Heterogeneity in tribologically transformed structure (TTS) of Ti-6Al-4V under fretting

Fretting wear is a surface degradation process caused by oscillatory motion and contact slipping. During gross slip, high local stresses and plastic deformation in the surface and subsurface can lead to the creation of a nanosized grained structure called Tribologically Transformed Structure (TTS). The current paper studies the formation of TTS in an alpha-beta Ti-6Al-4V alloy under fretting loading while changing the contact pressure and the number of fretting cycles.Cross-sections of wear scars are observed after polishing and chemical etching. Above a threshold pressure of 300 MPa, TTS appears early in the contact (before 1000 cycles) along with two other structures: a Third Body Layer (TBL) made of compacted debris and a General Deformed Layer (GDL) which is the plastic zone under the TTS. TTS first appears as islands and merges in the middle of the contact after enough cycles. Below 200 MPa, only TBL and GDL are formed. At 200 MPa, only small, localized TTS is found. All structures have the same chemical compositions as the initial bulk material except for the nitrided TBL. TTS has a very high hardness compared to the bulk. TTS was carefully extracted using a Focused Ion Beam (FIB) and its microstructure was observed with a Transmission Electron Microscope (TEM). It shows extreme grain refinement and is composed of two alternated zones. The first zone I is composed of $\alpha$ grains with a size of 20 to 50 nm with crystallographic texture. Zone II comprises nanosized equiaxed grains whose sizes range from 5 to 20 nm without texture. The results made it possible to establish a scenario of the appearance of the TTS according to the conditions of contact pressure and number of fretting cycles

A review of non-destructive testing techniques for the in-situ investigation of fretting fatigue cracks

© 2020 The Authors Fretting fatigue can significantly reduce the life of components, leading to unexpected in-service failures. This phenomenon has been studied for over a century, with significant progress being made during the past decade. There are various methods that have been used to study fretting fatigue cracks in order to gain a greater understanding of the effects of fretting fatigue. Destructive methods are traditionally used to observe fretting fatigue cracks. Although useful in determining crack location, crack length, crack propagation modes, crack path and shape, it is not efficient or reliable for time based measurements. Non-destructive testing has developed in recent years and now in-situ monitoring can be used during testing in order to increase the understanding of fretting fatigue. This paper presents a review of non-destructive testing techniques used in-situ during fretting fatigue testing, which are compared in order to conclude the suitability of each technique. Recent developments in non-destructive techniques that could be also applied for fretting fatigue tests are also discussed, as well as recommendations for future research made

Étude et modélisation de l'endurance en fretting fatigue: effet de la plasticité et des sollicitations variables

This work concerns the prediction of fretting fatigue cracking risk in the Teta/Frette contacts of flexible pipes used for oil&gaz transportation. Swell movements produce micro-displacements between Teta wires and Frette wires, which are typical of fretting. Fretting problem in flexible pipes is particularly complex as it involves taking account of high pressure levels inducing plasticity and variable loadings. To answer this problem, exerimental and numerical studies have been carried out.Cracking study is divided between crack nucleation risk prediction and the study of crack propagation conditions. Crack nucleation is studied using Crossland multiaxial fatigue criterion applied at a critical distance to address fretting stress gradient effect. This critical distance is optimized to be representative of the wide spectrum of stress gradient studied. It is shown that if the critical distance is constant whatever the stress gradient, it is combined with a crack nucleation length which depends on the stress gradient (ℓopt-bopt couple). This optimized approach, combined with a representative elastic-plastic material law, allows very precise estimates of crack nucleation conditions.Crack arrest condition ∆K_th is deduced by inverse analysis of plain fretting tests. We show that, whatever stress gradients and elastic or elasto-plastic loadings, this method remains stable. The crack arrest condition allows conservative predictions of crack arrest conditions as long as an elastic-plastic material law is considered. Indeed, accurate predictions of crack arrest conditions require good reproduction of the geometrical accommodation of the contact and of the residual stress generated by the plasticityKnowing ∆K_th and optimized ℓopt-bopt couple, the fretting fatigue map, which, for a given contact, gives the non-cracking domain and non-propagation domain as function of macroscopic fretting and fatigue loadings, is established.Crack propagation situation is also examined. Crack propagation kinetik is estimated by coupling Kujawski’s fatigue crack driving force parameter, K ̅*, with the Paris law of the material. K ̅* allows taking account of the stress ratio evolution below the contact. Thus we predict lifetime and crack propagation kinetik of tests under constant as well as variable fretting fatigue loadings. Loading history is taken into account by the successive application of the loading blocks in the finite element modelling.We show that the developed methods allow adequate estimates of the fretting fatigue cracking response for a varied panel of constant loadings and blocks loadings, even if they tend to overestimate crack propagation. However the methods allow obtaining reasonably conservative predictions, which is the aim of the inductrial partner.Ce travail porte sur la prévision du risque de fissuration en fretting fatigue des contacts Teta/Frette des conduites flexibles servant à l’acheminement du pétrole et du gaz. Les mouvements de la houle produisent des micros déplacements typiques du fretting entre les fils de Teta et de Frette. Le problème des conduites flexibles est particulièrement complexe puisqu’il implique de prendre en compte des forts niveaux de pressions induisant de la plasticité et des sollicitations variables. Pour répondre à ce problème, des études expérimentales et numériques ont été menés en parallèle. L’étude de fissuration est divisée entre prévision du risque d’amorçage des fissures et étude des conditions de propagation. L’amorçage est étudié avec le critère de fatigue multiaxial de Crossland appliqué à distance critique pour prendre en compte les gradients de contrainte spécifiques au fretting. Cette distance critique est optimisée pour être représentative du large spectre de gradient des contraintes étudié. Il est montré que si la distance critique est constante quelle que soit le gradient des contraintes, elle est associée à une longueur de fissure amorcée dépendante du gradient (couple ℓopt-bopt). Cette approche optimisée, combinée à une loi élastoplastique représentative permet des estimations très précises des conditions d’amorçage des fissures. La condition d’arrêt de fissuration ∆K_th est déduite par analyse inverse d’essais de fretting simple. Nous montrons que quels que soient les gradients de contrainte, et le caractère élastique ou élastoplastique de la sollicitation, cette méthode reste stable. Cette valeur permet des prévisions conservatives des conditions d’arrêt de propagation des fissures tant qu’une loi élastoplastique est considérée. En effet, une prévision correcte des conditions d’arrêt de propagation nécessite de bien reproduire à la fois l’accommodation géométrique du contact et les contraintes résiduelles générées par la plasticité.Connaissant ∆K_th et des paramètres ℓopt-bopt optimisés, la carte de fretting fatigue, qui donne pour un contact donné les domaines de non amorçage et de non propagation des fissures en fonction des chargements macroscopiques de fretting et de fatigue, est établie.Le cas de fissures propageantes est également examiné. La cinétique de propagation est estimée en couplant le paramètre moteur de propagation de Kujawski K ̅* avec une loi de Paris. K ̅* permet de prendre en compte l’évolution du rapport de charge sous le contact. Ainsi nous prévoyons la durée de vie et la cinétique de propagation d’essais sous sollicitations constantes de fretting fatigue comme sous sollicitations variables de fretting fatigue. L’historique de chargement est pris en compte par l’application successive des blocs dans le calcul éléments finis. Nous montrons que les méthodes développées permettent d’estimer de manière adéquate la réponse en fretting fatigue d’un panel varié de conditions de chargements constants et de chargements blocs, même si elles ont tendances à surestimer la propagation des fissures. Toutefois, cela permet d’obtenir des prévisions raisonnablement conservatives, ce qui est l’objectif visé par l’industriel

Fatigue Stress Ratio Effect on Fretting-Fatigue Crack Nucleation: Comparison between Multi-Axial and Uni-Axial Predictions

International audienceThis research focuses on high cycle fretting fatigue crack nucleation prediction. A plastic steel/steel cylinder/plane contact was investigated keeping constant the normal force and the maximum fatigue stress ratio but varying the fatigue stress ratio (RF=0.6 to 1). The evolution of the crack length as function of the applied fretting tangential force amplitude at 10^6 cycles allows us to formalize the crack nucleation condition. It shows that the threshold tangential force marking the crack nucleation (i.e. bp_th=0μm) is not affected by the fatigue stress ratio. However an increase of the fatigue stress amplitude sharply increases the crack extension. To model the experiments, a 2D plastic plain strain FEM modeling is performed. As expected the computed stress field description is mesh dependent. However we demonstrate that a representative stress description is obtained at the 3rd node (i.e. 2nd node below the surface). By coupling this mesh condition and non-local critical distance approach, reliable prediction of the crack nucleation risk can be achieved either considering a Crossland multi-axial fatigue analysis or using a basic uni-axial Haighs description

Stability of critical distance approach to predict fretting fatigue cracking: a “ℓ_opt – b_opt” concept

International audienceFretting fatigue life prediction is a strategic issue for modern industry. Accurate prediction of totallifetime depends on crack nucleation time prediction. However, fretting fatigue is characterized by a high stress gradient which overestimates cracking risk at the hotspot. Hence non-local analyses are required. The present study focused on the critical distance method. However, no defined method is currently established to obtain this value, which has been shown to depend on microstructure or stress gradient. The present study investigated the influence of incipient crack length on critical distance crack nucleation prediction. An optimal ‘opt–bopt condition was introduced enabling accurate prediction of the crack nucleation condition whatever the loading condition. This approach was then applied to fretting endurance experiments, allowing to correlate fretting cracking endurance with shear fatigue data. Finally it was used on fretting fatigue lifetime experiments, showing good endurance prediction

Fretting fatigue crack propagation rate under variable loading conditions

International audienceFretting fatigue experiments aim to represent industrial problems and most of them endure variable loading. Being able to assess lifetime of assemblies, especially for low propagation rate conditions, is essential as experimental validation is often too expensive. Both experimental and numerical approaches are proposed to follow the crack propagation rate of steel on steel cylinder/plane fretting fatigue contact submitted to variable loading conditions. An original experimental monitoring has been implemented on the fretting-fatigue test device to observe crack propagation using a potential drop technique. A calibration curve relating crack length and electrical potential was established for the studied contact. It allows direct knowledge of the crack length and crack propagation rate. It was applied to mixed load test showing crack arrest for the last loading condition. To explain this behavior, a 2-dimensional FE modeling was implemented to simulate the complexes multi-axial contact stressing. The crack propagation rate was formalized using an effective stress intensity factor amplitude ΔKeff coupled with Paris law of the material. The crack arrest condition for a given loading was related to ΔKeff along the expected crack path crossing the material crack arrest threshold ΔK0. The failure was related to ΔKeffreaching the critical stress intensity factor KIC. A good correlation with experiments was observed allowing to predict the crack arrest condition although the model tends to overestimate the final crack length extension

FEM modeling of crack nucleation and crack propagation fretting fatigue maps: Plasticity effect

International audienceFretting fatigue life is a strategic issue for modern industries. Various strategies have been proposed to predict finite endurance fretting fatigue behavior. For long term assemblies, « infinite » endurance is preferred. This concept was rationalized using a so-called fretting-fatigue map which plots crack nucleation and crack arrest boundaries as functions of fatigue stress and fretting force. This mapping concept was established experimentally coupling plain fretting, fatigue and fretting fatigue experiments. It was also simulated using FEM elastic simulation. The correlation was good as long as elastic stress conditions were imposed. However, once significant plastic deformation was generated in the contact the elastic prediction induced a significant discrepancy. In the frame of this work we investigated how the integration of plasticity in the modeling strategy could improve respectively the crack nucleation and crack arrest boundaries predictions. The influence of plastic deformation on contact pressure profiles and the role of residual stresses were discussed

Conception d'un test de fretting in-situ pour la tomographie à rayons X

International audienceLes assemblages mécaniques sont souvent soumis à des vibrations qui génèrent un micro-frottement au niveau des contacts : du fretting. Ces légers déplacements peuvent générer des fissures de et/ou une usure qui réduisent la durée de vie de l'assemblage. De nombreux modèles et tests existent pour prédire la durée de vie des contacts, cependant il existe encore une marge d'amélioration importante basée sur une mauvaise compréhension de ce qui se passe dans le contact. En effet, la plupart des modèles utilisent une interface continue et homogène alors que l'air réel du contact, la distribution de la pression et le coefficient de friction dépendent de l'état local du contact. L'accès à l'évolution de l'interface dans le temps, ainsi qu'à son endommagement par usure et fissuration sont donc nécessaires pour aller plus loin dans la compréhension et la simulation des assemblages mécaniques. La réalisation d'un essai in-situ en tomographie permettrait d'accéder à ces informations. Ce travail a pour but de présenter le développement d’un montage de fretting in-situ pour tomographie aux rayons X. Cette conception présente de nombreux défis : Le challenge principal consiste à assurer des efforts de contact quantifiable tout en mettant le moins de matière possible au niveau de la source afin d’assurer une transmission suffisante. Le design final combine des choix techniques et matériaux permettant une bonne reconstruction

Conception d'un test de fretting in-situ pour la tomographie à rayons X

International audienceLes assemblages mécaniques sont souvent soumis à des vibrations qui génèrent un micro-frottement au niveau des contacts : du fretting. Ces légers déplacements peuvent générer des fissures de et/ou une usure qui réduisent la durée de vie de l'assemblage. De nombreux modèles et tests existent pour prédire la durée de vie des contacts, cependant il existe encore une marge d'amélioration importante basée sur une mauvaise compréhension de ce qui se passe dans le contact. En effet, la plupart des modèles utilisent une interface continue et homogène alors que l'air réel du contact, la distribution de la pression et le coefficient de friction dépendent de l'état local du contact. L'accès à l'évolution de l'interface dans le temps, ainsi qu'à son endommagement par usure et fissuration sont donc nécessaires pour aller plus loin dans la compréhension et la simulation des assemblages mécaniques. La réalisation d'un essai in-situ en tomographie permettrait d'accéder à ces informations. Ce travail a pour but de présenter le développement d’un montage de fretting in-situ pour tomographie aux rayons X. Cette conception présente de nombreux défis : Le challenge principal consiste à assurer des efforts de contact quantifiable tout en mettant le moins de matière possible au niveau de la source afin d’assurer une transmission suffisante. Le design final combine des choix techniques et matériaux permettant une bonne reconstruction

Coupling 1D beam elements with 3D solid elements for the modelling of fibre-reinforced composites

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