Dissertations / Theses on the topic 'Fatigue à grand nombre de cycles (HCF)'
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Cervellon, Alice. "Propriétés en fatigue à grand et très grand nombre de cycles et à haute température des superalliages base nickel monogranulaires." Thesis, Chasseneuil-du-Poitou, Ecole nationale supérieure de mécanique et d'aérotechnique, 2018. http://www.theses.fr/2018ESMA0009/document.
Full textHigh and very high cycle fatigue properties of Ni-based single crystal superalloys have been studied at 1000°C using three frequencies (0.5, 70, 20000 Hz), four stress ratios (-1, 0.05, 0.3, 0.8) and CMSX-4 alloy as reference.Firstly, the very high cycle fatigue regime (VHCF) has been studied at 20 kHz. At R = -1, fatal crack always initiates between 106 and 109 cycles from an internal casting pore et then propagates in a crystallographic mode, despite the presence of other metallurgical defects such as eutectics and carbides. The elaboration process controls VHCF life under these conditions as it directly affects casting pore size, whereas microstructure has no relevant influence. At R = 0.8, creep controls VHCF life and makes this regime sensitive to microstructure and alloy’s chemical composition. Long term tests (> 109 cycles) have also been characterized and have enabled to propose a crack initiation mechanism operating in the VHCF regime.Secondly, the influence of frequency on time-dependent damages such as oxidation and creep, and their interaction, has been studied. At R ≤ 0, reducing frequency inhibits the harmfulness of internal critical defects by promoting oxidation surface damage. At R ≥ 0.8, creep damage leads to specimens rupture for all frequency test and loading conditions. Intermediate stress ratios present an important interaction between oxidation, fatigue and creep damage according to the frequency and loading conditions. Based on these observations, a damage model that predicts CMSX-4 fatigue life has been proposed and presents satisfying results in comparison to experimental data
Barbier, Grégory. "Fatigue biaxiale à grand nombre de cycles : étude expérimentale et modèle d'endommagement à deux échelles probabiliste." Phd thesis, École normale supérieure de Cachan - ENS Cachan, 2009. http://tel.archives-ouvertes.fr/tel-00572928.
Full textLe, Viet Duc. "Etude de l’influence des hétérogénéités microstructurales sur la tenue en fatigue à grand nombre de cycles des alliages d’aluminium de fonderie." Thesis, Paris, ENSAM, 2016. http://www.theses.fr/2016ENAM0012/document.
Full textThis work treats the influence of the microstructural heterogeneities on the multiaxial high cycle fatigue (HCF) strength of cast aluminium alloys used in an automobile context. The characteristic microstructural heterogeneities present in this family of materials are the aluminium matrix (often characterised by the SDAS and/or the DAS and the precipitation hardening level), inclusions (silicon particles and intermetallics) and casting defects (oxide films and casting porosity).In order to clearly decouple these effects, three cast Al-Si alloys, obtained thank to different casting processes (gravity die casting and lost foam die casting) and associated with several heat treatments (T7 and Hot isostatic pressing-HIP), have been investigated. The HIP treatment is used in order to obtain a porosity free alloy. A vast experimental HCF campaign, including four loading modes (uniaxial (R=-1), torsion (R=-1), combined tension-torsion (R=-1) and equibiaxial tension (R=0.1)) has been undertaken. The following effects on the HCF behaviour have been characterised for the porosity free alloy as well as porosity containing alloys: (a) the effect of the multiaxiality (for the loading modes at R=-1), (b) the effect of the mean stress and (c) the effect of the biaxality (for equibiaxial tensile loads at R=0.1). The fatigue damage mechanisms have been studied in order to highlight the roles of the casting pores, the aluminium matrix and the inclusions on the fatigue damage mechanisms.Two analytical fatigue models are proposed. The first one concerns the effect of the loaded volume on the uniaxial fatigue strength of the porosity containing alloys using an approach to predict of the maximum pore size in a given volume. The second model, based on a probabilistic approach, takes into account the competition between the different observed damage mechanisms and leads to a Kitagawa-Takahashi type diagrams for different loading modes. It is shown that these analytical models result in good predictions for the three materials investigated and the four loading modes.A numerical study, presented in the last section, is related to the 3D finite element analysis of real pores. Real pore geometries are obtained thank to micro-tomography observations. The principal aim of this study is to evaluate the possibility of predicting the fatigue strength at the macroscopic scale thanks to the local mechanical behaviour around critical pores
Guellec, Corentin. "Caractérisation en fatigue à grand nombre de cycles par mesures d’auto-échauffement des aciers d’arbres de transmission pour application navale sous chargements cycliques complexes." Electronic Thesis or Diss., Brest, École nationale supérieure de techniques avancées Bretagne, 2023. http://www.theses.fr/2023ENTA0001.
Full textMarine shaft lines undergo complex cyclic loadings. In order to characterize these structures in fatigue, two complementary objectives are achieved. Firstly, the marine shaft’s loads are characterized in order to identify relevant fatigue cycles. During this step, an original parametric fatigue design method is implemented. This method is based on the definition of an equivalent load considering multiaxiality, variability, non-proportionality and mean stress. This method allows to distinguish two damage modes with a mode associated with the established cyclic regime of rotary bending and a mode associated with the ship’s maneuvers. Secondly, a rapid characterization method is implemented to characterize in fatigue the marine shafts’ steels for a large number of loading configurations. The method is based on a model which enables fatigue identification behaviors from self-heating measurements. The steels of the study are characterized in tension-torsion for various configurations of mean stress and nonproportionality of the loading. In this context, the notion of iso-self-heating surfaces is introduced. It allows, for a specimen, to model the elastoplastic and dissipative behavior of the material in the dedicated stress space. Their use makes it possible to define a multiscale fatigue criterion based on the invariants of the stress tensor. In the study, a modeling of the behavior of the material is proposed for very low amplitude loads (VHCF) exhibiting a strong difference in dissipation compared to the regime of higher amplitudes (i.e. HCF domain)
Abroug, Foued. "Effet des défauts d’usinage sur la tenue en fatigue de pièces aéronautiques." Thesis, Paris, ENSAM, 2018. http://www.theses.fr/2018ENAM0016/document.
Full textThe size of a component has been proved in several studies of the literature to affect the fatigue strength and this trend is known to be more pronounced in the High Cycle Fatigue regime. More exactly a drop of the fatigue limit is observed and this evolution is very often explained by the probability to find a critical defect or a weakest zone in the material as the stressed volume rises. The present manuscript is part of a French research project gathering several industrial and academic partners that aims to control the machining quality of aircraft structural components. For one part of the project the challenge is to define a proper defect acceptability criterion for HCF design purpose. It must be able to account for a large range of surface defects and of component sizes and geometries. Even though the primary objective was to better understand the impact of periodic surface micro-geometry patterns (characteristic of the type of machining used) on the fatigue limit, we thought that it was first necessary to check if a size effect can be observed when an increasing number of artificial simplified surface defects are introduced at the surface of smooth specimens. The aeronautical material under investigation is a 7050 Aluminum alloy (Al Zn6CuMgZr). A large fatigue testing campaign under fully reversed plane bending loading is undertaken on specimens with artificial surface hemispherical defects. Defect number varies from 1 to 44 per specimen whereas their size ranges from 60 µm to 800 µm. Testing results allow the characterization of both Kitagawa effect and scale effect on the fatigue response. A probabilistic approach based on the weakest link concept together with a proper fatigue crack initiation criterion is used to account for the stress distribution and the size of the highly stressed volume. Predictions using FE simulations show a good agreement with experimental results and illustrate the importance of taking the scale effect into account while designing components containing different types of surface defects or roughness patterns.Keywords : Surface defect, HCF, Kitagawa-Takahashi diagram, Weakest link concept, AA7050 alloy
Alarcon, Tarquino Eduardo Augusto. "Structural fatigue of superelastic NiTi wires." Thesis, Brest, 2018. http://www.theses.fr/2018BRES0019/document.
Full textThis Ph.D. dissertation thesis addresses the conditions and mechanisms that lead superelastic NiTi wires to fail under cyclic mechanical loads. NiTi shape memory alloys exhibit functional thermomechanical properties (superelasticity, shape memory effect, thermal actuation) due to martensitic phase transformations caused by a change of the applied stress and temperature. These phase transformations are though as fully reversible damage-free processes, however, when NiTi is subjected to repetitive stress-induced phase transformations its fatigue performance drops drastically in comparison to non-transforming NiTi. Most of fatigue S-N curves reporting this drop were measured on straight NiTi wires in which martensitic transformations proceed heterogeneously through nucleation and propagation of shear bands. Moreover, from our experience fatigue testing straight wire samples results in undesired failure inside the testing machine clamps. Hence, the reported stress-strain values in S-N curves are not necessarily representative of the critical mechanical conditions that lead the material to failure. With the aim of better characterize the fatigue performance of NiTi wires, we started by carrying out a series of pull-pull fatigue tests using hourglass-shaped samples. This sample geometry allowed us to confine all martensitic transformation and related material fatigue processes into a well-defined gauge volume. The samples’ characterization was performed by combining several experimental and analysis techniques such as Digital Image Correlation, Infrared Thermography, Synchrotron-source X-ray diffraction, Optical Microscopy, Scanning Electron Microscopy and Finite Element Analysis. A special attention was paid to the High Cycle Fatigue (HCF) performance of NiTi in which the material shows elastic behavior and/or an intermediate phase transformation (so-called R-phase). The results from HCF tests allowed us to distinguish crack nucleation and crack propagation stages during the total life of our NiTi samples. In order to get a better understanding of the mechanisms that lead to crack nucleation, we applied the nonconventional Self-Heating fatigue assessment method, which has shown efficiency in the case of aluminum and steel alloys. This method correlates the temperature elevation of a sample subjected to different cyclic load amplitudes with energy dissipating mechanisms that contribute to accumulating local damage in the material. The Self-Heating method was performed using full-field thermal measurements of cyclically loaded NiTi hourglass-shaped samples
Blanche, Antoine. "Effets dissipatifs en fatigue à grand et très grand nombre de cycles." Phd thesis, Université Montpellier II - Sciences et Techniques du Languedoc, 2012. http://tel.archives-ouvertes.fr/tel-00784772.
Full textDoudard, Cédric. "Détermination rapide des propriétés en fatigue à grand nombre de cycles à partir d'essais d'échauffement." Cachan, Ecole normale supérieure, 2004. http://www.theses.fr/2004DENS0036.
Full textQilafku, Genti. "Effet d'entaille à la fatigue en grand nombre de cycles : rôle du gradient de contrainte." Metz, 2000. http://docnum.univ-lorraine.fr/public/UPV-M/Theses/2000/Qilafku.Genti.SMZ0010.pdf.
Full textQILAFKU, GENTI Pluvinage Guy. "EFFET D'ENTAILLE A LA FATIGUE EN GRAND NOMBRE DE CYCLES. ROLE DU GRADIENT DE CONTRAINTE /." [S.l.] : [s.n.], 2000. ftp://ftp.scd.univ-metz.fr/pub/Theses/2000/Qilafku.Genti.SMZ0010.pdf.
Full textMarti, Nicolas. "Effets de la fréquence et de la température sur les mécanismes de microplasticité en fatigue à grand et très grand nombre de cycles." Thesis, Paris, ENSAM, 2014. http://www.theses.fr/2014ENAM0041/document.
Full textNowadays there is a growing demand for the development of fast and robust fatigue life prediction methods in the very high cycle fatigue domain. In this way, ultrasonic fatigue technique which appeared in 1950 is very interesting for manufacturers. Because the typical frequency of these tests is 20 kHz, this technique is efficient to perform tests up to a very high number of cycles in a reasonable time (109 cycles are reached in 14 h). However, the frequency domain of these fatigue tests facilities raises the issue of the effect of frequency and more generally the validity of the obtained results for estimating fatigue life of structures loaded at frequencies three or four order of magnitude below ultrasonic frequencies. The objective of this work is to evaluate the effect of the loading frequency on the precursors of fatigue damage, namely the microplasticity at the grain scale. This thesis work deals with the case of polycrystalline pure copper loaded in fully reversed tensioncompression. To show the effect of frequency, the Wöhler or S-N curves were constructed at different frequencies. Then, the study focused on the mechanisms of microplasticity preceding crack initiation and several criteria were investigated: the morphologies of the slip bands and their locations in the microstructure, the thresholds of appearance of the slip bands, the evolution of the slip bands amount with the number of cycles, the distribution of the microplasticity in the grains, the dissipated energy during a fatigue cycle. Cross slip and vacancies production and diffusion are two mechanisms which play a part in the formation of slip bands and extrusions in surface. Their respective roles on the effects of frequency observed are discussed
Berthel, Bruno. "Mesures thermographiques de champs de dissipation accompagnant la fatigue à grand nombre de cycles des aciers." Phd thesis, Université Montpellier II - Sciences et Techniques du Languedoc, 2007. http://tel.archives-ouvertes.fr/tel-00410074.
Full textWang, Chong. "Microplasticité et dissipation en fatigue à très grand nombre de cycles du fer et de l’acier." Paris 10, 2013. http://www.theses.fr/2013PA100077.
Full textThis thesis is part of research conducted by the team of Professor Claude Bathias, since 1976, on the subject of Very High Cycle Fatigue (VHCF), also called "fatigue gigacyciique. " lt is obvious that many of machinery such as bearings,1 springs, turbine disks, rails, cylinder heads, etc. , must reach the billion cycles without failure. And also there are obvious scientific interests to understand and predict the fatigue crack initiation at stress just half the yield, while the damage in continuum mechanics is excluded. Contrary to what has been believed, since the early work of Wôhler [Wôhler, 1867], the SN curve of iron and steel is not a hyperbolic function and has no horizontal asymptote after 106 or 101 cycles. But Wôhler never conclude more than that. It is Basquin in 1910, who introduced the concept of "infinite life time” associated with the definition of "fatigue limit". However the concept of fatigue limit has been questioned since the 80s, when the research in cooperation with several Chinese scientists [Ni and Bathias 1992, Kong et al 1991, Wu et al 1993, Wang et al 1999] and the laboratory of C. Bathias led to show that all metals and alloys can fatigue crack up to 1010 cycles, a way to define ’infinity' in reasonable scale. In regimes of low cycle fatigue and high cycle fatigue lot of work, including the analysis and modeling [Mughrabi and al 1 976; Mughrabi et al, 1979; Mughrabi, 1980], were performed and a proper understanding has emerged over the years. Formation of persistent slip bands (PSB) on the surface has been recognized in the single—phase material and alloys as nucleation of fatigue cracks. The PSB is resulted by localized cyclic plasticity. Formation of PSBs is accompanied with the development of the extrusions and intrusions. Base on the geometry explaining, the intrusion on the surface introducing stress concentrations is the reason of cracks creation. However, it is still remains how the mechanisms of the low cycle fatigue characteristics persisting in VHCF regime. Part of this thesis is dedicated to highlighting the formation of PSB in the Armco iron when crack occurs beyond 107 cycles. At same time, experience has shown that, in industrial multiphase alloys, the fatigue crack initiation at VHCF regime is often internal. Lt was first present by the work of Y. Murakami [Murakami et al, 1999] and C. Bathias [Bathias, 1999] in the 90s. They recognized that fatigue cracking of metals does not initiate from surface
Poncelet, Martin. "Multiaxialité, hétérogénéités intrinsèques et structurales des essais d'auto-échauffement et de fatigue à grand nombre de cycles." Phd thesis, École normale supérieure de Cachan - ENS Cachan, 2007. http://tel.archives-ouvertes.fr/tel-00200459.
Full textNguyen, Hoang Quan. "Modélisation numérique de la propagation d’une fissure lors d’une rupture par fatigue à très grand nombre de cycles." Paris 10, 2013. http://www.theses.fr/2013PA100104.
Full textThis manuscript is a contribution to the study of the crack propagation in very high cycle fatigue domain. We develop a numerical tool which allows to model this crack propagation in mode I and to study the formation of fish eye crack, a characteristic of this failure. Firstly, an iterative numerical procedure based on three dimensional finite element method is developed to study the evolution of crack form during the crack propagation and to calculate the number of cycles to failure when it is not possible to find an analytical solution. The virtual crack closure technique is used to calculate the stress intensity factor in each step of crack growth process. Secondly, a coupled thermomecanical finite element model in 3D is proposed to estimate the evolution of the temperature field during the crack propagation. The problem is solved into 2 steps. In the first step, we calculate the plastic energy per cycle from the cyclic plastic zone at the crack front. In the second step, we use a fraction of this energy as a mobile heat source to estimate the temperature field evolution. The plastic energy per cycle is calculated by two methods. The first one is based on an elastic-plastic finite element analysis with a perfectly elastic-plastic constitutive law. The second one is based on an empirical relation between the energy plastic per cycle per unit of crack length and the range of stress intensity factor. Numerical results obtained agree fairly well with experimental results
Bracquart, Benoît. "Etude des interactions défaut géométrique / microstructure dans les mécanismes d’endommagement en fatigue à grand nombre de cycles d’alliages métalliques." Thesis, Angers, 2018. http://www.theses.fr/2018ANGE0009.
Full textThe aim of this study is to analyze the influence of microstructure and geometrical surface defects on the high cycle fatigue (HCF) behavior of acommercial purity polycrystalline aluminium. This work relies on an experimental test programme and numerical Finite Element (FE) simulations, in order to better understand microstructural parameters governing crack initiation at defects. To this end, both caracteristic dimensions (grains and defect) are varied. Thermomechanical treatments are set up to control the grain size, and obtained microstructures are caracterized monotonically and cyclically. Then, uniaxial fully reversed tension-compression HCF tests are carried out on specimens with defects, either with an in-situ surface crack monitoring, or a study of the crack initiation at the defect root after cycling. Results are then analyzed in order to determine the influence of different characteristic lengths, as well as crystalline orientation, via EBSD maps. Numerical FE simulations are then carried out to detetermine precisely local mechanical fields. To this end, polycrystalline aggregates 3D meshes representative of experimental configurations are set up. A crystal plasticity constitutive model with gradient is used in order to reproduce the plastic slip at the glide system scale, and the grain size effect. The distribution of afatigue indicator parameter in the different configurations is studied, in order to complement experimental results
Simon, Julien. "Influence de micro-entailles sur le comportement en fatigue à grand nombre de cycles d'un alliage de TA6V : Comparaison avec le fretting-fatigue." Thesis, Chasseneuil-du-Poitou, Ecole nationale supérieure de mécanique et d'aérotechnique, 2017. http://www.theses.fr/2017ESMA0034/document.
Full textThis study is about the influence of micro-notches on the high cycle fatigue behavior of a Ti64 alloy and the comparison with similar fretting fatigue tests. Preliminary studies showed that fretting-fatigue stress fields can be reproduced by micro-notched. The COGNAC project leaded by Safran Aircraft Engines was built to verify if a unique model can reproduce both fretting-fatigue and notched samples fatigue tests. The scope of this experimental study is limited to the notched samples. In this PHD thesis, the notches are grinded and the results are compared with fretting-fatigue tests on grinded surfaces. V-notched with notch root radius from 50 to 500 μm were chosen to reproduce the stress gradient of fretting-fatigue tests. A geometry with two inclined notches facing each other allows to reproduce shear stress and stress triaxiality from fretting-fatigue tests. Three different experimental studies were performed. First on un-notched specimens with a grinded surface state to produce a reference fatigue limit for grinded surface state without stress concentration. The second study the notch root radius effect on the fatigue limit with samples with one non-inclined notch of 500 μm of depth. The last one is a series of fatigue tests on samples with two inclined notches that reproduce the stress distribution of fretting-fatigue tests. Pseudo-cleavage facets, multi cracks initiations in the notch root and the presence of two propagation stages is similar to the fretting-fatigue tests mechanisms. However, non propagating cracks were not observe at the notch root while many of them were present during the fretting-fatigue test. The initiation sites and the early stages of propagation seem to be controlled by grinded affected areas. Finally, three fatigue criteria were used to try to reproduce the experimental results. The criteria based on the critical distance theory and gradient theory –with a local gradient and an affine effect of the gradient term- can reproduce the fatigue limits of notched samples but fail to predict both notched and un-notched specimen fatigue limits with a unique data set. The third one is a probabilistic criterion which success to predict the experimental fatigue limits not only of notched samples but also smooth ones. The comparisons between the experimental results of the fatigue on the specimens with two inclined notches and the fretting-fatigue shows a difference of 20%. The distribution of the Crossland stress is quite similar. In the future, it would be useful to make a new comparison between fretting-fatigue and fatigue on notches using the probabilistic criterion to design the tests. This new comparison could lead to the proposal of a unique methodology to take into account the fretting-fatigue and the fatigue on stress concentrators
Zghal, Jihed. "Etude du comportement en fatigue à grand nombre de cycles d'un acier à haute limite d'élasticité HC360LA : endommagement, plasticité et phénomènes dissipatifs associés." Thesis, Paris, ENSAM, 2016. http://www.theses.fr/2016ENAM0004/document.
Full textThe present work, which is integrated in the IRT Jules Verne APSTRAM project, focuses on the high cycle fatigue behavior of a ferritic high-strength low-alloy steel (HC360LA). First, different stress-controlled cyclic tests are carried out to study the influence of loading conditions and pre-straining on the fatigue behavior. According to the experimental results, a uniaxial tension pre-straining allows for a significant increase of the fatigue strength. Using the experimental dataset (force, elongation and temperature), an important effort is made to estimate the fraction of strain energy that is either dissipated into heat or stored within the material during cyclic tests. The strong correlation between the number of cycles to failure and heat dissipated energy emphasizes the importance of plasticity in the process driving to fatigue failure. Second, a polycrystalline model is proposed to describe the fatigue behavior of metallic materials in the high cycle fatigue regime. To consider the anisotropy of plastic properties, the constitutive model is developed at the grain scale within a crystal plasticity framework. It uses continuum damage mechanics to describe the progressive degradation of mechanical properties within an anisotropic context. The constitutive model is then integrated within a self-consistent formulation to consider the polycrystalline nature of metallic materials. Finally, the proposed model allows for investigating the fatigue behavior of the HC360LA steel at a microscopic scale. Damage is found to be highly localized in some specific grains. As a result, while fatigue damage results in a progressive decrease of elastic stiffness at the crystal scale, the elastic properties are not significantly affected at the macroscopic scale. Also, the contribution of damage to heat dissipation is negligible. The correlation between energy dissipation and fatigue failure is therefore a consequence of the strong coupling between plasticity and damage
Guerchais, Raphaël. "Influence d'accidents géométriques et du mode de chargement sur le comportement en fatigue à grand nombre de cycles d'un acier inoxydable austénitique 316L." Thesis, Paris, ENSAM, 2014. http://www.theses.fr/2014ENAM0020/document.
Full textThe aim of this study is to analyze the influence of both the microstructure and defects on the high cycle fatigue (HCF) behaviour of a 316L austenitic stainless steel thanks to finite element (FE) simulations of polycrystalline aggregates.%The scatter encountered in the HCF behavior of metallic materials is often explained by the anisotropic elasto-plastic behavior of individual grains leading to a highly heterogeneous distribution of plastic slip.Since fatigue crack initiation is a local phenomenon, intimately related to the plastic activity at the crystal scale, it seems relevant to rely on this kind of modeling to evaluate the mechanical quantities.A preliminary numerical study, based on experimental data drawn from the litterature, was conducted on an electrolytic copper using simulations of 2D polycrystalline aggregates. The effect of the loading path and small artificial defects on the mesoscopic mechanical responses have been analyzed separately. Moreover, the predictive capabilities of some fatigue criteria, relying on the mesoscopic mechanical responses, has been evaluated. It was shown that the macroscopic fatigue limits predicted by a probabilistic fatigue criterion are in accordance with the experimental trends observed in multiaxial fatigue or in the presence of small defects.An experimental campaign is undertaken on an austenitic steel 316L. Low cycle fatigue tests are conducted in order to characterize the elasto-plastic behavior of the material. Load-controled HCF tests, using both smooth specimens and specimens containing an artificial hemispherical surface defect, are carried out to estimate the fatigue limits under various loading conditions (tension, torsion, combined tension and torsion, biaxial tension) and several defect radii. To complete the characterization of the material, the microstructure is studied thanks to EBSD analyzes and the cristallographic texture is measured by X-ray diffraction. These experimental data are used to reproduce, with FE simulations, the HCF tests on 2D and 3D microstructures representative of the austenitic steel. The heterogeneity of the mesoscopic mechanical quantities relevant in fatigue are discussed in relation to the modeling. The results from the FE models are then used along with the probabilistic mesomechanics approach to quantify the defect size effect for several loading paths. The relevance, with respect to the experimental observations, of the predicted fatigue strength distributions is assessed
Nguyen, Thi Thu Huyen. "Effet des hétérogénéités microstructurales sur le comportement en fatigue multiaxiale à grand nombre de cycles : application à l'usinage assisté laser." Phd thesis, Paris, ENSAM, 2008. http://pastel.archives-ouvertes.fr/pastel-00004274.
Full textPetitjean, Sébastien. "Influence de l'état de surface sur le comportement en fatigue à grand nombre de cycles de l'acier inoxydable austénitique 304L." Poitiers, 2003. http://www.theses.fr/2003POIT2269.
Full textThis work has dealt with the influence of surface finish on the high cycle fatigue behavior of a 304L. The role played by roughness, surface hardening and residual stresses has been particularly described. First part of this study has consisted of the production of several surface finishes. These latter were obtained by turning, grinding, mechanical polishing and sandblasting. The obtained surfaces were then characterised in terms of roughness, hardening, microstructure and residual stresses. Fatigue tests were finally conducted under various stress ratios or mean stresses at two temperatures (25ʿC and 300ʿC). Results clearly evidenced an effect of the surface integrity on the fatigue resistance of the 304L. This influence is nevertheless more pronounced at ambient temperature and for a positive mean stress. For all explored testing conditions, the lowest endurance limit was obtained for ground specimens whereas polished samples exhibited the best fatigue strength. Results also cleared out a detrimental influence of a positive mean stress in the case of specimens having surface defaults of a great acuity. The study of the relative effect of each of the surface parameter, under a positive stress ratio and at the ambient temperature, showed that roughness profile and surface hardening are the two more influent factors. The role of the residual stresses remains negligible due to their rapid relaxation during the application of the first cycles of fatigue. The estimation of the initiation and propagation periods showed that mechanisms differed as a function of the applied stress ratio. Crack propagation is governed by the parameter DK at a positive stress ratio and by Dep/2 in the case of tension-compression tests
Dehmani, Helmi. "Étude de l’influence du procédé de poinçonnage sur la tenue en fatigue à grand nombre de cycles de tôles minces ferromagnétiques." Thesis, Paris, ENSAM, 2016. http://www.theses.fr/2016ENAM0014/document.
Full textNew electrical steel grades, with improved magnetic properties, are used to build electric motors. For these steel grades, the iron losses are reduced by adjusting the chemical composition (mostly the Si content), decreasing the thickness below 0.5 mm and increasing the grain size. The punching is used to produce electric motor components because it generates important alterations of sheet edges, this work aims at elaborating a HCf fatigue design strategy for thin punched electrical steel sheets. First, the quasi-static and cyclic behavior of this electric steel was studied through monotonic and cyclic tests. The behavior model of this material, which will be used in FE simulation, is then identified. The study of the high cycle fatigue (HCF) resistance of this material is performed using smooth and notched specimen’s geometries. The effect of stress ratio, temperature (180°C) and the punching process are considered. Due to its influence on the fatigue resistance, the effect of the punching process is finely investigated. Different experimental techniques such as microscopic observations, 3D surface topography, micro‒hardness and X‒ray diffraction are combined to characterize the specimen’s edges. To dissociate the respective influences of strain hardening, residual stresses and geometrical defects induced by the punching process, and to quantify the contribution of each parameter to the HCF resistance, different specimen’s configurations were tested. A strategy allowing the identification of the critical defects, on which fatigue crack initiation occurs, was adopted. The stress distribution around defects is determined from finite element analyses (FEA) on real defect geometries. A non‒local high cycle fatigue criterion is finally used as post‒processing of FEA to consider the effect of defects and the associated stress-strain gradients in the HCF strength assessment
Pessard, Etienne. "Comportement anisotrope en fatigue des composants mécaniques forgés." Phd thesis, Université d'Angers, 2009. http://tel.archives-ouvertes.fr/tel-00464089.
Full textMilesi, Marc. "Impact des caractéristiques microstructurales des pièces forgées sur leur tenue en fatigue à grand nombre de cycles : modélisation multi-échelles et validation expérimentale." Phd thesis, École Nationale Supérieure des Mines de Paris, 2009. http://tel.archives-ouvertes.fr/tel-00408851.
Full textGraux, Nicolas. "Caractérisation et modélisation des propriétés à la fatigue à grand nombre de cycles des aciers cémentés à partir d'essais d'auto-échauffement sous sollicitations cycliques." Thesis, Brest, 2017. http://www.theses.fr/2017BRES0104.
Full textThe rolling contact fatigue prediction between two carburizing part quickly becomes complex.On one hand, the carburizing treatment give heterogeneous properties in surface layer depending on the treatment protocol. On the other hand, the rolling contact load is a complex load with a fatigue initiation in the sub-layer. To limit the duration of the field fatigue properties characterization, self-heating measurements under cycle load are used and their interpretation by a probabilistic two scales model is proposed. Nevertheless applying this fatigue evaluation method on heterogeneous material and for rolling contact load can be difficult. ln first approach those difficulties are split.To take into account the material heterogeneity, an analysis based on a variation of one probabilistic two scales model and on carbon rate measurement is proposed. Model parameters are identified on one steel class with self-heating measurement made on specimens representative of carburizing material heterogeneity. Finally the model is validated by comparison with experimental fatigue point.Making self-heating measurement for rolling contact load is complex. Consequently a first self-heating measurement campaign is made on the intermediary case of repeated contact. With a simple analytic model, the temperature field evolution can be linked to a mean heat source whose link with fatigue mechanism must be proven. Finally, rolling contact machine prototypes are proposed. Self-heating measurement made on those prototypes and their interpretation suggest that it will be possible to identify fatigue properties with self-heating measurement
De, Carvalho Pinheiro Bianca. "Étude par diffraction des rayons X des modifications microstructurales en cours de fatigue." Thesis, Lille 1, 2011. http://www.theses.fr/2011LIL10063/document.
Full textThe present work aims to evaluate the microstructural mechanisms associated with the initiation of fatigue damage of steels used in the oil and gas industry. Microdeformations and residual stresses (macrostresses) were evaluated by X-ray diffraction in real time during alternating bending fatigue tests performed on flat test pieces taken from a pipe sample. Microdeformations were estimated from measurements of the full width at half maximum (FWHM) of the diffraction peak and residual stresses from the peak displacement. The fatigue tests were performed at five different stress levels. Three stages of changes during the evolution of microdeformation were detected. We show that their amplitude and duration are proportional to the level of alternating stress. Similar variations were observed for the residual stresses, with duration identical to those of the microdeformation. Changes in the density and distribution of dislocations were observed by transmission electron microscopy using the technique of focused ion beam. To understand the role of the initial structure, fatigue tests on annealed samples were performed under the same test conditions. Again, three stages of changes are observed but with an increase of the microdeformations instead of a decrease during the first stage due to the initial state of the dislocation network. The results are very encouraging for the consideration of the microstructural evolutions in the construction of a future counter of fatigue damage initiation in materials
Ezanno, Anthony. "Caractérisation rapide des propriétés à la fatigue à grand nombre de cycles des matériaux de fonderie à partir d'essais d’auto-échauffement : application aux alliages d'hélices marines." Brest, 2011. http://www.theses.fr/2011BRES2028.
Full textMarine propellers are subjected to cyclic loadings which can lead to fatigue failure of the blades. Large ship propellers are typically obtained by a sand casting process, and are generally made of aluminium bronze. As per all cast components, propellers content casting flaws such as micro-shrinkage pores which act as fatigue crack initiation sites. The objectives of this thesis are to firstly develop a numerical tool that aids the fatigue conception of cast components and to validate it using full-scale tests. A High Cycle Fatigue model is proposed. In this model, it is assumed that fatigue failure is due to microplasticity which occurs around casting defects and results in the dissipation of energy. A two-scale probabilistic approach is adopted. A set of elasto-plastic sites with variable activation threshold and a random distribution in the material is considered. Self-heating tests under cyclic loadings are used to identify the mechanical behavior of a site and the variability of the population of sites. Castings defects are taken into account indirectly, without considering a precise geometrical description, thanks to the thermal response of the material under cyclic loadings. The predictions from the model are compared to experimental results from full-scale fatigue tests on propellers. These tests required the design of a specific experimental set-up
Vincent, Matthieu. "Interaction entre défaut de surface et taille de grain en fatigue à grand nombre de cycles : approche expérimentale et numérique sur un fer pur Armco." Thesis, Chasseneuil-du-Poitou, Ecole nationale supérieure de mécanique et d'aérotechnique, 2018. http://www.theses.fr/2018ESMA0018/document.
Full textThe objective of this thesis is to study the influence of the ratio between the defect size and themicrostructure characteristic length on the fatigue limit (for a fixed fatigue life) of a metallic material, under highcycle fatigue (HCF). Pure Armco iron is chosen because its simple microstructure has a single characteristic lengthat the mesoscopic scale (grain scale) : the grain size. The aim of the study is thus to study the competition betweena structural effect (surface defect) and a material effect (grain size) in the context of mechanical stresses in HCF.In order to obtain a comparable different grain size, a thermomechanical protocol has been developed. HCF tests,using specimens from both microstructure sizes (initial material and processed hardened material) in which hemisphericaldefects of different sizes were introduced, were performed to estimate the fatigue limits for different defectsize / grain size ratios. When Kitagawa diagrams are presented in relative values (fatigue limit / fatigue limit ofdefect free material versus defect size / grain size), there is a single curve that combines the two microstructures.This dimensionless Kitagawa diagram thus makes it possible to analyze the reduction of the fatigue limit inducedby a defect. The use of the relative size of the defect with respect to the characteristic microstructural dimensionappears to be more relevant than the use of the physical size of the defect.These experimental results are used to reproduce the HCF tests with Finite Element simulations on 3D microstructuresrepresentative of Armco iron. The competition existing between the stress concentration induced by thegeometrical defect and the highly stressed regions of the microstructure generated by the anisotropy of the mechanicalbehavior of the grains is studied. A mesoscopic criterion (involving mechanical quantities averaged by grain)based on a statistical approach allows to find the evolution of the dimensionless Kitagawa diagram, ie the relativesize of the critical defect from which it predominates over the response heterogeneity of the microstructure and thusgoverns the fatigue behavior of the polycrystal. The modification of the mesoscopic criterion by taking into accountintragranular heterogeneities (with the standard deviation per grain of mechanical quantities) is discussed
Wang, Yanjun. "Fatigue Thermique à grand nombre de cycles d’un acier inoxydable austénitique : apport des mesures de champs pour l’identification du chargement et le suivi in-situ de l’endommagement." Thesis, Université Paris-Saclay (ComUE), 2019. http://www.theses.fr/2019SACLN037/document.
Full textThis PhD work is devoted to the study of thermal fatigue damage of AISI 316L(N) austenitic stainless steel, a candidate material to make the primary cooling system of Sodium-cooled Fast Reators (SFRs). Initiation and propagation of crack networks can be induced by locally constrained thermal expansions or contractions of the component surface subjected to repeated thermal shocks of turbulent coolants.A campaign of high cycle thermal fatigue tests on AISI 316L(N) austenitic stainless steel has been carried out with the FLASH facility. Full field measurements have been performed to capture thermomechanical fields of monitored surfaces in thermal fatigue tests. An original procedure based on hybrid multiview correlation (HMC) uses images acquired by two visible light cameras and one infrared camera. With such a system, Lagrangian temperature fields can be measured and experimental strain or displacement fields can be used to calibrate Finite Element analyses to reproduce the thermomechanical cyclic response of the material in the region of interest. One additional benefit of the spatiotemporal synchronization of the HMC system is that the entire fatigued region has been monitored in-situ during the whole test, without interruptions, which enables crack initiation and propagation to be tracked thanks to the different modalities of the three cameras
Florin, Pierrick. "Caractérisation rapide des propriétés à la fatigue à grand nombre de cycle des assemblages métalliques soudés de type automobile : vers une nouvelle approche basée sur des mesures thermométriques." Thesis, Brest, 2015. http://www.theses.fr/2015BRES0100/document.
Full textFatigue design of weld assemblies still remains of prior concern in the automotive industry, despite several decades of experimental, theoretical and numerical body of work. Actually, many complex components (e.g. front and rear axles) are embedded thanks to welding process. Such welded assemblies are expected to be designed as high-safety parts, for which any fatigue failure is supposed to be prevented. Despite numerical modelling is more and more effective, experimental tests are still necessary in order to provide basic design data and at last to prove the design reliability. Standard fatigue tests procedures are sensibly time consuming, are usually destructive and need for several specimens in order to manage reliable results. Conversely, the so-called self-heating tests offer the opportunity to dramatically shorten the test duration and save specimens because it is a non-destructive method. It consists in measuring the temperature evolution of the structure surface during cyclic loading. Such an approach allows to take advantage of the macroscopic thermal signature of microscopic plasticity processes responsible for fatigue damage. The purpose of the present work is the determination of a correlation between thermal measurement and fatigue properties of welded structures, and then studying the influence of parameters on fatigue properties with the developed method. An experimental protocol is first proposed to measure the temperature of the tested specimen under cyclic loading. A first analysis of the thermal response of standard steel sheet specimen under low load amplitude is proposed in order to validate a deterministic heat source model for both tensile and bending tests. These tests allow us to correctly describe the thermal behavior of the base material of welded specimen away from the joint. Then, the model is extended to welded specimen in order to take into account the dissipation of the welded area. Once the dissipative heat source is identified thanks to the model, its evolution with the applied loading leads to a self-heating curve allowing an estimation of the fatigue limit of the welded specimen after only few hours of test. The method is then applied to other configurations of welded specimens in order to study the effects of shot peening and stress gradient on fatigue properties. Finally, the testing procedure is applied for the determination of an industrial component fatigue properties, a welded car wishbone. Due to the bad heat boundary condition, another analysis is proposed to estimate the heat source along the entire component. This estimation leads to the determination of the weakest area of the structure concerned by fatigue and a first good estimation of its fatigue limit
Zhao, Mengxiong. "Ultrasonic fatigue study of Inconel 718." Thesis, Paris 10, 2018. http://www.theses.fr/2018PA100063/document.
Full textInconel 718 is widely used in turbine disk of aeronautic engines, due to its high resistance to corrosion, oxidation, thermal creep deformation and high mechanical strength at elevated temperature. The total cycle of these mechanical components is up to 109~1010 during its whole lifetime. It endures high-amplitude low-frequency loading including centrifugal force or thermal stress, and also low-amplitude high-frequency loading came from vibration of blade.In this work, the very high cycle fatigue (VHCF) behaviour of Inconel 718 with self-heating phenomenon without any cooling is studied using ultrasonic fatigue system at 20KHz. Acquisition system is improved using NI capture card with LabView for monitoring the frequency, temperature, displacement and so on during all the tests. Keyence laser sensor with two probes at the top and bottom surfaces of the specimens is used to reveal the frequency and vibration mode. The difference of mean values between these two probes is the elongation of the specimen caused by self-heating phenomenon.Three sets of materials with different heat treatment, As-Received (AR), Directly Aged (DA) and Directly Aged High Quality (DAHQ) from ONERA and SAFRAN are compared. The difference of grain size, phase, precipitate particle, etc. is investigated by metallographic micrograph using optical microscope (OM) and scanning electron microscope (SEM). Quasi-static uniaxial tensile property and cyclic stress-strain response is also proposed. The transition from cyclic hardening to cyclic softening appears after aged heat treatment. Finally, fracture surfaces are observed using optical camera and scanning electron microscope in order to identify the mechanism of fracture of Inconel 718 in the VHCF domain
Bidouard, Hadrien. "Etude de l'effet de surcharges sur la tenue en fatigue à grande durée de vie d'un acier ferrito-bainitique sous chargement d'amplitude variable." Phd thesis, Paris, ENSAM, 2009. http://pastel.archives-ouvertes.fr/pastel-00005937.
Full textMa, Zepeng. "Fatigue models for life prediction of structures under multiaxial loading with variation in time and space." Thesis, Université Paris-Saclay (ComUE), 2017. http://www.theses.fr/2017SACLX117/document.
Full textThe aim of this work is to propose a multi-scale approach to energy-based fatigue, which can estimate lifetimes associated with variable multidimensional loading. The foundation of the approach is to assume that the energy dissipated on a small scale governs the fatigue behavior. Each material point is associated to a stochastic distribution of weak points that are likely to plasticize and contribute to the dissipation of energy without affecting global macroscopic stresses. This amounts to adopting Dang Van's paradigm of high cycle fatigue. The structure is supposed to be elastic (or adapted) on a macroscopic scale. In addition, we adopt on the mesoscopic scale an elastoplastic behavior with a dependence of the plastic load function not only of the deviatoric part of the stresses, but also of the hydrostatic part. Linear kinematic hardening is also considered under the assumption of an associated plasticity. Instead of using the number of cycles as an incremental variable, the concept of temporal evolution of the load is adopted for a precise follow-up of the history of the actual loading. The effect of mean stress is taken into account in the mesoscopic yield function; a law of nonlinear accumulation of damage is also considered in the model. Fatigue life is then determined using a phenomenological law based on mesoscopic energy dissipation from the plastic accommodative cycle. The first part of the work focused on a proposal for a fatigue model with a simpler implementation gradient than the previous models
Liang, Xiaoyu. "Comportement en fatigue à grand nombre de cycle d’un acier inoxydable 316L obtenu par fabrication additive : effets de la microstructure, de la rugosité et des défauts." Thesis, Paris, HESAM, 2020. http://www.theses.fr/2020HESAE017.
Full textThis study aims to investigate the influence of both the microstructure and surface defects on the high cycle fatigue (HCF) behavior of a 316L stainless steel obtained by additive manufacturing (AM). Surface defects and microstructure are dominant factors of fatigue behavior, while the AM materials often exhibit distinguished surface state and microstructure compared to conventional materials. The current study begins with an investigation of the material properties that are related to fatigue behavior. Microstructure observations of the powder and fabricated specimens are undertaken. Profilometry and tomography analyses make the inherent defects visible. The hardness, elastic behavior and elastic-plastic behavior are studied via mechanical tests. Then, load-controlled fatigue tests concerning different surface-treated specimens under different loading types are conducted. To reveal the mechanism of fatigue failure in the studied specimens, a comprehensive fractography analysis is carried out. Experimental research reveals the weakening of fatigue strength due to lack-of-fusion defects. Yet, the effect of the microstructural attributes is difficult to evaluate without numerical tools. A preliminary numerical study about the application of the non-local method in an explicit microstructure sensitive model is undertaken to complement the microstructure-sensitive modeling framework. Based on the data collected in the experimental campaign, a finite element model that can take into consideration of the defects and the microstructure of the SLM SS 316L is built up. Finite element analyses are performed with both cubic elasticity and polycrystal plasticity constitutive laws. With the help of the statistical method, the results from the FE model are used to quantitatively assess the influence of surface roughness and microstructural attributes on the fatigue performance of SLM SS 316L
Godet, François. "Influence de la microstructure sur le comportement mécanique d’une couche nitrurée de l’acier de nuance 33CrMoV12-9 en vue de l’optimisation des propriétés en fatigue." Thesis, Paris, ENSAM, 2018. http://www.theses.fr/2018ENAM0046/document.
Full textDesign of high-performance and lighter mechanical systems needs a perpetual work on materials used. In the power transmission field, gaseous nitriding thermochemical treatment enhances significantly the life-cycle of the parts which are higher stressed on the surface. While the contribution of the mechanical property gradients due to the treatment on the fatigue life is well-known, the microstructure effects on the (micro-)plasticity mechanisms remains undetermined. This work concerns exclusively the nitrided 33CrMoV12-9 graded steel, used for gears and bearings in the naval and aeronautical industry. It aims particularly at defining mechanical properties linked to crack initiation mechanisms during high-cycle fatigue. Properties such as Young modulus, hardness, yield stress and fatigue limits are looked at several scales, from microstructural ones to hardened case one. In-situ and post-mortem investigation on loaded and damaged nitrided layers leads to estimating the microstructural influence on local deformations, microplasticity and damage but no particular mechanisms were found. Special fully-nitrided parts were designed to perform the experiments. This tool helps to carry out simple loadings and measure the strain-hardening curve and fatigue limits. Finally, it as been attested that measured properties depend on scales, means and methodology
Gerin, Benjamin. "Modélisation des effets de défauts et d’intégrité de surface sur la tenue en fatigue dans les composants forgés." Thesis, Paris, ENSAM, 2016. http://www.theses.fr/2016ENAM0040/document.
Full textThis study is part of the DEFISURF project, with the goal of analysing and modelling the various effects of the surface on the fatigue strength of forged components. Two components are studied: a cold-extruded specimen and a connecting rod which is hot-forged and then cleaned by shot-blasting. For each component, different batches are studied in order to quantify the effects of the process on the mechanical characteristics and the fatigue behaviour. Two different reduction of sections are used for the cold-extruded specimens and shot-peening is used to obtain various surface states for the connecting rods.The various batches are thoroughly characterised and fatigue tests are performed. Traction, torsion and plane-bending tests are conducted on the cold-extruded specimens. Fatigue specimens are machined from the central part of the connecting rods and loaded in plane-bending.For cold-extrusion, the parameters with the most influence in fatigue are the prestrain induced during forging and the residual stresses. In hot-forging, the surface defects produced during forging and the residual stresses induced by the shot-blasting have the most influence.For each component, a fatigue model is suggested. For the cold-extruded specimens, a multiaxial fatigue criterion is used to take into account the various fatigue loadings and the effect of the residual stresses. Two approaches are used for the connecting rods. The first is a geometrical analysis of the defect which approximates them with an ellipsoid. The second approach uses finite element simulations of the critical defects. The shot-peening residual stresses are integrated in the simulations with a multiaxial criterion. These simulations predict the fatigue limit of the specimens with an error of around 15%
Mille, Pierre. "Comportement mecanique et metallurgique de l'alliage de titane ta6v dans le domaine de la resistance a la fatigue a grand nombre de cycles : etude de modifications de surface et application a l'amorcage de fissures." Université Louis Pasteur (Strasbourg) (1971-2008), 1987. http://www.theses.fr/1987STR13132.
Full textMille, Pierre. "Comportement mécanique et métallurgique de l'alliage de titane TA6V dans le domaine de la résistance à la fatigue à grand nombre de cycles étude de modifications de surface et application à l'amorçage de fissures /." Grenoble 2 : ANRT, 1987. http://catalogue.bnf.fr/ark:/12148/cb37608020x.
Full textChamat, Abderrahim. "Prévision de la durée de vie en fatigue des roues ferroviaires sous ollicitations multiaxiale proportionnelle et non-proportionnelle." Metz, 2005. http://docnum.univ-lorraine.fr/public/UPV-M/Theses/2005/Chamat.Abderrahim.SMZ0509.pdf.
Full textThe experience shows that the fracture of the components or structures in normal working conditions is often due to fatigue. The aim of this works is to propose a criterion which predicts the life duration in endurance fatigue. For this purpose, a signal has been raised during the tests. In order to apply this signal the specimens are extracted from the wheel. Annexe Fatigue Multiaxiale en endurance illimitée 145. Our Instron machine is not predisposed for the realization of this type of loading ; therefore a Labview program has been set which enables to introduce all points of the cycle. The experimental results are compared to the prediction of the various criteria. The application of the various criteria predicted results different from the experimental data. In this perspective, a semi local approach based on the application of the volumetric method has been proposed. Two methods have been proposed to depict the test results in fatigue strength diagram. The first consists in depicting a load factor as a function of the number of cycles to fracture. While the second consists in representing the maximal von Mises stress versus the number of cycles to fracture diagram. Numerous studies showed the influence of the signal in the endurance domain. Experimental results of the present study confirmed these findings. It is noted, the effect of the loading on the life duration becomes negligible from a life duration of 10-7cycles i. E. Low values
Torabiandehkordi, Noushin. "High and very high cycle fatigue behavior of DP600 dual-phase steel : correlation between temperature, strain rate, and deformation mechanisms." Thesis, Paris, ENSAM, 2017. http://www.theses.fr/2017ENAM0020/document.
Full textThis work is an attempt towards a better understanding of the high cycle and very high cycle fatigue behaviors of a ferritic-martensitic dual-phase steel, with a regard to temperature and strain rate effects, resulting from accelerated fatigue loading frequencies. The influence of frequency on fatigue response of DP600 steel was investigated by conducting ultrasonic and conventional low frequency fatigue tests. Fractography studies and microscopic observations on the surface of specimens were carried out to study the deformation and fracture mechanisms under low and ultrasonic frequencies. Moreover, in situ infrared thermography was carried out to investigate the thermal response and dissipative mechanisms of the material under fatigue tests. The S-N curves were determined from ultrasonic 20-kHz fatigue loadings and conventional tests at 30 Hz. Fatigue life for a given stress amplitude was found to be higher in the case of ultrasonic fatigue whereas the fatigue limit was the same for both cases. Moreover, crack initiation was always inclusion-induced under ultrasonic loading while under conventional tests it occurred at slip bands or defects on the surface. The inevitable temperature increase under ultrasonic fatigue at high stress amplitudes along with the rate dependent deformation behavior of ferrite, as a body centered cubic (BCC) structure, were found as the key parameters explaining the observed fatigue behavior and thermal response under low and ultrasonic frequencies. The discrepancies observed between conventional and ultrasonic fatigue tests were assessed through the mechanisms of screw dislocation mobility in the ferrite phase as a BCC structure. The higher fatigue life and inclusion-induced crack initiations in the case of ultrasonic loading were attributed to the dynamic strain aging, which resulted from the high temperature increases at high stress amplitudes. The existence of a transition in deformation regime from thermally-activated to athermal regime under ultrasonic fatigue loading by increasing the stress amplitude was confirmed. Below the fatigue limit, deformation occurred in thermally-activated regime while it was in athermal regime above the fatigue limit. Under conventional loading deformation occurred in athermal regime for all stress amplitudes. From the analysis of the experimental data gathered in this work, guidelines were given regarding the comparison and interpretation of S-N curves obtained from conventional and ultrasonic fatigue testing. A transition map was produced using the experimental results for DP600 steel as well as data available in the literature for other ferrite based steels, showing the correlation between thermally-activated screw dislocation movement and the absence of failure in very high cycle fatigue
Maquin, François. "MÉTHODOLOGIE EXPÉRIMENTALE D'ÉTUDE DU COMPORTEMENT THERMO-MÉCANIQUE DES MATÉRIAUX SOUS SOLLICITATIONS CYCLIQUES." Phd thesis, Paris, ENSAM, 2006. http://tel.archives-ouvertes.fr/tel-00351687.
Full textKoutiri, Imade. "Effet des fortes contraintes hydrostatiques sur la tenue en fatigue des matériaux métalliques." Phd thesis, Paris, ENSAM, 2011. http://pastel.archives-ouvertes.fr/pastel-00599125.
Full textPu, Xiaoxue. "Thermomechanical study of the gigacycle fatigue behavior of pure iron and carbon-manganese steels : influence of chemical composition and microstructure on damage and crack initiation mechanism." Thesis, Paris 10, 2019. http://www.theses.fr/2019PA100051.
Full textThis work attempts to a better understanding of the fatigue damage in ferrite-pearlite steels in the Very High Cycle Fatigue (VHCF) domain. The influences of two parameters, pearlite phase percentage and free interstitial atoms percentage in solid solution, are investigated to understand dissipative mechanisms under 20 kHz high frequency fatigue loading. In-situ infrared thermography is carried out to record the temperature changes, while fractography studies and microscope observations are conducted to investigate the dissipative mechanism on the surface of specimens.For body centered cubic (BCC) materials, under high stress amplitudes, a sudden increase of the temperature occurs without a crack initiation and fracture. The inevitable temperature increase up to hundreds of degrees at high stress amplitudes, is caused mainly by the screw dislocations mobility, which is the key to explaining the observed fatigue behavior and thermal response of BCC structure under high frequency loading. Therefore, PSBs on surface and micro-voids in matrix emerge massively, accompanying with this abrupt temperature increase. These phenomena are considered as transition of deformation mechanism from thermal regime to athermal regime. At low amplitudes, few PSBs or surface roughness are still observed on the specimen surface. Through the cycles of PSB appearance on armco-iron, it’s found that PSBs are inclined to appear before 1x10(7)cycles, and PSB threshold lies below the conventional fatigue limit. The increase of pearlite phase content weakens the temperature elevation, and strengthens the fatigue properties. The presence of free interstitial atoms in steels results in appearence of a secondary temperature increase in the stabilized temperature part (100-200 degree). This behavior seems to be related to the interaction of edge dislocations with free interstitial atoms. Moreover, the remarkable hardening-softening-hardening phenomenon after the sudden temperature elevation to above 300 degree is thought as the interaction of multiplicated screw dislocations and free interstitial atoms
Refai, Khalil. "Effet de la méso-architecture sur le comportement en fatigue des structures lattices optimisées obtenues par fabrication additive." Thesis, Paris, HESAM, 2020. http://www.theses.fr/2020HESAE028.
Full textA numerical approach is proposed to assess the high cycle fatigue strength of periodic cellular structures produced by SLM under multiaxial loads. The model is based on a general numerical homogenisation scheme and an explicit description of the Elementary Cell combined to an extreme values analysis making use of a fatigue indicator parameter based on Crossland’s criterion. Also, geometric discrepancy and surface roughness are experimentally characterised and considered in the numerical model using three methods which are compared to the experimental fatigue strength. Topology optimisation (TO) pushes the boundaries of design freedom even further. In our study, Topology Optimisation was developed to prevent fatigue failure using SIMP method revisited and reformulated within the mathematical framework of Non-Uniform Rational BSpline functions
Louge, Julien. "Apports des mesures d’auto-échauffement pour l’étude de la fatigue des aciers : effets d’histoire et cisaillement pur." Thesis, Brest, École nationale supérieure de techniques avancées Bretagne, 2019. http://www.theses.fr/2019ENTA0001.
Full textIn the automotive industry, rolled steels are widely used to produce constituent parts of a car. The high cycles fatigue properties of these flat steel are usually determined in a virgin state of the material (i.e., without preliminary loading) under uniaxial cyclic loading with a constant stress amplitude. However, production of structural parts mostly involves some forming processes that have for consequence to modify the material state and thus to impact the fatigue properties. Furthermore, operational loads condition actually incurred by such a part is often multiaxial and with variable amplitude. The aim of this study is to take into account multiaxial and load history aspects in the fatigue characterization by means of self-heating measurements under cyclic loading. Multiaxial fatigue characterization usually involves a fatigue criterion that required the endurance limit of the material under a shear loading. But this limit is difficult to obtain for flat steel. That why a new shear test and two new disk-shaped specimens were proposed, permitting the use of self-heating measurements. Thus, the endurance limit of the material under a shear loading was determined. In order to study the impact of load history effects on fatigue properties, various self-heating tests were performed considering several initial pre-straining levels, several cyclic pre-loading and several load ratios. From these results, a new probabilistic two-scale model was proposed capable of describes selfheating curves with any load history considered and provides a reliable estimation of these load history effects on S-N-P curves