[Show abstract][Hide abstract] ABSTRACT: The surface state of mechanical components differs according to applied loadings. Industrial processes may produce specific features at the surface, such as roughness, local hardening, residual stresses or recrystallization. Under fatigue loading, all these parameters will affect the component lifetime, but in different manner. A better understanding of each surface state parameter, separately first and then all combined, will provide a better prediction of fatigue life. The study focuses on the effect of surface roughness. Crystal plasticity finite element computations have been carried out on three-dimensional polycrystalline aggregates with different roughness levels. Local mechanical fields have been analyzed both at the surface and inside the bulk to highlight the competition between crystallography and roughness to impose localization patterns. As soon as surface roughness is strong enough, classical localization bands driven by grains orientation are replaced by localizations patterns driven by the local roughness topology. Nevertheless, this effect tends to decrease gradually under the surface, and it becomes usually negligible after the first layer of grains. The discussion allows us to characterize the influence of the surface state on the local mechanical fields.
FDMD II - JIP 2014 - Fatigue Design & Material Defects, PAris, France; 06/2014
[Show abstract][Hide abstract] ABSTRACT: The influence of porosity on the wear behaviour of a reactive plasma spray Ti-6Al-4V/TiN coating is investigated through a comparison between experimental results and numerical simulations by finite elements. Samples have been coated using optimized plasma spray conditions and then tested in linear reciprocating sliding against a Ti-6Al-4V ball under different normal loads and number of cycles. Wear tracks were investigated using a combination of scanning electron microscopy, wavelength dispersive spectroscopy and profilometry to assess friction and wear mechanisms. Finite element analysis of the wear problem has been tackled with an iterative 2D model using remeshing to simulate wear and including some of the microstructural features of the coating such as the actual porosity shape and size distribution. Finite elements simulations are able to reproduce the wear kinetics observed experimentally. In addition both experimental and numerical analysis reveal that pores within the coating layer may represent weak points for the wear resistance.
[Show abstract][Hide abstract] ABSTRACT: Fatigue crack initiation is classically predicted by macroscopic models, where the critical variables are combinations of macroscopic stress and strain. Nevertheless, the local mechanisms responsible for crack initiation in metals and alloys are persistent slip bands (PSB), which develop on a microscale, in each grain. Having in view a better understanding of the local mechanisms, the purpose of this work is to get an improved knowledge of the local stress and strain fields and to examine the result obtained by using the relevant variables in local crack initiation models.
[Show abstract][Hide abstract] ABSTRACT: This study is devoted to the computation of realistic stress and strain fields at a local scale in fretting. Models are proposed to improve surface and volume modelling, by taking into account the heterogeneity of stress fields due to the irregular interface. This gives a new view toward damage mechanisms. The surface heterogeneity which is considered here, results from the third body trapped in the contact zone. This third body is known to drastically influence the contact conditions. The competition between wear and crack initiation is investigated with respect to local stress fields. The first model is used to study the evolutions of particles and the contact stress according to the loading conditions. Then, Dang Van's multiaxial fatigue model is used to predict crack initiation during the fretting test. This criterion may highlight the presence of microcracking everywhere in the contact zone.
[Show abstract][Hide abstract] ABSTRACT: In this work, the finite elements method (FEM) is used to analyse the growth of fretting cracks. FEM can be favourably used to extract the stress intensity factors in mixed mode, a typical situation for cracks growing in the vicinity of a fretting contact. The present study is limited to straight cracks which is a simple system chosen to develop and validate the FEM analysis. The FEM model is tested and validated against popular weight functions for straight cracks perpendicular to the surface. The model is then used to study fretting crack growth and understand the effect of key parameters such as the crack angle and the friction between crack faces. Predictions achieved by this analysis match the essential features of former experimental fretting results, in particular the average crack arrest length can be predicted accurately.
[Show abstract][Hide abstract] ABSTRACT: Fatigue crack initiation in ductile alloys like austenitic stainless steels is mainly due to the occurrence of localized deformation in persistent slip bands (PSB). The presence of PSB is classically related to the orientation of the surface grains. In fact, the local fields in a grain does not depend on the local orientation only. The aim of the present paper is to investigate the consequences of this observation, and to propose an analysis, where the neighborhood of the grain also plays a significant role. The study is made on a 316 stainless steel. Finite element computations using a crystal plasticity model are performed to simulate an aggregate submitted to a cyclic tension–compression loading. Various configurations of grain orientations ("clusters") are studied at the free surface of the aggregate. A statistical analysis of the results is carried out to extract significant information concerning the local strain and stress fields, including the most critical arrangements of grain orientations. The introduction of local fields in classical fatigue life prediction models provides an explanation of the experimental scatter.
International Journal of Fatigue 01/2010; 32:1748-1763. · 1.98 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The damage mechanisms in fretting tests are wear and fatigue cracking. The gradients of the stress and strain fields are quite high under the contact, so that the typical associated lengths can be compared to grain size. Since the microstructure size is not negligible when compared to the length associated to the loadings, it seems reasonable to explicitly represent the grains in the computations. This is proposed in this paper: a polycrystal plasticity model is used for the case of a disk-plane contact with two bodies made of titanium alloy. The simulations corresponding to a material response fretting map are compared to fretting experiments. The fatigue prediction is made by means of the Dang Van high cycle fatigue parameter.
[Show abstract][Hide abstract] ABSTRACT: Résumé — Le fretting est un endommagement insidieux qui peut prendre la forme de petites fissures s'amorçant en bordure de contact et se propageant sous la surface. Ces fissures peuvent grandement affecter les propriétés de services de pièces en abaissant la limite de fatigue, il est donc très important de comprendre et prédire leur développement. Dans ce travail, la méthode des éléments finis est utilisée pour prédire la croissance de fissures de fretting en lien avec leur trajet de fissuration. L'effet du mode mixte imposé par le contact ainsi que celui de la microstructure du matériau sont étudiés par simulation. Mots clés — fretting, propagation de fissure, éléments finis, microstructure.
[Show abstract][Hide abstract] ABSTRACT: The aim of the present paper is to investigate the consequences of the loading on the free surface response in polycrystalline aggregates. The study is made on a 316L stainless steel. Finite element computations using a crystal plasticity model are performed to simulate a polycrystalline aggregate submitted to different cyclic loadings. A statistical analysis of the results is carried out to extract information concerning the local stress and local strain fields at the free surface. The analysis of plastic strain localization on surface maps and inside the bulk through transparent volumetric views allows to exhibit the effects of the grain orientation and of the loading on local mechanical fields. The computation of an indicator characterizing extrusion/intrusion steps give some information on the initiation sites.