Jamal Fajoui

University of Nantes | UNIV Nantes · Département de Physique

A multi-layer structure by sandwiching rigid cellular PVC (polyvinyl chloride) foam between two GFRP (glass-fibre reinforced plastic) faces is studied in order to investigate the sample edge effects on fatigue life of the sandwich structure. Therefore, a new geometry of specimen is developed based on numerical simulation approach. Compared to the r...

The objective of this paper is to contribute to a better understanding of the deformation mechanisms of a rolled polycrystalline cobalt sheet. In order to achieve this goal, a particular attention is given to the strain-induced phase transformation and the forming parameters on which it may depend such as the plastic anisotropy or the loading path....

The sequence of structural changes produced in two deformed microstructures of austenitic stainless steel elaborated by a multipass unidirectional cold rolling (CR) and a two‐step one, to a 75% thickness reduction, is followed by dilatometric experiments. The two materials show different dilatometric behaviors. X‐ray diffraction and microhardness m...

Predicting temperature field in a Gas Tungsten Arc Welding process is essential and precondition for any study of thermal or mechanical behavior of the weld. In fact, thermal, material proprieties and physical phenomena have to be taken into account in order to predict welding morphology, residual stresses and the final product quality of the weld....

Plastic deformation of titanium alloys depends on the temperature and the mechanical loading mode. It is accommodated by a complex mixture of slip and twinning systems. It remains nevertheless unclear which deformation modes are activated in the polycrystal during loading, especially with the temperature. To better understand the mechanical behavio...

Metal additive manufacturing is an emerging advanced technology, it differs from conventional manufacturing methods as machining, casting, and forging, which are either subtractive or forming. Firstly, the objective of this work is to elaborate a new 316L stainless steel material by selective laser melting (SLM) from metallic powder according to sp...

The development of modern technologies requires the use of materials with high mechanical properties specific to their use, but with low densities. Composite materials with sandwich structure meet the above requirements, due to their low density, high strength, high rigidity and excellent durability. Despite of their advantages, sandwich structure...

Le cobalt polycristallin est un métal de structure hexagonale compacte (HC) qui présente un rapport c/a proche de celui d’une structure compacte idéale et une énergie de faute d’empilement faible, lui conférant des mécanismes de déformations particuliers. Dans cette étude, le comportement mécanique d’une tôle de 500 μm d’épaisseur brute de laminage...

Metal additive manufacturing is an emerging advanced technology, it differs from conventional manufacturing methods as machining, casting and forging, which are either subtractive or forming. Firstly, the objective of this work is to elaborate a new 316L stainless steel material by Selective Laser Melting (SLM) from metallic powder according to spe...

Neutron and high-energy X-ray diffraction methods have been used to investigate deformation mechanisms (slip and twinning activities) during uniaxial tensile tests of Grade 1 commercially pure titanium (Ti-α). These two diffraction techniques have provided valuable and complementary information. The predictions made by an Elasto-Plastic Self-Consis...

The allotropic phase transformation in polycrystalline high-purity cobalt is incompletely reversible and exhibits a temperature hysteresis. This leads to the presence of a FCC metastable phase at room temperature, which alters the mechanical properties. Moreover, this phase transformation seems to be induced by the plastic deformation. The influenc...

L’effet de la microstructure et du chargement thermomécanique sur les transformations de phases du cobalt polycristallin de haute pureté (99,9 %) a été investigué via diverses techniques expérimentales. Les recuits thermiques permettent une homogénéisation de la microstructure sans provoquer l’achèvement de la transformation martensitique. La trans...

The development of modern technologies requires the use of materials with high mechanical properties and lightest weight structure. With their low density, high strength, high rigidity and excellent durability, the sandwich structures are one of the best materials to meet these requirements. Despite of these advantages, sandwich structure is rather...

Thermal and mechanical effects on the phase transformation of polycrystalline two-phase HCP/FCC cobalt have been studied in coupled and decoupled ways. This experimental work has involved in situ X-ray and neutron diffraction analysis to determine phase proportions under various thermomechanical conditions, those which are not referenced in the lit...

In situ neutron diffraction method has been used to investigate deformation mechanisms (slip and twinning deformation modes) during uniaxial tensile tests of extruded a-titanium alloy at room temperature, 100 and 300 °C. For this purpose, the prediction of an Elasto-Plastic Self-Consistent (EPSC) model was compared with the experimental data. Resul...

MLZ Large Facility

In practice for all metallic materials, damage by fatigue usually takes in two steps, the appearance of an initial crack which then grows as a function of the present microstructure. The objective of this study is to identify the elements influencing the fatigue crack growth rate on aluminum alloys of different microstructures. Characterization tes...

Institut Laue-Langevin (ILL) Large Facility

In the present paper, the impact of exfoliation corrosion (EFC) of 2024 and 5083 aluminum alloys on the residual mechanical properties was investigated. Tensile test had been carried out on pre-corroded specimens, exposed to laboratory accelerated exfoliation corrosion solution. The analysis suggests that exfoliation exposure leads to decrease the...

In this paper, quantitative investigation of localized corrosion, in particular, exfoliating corrosion (EXCO) of aluminum alloys 2024 T3 and 7075 T6, is carried out. The mechanism of EXCO focusing on the effects of the combination of materials/environment/mechanical stress was also considered. An optical microscope (MO) and a scanning electron micr...

ISIS large facility

In the present work, the influence of heat input on phase balance, residual stresses and corrosion resistance of UNS 32101 lean duplex stainless steel welds has been investigated. Three weldment types were obtained with different heat inputs by varying welding current, using gas tungsten arc welding process. The results revealed that increasing hea...

Automotive brake lining materials are composite materials of very complex formulation, highly heterogeneous. They help to carry out the desired combination of braking performance properties. Obviously, it requires that the friction material exhibits good complementarities and adequate combination of physico-chemical, thermal properties that act syn...

In the first part of this paper, a viscoelastic model is used to numerically study the cellular foam core fatigue behaviour. Material’s temperature evolution is quantified with a numerical/experimental confrontation. In the second part, the slamming phenomenon is discussed by measuring the panel flexural strain on a racing sailboat during navigatio...

Le durcissement structural des alliages d'aluminium c’est l’une des solutions que les métallurgistes ont adopté pour améliorer les propriétés mécaniques des alliages d’aluminium. Mais la corrosion exfoliante et la corrosion intergranulaire dans ces alliages d'aluminium ont un processus très complexe qui peut être affecté par divers facteurs tels qu...

In practice for all metallic materials, damage by fatigue usually takes in two steps, the appearance of an initial crack which then grows as a function of the present microstruc-ture. The objective of this study is to identify the elements influencing the fatigue crack growth rate on aluminum alloys of different microstructures. Characterization te...

The analysis and design of offshore monopile foundations are generally undertaken using deterministic approaches. In this paper, a probabilistic analysis is performed taking into account the soil spatial variability. The aim is to compute the failure probability against exceeding a threshold value on the pile head displacement. The mechanical model...

L'objectif de ce travail était d'étudier l'influence de la corrosion exfoliante (EFC) des alliages d'aluminium 2024 et 5083 sur la dégradation des propriétés mécaniques. L'essai de traction a été effectué sur des échantillons pré-corrodés, exposés à une solution de corrosion exfoliante accélérée en laboratoire. L'analyse suggère que l'exposition à...

This article describes and explains the tribological tests and methods for the evaluation of the performance of the brake friction materials. It starts by discussing the particularities of these materials and the variation of characterization tests, which can experimentally simulate many aspects of brake situation but with a large field of tribo-te...

A theoretical and experimental study was carry out to investigate deformation mechanisms in a textured titanium alloy. In situ neutron diffraction measurements were performed to analyze different {hk.l} family planes ({10.0}, {10.1}, {11.0} and {00.2}) and determine the corresponding internal strain pole figures. This method was applied to a pure t...

The purpose of this study is to investigate the influence of a spatially moving load and edge effects on the fatigue life of the foam-cored sandwich structures. A spatially moving load can be observed in structures subjected to slamming. A new geometry of specimen is developed to reduce the influence of edge effects in the test specimen. Numerical...

L’objectif de cette étude est de voir l’influence des éléments d'alliage et de la microstructure sur les propriétés mécaniques et sur le comportement en fatigue sur des différentes nuances d’alliages d’aluminium. Passant par des essais de caractérisations mécaniques et d'analyse microstructurale sur les nuances (2024-T3, 5083-H111, 6082-T6 et 7075-...

In marine construction, ship hulls are subjected to high fatigue stresses due to the impact of waves and their cyclic solicitations. However, fatigue behavior evaluation of the constitutive materials of the hull is based on standard tests. The objective of this study is to take a critical view of the current standard test method set to investigate...

In marine construction, ship hulls are subjected to high fatigue stresses due to the impact of waves and their repetitive actions. However, fatigue life data of the constitutive materials of hull are based on standard test [1]. This paper explains the design of a sandwich specimen under fatigue from 4-points bending test. Following the work from [2...

The generation of internal stresses/strains arising from mechanical deformations in single-phase engineering materials was studied. Neutron diffraction measurements were performed to study the evolution of intergranular strains in austenitic steel during sequential loadings. Intergranular strains expand due to incompatibilities between grains and a...

Noise in vehicle is induced by compounds and surface characteristics particularly in the friction contact. The purpose of the present work is to study the influence of brake pads containing steel fibers as well as to relate surface roughness on the generation of brake squeal. Two brake pads, steel fiber-free and with steel fiber materials, were stu...

In this work, we demonstrate that current fatigue characterization tests with high density foam suffer from edge effects within the specimen, probably leading to early failures. Then a new specimen is designed to avoid this artifact. With this developed geometry, the stress is concentrated in the center of the specimen and not in the edge as given...

During hot forming process, tools are exposed to severe and complex loads. As result, damages by oxidation and thermal and mechanical fatigue are generated which cause tools failure. The aim of this work is a multi-scale analysis of damaged hot forging tools. An expertise of main failures on industrial hot-forging die, made of hot-working die steel...

To understand how internal strains evolve for textured material, it is necessary to obtain large data set concerning the behavior of a grain group from a representative volume of the material under loading conditions. In this work, in situ neutron diffraction technique was used to analyze different {hk.l} reflections ({10.0}, {10.1}, {11.0} and {00...

For the first time, Eshelby-Kröner self-consistent model extended to thermo-elastic loading is developed using the geometric mean for performing the set average operations. A new homogenization Eshelby-Kröner self-consistent approach is developed using a geometric average during thermo-mechanical loading. The description of the material thermo-elas...

Intergranular strains due to tensile plastic deformation were investigated in a commercially pure Ti. Neutron diffraction has been used to characterize the evolution of residual elastic strain in grains with different crystallographic orientations. Experimental data have been obtained for the macroscopic stress-strain curve and the intergranular st...

Intergranular strains due to tensile plastic deformation were investigated in a commercially pure Ti. Neutron diffraction has been used to characterize the evolution of residual elastic strain in grains with different crystallographic orientations. Experimental data have been obtained for the macroscopic stress-strain curve and the intergranular st...

A self-consistent scheme taking into account the intragranular microstructure is applied for the micromechanical modelling of the elastoplastic material behaviour during monotonic and sequential loading paths. The intragranular description used in the model is initially based on experimental observations of the dislocations evolution in body-centre...

Foam-cored sandwich yacht hulls are subjected to high core shear stresses during slamming events. As slamming is repetitive by nature, failures observed on boats may be due to fatigue. This study aims to investigate possible improvements to fatigue testing of both foam core materials and sandwich specimens. In general test setup induces differences...

A two-level homogenization approach is adopted in order to predict the stress state of deformed polycrystals in correlation with the microstructure. This study is devoted to complex loading paths behaviour of cubic material. A mechanical description of the grain is developed through a micro-meso transition based on elastoplastic Kröner’s model. The...

A new theoretical approach is developed to simulate the elastoplastic behaviour in cubic alloys during various strain-path changes. The polycristal is considered as a composite consisting of hard dislocation walls of high local dislocation density which are separated by soft regions of low local dislocation density. The improved elasto-plastic self...

An experimental and theoretical investigation was carried out to study the in-depth distribution of residual stress after a cold-rolling test in a zirconium alloy. Grazing incidence X-ray diffraction was used to analyze the heterogeneous stress field with regard to different diffraction volumes below the surface of the sample. An interpretation of...

Two complementary methods were used to analyse the deformation mechanisms involved in the plasticity of commercially pure titanium during compressive loading: neutron diffraction and the self-consistent model. The strain response of 15 crystallographic planes was tracked during the measurement, in directions both parallel and transverse to the stra...

Dans ce travail, un matériau composite à fibres de verre et à matrice vinylester, obtenu par pultrusion, est soumis à un chargement en flexion 3 points, d'abord sous chargement quasi statique, et ensuite en fatigue à fréquence relativement faible. La technique d'émission acoustique est utilisée lors des essais afin de préciser le seuil d'endommagem...

Deux méthodes complémentaires ont été utilisées pour analyser les gradients de contraintes résiduelles dans des plaques de Zy-4 laminées à froid à différent taux de déformation : la diffraction des rayons X et le modèle autocohérent. Une méthode d'analyse du gradient de contraintes par diffraction des rayons X en faible incidence (méthode « multiré...

Scale-transition models, such as Eshelby-Kröner self-consistent framework, which are often used for predicting the effective behavior of heterogeneous materials or estimating the distribution of local states from the knowledge of the corresponding macroscopic quantities, require the extensive use of set averages. The present paper is devoted to the...

Scale-transition models, such as Eshelby-Kröner self-consistent framework, which are often used for predicting the effective behavior of heterogeneous materials or estimating the distribution of local states from the knowledge of the corresponding macroscopic quantities, require the extensive use of set averages. In the present work, the fundamenta...

Le comportement mécanique en plasticité d'un acier inoxydable a été étudié durant des essais Bauschinger (traction/compression) au travers d'un couplage entre des mesures in situ de déformations intergranulaires et un modèle à double transition d'échelles. L'effet Bauschinger a été caractérisé expérimentalement par l'analyse des différentes courbes...