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Fatigue is the main phenomenon causing the failure of mechanical structures. For very long service lives the initiation of cracks leading to the failure of in service mechanical parts changes from surface to volume. Due to experimental difficulties the propagation of internal cracks has only been studied to a limited extent in situ and in 3D. However, it is crucial from a dimensional point of view to know the behaviour of such cracks. In this work, in situ monitoring of the initiation and propagation of internal cracks was carried out by X-ray tomography (synchrotron source). Firstly, a method for manufacturing specimens was developed to enable systematic, non-destructive, and 3D visualization of the propagation of internal cracks in a titanium alloy (Ti-6Al-4V) at 20 Hz. For this purpose, an artificial defect was drilled on the surface of a rolled sheet of this material. A second sheet, placed on top of the first one, was diffusion bonded by spark plasma sintering, making it possible to make the defect volumetric. In situ fatigue tests showed the crucial role of the environment (vacuum) and, to a lesser extent, that of the crystallographic texture on the internal crack propagation rates. Secondly, an ultrasonic fatigue machine (cycling frequency of 20 kHz) allowing in situ testing at the synchrotron was used to study the 3D propagation of internal cracks in A357-T6 aluminium alloy specimens containing an artificial internal defect. The 3D grain distribution obtained by Diffraction Contrast Tomography (DCT) was used to study the interaction mechanisms between the microstructure and the 3D crack path., La fatigue est le principal phénomène causant la rupture de structures mécaniques. Pour les très grandes durées de vie, l’amorçage des fissures menant à la rupture des pièces en service passe de la surface au volume. Du fait des difficultés expérimentales, la propagation des fissures internes n’a que très peu été étudiée in situ et en 3D. Il est cependant crucial du point de vue du dimensionnement de connaître le comportement de telles fissures. Dans ces travaux, le suivi in situ de l’amorçage et de la propagation de fissures internes a été réalisé par tomographie aux rayons X (source synchrotron). Premièrement, une méthode de fabrication d’éprouvettes a été élaborée pour permettre de visualiser de manière systématique, non destructive et en 3D la propagation à 20 Hz de fissures internes dans un alliage de titane (Ti-6Al-4V). Un défaut artificiel a pour cela été usiné à la surface d’une tôle laminée de ce matériau. Une seconde tôle, placée sur la première, a été soudée par diffusion au frittage flash, permettant ainsi de rendre le défaut volumique. Des essais de fatigue in situ ont permis de montrer le rôle crucial de l’environnement (vide) ainsi que, dans une moindre mesure, celui de la texture cristallographique sur les vitesses de propagation des fissures internes. Deuxièmement, une machine de fatigue ultrasonique (fréquence de cyclage de 20 kHz) permettant de réaliser des essais in situ au synchrotron a été utilisée pour étudier la propagation 3D de fissures internes dans des éprouvettes d’alliage d’aluminium A357-T6 contenant un défaut interne artificiel. L’obtention préalable de la distribution 3D des grains par tomographie en contraste de diffraction (DCT) a permis d’étudier les mécanismes d’interaction entre la microstructure et le chemin de fissuration en 3D.

International audience; This paper is focused on variable amplitude loading spectra applied in the automotive industry, more specifically for the chassis system parts with respect to high cycle fatigue design. A first analytical model referred to as Heuler's model is considered. It is of the simplest use, with one parameter to be identified. A whole identification process, based on road measurement realized on carmaker's proving ground with instrumented vehicles, is developed and discussed. Once the model is identified, the cycles influence on damage production is also investigated. In the light of the obtained results with this model, a second one, noted modified Heuler's model, is introduced and investigated. This model, similar to the first, requires the identification of two parameters. Once again, the whole model identification process is applied, and the lifetime accuracy is assessed. A new tool is presented, enabling the visualisation on the loading spectrum representation of the most and less damaging cycles. Hence, a methodology is set to legitimize a gate implementation when studying a variable amplitude loading spectrum.

International audience; In recent years, self-heating method, which is based on temperature measurements, has been proposed as a faster and cheaper alternative to classical fatigue tests. In the present study, one-dimensional thermomechanical constitutive equations of shape memory alloys (SMAs) were implemented into the self-heating method by using a probabilistic two-scale model in order to study fatigue of these alloys. This model was shown to be capable of predicting the S/N curve of these alloys as well as its scatter by using probability relations. To identify parameters of the two-scale model, self-heating experiments were conducted. The presented approach was developed to predict the S/N curve of both raw specimens and heterogeneous ones with surface treatments for any failure probability in a much shorter time compared to the classical methods. The heterogeneous samples were made by hydrogenating the surface of virgin specimens. To investigate validity of the numerical results, the theoretical predictions were compared with experimental data and a good agreement was observed.

cited By 0; International audience; Fatigue specimens designed with controlled internal defects were obtained by diffusion bonding of Ti-6Al-4V sheets containing femtosecond laser drilled notches. Crack initiation from the internal defect occurred systematically. The cracks were characterised by in situ synchrotron X-ray tomography. Propagation rates are lower than those of surface cracks of comparable sizes in the same material and fall between experimental data obtained for fatigue cracks propagating in air and in vacuum. © 2019 Acta Materialia Inc.; Fatigue specimens designed with controlled internal defects were obtained by diffusion bonding of Ti-6Al-4V sheets containing femtosecond laser drilled notches. Crack initiation from the internal defect occurred systematically. The cracks were characterised by in situ synchrotron X-ray tomography. Propagation rates are lower than those of surface cracks of comparable sizes in the same material and fall between experimental data obtained for fatigue cracks propagating in air and in vacuum.

cited By 0; International audience; Ti-6Al-4V thin struts, built by selective electron beam melting have been submitted to hot isostatic pressing and further processed with ultrasonic shot peening to investigate the effect of this surface treatment on surface defects. The consequence of those surface defects on the fatigue resistance of thin struts has been evaluated before and after treatment. X-ray microtomography has been used to track the defect population with repeated scans before and after ultrasonic shot peening as well as after fatigue tests conducted to failure. Our results show that the fatigue resistance of surface-treated struts (measured at 105 cycles) is doubled compared to the fatigue resistance of struts with an as-built surface. This enhancement in fatigue resistance is attributed to a refinement of the sub-surface microstructure, to the introduction of local compressive residual stresses and to a significant surface smoothing effect induced by ultrasonic shot peening. Ultrasonic shot peening fails to “heal” the most tortuous surface defects observed in the as-built condition, leaving residual defects in the sub-surface region. The presence of those residual surface defects limits the fatigue resistance when compared to struts with a machined surface. © 2019 Elsevier B.V.

cited By 0; International audience; Predicting the fretting fatigue crack propagation is complex and uncertain. It implies to consider stress gradient and short crack behavior. To address this challenge former in situ synchrotron X-ray imaging of micro-fretting fatigue experiments are simulated. First a Pugno et al./Fleury et al. short crack correction (SCC) approach is considered. Then a microstructural Navarro-Rios (NR) model originally developed to analyze fatigue crack growth for short crack is transposed. Both strategies are compared. Very good agreement with experiments are observed. However the NR model provides a more physical description of the short crack behavior allowing the quantification of the non-monotonous crack growth rate when this later shifts from fretting dominant to fatigue dominant stress domain. © 2018 Elsevier Ltd

cited By 0; International audience; The influence of defects parameters (i.e., size, shape and location) on early crack nucleation in an A357-T6 cast aluminium alloy is studied by means of synchrotron in situ fatigue testing and 3D microstructure based FE simulations. An experimental procedure using laboratory tomography is developed to produce specimens containing controlled internal defects. Two types of defects are investigated: natural shrinkages and artificial defects. Results show that is not possible to assert the harmfulness of a defect if only its size (using A parameter) is taken into account. Location is critical: it is observed that ten times smaller surface defects are able to lead to final failure even when a ∅ = 1 mm internal artificial defect is present within the specimen gauge. Regardless the size, location and nature (artificial or shrinkages) of the defect, the shape is also a factor to take into account because the earliest crack nucleation are observed to occur at regions were the Kt values are the highest. For defects of similar nature and size, Kt evaluations give good indication of the defect tendency to early nucleation. © 2018 Elsevier Ltd

cited By 0; International audience; The influence of forging conditions on the propagation of physically small fatigue cracks has been studied for two high strength steels. Two surface conditions were produced after the forging process. The subsurface microstructure of the materials has been characterized by EBSD. Small samples extracted from the original specimens were used to perform in situ fatigue tests monitored by high resolution synchrotron X-ray tomography. Fatigue cracks were initiated from an artificial defect (100 μm × 50 μm deep) introduced in the forging skin by laser machining. 3D images of the initiation and growth of those physically small fatigue cracks have been obtained. It was found that the presence of a shot-blasted skin containing a hardness and microstructure gradient influences the 3D crack shape during propagation in comparison with the materials without material properties gradient. The 3D crack shapes are rationalized in terms of crack closure effects induced by the forging processes, close to the surface. © 2018 Elsevier Ltd

Hot-forging is a process commonly used in the manufacturing of automobile parts such as connecting rods. Most hot-forged components are shot-blasted after forging in order to clean off the forging scale. The shotblasting process is akin to shot-peening and greatly affects the surface integrity of the components by introducing hardening and residual stresses. This study focuses on the influence of the surface integrity on the fatigue behaviour of a hot-forged C70 steel connecting rod. Specimens were machined out of the connecting rods in order to perform fatigue testing on the forged surface. Several surfaces states, including shot-peened surfaces, were studied in order to quantify the influence of the surface integrity. A thorough characterisation of the surface integrity of the specimens was first performed. The hardness, residual stresses and microstructure gradients were analysed, and the surface of each specimen was scanned using a profilometer. The specimens show complex networks of surface defects introduced during forging, and the shot-blasting process introduces important surface residual stress and microstructure gradients. High cycle fatigue tests in plane bending were then performed on the specimens, with the surface scans helping to identify the critical defect on which crack initiation occurred. The fatigue results, presented in the form of a Kitagawa diagram, are analysed in order to determine which surface integrity parameters are the most influential on fatigue behaviour. The forging defects have an important negative effect on fatigue strength. After shot-blasting, the fatigue strength increases considerably because of the large compressive residual stresses introduced by the shot-blasting process. This work has been performed within the ANR (National Research Agency) DEFISURF project, in a partnership including several industrial (Ascometal, CETIM, Renault, Transvalor, Atelier des Janves, Gévelot Extrusion) and academic (INSA Lyon MATEIS, ENSMPCEMEF, Arts et Métiers ParisTech LAMPA) institutions.

cited By 2; International audience; Among the prevalent tribological failures affecting rolling element bearings, an unconventional rolling contact fatigue mode has been identified as white etching cracks. Those correspond to three-dimensional branching crack networks partially bordered by white etching microstructure, eventually leading to premature and unpredictable failure. Recent work supports that this failure mode may be associated with various combinations of operating conditions depending on the application or test rig, but that all seem to converge towards similar tribological drivers related to surface-affected hydrogen evolution at asperity scales, which is known to embrittle the bearing steel. Nevertheless, as white etching cracks remain delicate to reproduce without artificial hydrogen charging, the underlying formation mechanisms remain unsettled. The present work aims to better understand how some of the main tribomechanical and tribochemical drivers may trigger white etching cracks and premature failures. In this study drivers such as sliding kinematics, water contamination, and electrical potential and lubricant additives are progressively transposed on a twin-disc machine that provides an enhanced control of contact parameters. Various attempts advocate that the tested drivers are not self-sufficient to reproduce the failure mode in such apparatus, but confirm that specific lubricant additives may reduce the fatigue life by promoting surface-initiated embrittled cracking similar to white etching cracks. A local criterion accounting for the local sliding frictional power dissipation and the lubrication regime is further proposed to assess the risk of white etching cracks based on the analysis of various reproduction and occurrences. © IMechE 2016.

cited By 0; International audience; In this paper, the tension-tension fatigue behavior of unidirectional SiC/Si3N4 ceramic-matrix composite with strong and weak interface bonding at room temperature has been investigated using a micromechanical approach. The hysteresis loops models considering different interface slip cases have been developed to establish the relationships between fatigue hysteresis loops, hysteresis dissipated energy, hysteresis modulus, and the interface shear stress. The damage evolution process under tension-tension fatigue loading has been analyzed using hysteresis loops. By comparing experimental fatigue hysteresis dissipated energy with theoretical computational values, the interface shear stresses of SiC/Si3N4 composite with weak and strong interface bonding were obtained for different cycle numbers. The fatigue life S‒N curves and broken fibers fraction versus cycle number curves corresponding to different fatigue peak stresses have been predicted. For SiC/Si3N4 with strong interface bonding, the fatigue limit stress approaches to 75% tensile strength, which is much higher than that of composite with weak interface bonding, i.e., 58% tensile strength, due to the higher interface shear stress degradation rate for weak bonding interface. © 2017 Elsevier Ltd and Techna Group S.r.l.

cited By 0; International audience; The intent of this paper is to identify the damage phenomena controlling the life of composites at high temperatures (> 600°C), and to measure the kinetics of these phenomena during mechanical tests. The analysis of AE data provides a classification into groups of signals having similar characteristics that can be afterward associated to one or to several damage mechanisms. A method of unsupervised classification was used to differentiate the signals resulting from various mechanisms of damage and from failures. Then, a method of supervised classification was developed, so as to establish a classification in real-time whatever the temperature, the applied constraint and the mode loading (static or cyclic). © 2013 Lavoisier.

cited By 0; International audience; The fracture of fibres dictates the fracture of composites reinforced by long fibres. The ceramic matrix composites that are made of a ceramic matrix reinforced by ceramic fibres, display features that are interesting to address lifetime issues. The flaws they contain have a random distribution, and act as stress concentrators. The resistances to failure display a statistical distribution. The effects of microstructural defects on lifetime are discussed in the present paper. A multi scale approach based on modelling of the damage modes in the matrix and the fibers is outlined. The models integrate facture probabilities owing to stochastic phenomena induced by the statistical distribution of microstructural flaws. © 2013 Lavoisier.

International audience; Forged components exhibit good mechanical strength, particularly in terms of high cycle fatigue properties. This is due to the specific microstructure resulting from large plastic deformation as in a forging process. The goal of this study is to account for critical phenomena such as the anisotropy of the fatigue resistance in order to perform high cycle fatigue simulations on industrial forged components. Standard high cycle fatigue criteria usually give good results for isotropic behaviors but are not suitable for components with anisotropic features. The aim is to represent explicitly this anisotropy at a lower scale compared to the process scale and determined local coefficients needed to simulate a real case. We developed a multi-scale approach by considering the statistical morphology and mechanical characteristics of the microstructure to represent explicitly each element. From stochastic experimental data, realistic microstructures were reconstructed in order to perform high cycle fatigue simulations on it with different orientations. The meshing was improved by a local refinement of each interface and simulations were performed on each representative elementary volume. The local mechanical anisotropy is taken into account through the distribution of particles. Fatigue parameters identified at the microscale can then be used at the macroscale on the forged component. The linkage of these data and the process scale is the fiber vector and the deformation state, used to calculate global mechanical anisotropy. Numerical results reveal an expected behavior compared to experimental tendencies. We proved numerically the dependence of the anisotropy direction and the deformation state on the endurance limit evolution

Numerical modelling of fatigue behavior for anisotropic structures has become critical for design applications. This is particularly true for forged components due to the intrinsic anisotropy of the material resulting from the process. The aim of this study is to relate the microstructure scale to the process scale, i. e. the engineer scale. Anisotropy induced by the forming process and the most relevant feature which results from forging, is the preferential orientation of structural defects and grains in the direction of the deformation. Grain flow is modelled using a fiber vector at the level of the representative elementary volume. It can then be used to improve and refine the Papadopoulos fatigue criterion by taking into account fatigue limits for each direction of anisotropy. In practice, it is very tedious to determine precisely these fatigue limits and impossible to obtain experimentally all of them for each direction of uniaxial loading. To circumvent this difficulty, we simulate the problem at the microstructure scale by considering fiber vector as the result of the inclusion and grain orientation. Microstructures are then precisely modelled using DIGIMICRO software. A representative elementary volume including inclusions is meshed and high cycle fatigue simulation is performed. The results can be used in order to optimize the preform of the component before simulation. © Springer/ESAFORM 2008.

International audience; Cracking is a dangerous degradation mode under fretting loading and the understanding of crack initiation and propagation is thus a necessity. A double experimental and theoretical approach has been undertaken to deal with crack initiation during Stage I, Stage I/Stage II transition and Stage II propagation. The work presented here is related to the latter point and aims at determining the direction and the propagation mode of cracks. A new approach is proposed here to account for nonproportional mixed mode conditions such as those encountered under fretting conditions at crack tips. Propagation directions during Stage II are derived from Δσθθ*max, the maximum effective amplitude of the tangential stress perpendicular to the crack trajectory. They correlate well with experimental data. The stress field analysis shows that the trajectory of cracks borders the tensile-compressive and the tensile zones existing around the crack tip over a loading cycle.