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[en] ANALYSIS OF MICROSTRUCTURAL DAMAGE IN AN INTERSTITIAL FREE STEEL AFTER VERY HIGH CYCLE FATIGUE / [pt] ANÁLISE DE DANOS MICROESTRUTURAIS EM UM AÇO INTERSTITIAL FREE APÓS FATIGA DE ALTÍSSIMO CICLOHELDER KEITARO ARCARI AMBO 07 July 2022 (has links)
[pt] Pesquisas atuais evidenciam que diversos materiais com aplicação nas
indústrias automotiva, ferroviária, aeronáutica e aeroespacial não apresentam um
limite de fadiga. Porém, em torno de 1860 Wohler propôs a curva S-N (tensão -
número de ciclos), admitindo um conceito que há um limite de fadiga, estipulado
na época como 1E5 ciclos e considerado como vida infinita em fadiga. Contudo,
com os avanços tecnológicos, inúmeros componentes falham na condição de vida
infinita, de maneira contrária ao proposto por Wöhler. Atualmente, se considera 3
regimes distintos de fadiga: fadiga de baixo ciclo, fadiga de alto ciclo e fadiga de
altíssimo ciclo. Nas indústrias automotivas há um extensivo uso de chapas de aço
para a fabricação de componentes de geometria complexa. Neste contexto, o aço
interstitial free (IF – aços com baixa quantidade de átomos intersticiais), é um dos
materiais de largo emprego, em função da sua excelente estampabilidade. O aço IF
apresenta uma microestrutura ferrítica com uma baixíssima quantidade de
elementos intersticiais. Interstícios com carbono e nitrogênio são estabilizados pelo
titânio (Ti) ou nióbio (Nb) formando frações de volume extremamente pequenas na
matriz ferrítica de carbetos e nitretos. O trabalho baseia-se na investigação
experimental dos danos microestruturais no aço IF após carregamentos cíclicos de
longa duração. Amostras do aço IF estabilizado pelo Ti foram submetidas a ensaios
ultrassônicos de fadiga com carregamentos superiores a 1E7 ciclos. Se observou o
aparecimento de danos microestruturais (persistent slip bands, PSB s) após
determinado número de ciclos. Por meio de análise no microscópio ótico foi
estimado que 97,76 por cento da vida em fadiga foi destinado para nucleação de trinca
enquanto 2,23 por cento para a propagação de trinca. Houve o projeto de um novo corpo de
prova para a máquina de fadiga de altíssimo, com o intuito de acompanha a
superfície do material durante os ensaios. É proposto a equação da curva S-N fadiga
de altíssimo ciclo com os dados extraídos dos testes. E por fim, é sugerido um fator
de correção na equação de deformação plástica cisalhante nos PSB s formados / [en] Current research shows that several materials with application in the
automotive, railway, aeronautical and aerospace industries do not have a fatigue
limit. However, around 1860 Wohler proposed the S-N curve (stress - number of
cycles), admitting a concept that there is a fatigue limit, stipulated at the time as
1E5 cycles and considered as infinite fatigue life. Although, with technological
advances, numerous components fail in the condition of infinite life, contrary to
what Wöhler proposed. Currently, 3 different fatigue regimes are considered: low
cycle fatigue, high cycle fatigue and very high cycle fatigue. In the automotive
industries there is an extensive use of sheet steel for the manufacture of components
with complex geometry. In this context, interstitial free steel (IF steel) is one of the
widely used materials, due to its excellent stampability. IF steel has a ferritic
microstructure with a very low amount of interstitial elements. Interstices with
carbon and nitrogen are stabilized by titanium (Ti) or niobium (Nb) forming
extremely small volume fractions in the ferritic matrix of carbides and nitrides. The
work is based on the experimental investigation of microstructural damage in IF
steel after long-term cyclic loading. Samples of Ti-stabilized IF steel were subjected
to ultrasonic fatigue tests with loads greater than 1E7 cycles. It was observed the
arising of microstructural damage (persistent slip bands, PSB s) after a certain
number of cycles. By means of image analysis in the optical microscope, it was
estimated that 97.76 percent of the fatigue life was destined for crack nucleation while
2.23 percent for crack propagation in very high cycle fatigue regime. There was the
project of a new specimen for the very high fatigue machine, in order to accompany
the surface of the material during the tests. It is proposed the equation of the curve
S-N fatigue of very high cycle os with data extracted from the tests. Finally, a
correction factor is suggested in the plastic shear deformation equation in PSB s
based on experimental data.
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Approches expérimentales et multi-échelles des processus d'amorçage de fissures en fatigue sous chargements complexes / Experimental and multi-scale approaches of fatigue crack initiation process under complex loading conditionsAgbessi, Komlan 21 March 2013 (has links)
Les méthodes de calcul en fatigue à grande durée de vie sont en cours de développement depuis des décennies et sont utilisées par les ingénieurs pour dimensionner les structures. Généralement, ces méthodes se basent sur la mise en équations de quantités mécaniques calculées à l'échelle macroscopique ou mésoscopique. Les critères de fatigue multiaxiale reposent généralement sur des hypothèses de changement d'échelle dont l'objectif est d'accéder à l'état de contraintes ou de déformations à l'échelle du grain. Dans les approches de type plan critique (Dang Van, Papadopoulos, Morel), l'amorçage d'une fissure de fatigue est considéré comme piloté par une quantité mécanique liée à une orientation matérielle particulière (plan critique). Si ces phénomènes sont bien établis dans le cas des chargements uniaxiaux, la nature des mécanismes liés à l'activation des systèmes de glissement, à la multiplicité du glissement et aux différents sites préférentiels d'amorçage de fissures sous chargements complexes reste peu connue.Afin de mieux comprendre les mécanismes d'endommagement en fatigue multiaxiale, les techniques d'analyse et de caractérisation de l'activité plastique (activation des systèmes de glissements, bandes de glissement persistantes) et d'observation de l'endommagement par fatigue ont été mises en place en se basant principalement sur des observations MEB et analyses EBSD. Ces investigations ont permis de mettre en lumière les effets des chargements non proportionnels sur la multiplicité du glissement sur du cuivre pur OFHC. L'étude statistique des sites préférentiels d'amorçage de fissures montre que les grains à glissement multiple présentent une forte probabilité d'amorçage de fissures, surtout sous les chargements non proportionnels. Nous avons également mis en évidence le rôle des joints de grains et des joints de macle sur le développement de la plasticité à l'échelle de la microstructure. Les résultats expérimentaux sont confrontés à ceux du calcul éléments finis (EF) en plasticité polycristalline sur des microstructures synthétiques 3D semi-périodiques. L'application du critère de Dang Van à l'échelle mésoscopique (le grain) montre une forte variabilité de la contrainte hydrostatique et du cisaillement. Cette variabilité est plus importante pour un modèle de comportement cristallin élastique anisotrope. Le rôle de la plasticité cristalline se révèle secondaire. Ces analyses permettent de remettre en perspective les hypothèses usuelles de changement d'échelle utilisées en fatigue multiaxiale. Enfin, une méthode basée sur la statistique des valeurs extrêmes est proposée pour le dépouillement des calculs EF sur agrégats. Cette analyse a été appliquée sur la contrainte équivalente associée au critère de fatigue de Dang Van pour les calculs d'agrégats polycristallins avec différentes morphologies et orientations des grains. Les effets de la surface libre, du type de chargement et du modèle de comportement mécanique des grains ont été analysés. Les résultats offrent des perspectives intéressantes sur la modélisation de l'amorçage des fissures en fatigue multiaxiale des matériaux et des structures avec une prise en compte de la microstructure. / The development of high cycle fatigue (HCF) strength assessment methods has now been running for more than a century, leading to relatively efficient methods for engineers. Generally, these methods are based on mechanical quantities calculated at macroscopic or mesoscopic scales and validated by the model's ability to accurately reproduce experimental results. Multiaxial fatigue strength criteria are usually based on scaling transition assumptions aiming at capturing the stress or strain state in the grain. In the case of critical plane based criteria (Dang Van, Papadopoulos, Morel), fatigue crack initiation is supposed to be controlled by a mechanical quantity linked to a particular orientation (critical plane). If fatigue crack initiation phenomena are well established in the case of uniaxial loadings, the nature of the mechanisms involved in the activation of slip systems, multiple slip and preferential sites of rack initiation under complex loadings remains little known.To better understand the mechanisms of multiaxial fatigue crack initiation, analysis and characterization of the plastic activity (e.g. activation of slip systems, persistent slip bands) and observations of fatigue damage have been carried out on pure OFHC copper, using SEM and EBSD analyses. These investigations enabled to highlight the effects of non-proportional multiaxial loadings through the induced multiplicity of slip. The statistical study of preferential crack initiation sites shows that grains with multiple slip have a high probability of crack initiation, especially under non-proportional loading. We also highlighted the role of grain boundaries and twin boundaries on the development of plasticity across the microstructure. The experimental results were compared with those of finite element crystal plasticity computations on synthetic 3D semi-periodic microstructures. The application of the Dang Van criterion at the mesoscopic (grain) scale showed a strong variability of the hydrostatic stress and the shear stress. This variability was greater for anisotropic elastic behavior, while the role of crystal plasticity seemed to be secondary. These analyses allowed putting into perspective the usual assumptions of scaling transition rules used in multiaxial fatigue. Finally, a method based on the extreme values statistics was proposed and applied to the equivalent stress associated to the Dang Van fatigue criterion for polycrystalline aggregate computations with different morphologies and grains orientations. The effects of the microstructure, free surface, loading types and mechanical behavior were analyzed. The results offered interesting insights into the multiaxial fatigue modeling of metals and structures taking into account the microstructure.
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