SAM

A mechanistic approach to the Kitagawa-Takahashi diagram using a multiaxial probabilistic framework

PESSARD, Etienne — Tue, 01 Jan 2013 00:00:00 GMT

A mechanistic approach to the Kitagawa-Takahashi diagram using a multiaxial probabilistic framework PESSARD, Etienne; BELLETT, Daniel; MOREL, Franck; KOUTIRI, Imade The aim of this paper is to propose a flexible multiaxial modelling framework that is capable of combining two fatigue damage mechanisms so as to continuously describe the Kitagawa-Takahashi diagram. It is proposed that this diagram represents two distinct fatigue damage mechanisms: one associated with crack initiation (or microstructurally small cracks) and the other with crack propagation (or long cracks). It is further postulated that these damage mechanisms are more appropriately modelled using di erent fatigue criteria. A probabilistic modelling framework is proposed in which any two suitable fatigue criteria can be combined in order to simultaneously model both damage mechanisms and the transition between them. This framework is based on the weakest link hypothesis and results in a probabilistic Kitagawa-Takahashi type diagram.

Microstructural-based analysis and modelling of the fatigue behaviour of cast Al-Si alloys

LE, Viet-Duc — Thu, 01 Jan 2015 00:00:00 GMT

Microstructural-based analysis and modelling of the fatigue behaviour of cast Al-Si alloys; Analyse et modélisation du comportement en fatigue des alliages d'aluminium de fonderie LE, Viet-Duc; MOREL, Franck; BELLETT, Daniel; PESSARD, Etienne; SAINTIER, Nicolas; OSMOND, Pierre This paper describes a microstructural-based high cycle fatigue behaviour model applied to cast Al-Si alloys used in an automobile context. These materials are characterized by the presence of different microstructural heterogeneities at different scales: the aluminium matrix (DAS/SDAS and the precipitation hardening level), inclusions (Si particles and intermetallic) and casting defects (porosity). It is shown that the effects of these factors on the HCF damage mechanisms are important and can depend on the loading mode. A multiaxial fatigue test campaign has been carried out using three cast aluminium alloys, fabricated by different casting processes (gravity die casting and lost foam casting), associated with several heat treatment(T7 and Hot Isostatic Pressing-HIP). The HIP treatment is used to eliminate or minimise the porosity. The first part of the article is dedicated to the experimental characterization of the HCF damage mechanisms. With regard to the effect of the casting defects, a study of natural fatigue crack growth and artificial long crack growth is presented and subsequently used to choose an appropriate fatigue strength criterion to take into account the effect of defects, for different loading modes (tension, torsion and combined tension-torsion). Finally, a flexible modelling framework, providing the possibility of combining any two suitable criteria, which leads to the construction of a multiaxial Kitagawa-Takahashi diagram, is used.

A new approach to model the fatigue anisotropy due to non-metallic inclusions in forged steels

BELLETT, Daniel — Sun, 01 Jan 2012 00:00:00 GMT

A new approach to model the fatigue anisotropy due to non-metallic inclusions in forged steels BELLETT, Daniel; MOREL, Anne; MOREL, Franck; PESSARD, Etienne The objective of this work is to propose an anisotropic fatigue criterion for the sizing of industrial forged components. The results from different experimental campaigns using three different rolled steels are first presented. The effect of inclusions and the microstructure on the fatigue behaviour are investigated. For the two ferrite-pearlitic steels tested, the presence of a microstructure consisting of elongated grains has no observable effects on the fatigue behaviour. For two of the three steels studied the presence of non-metallic inclusions, elongated in the rolling direction, form the origin of the anisotropic fatigue behaviour. The proposed probabilistic model is based on the competition between two possible fatigue crack initiation mechanisms. The anisotropic character of the fatigue resistance of forged components is taken into account by the definition of the geometry and the orientation of the non-metallic inclusion. This criterion results in the establishment of a probabilistic Kitagawa type diagram.

Effet bénéfique de l'extrusion à froid sur la tenue en fatigue de pièces automobiles

GERIN, Benjamin — Fri, 01 Jan 2016 00:00:00 GMT

Effet bénéfique de l'extrusion à froid sur la tenue en fatigue de pièces automobiles GERIN, Benjamin; VERDU, Catherine; MARY, Alain; MOREL, Franck; PESSARD, Etienne La déformation introduite dans le matériau par les procédés de forgeage à froid peut être considérée comme un type de pré-déformation, appliquée avant l’utilisation en fonctionnement du composant. Cette déformation modifie le matériau, en introduisant écrouissage et contraintes résiduelles. Les caractéristiques mécaniques du matériau en sont donc modifiées, ainsi que la dureté et la microstructure. De plus, la surface du matériau est affectée, l’extrusion permet d’obtenir par exemple un très bon état de surface (Ra = 0,1 à 0,5μm) [Caminaga et al., 2007]. Les modifications que subit le matériau dépendent des conditions de forgeage : taux de corroyage, forme de la matrice, type de lubrification… Le pivot de direction présenté en figure 1 est un exemple de pièce extrudée à froid. Seules les zones fonctionnelles du composant ont été usinées (gorges, rotule...), le reste de sa surface a été laissée brut de forge. Le composant présente plusieurs réductions de sections consécutives. Ces réductions de section témoignent des différents niveaux de prédéformation introduits lors de la mise en forme. Les caractéristiques mécaniques et la tenue en fatigue dépendent de l’intensité de cette prédéformation, et donc de la réduction de section (aussi appelé taux de corroyage).

The effect of machining defects on the fatigue behaviour of the Al7050 alloy

ABROUG, Foued — Fri, 01 Jan 2016 00:00:00 GMT

The effect of machining defects on the fatigue behaviour of the Al7050 alloy ABROUG, Foued; PESSARD, Etienne; GERMAIN, Guénaël; MOREL, Franck; CHOVE, Etienne During the High Speed Machining (HSM) of aircraft components, geometrical defects, such as mismatches or chatters, can be created. To obtain a high surface quality, an expensive manual grinding operation is systematically done to remove these defects. The aim of this study is to identify the impact of HSM defects on the fatigue behaviour of the aluminium alloy Al7050. After listing and reproducing the most frequently observed surface defects, fatigue tests are conducted under fully reversed plane bending loads. Investigations carried out in previous work showed that residual stresses and the strain hardening introduced by machining under these conditions can be neglected. Therefore, only the geometric aspect of the surface integrity is considered in this study. The results show that the fatigue strength decreases only when the surface roughness is significantly degraded. It is also pointed out that manual grinding allows the effect of the machining defects to be removed from the fatigue behaviour. In order to predict the influence of the surface condition on the fatigue behaviour, a numerical approach based on the real surface topology is also developed. Crack initiation sites that are numerically identified are in agreement with experimental results. Numerical simulation results are compared to the predictions of different fatigue criteria from the literature and discussed over a wide range of surface defects.

Influence of SLM process parameters on the surface finish, porosity rate and fatigue behavior of as-built Inconel 625 parts

KOUTIRI, Imade — Mon, 01 Jan 2018 00:00:00 GMT

Influence of SLM process parameters on the surface finish, porosity rate and fatigue behavior of as-built Inconel 625 parts KOUTIRI, Imade; PESSARD, Etienne; PEYRE, Patrice; AMLOU, Ouafae; DE TERRIS, Thibaut This paper is dedicated to understanding fatigue crack initiation for an Inconel 625 manufactured by SLM, using a hatching + contour procedure. In the first part of the paper, an optimum set of parameters was found to deliver the best surface roughness combined with low porosity. This process optimization, mostly focused on adjusting the volume energy density aimed at finding a compromise between an optimum densification state and a minimum number of contaminating spatters. Secondly, a fatigue test campaign has been conducted on as-built SLM samples or polished samples. The analysis of failure surfaces allowed identifying different heterogeneities at the origin of the fatigue damage for each configuration. The embedded particles on the surface of as-build specimens have been shown to play an important role in fatigue and need to be optimized or taken into account in the fatigue strength design of SLM components.

A non-local approach to model the combined effects of forging defects and shot-peening on the fatigue strength of a pearlitic steel

GERIN, Benjamin — Mon, 01 Jan 2018 00:00:00 GMT

A non-local approach to model the combined effects of forging defects and shot-peening on the fatigue strength of a pearlitic steel GERIN, Benjamin; VERDU, Catherine; MOREL, Franck; PESSARD, Etienne This study focuses on the influence on fatigue behaviour of the surface integrity of a steel connecting rod. The component is hot-forged and shot-blasted, producing a complex surface state with large surface defects and high residual stresses. In a prior study, the surface was thoroughly characterized and fatigue tests were performed. Several different surface states were analysed in order to quantify the influence of the various surface aspects. These tests showed that the forging defects and the residual stresses are the most influential aspects of the surface. The goal of this paper is to develop an approach capable of taking into account the influence of both these aspects on fatigue behaviour. Two methods were developed. First, using surface scans of the fatigue specimens, the forging defects were fitted with ellipsoids so as to determine their size and shape. This allows to easily compare the numerous defects and test various criteria in order to identify the critical defect of each specimen. The second method used was the finite element simulation of the defects based on real topography scans. Using a non-local approach based on the theory of critical distances, the simulations were used to accurately predict the influence of the defects’ geometry. The residual stress profiles were integrated in the simulations using Dang Van’s criterion. The predictions are accurate and show the importance of taking into account the real defect geometry when estimating the fatigue strength.

HCF of AA7050 alloy containing surface defects: Study of the statistical size effect

ABROUG, Foued — Mon, 01 Jan 2018 00:00:00 GMT

HCF of AA7050 alloy containing surface defects: Study of the statistical size effect ABROUG, Foued; PESSARD, Etienne; GERMAIN, Guénaël; MOREL, Franck This work investigates the effect of artificial surface defects on the fatigue limit of a 7050 Aluminum alloy (AlZn6CuMgZr). A large fatigue testing campaign under fully reversed plane bending loading is undertaken on specimen with artificial surface hemispherical defects. The defect number was varied from 1 to 44 defects per specimen and the diameter size ranged from 60 μm to 800 μm. The test results allow the characterization of both the defect effect and scale effect on the fatigue behavior of the material. A probabilistic approach based on the weakest link concept together with a fatigue crack initiation criterion are used to account for the stress distribution and the size of the highly stressed volume. This approach leads to a probabilistic Kitagawa-Takahashi type diagram, which in this case explains the relationship between the defect size and the scale effect on the fatigue strength. The predictions show good agreement with the experimental results and illustrate the importance of taking the scale effect into account when designing components containing different surface defects types or roughness patterns.

Texture control of 316L parts by modulation of the melt pool morphology in selective laser melting

ANDREAU, Olivier — Tue, 01 Jan 2019 00:00:00 GMT

Texture control of 316L parts by modulation of the melt pool morphology in selective laser melting ANDREAU, Olivier; PEYRE, Patrice; PENOT, Jean-Daniel; DE TERRIS, Thibaut; DUPUY, Corinne; BAUDIN, Thierry; KOUTIRI, Imade; PESSARD, Etienne; SAINTIER, Nicolas In this study, 316L parts were fabricated with the selective laser melting additive layer manufacturing process using unidirectional laser scan to control their texture. The melt pool shape, microstructure and texture of three different cubic samples were analyzed and quantified using optical microscopy and electron back-scattered diffraction. The samples scanned along the shielding gas flow direction were shown to exhibit shallow conduction melt pools together with a strong {110}<001> Goss texture along the laser scanning direction. The sample prepared with a laser scan perpendicular to the gas flow direction had deeper melt pools, with a weaker {110} <001> Goss texture in addition to a <100> fiber texture parallel to the scanning direction. Correlations were proposed between the melt-pool geometry and overlap and the resulting texture. The decrease of the melt pool depth was assumed to be linked to local attenuation of the laser beam effective power density transmitted to the powder bed.

Contribution à la prise en compte des effets du procédé de mise en forme sur la tenue en fatigue des matériaux métalliques

PESSARD, Etienne — Wed, 01 Jan 2020 00:00:00 GMT

Contribution à la prise en compte des effets du procédé de mise en forme sur la tenue en fatigue des matériaux métalliques PESSARD, Etienne Les travaux de recherche présentés dans ce mémoire se concentrent sur la prise en compte de l’effet du procédé sur la tenue en fatigue à grand nombre de cycles des matériaux métalliques (entre 10⁵ et 10⁷ cycles). Ces travaux ont pour originalité de balayer une large gamme de couple procédé-matériau avec pour objectif de développer des critères capables d’intégrer l’effet du procédé sur la tenue en fatigue. Pour les différents couples procédé-matériau abordés, des essais de fatigue originaux visant à découpler les effets des paramètres procédés sont développés et conduits. La compréhension fine des mécanismes d’endommagement permet ensuite de proposer des approches le plus souvent probabilistes physiquement justifiées. Des approches numériques basées sur la méthode des éléments finis intégrant les spécificités des couples matériaux-procédé étudiés (topologie de surface, gradient de chargement, défaut…) sont parfois conduites pour analyser les résultats obtenus. Quand les liens entre les caractéristiques sensibles en fatigue et les conditions procédé sont connus, un chainage numérique complet procédé-tenue en fatigue est proposé. Plus précisément, mes travaux se sont tout d’abord intéressés aux procédés modifiant la microstructure du matériau par déformation plastique à chaud et à froid mais également par traitement thermique. Mes activités se sont ensuite tournées vers la prise en compte de l’intégrité de surface des pièces forgées, usinées et issues de fabrication additive sur la tenue en fatigue. Différentes approches numériques intégrant la topologie de surface 3D réelle ont pour ces besoins été développées. La prise en compte des populations de défauts (pores) dans les matériaux métalliques issus de fabrication additive est ensuite devenue l’une de mes activités de recherche importante. Ces différents travaux ont nécessité le développement d’approches probabilistes et transverses entre les différentes problématiques abordées.