue was investigated on the basis of the stress distribution near the contact edge. The local stress distribution near the contact edge was experimentally measured using a small multi-element stress concentration gauge. It was shown that the magnitude of the stress concentration at the contact edge varied significantly depending on these factors. It was found that the S – N curve was expressed uniquely on the basis of the local stress amplitude at the contact edge     

Influence of residual stresses from shot peening on fretting fatigue crack growth

One method to improve fretting fatigue life is to shot peen the contact surfaces. Experimental fretting life results from specimens in a Titanium alloy with and without shot peened surfaces were evaluated numerically. The residual stresses were measured at different depths below the fretting scar and compared to the corresponding residual stress profile of an unfretted surface. Thus, the amount of stress relaxation during fretting tests was estimated. Elastic–plastic finite element computations showed that stress relaxation was locally more significant than that captured in the measurements. Three different numerical fatigue crack growth models were compared. The best agreement between experimental and numerical fatigue lives for both peened and unpeened specimens was achieved with a parametric fatigue growth procedure that took into consideration the growth behaviour along the whole front of a semi‐elliptical surface crack. Furthermore, the improved fretting fatigue life from shot peening was explained by slower crack growth rates in the shallow surface layer with compressive residual stresses from shot peening. The successful life analyses hinged on three important issues: an accurate residual stress profile, a sufficiently small start crack and a valid crack growth model.     

Non-propagating crack behaviour at giga-cycle fretting fatigue limit

Fretting fatigue fracture of industrial machines is sometimes experienced after a long period of operation. It has been a question whether the fatigue limit which means infinite life really exists in fretting fatigue or not. Fretting fatigue tests in ultra high cycle region up to 10⁹ cycles were performed. Test results showed that the S‐N curve had a knee point around 2 × 10⁷ cycles and a clear fatigue limit was observed in the giga‐cycle regime for partial slip conditions. An electropotential drop technique was applied to detect the crack growth behaviour under the contact pad. The real‐time measurement of crack depth during the fretting fatigue test at the fatigue limit showed that a crack initiated at an early stage and then ceased to grow after 2 × 10⁷ cycles and the crack became a non‐propagating crack. These results indicated that the fatigue limit exists in fretting fatigue and infinite endurance is achieved by the mechanism of forming a non‐propagating crack.     

Influence of contact pressure on fretting fatigue behaviour of AA 2014 alloy with dissimilar mating material

The effect of contact pressure on the fretting fatigue behaviour of 2014 Al alloy which has been solution heat treated and age hardened (T6 heat treatment) with dissimilar mating materials, was investigated. The fretting fatigue configuration involved bridge‐type contact pads on a flat fatigue specimen. Specimens were made of 2014 Al alloy and bridge‐type pads were made AISI 4140 steel. All the fatigue tests were conducted at a rotational speed of 5000 rpm with a rotating bending fatigue machine (R=?1), using S–N curves to evaluate the fatigue and fretting fatigue properties. The fretting fatigue strength of the material subject to a T6 heat treatment condition at 1 × 10⁷ cycles was dramatically reduced, as compared to that without fretting and with as‐cast. The fretting fatigue life exhibited a variable behaviour with an increase in the contact pressure. A scanning electron microscope was employed to observe the fretting scars and fracture surfaces of the specimens. This analysis showed that cracks originated at the contact surface and crack orientations were approximately ±56 ° from perpendicular to the loading direction.     

Effect of contact stress on fretting fatigue behaviour of 35CrMoA steel under multiaxial nonproportional paths

Fretting fatigue often occurs in contact couples subjected to both bulk oscillatory loads and normal loads, which accelerates the initiation and propagation of fatigue cracks. This study explores the multiaxial fretting fatigue behaviour of 35CrMoA steel with different contact stresses under diamond and square loading paths by experimental methods. From experimental results, the fracture mechanisms with the increase of contact stress have been summarized by combining the analysis of macrorupture and microrupture surface. The strain response behaviour, characteristics of fretting region and crack propagation direction are also analysed and concluded considering the influence of loading path and contact stress in this paper, which have been rarely mentioned in the previous literature. In addition, debris composition of stick and slip regions is investigated as well.     

Fretting fatigue under variable loading below fretting fatigue limit

The fatigue limit diagram provides the critical condition of non‐failure against fatigue under constant amplitude loading. The fatigue limit diagram is usually considered to give the allowable stress if every stress component is kept within the fatigue limit diagram. In the case of variable amplitude fretting fatigue, however, this study showed that fatigue failure could occur even when all stresses were within the fatigue limit diagram. An example of such a condition is a repeated two‐step loading such as when the first step stress is R=?1 and the second step stress has a high mean value. The reason why such a phenomenon occurs was investigated. A non‐propagating crack was formed by the first step stress even when well below the fatigue limit. The resultant non‐propagating crack functioned as a pre‐crack for the second step stress with a high mean value. Consequently, fatigue failure occurred even when every stress was within the fatigue limit diagram of constant amplitude loading. The fatigue limit diagram obtained in constant amplitude fatigue test does not necessarily guarantee safety in the case of variable amplitude loading in fretting fatigue.     

Effects of cyclic frequency and contact pressure on fretting fatigue under two-level block loading

Fretting fatigue tests involving the contact of flat and cylindrical titanium alloy Ti–6Al–4V surfaces, and constant- and two-level block remote bulk stresses are described. The constant-amplitude tests have been performed at cyclic frequencies of 1 and 200 Hz. The two-level block spectra involve the superposition of a 1-Hz, low-cycle fatigue (LCF) constant-amplitude component and a 200-Hz, high-cycle fatigue (HCF) component. Two values of contact pressure are considered. The cyclic frequency of 200 Hz is found to curtail the constant-amplitude fretting fatigue life regardless of the contact pressure applied. Increasing the contact pressure reduces life at 1 Hz but does not have any effect at 200 Hz. Under two-level block loading, the fretting fatigue life is determined primarily by the stress amplitude and high-cyclic frequency of the HCF component of the load spectrum. The LCF component is found to play a secondary role in the determination of the two-level block fretting fatigue life. Fracture topographies for the different test conditions are documented.     

Similarities of stress concentrations in contact at round punches and fatigue at notches: implications to fretting fatigue crack initiation

A linear elastic model of the stress concentration due to contact between a rounded flat punch and a homogeneous substrate is presented, with the aim of investigating fretting fatigue crack initiation in contacting parts of vibrating structures including turbine engines. The asymptotic forms for the stress fields in the vicinity of a rounded punch-on-flat substrate are derived for both normal and tangential loading, using both analytical and finite element methods. Under the action of the normal load, P , the ensuing contact is of width 2 b which includes an initial flat part of width 2 a . The asymptotic stress fields for the sharply rounded flat punch contact have certain similarities with the asymptotic stress fields around the tip of a blunt crack. The analysis showed that the maximum tensile stress, which occurs at the contact boundary due to tangential load Q , is proportional to a mode II stress intensity factor of a sharp punch divided by the square root of the additional contact length due to the roundness of the punch, Q /(√( b − a )√ π b ). The fretting fatigue crack initiation can then be investigated by relating the maximum tensile stress with the fatigue endurance stress. The result is analogous to that of Barsom and McNicol where the notched fatigue endurance stress was correlated with the stress intensity factor and the square root of the notch-tip radius. The proposed methodology establishes a 'notch analogue' by making a connection between fretting fatigue at a rounded punch/flat contact and crack initiation at a notch tip and uses fracture mechanics concepts. Conditions of validity of the present model are established both to avoid yielding and to account for the finite thickness of the substrate. The predictions of the model are compared with fretting fatigue experiments on Ti–6Al–4V and shown to be in good agreement.     

A 'crack-like' notch analogue for a safe-life fretting fatigue design methodology

Various analogies have recently been proposed for comparing the stress fields induced in fretting fatigue contact situations, with those of a crack and a sharp or a rounded notch, resulting in a degree of uncertainty over which model is most appropriate in a given situation. However, a simple recent approach of Atzori–Lazzarin for infinite‐life fatigue design in the presence of a geometrical notch suggests a corresponding unified model also for fretting fatigue (called Crack‐Like Notch Analogue model) considering only two possible behaviours: either ‘crack‐like’ or ‘large blunt notch.’ In a general fretting fatigue situation, the former condition is treated with a single contact problem corresponding to a Crack Analogue model; the latter, with a simple peak stress condition (as in previous Notch Analogue models), simply stating that below the fatigue limit, infinite life is predicted for any size of contact. In the typical situation of constant normal load and in phase oscillating tangential and bulk loads, both limiting conditions can be readily stated. Not only is the model asymptotically correct if friction is infinitely high or the contact area is very small, but also remarkably accurate in realistic conditions, as shown by excellent agreement with Hertzian experimental results on Al and Ti alloys. The model is useful for preliminary design or planning of experiments reducing spurious dependences on an otherwise too large number of parameters. In fact, for not too large contact areas (‘crack‐like’ contact) no dependence at all on geometry is predicted, but only on three load factors (bulk stress, tangential load and average pressure) and size of the contact. Only in the ‘large blunt notch’ region occurring typically only at very large sizes of contact, does the size‐effect disappear, but the dependence is on all other factors including geometry.     

A procedure for estimating the total life in fretting fatigue

ABSTRACT This paper proposes a procedure for estimating the total fatigue life in fretting fatigue. It separately analyses the fatigue crack initiation and propagation lives. The correlation between crack initiation and propagation is made considering a non‐arbitrary crack initiation length provided by the model. The number of cycles to initiate a crack is obtained from the stress distribution beneath the contact zone and a multiaxial fatigue crack initiation criterion. The propagation of the crack is considered using different fatigue crack propagation laws, including some modifications in order to take the short crack growth into account. The results obtained by this method are compared with the fatigue lives obtained in various fretting fatigue tests under spherical contact with 7075‐T6 aluminium alloy.     

On the design and test of equivalent configurations for notch and fretting fatigue

This work investigates the possibility of designing fretting and notch fatigue experiments that are nominally equivalent in terms of damage evaluated by a multiaxial fatigue model. The methodology adopted to carry out this search considered a cylinder‐on‐flat contact geometry and a V‐notched plate. The loading conditions and geometries of the experimental configurations were adjusted to obtain the same decay of the multiaxial fatigue index from the hot spot up to a critical distance. Aluminium alloy 7050‐T7451 was used in the experimental evaluation of the methodology. Considering the well‐known scatter of fatigue data and the limited number of specimens available, the obtained results suggest that the use of the notch analogy in fretting fatigue is appropriate.     

Characterization of fretting fatigue in self-piercing riveted aluminium alloy sheets

A study was conducted to characterize fretting fatigue in self‐piercing riveted single‐lap joints of aluminium alloy 5754 sheets. The experimental results showed that fretting occurred at three different positions in the joint. It was established that fretting led to surface work‐hardening and crack initiation as well as early stage crack propagation. Crack initiated at the surface of the riveted sheets as a result of high stress concentration and propagated oblique to the mating surface under the effect of fretting fatigue. The depth of damage due to fretting depended on the applied load and the cycle time. Microhardness measurements allowed the estimation of the depth of damage due to fretting. These results were observed to correlate well with the length of crack propagation.     

A refined CLNA model in fretting fatigue using asymptotic characterization of the contact stress fields

Using the Atzori–Lazzarin criterion, the first author has recently proposed a unified model for fretting fatigue (FF), called the crack‐like notch analogue (CLNA) model. Two possible types of behaviour were suggested: either ‘crack‐like’ or ‘large blunt notch,’ and these are immediately studied in the typical condition of constant normal load and in phase oscillating tangential and bulk loads. The former condition (‘crack‐like’) was treated by approximating the geometry to the perfectly flat one of the crack analogue (CA), improved in some details, reducing all possible geometries to a single contact problem. The latter (‘large blunt notch’), with a simple peak stress condition i.e. a simple notch analogue model. In the present paper, the calculation of the ‘crack‐like’ behaviour is improved using the recent asymptotic characterisation developed by Dini, Hills and Sackfield, which extracts the asymptotic singular stress field of the fretting contact. A significant difference is found in the ‘equivalent’ geometric factor obtained for the Hertzian geometry, particularly near full sliding, where the new criterion is more conservative, but still not large enough to permit to find, for example in Nowell's FF experimental data, if the refinement is an improvement of predictive capabilities. In flatter geometries, the difference is expected to be even smaller than in the case of the Hertzian geometry, and in this case, the original CLNA model, for its simplicity, remains a very convenient simple closed form criterion.     

On the estimation of fatigue failure under fretting conditions using notch methodologies

ABSTRACT According to experimental evidence, the early stages of fatigue crack propagation under fretting conditions are strongly influenced by the stress gradient generated in the material near the contact zone. This suggests that the crack growth process can be analysed using methodologies similar to those employed to predict the fatigue behaviour of notched elements. This paper assesses the applicability of a number of models originally developed for notched components to fretting fatigue problems. The ability of such models to predict fatigue failure is discussed and compared with experimental results for Al 7075‐T6 specimens that were subjected to fretting fatigue under spherical contact.     

Fretting fatigue behaviour of Ni-free high-nitrogen stainless steel in a simulated body fluid

Abstract

Fretting fatigue behaviour of Ni-free high-nitrogen steel (HNS) with a yield strength of about 800 MPa, which was prepared by nitrogen gas pressurized electroslag remelting, was studied in air and in phosphate-buffered saline (PBS(-)). For comparison, fretting fatigue behaviour of cold-rolled SUS316L steel (SUS316L(CR)) with similar yield strength was examined. The plain fatigue limit of HNS was slightly lower than that of SUS316L(CR) although the former had a higher tensile strength than the latter. The fretting fatigue limit of HNS was higher than that of SUS316L(CR) both in air and in PBS(-). A decrease in fatigue limit of HNS by fretting was significantly smaller than that of SUS316L(CR) in both environments, indicating that HNS has better fretting fatigue resistance than SUS316L(CR). The decrease in fatigue limit by fretting is discussed taking into account the effect of friction stress due to fretting and the additional influences of wear, tribocorrosion and plastic deformation in the fretted area.     

椭圆形路径载荷下的微动疲劳失效特征

为了解微动疲劳失效机理,通过柱面对柱面的接触方式,研究了60Si2Mn钢在椭圆形路径、拉扭耦合作用下的多轴低周微动疲劳特性,深入分析讨论了不同轴向循环拉伸应力幅值对摩擦磨损表面和断口形貌的影响.结果认为：磨损区产生的氧化物磨屑对微动区磨擦损伤行为具有显著影响;微动摩擦磨损对试样表面的影响深度只有数十微米;微动疲劳裂纹源...     

Multi‐axial fretting fatigue behaviour of 35CrMoA steel

The fretting fatigue behaviours in 35CrMoA steel were investigated under conditions of the various contact pressure and the same maximum equivalent stress of cyclic multi‐axial loading. The specimens were characterized by optical microscopy and scanning electron microscopy. Results showed that the fretting fatigue life has complex variation with the increase of contact pressure. The different contact pressure also played very important role in fretting wear model and various wear scars were formed. The influence of them at the fretting surface on the fretting fatigue life was finally discussed by the comparison of the experimental and numerical analysis results.     

Computational fretting fatigue maps for different plasticity models

This paper presents qualitative elastoplastic simulations and analyses of fretting fatigue. Three hardening constitutive models are considered, and their effects on stick–slip conditions and lifetime prediction are compared. The computational analysis consists of the estimation of the shakedown limit cycle and the fatigue prediction using Dang Van or Crossland criterion. A particular configuration, the interaction of a flat pad with rounded corners in contact with a flat substrate made, respectively, of Inconel In718 and Titanium Ti64 alloys, is studied. The shakedown state is analysed using the cyclic and ratcheting equivalent strain concepts already discussed in the literature. In the paper, different fretting maps, based on slip, shakedown and fatigue regimes, are numerically produced and analysed. A new variable, the global slip percentage, is proposed for the characterization of the stick–slip regimes. Analyses of a series of slip maps show that the different hardening models do not introduce significant changes in the stick–slip conditions. Using finite element method simulations combined with fatigue limit criteria (Dang Van and Crossland), fretting fatigue maps are qualitatively reproduced. The main contribution of this work is a comparative discussion on the influence of the hardening models complexity on such maps.