Fretting Crack Nucleation Research Articles

Fretting fatigue behavior of metal injection molded stainless steel MIM 316L with small pores and low porosity

The fretting fatigue of metal injection molded stainless steel (MIM 316L) with a mean pore diameter of 3 μm and porosity of 5.1% was investigated both experimentally and numerically. The granular-like wear debris accumulated in pores on the fretted surface of the MIM 316L specimen was the cause of the reduction in the tangential force coefficient, potentially improving fretting fatigue strength. These pores did not significantly influence the location of fretting crack nucleation and crack path due to high stress concentration at the fretting contact edge. Under the large-scale yielding at the crack tip and fretting-contact-induced crack opening/closure, a continuum body finite element model can be applied to estimate crack nucleation and propagation behavior, as well as fretting fatigue life.

The cohesive zone crack analogue for incomplete contacts under mild wear conditions

Fretting fatigue may appear in mechanical components that are in contact under vibrational loading, inducing oscillatory frictional stresses, often combined with alternating bulk stresses. Fretting fatigue cracking is important, with its initiation being difficult to estimate. This work extends a well-known fretting fatigue model, the crack analogue model that examines fretting fatigue induced by incomplete contacts of metallic surfaces. This is done by inserting a mode II type cohesive zone at the contact edges. In our previous work on fretting fatigue of complete contacts, we proposed mixed mode I and II cohesive zones because of the nature of the stress fields around the contact edges. The proposed cohesive zones address several issues related to the mechanics of friction and wear, by combining them in the case of mild wear where asperity plastic shakedown is the dominant inelastic mechanism. Thus, the micromechanical behaviour of the surface topology and its evolution are naturally introduced into the analysis. The surface roughness becomes an internal variable that evolves with the cyclic loading, establishing a steady state friction coefficient. Alternatively, friction evolution can be established pointwise by monitoring the accumulated micro-slip. The work of friction dissipates the mechanical work and at the same time becomes the precursor of large scale wear. A predictive methodology for fretting crack nucleation is proposed through the fatigue threshold stress intensity factor used in ordinary fatigue.

Stability of the critical distance method for fretting cracking prediction: influence of microstructure and crack nucleation size

To provide lifetime predictions of metal pieces under fretting-fatigue loading, the crack initiation process has to be understood. However, there is no consensus in the literature over the crack nucleation length. The objective of this study is to understand the link between fretting crack nucleation and microstructure to resolve this question of the critical crack nucleation size. For this purpose, cylinder/plane fretting tests were performed to plot the evolution of the crack length as a function of tangential load amplitude. Finally, a link between the critical distance, evaluated by inverse method, and the grain size is established for different crack nucleation lengths. The investigation suggests that the critical distance evolution depends on the considered defect size.

A Surface Skewness and Kurtosis Integrated Stress Concentration Factor Model

Abstract Surface skewness and kurtosis are two crucial topography property indexes that greatly influence the functional performance of the machined surface. This paper proposes a modified model of stress concentration factor (SCF), which integrates these two surface texture parameters with the well-known standard surface roughness parameters (Arola's model). The relative weight of positive and negative heights of the surface is considered to describe the influence of the shape of the peaks and valleys on the stress concentration of the surface profile for the first time, meanwhile, without losing the effect of the standard surface roughness parameters. The performance of the modified model is studied by comparing it with the other two models involving various aspects of the functional performance of machined surfaces, including fatigue life, wear resistance, fretting crack nucleation, and initiation behaviors, as well as the surface bearing capability. The results indicate that by accounting for the surface skewness and kurtosis parameters, the modified model is more suitable for evaluating the SCF of machined surfaces, appropriately describing the correlation between surface texture and fatigue life and achieving a good prediction of fatigue life compared with the experimental results.

The cohesive zone crack analogue for fretting fatigue based on mild wear

Fretting fatigue is an important problem that plagues many mechanical components that are in contact under vibrational loading. Life assessment of fretting fatigue cracking is important, with the damage initiation being difficult to estimate. This work extends a well-known fretting fatigue model, the crack analogue model that examines fretting induced by complete contacts or by adhering incomplete contacts of metallic surfaces. This is done by introducing cohesive zones at the contact edges. These zones address several issues related to friction and wear, by combining them in the case of mild wear where asperity plastic shearing is the dominant inelastic mechanism. Thus, the micromechanical behaviour of the surface topology is naturally introduced into the analysis. The surface roughness becomes an internal variable that evolves with the cyclic loading. The work of friction dissipates the mechanical energy and at the same time becomes the precursor of large scale wear. A predictive methodology for fretting crack nucleation is proposed through a novel cohesive model that incorporates friction and mild wear, lifting much pathology encountered by simple friction models at high normal loads.

On the fretting fatigue crack nucleation of complete, almost complete and incomplete contacts using an asymptotic method

In this work, the fretting crack nucleation of an aeronautical Ti-10V-2Fe-3Al titanium alloy is investigated. The main difficulty when designing complex industrial systems, especially helicopters rotor parts, lies in the prediction of the assembly interfaces behaviour. Those areas can be subjected to fretting loading characterized by a very confined multiaxial high stress gradient, leading to fatigue crack nucleation. This kind of failure reduces the parts potential lifetime and increases therefore the security margin that can be considered.The aim of this paper is to provide a criterion to prevent crack nucleation on industrial contacts configurations. To reach this objective, this study focuses on the geometrical conditions, loadings and material properties linked to crack initiation. Two dedicated fretting test campaigns are used to study crack initiation conditions with various contact shapes and loadings on the same material. The crack initiation conditions are compared with a contact mechanics representation based on asymptotic methods. This method allows the stress field evaluation at the hot spot location by using the problem’s eigenvalues.The combination of the asymptotic method with contact mechanics and the Goodman approach allows to represent the key parameters of the fretting fatigue crack initiation. They include the impacted material volume, the shear intensity and the bulk stress. A fretting fatigue crack initiation criterion is proposed. It is based on classical material properties and the bulk stress ratio Rσ0 only and is applicable to all sorts of contact shapes. The main interests of this prediction method are the absence of size effect parameters for industrial contacts and its efficiency, as it is only based on analytical formulations.

Effect of machined surface texture on fretting crack nucleation under radial loading in conformal contact

This paper presented the influence of machined surface texture on fretting cracks behaviors of Titanium alloy Ti–6Al–4V under radial loading in conformal contact. The contacted holes of specimen were machined with combined helical and flank milling before fretting fatigue tests, and a special fretting fatigue test apparatus was proposed to focus on conformal contact fretting fatigue (CCFF) test under radial loading. The test results showed that all the specimens failed at the contact area where characterizes an inner obtuse angle between the material flow direction (or cutting direction) and the direction of the tangential component of the contact force. The area with higher surface skewness and kurtosis values was also prone to nucleate fretting fatigue crack. These results suggested that the corresponding machining conditions in hole-making and surface strengthen processes should be adjusted to guide the production of surface texture properties, in terms of obtaining desirable lower skewness and kurtosis features to relieve nucleation of fretting cracks. Additionally, the debris in conformal contact tended to aggregate towards the partial slip zone (PSZ) due to the conformal geometry and form a compacted layer in the gross slip zone (GSZ) subjected to radial loading. These characteristics exacerbated and alleviated fretting damage in the PSZ and GSZ, respectively. In this research, a set of suitable milling parameters for the hole flank milling process in terms of prolonging fretting fatigue life were also defined.

Partial slip contact of a rigid pin and a linear viscoelastic plate

This paper analyzes partial slip contact problems in the theory of linear viscoelasticity under a wide variety of loading conditions, including cyclic (fretting) loads, using a semi-analytical method. Such problems arise in applications like metal-polymer contacts in orthopedic implants. By using viscoelastic analogues of Green’s functions, the governing equations for viscoelastic partial-slip contact are formulated as a pair of coupled Singular Integral Equations (SIEs) for a conforming (pin-plate) geometry. The formulation is entirely in the time-domain, avoiding Laplace transforms. Both Coulomb and hysteretic effects are considered, and arbitrary load histories, including bidirectional pin loads and remote plate stresses, are allowed. Moreover, the contact patch is allowed to advance and recede with no restrictions. Viscoelasticity necessitates the application of the stick-zone boundary condition in convolved form, and also introduces additional convolved gap terms in the governing equations, which are not present in the elastic case. Transient as well as steady-state contact tractions are studied under monotonic ramp-hold, unload-reload, cyclic bidirectional (fretting) and remote plate loading for a three-element solid. The contact size, stick-zone size, indenter approach, Coulomb energy dissipation and surface hoop stresses are tracked during fretting.Viscoelastic fretting contacts differ from their elastic counterparts in notable ways. While they shakedown just like their elastic counterparts, the number of cycles to attain shakedown states is strongly dependent on the ratio of the load cycle time to the relaxation time. Steady-state cyclic bulk hysteretic energy dissipation typically dominates the cyclic Coulomb dissipation, with a more pronounced difference at slower load cycling. However, despite this, it is essential to include Coulomb friction to obtain accurate contact stresses. Moreover, while viscoelastic steady-state tractions agree very well with the elastic tractions using the steady-state shear modulus in load-hold analyses, viscoelastic fretting tractions in shakedown differ considerably from their elastic counterparts. Additionally, an approximate elastic analysis misidentifies the edge of contact by as many as 7 degrees in fretting, showing the importance of viscoelastic contact analysis. The SIE method is not restricted to simple viscoelastic networks and is tested on a 12-element solid with very long time scales. In such cases, the material is effectively always in a transient state, with no steady edge-of-contact. This has implications for fretting crack nucleation.

An evaluation of the competition between wear and crack initiation in fretting conditions for Ti–6Al–4V alloy

The fretting process can lead to material removal (wear), nucleation and propagation of cracks (fatigue), or both. In the first step, the aim of the present paper is to investigate the influence of non-linear behaviour on the wear profiles, in order to evaluate the role of plasticity under different slip regimes for a cylinder-on-flat contact fretting wear. The material of the study is a classical titanium alloy, Ti–6Al–4V (noted TA6V in the following) that is represented by a von Mises equivalent stress, and a combination of non-linear kinematic hardening and isotropic hardening. Since wear is not the only damage mechanism, crack propagation is also considered in the second step. The competition between wear and crack initiation is investigated, thanks to two types of fatigue criteria, purely stress dependent models (Dang Van and Crossland), and a stress–strain dependent fatigue model (Smith–Watson–Topper). The contact evolution influences fretting crack nucleation. The objective is to determine the relevant fatigue criterion and compare the numerical results to the experimental fretting map.

Prediction of Fretting-fatigue Crack Nucleation Using a Surface Shear - Sliding Size Crack Analog Parameter

Fretting fatigue is a critical load that appears on many structures, such as the blade/disk contact of aircraft engines, train wheel assemblies, etc. Predicting crack nucleation risk is essential for safety, but is particularly complex. Fretting contact stress is multiaxial, with severe stress gradients. To palliate this difficulty, a common approach consists in computing multiaxial fatigue at a critical distance, thus correcting the stress gradient effect and achieving stable pertinent predictions. However, this strategy is very costly in FEM computation due to very fine FEM mesh size, which may be <10μm, and application needs to be limited in large 3D industrial contacts. Investigating the partial slip fretting crack nucleation boundaries of 35NCD16 steel (plane) fretted against 52100 steel cylinders, a new semi-empirical contact loading parameter was introduced, defined as the maximum shear stress generated in the interface (qmax) multiplied by the square-root of the sliding size (s) of the partial slip interface (i.e., ϕ = qmax ×√s). Using this very simple parameter inspired by “crack analog strategy”, it was found that all the crack nucleation data obtained for a wide spectrum of contact pressures and cylinder radii were aligned along a single master curve. Scatter was very low, even less than for the costly “fatigue-critical distance method”. The approach was extended to various fretting-fatigue loading conditions, confirming stability of prediction.

Three-dimensional simulation of fretting crack nucleation and growth

Fretting nucleation models and three-dimensional finite element analyses are used to compute the fretting fatigue life for metallic components. The models predict crack nucleation cycles and location(s). Discrete crack growth simulations provide stress intensity factor histories, with multiple, non-planar, three-dimensional cracks possible. The histories are input into crack growth rate model(s) to compute propagation cycles. The sum of fretting nucleation plus propagation cycles is the total life. Experimental fretting data, including crack nucleation location and cycles, and crack growth trajectory and propagation cycles, are used to validate the approach. Predictions for a realistic turbine blade/disk are comparable to field observations.

Analysis of Fretting Crack Propagation Behavior with X-FEM Method

Fretting fatigue cracks always initial at the tralling of contact region, because the stresses in the vicinity of the contact zone exhibit steep gradients. A fracture mechanics approach is usually used to estimate fretting fatigue propagation life. In this paper, extended finite element method combined with fracture mechanics is used to study fretting crack propagation behaviors. The computation results reveal that fretting crack nucleation is mainly decided by fretting, and the cycle bulk stress is the main reason for crack propagation. Also the X-FEM exhibits merits in fretting fatigue problem.

On the fretting crack nucleation with provision for size effect

A thermodynamically based continuum damage mechanics (CDM) approach is employed as a propitious method for analyzing fretting crack nucleation. Intense fretting stress gradients in the contact region are examined and believed to be highly influential in the crack nucleation process. A methodology is proposed that allows for identification of an averaging zone as a function of the contact and loading parameters. The results predicted by this approach are verified by comparing the CDM crack nucleation predictions with the published experimental results on crack nucleation lives. Good agreement is observed between the predictions and the experiments.

Experimental characterisation and numerical simulation of contact evolution effect on fretting crack nucleation for Ti–6Al–4V

Contact evolution effects during fretting can be divided into two categories, namely, change of contact geometry due to wear and near-surface damage. In this paper a study on the effect of contact evolution on fretting crack nucleation in Ti–6Al–4V is carried out, based on a cylinder-on-flat contact fretting wear configuration. The fretted surfaces are carefully profiled so that the evolution of the contact geometry around fretting-induced cracks is identified. The evolution of the near-surface microstructure is identified by scanning electron microscopy. According to the experimental results, a predictive framework is presented to capture the competition and interaction between the contact evolution and crack initiation. The modelling of fretting wear is central to this approach, since it predicts not only the extent of wear damage, but also the concomitant change in fatigue pertinent stresses. The analysis of fretting wear tests, including partial slip and gross sliding conditions, without the substrate fatigue loading of fretting fatigue tests, provides an opportunity to investigate the crack initiation and thus validate predictions. A multi-axial fatigue damage parameter is employed within a cumulative fatigue-damage methodology to predict the wear-induced crack nucleation behaviour under different slip regimes.

Threshold fatigue measurements and fractographic examination of fretting induced cracks in Ti-17

Fretting damage is a common problem for titanium alloys used in turbomachinery equipment, particularly in the blade and disc contact regions. Under careful laboratory controlled conditions at Purdue University, the fretting process was recreated in specimens known as contact pads. Fretting in the contact pads is recognized for its highly localized damage, which results in the formation of cracks, minor propagation, and then crack arrest. This project studied the threshold behavior of fretting induced cracks in the titanium alloy Ti-17 with and without shot-peening surface enhancements. The approach utilized a unique C-specimen (created from the contact pads) for fatigue threshold experiments in addition to fractography to compare bare (untreated Ti-17) to shot peened Ti-17 contact pads with fretting induced damage. The results show that shot-peening does not mitigate fretting crack nucleation but does cause them to nucleate at steeper angles and arrest at depths less than the untreated specimens. Fracture surfaces are also presented to show the nature and physical characteristics of fretting damage in Ti-17.

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.

A fretting crack initiation prediction taking into account the surface roughness and the crack nucleation process volume

This paper presents an experimental study of the fretting crack nucleation threshold, expressed in terms of loading conditions, with a cylinder/plane contact. The studied material is a damage tolerant aluminium alloy widely used in the aerospace application. Since in industrial problems, the surface quality is often variable, the impact of a unidirectional roughness is investigated via varying the roughness of the counter body in the fretting experiments. As expected, experimental results show a large effect of the contact roughness on the crack nucleation conditions. Rationalisation of the crack nucleation boundary independently of the studied roughnesses was successfully obtained by introducing the concept of effective contact area. This does show that the fretting crack nucleation of the studied material can be efficiently described by the local effective loadings inside the contact. Analytical prediction of the crack nucleation is presented with the Smith–Watson–Topper (SWT) parameter and size effect is also studied and discussed.

Fretting wear in self-piercing riveted aluminium alloy sheet

Fretting wear in aluminium alloy AA5754 joined by self-pierce riveting (SPR), a relatively new fastening technique increasingly used in vehicle structures, has been investigated in this paper. Surface damages have been induced when a joint experienced a sinusoidal cyclic tension–tension load. Various degradation zones have been characterised in the contact surfaces between the aluminium sheets, as well as the rivet and the locked sheet. Subsequent examination of the fretting scars at the contact surfaces and through the cross-section has been carried out using optical and scanning electron microscopies. The fretting patterns in the self-piercing riveted joints have been identified. It was noted that fretting wear was initially patchy and layers of compacted debris were created as fretting continued. An energy dispersive X-ray (EDX) analysis of the fretting debris from the interface between the rivet and the aluminium sheet revealed the presence of Al, Fe, Si, C and O so that it is suggested that intimate mixing of debris from the aluminium alloy and the steel rivet had occurred. The effects of testing conditions such as the load magnitude and the number of loading cycles on the fretting wear patterns were also studied. Metallographic examination has shown that these fretting zones are critical with respect to fretting crack nucleation. The test results have shown that joint fracture initiates at these critical zones.

Identification of the characteristic length scale for fatigue cracking in fretting contacts

Fretting damage consisting in cracking and wear generated by debris formation is induced by very small alternated displacements between contacting surfaces. It is often considered through the drastic drop in the fretting fatigue limit which is very detrimental for industrial applications. This paper focuses on the quantification of the fretting crack nucleation appearance by comparing fretting experiments obtained on well defined quenched 30NCD16 steel and a multiaxial fatigue approach. The plane/sphere configuration was studied under partial slip situations characterised by a central stick domain surrounded by an external sliding zone. Validated for classical fatigue conditions, the Dang Van's fatigue prediction is compared to fretting cracking mechanisms (10 6 cycles). The correlation is achieved according to some conditions: - the local friction coefficient operating in the annular partial slip contact has to be identified. It permits a more accurate estimation of the stress loading path evolution during the fretting cycle. - the loading states which are computed to determine the crack nucleation risk must be averaged on an elementary volume representative of the microstructure of the steel. It allows a convenient size effect consideration regarding the very small material volume stressed below the contact.

Observation, analysis and prediction of fretting fatigue in 2024-T351 aluminum alloy

Fretting is associated with microslip at the interface of contacts experiencing oscillatory loads. One consequence of fretting is the formation and subsequent growth of cracks at the edge of contact, a phenomenon known as fretting fatigue. Fretting fatigue is an important fatigue failure mechanism in aircraft structural lap joints and turbine blade/disk contacts. A well-characterized, integrated fretting test system has been developed in which both normal and cyclic tangential fretting loads are applied and monitored in conjunction with a bulk load on the specimen. The experimental data includes histories of the three applied forces and a detailed record of the evolution of interfacial friction coefficient, an evolution driven by surface microslip. The experimental system has been exercised to observe fretting crack nucleation and growth under a wide range of loading conditions in the context of a statistically-designed test matrix. An extensive multiaxial fatigue analysis based on the stress–strain cycle experienced by each point of the bodies subjected to the fretting loads reveals that the critical location for crack formation is the trailing edge of contact, consistent with observations made in the laboratory. The resulting stress–strain cycles are coupled with strain-life theory and literature values of uniaxial fatigue constants to predict fretting fatigue crack nucleation. The data collected for 2024-T351 aluminum alloy correlates very well with this prediction.