Papers by Author: Delphine Retraint

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Authors: Florent Cochennec, Emmanuelle Rouhaud, Delphine Retraint, Sébastien Rouquette, Arjen Roos
Abstract: Shot-peening is a surface treatment widely used in the industry to improve fatigue life of mechanical components by introducing compressive residual stresses. Ultrasonic shot-peening is a recent development of this process. While the classical shot-peening process uses pneumatic energy to project the shots, ultrasonic peening uses high-power ultrasounds. This energy source allows the use of larger shots projected at lower velocity as compared to classical shot-peening. This work aims at studying the mechanical response (restitution coefficient, residual stress field) of a surface impacted by a shot at low velocity using the finite element method and experimental analysis. This paper presents the simulation of a single elastic steel shot normally impacting an Aluminum alloy plate considered to exhibit a linear-elastic behavior and non-linear isotropic work hardening characteristics. The numerical simulations are carried out for different impact velocities in order to take into account the heterogeneous shot velocity field observed in an ultrasonic shot-peening chamber. We compare the simulated rebound energy and the indentation profiles obtained for different impact velocities to experimental results. The simulated residual stress field topology shows a strong dependence on the shot velocity. While numerical results obtained at high impact energy agree well with literature results, the residual stress distribution simulated for low impact energies shows a tensile layer below the impacted area. The restitution coefficients and the indentation profiles compare well with the experiments.
Authors: Clémence Demangel, A. Poznanski, V. Steenhout, Alexandra Levesque, Hicham Benhayoune, Delphine Retraint
Abstract: In this work, the use of a nanocrystallization surface treatment is considered on a cobalt based alloy. Since the Co28Cr6Mo is widely used as an articular bearing surface for artificial joints like hip and knee prostheses, the improvement of its tribological properties is a matter of concern to extend the lifespan of implants. By means of SMA (Surface Mechanical Attrition) treatment, a nanostructured layer is formed at the surface of a CoCrMo alloy establishing an optimized hardness gradient down to a depth of a few hundred of microns. Different treatment times (5, 15 and 25 minutes) are assessed comparatively and several surface polishing methods are studied: with clothes, brushes and a liquid filled with abrasive particles. The influence of processing parameters is discussed regarding hardness and topography. Moreover, the impact of surface modification is examined in terms of wear strengthening through scratch testing. The use of increasing loads mode gives some evidence of the benefit of SMAT. A significant decrease of penetration depth is noticed, from 30% to 60% on average. A straight correlation is also noticed depending on the surface finish method. This study illustrates both the ability of micro scratch testing to assess comparatively treated surface layer and to highlight the benefit of SMAT for wear strengthening in an abrasive wear mode.
Authors: Donaro Gallitelli, Delphine Retraint, Emmanuelle Rouhaud
Abstract: In this paper the residual stress states induced by conventional shot peening (SP) and surface mechanical attrition treatment (SMAT) are compared. The treated part correspond to plates made of a titanium alloy. Different intensities of these two mechanical treatments are first considered: their influence on the surface characteristics (roughness, hardness...) is studied. These experimental data are then used to develop a model for the residual stress profiles with dimensional analysis, Experimental and analytical approaches are then discussed.
Authors: T. Roland, Delphine Retraint, K. Lu, Jian Lu
Abstract: A novel surface treatment has been developed in the present work to enhance the tribological properties of 316L Stainless Steel. This Technique involves the formation of a nanocrystalline layer ascribable to a grain refinement mechanism induced by repeated impact loadings supported by the surface. The resultant system has a layered structure, comprising nanometric grains (less than 100 nm) at the top and a strain hardened transition layer in the subsurface. Such a microstructural feature has the potential to significantlty enhance the surface hardness and to create a high compressive residual stress state. The tribological properties of the stainless steel are thus improved in terms of lower friction coefficient and increased wear resistance. Detailed studies on the response of the nanocrystalline surface layer to annealing at temperatures between 400°C and 600°C showed that an annealing at high temperature can offer much better tribological enhancement than low temperature annealings due to enhanced martensitic transformation.
Authors: C. Pilé, Delphine Retraint, Manuel François, Jian Lu
Abstract: The aim of this work is to use ultrasonic shot peening, a mechanical surface treatment derived from conventional shot peening, in order to increase the fatigue life of TiAl alloys. The goal of this treatment is to generate compressive superficial residual stresses which are aimed to enhance fatigue crack initiation and growth resistance. For this purpose, different ultrasonic shot peening tests have been carried out on Ti-48Al-2Cr-2Nb samples in order to optimise treatment conditions. The first results reveal that it is possible to generate very high stress levels (»1000 MPa) beneath the surface, far much higher than the tensile yield stress of the material which is in the range 350-600 MPa. Such a phenomenon was also observed in ultrasonic shot peened iron or stainless steel and seems to be associated to the creation of a new homogeneous and nanometric structure below the surface of the alloy [1, 2]. In the light of these encouraging results, the shot peening treatment was optimised in terms of residual stresses profile and surface quality. The influence of different parameters of shot peening like the treatment time, the shot diameter as well as the specimen-sonotrode distance were studied. S-N curves were realised on polished specimens as well as on shot peened samples in order to study the effect of the treatment on the fatigue life of this intermetallic alloy.
Authors: Joël Faure, Richard Drevet, Nader Ben Jaber, Sylvain Potiron, Clémence Demangel, Delphine Retraint, Hicham Benhayoune
Abstract: Hydroxyapatite (HAP) and 58S Bioactive Glasses (BG) coatings are successfully synthesized by Electrophoretic Deposition (EPD) on Ti6Al4V alloy subjected to Surface Mechanical Attrition Treatment (SMAT). This process uses steel balls impacts on the Ti6Al4V surface to improve its mechanical properties. However when the Ti6Al4V substrate is treated by SMAT the industrial plasma spray technique is not efficient to obtain adherent HAP coatings. This problem is mainly related to the modifications of the Ti6Al4V surface topography due to the SMAT process. Therefore, in this work we demonstrate that EPD offers an efficient solution to solve this technical problem. Indeed we obtain a homogeneous and adherent HAP coating on the SMATed Ti6Al4V surface from a suspension of nanoparticles in ethanol. Moreover EPD is successfully employed to produce a 58S BG coating on the SMATed Ti6Al4V surface. Scanning Electron Microscopy (SEM) associated to Energy Dispersive X-Ray Spectroscopy (EDXS) reveals that the coatings obtained by EPD are adherent and compact without alteration of their chemical composition.
Authors: J. Uusitalo, L. Pentti Karjalainen, Delphine Retraint, M. Palosaari
Abstract: Severely deformed surface layers have been created by ultrasonic attrition technique on four steel sheets to investigate their influence on fatigue behaviour. A low-carbon (0.05%) ferritic steel and a medium-carbon (0.47%) normalized ferritic-pearlitic steel were selected to study the effect of carbon content on fatigue properties of carbon steels. Two stainless steels, Type 316L and Type 301LN, were also tested to study the influence of stability of the austenitic structure. Microstructural features were characterized by hardness measurements, X-ray diffraction and optical and electron microscopy. Fatigue properties were determined in flexural bending in the range 104 to 107 cycles. Crack nucleation and propagation stages were followed. In the attrition treatment thin severely deformed surface layers were found to form. Highly increased hardness was measured in these layers, especially for stainless steels, where also strain-induced martensite was formed. Drastic improvement in fatigue resistance was observed for all steels due to the surface nanocrystallization treatment.
Authors: C. Pilé, Delphine Retraint, Manuel François, Jian Lu
Authors: T. Roland, Delphine Retraint, K. Lu, Jian Lu
Abstract: Improved mechanical behavior of surface nanostructured metallic materials produced by means of a surface mechanical attrition treatment (S.M.A.T) is investigated experimentally. Based on microscopic observations and residual stress measurements, factors leading to the high strength and yielding are discussed. The effects due to treatment, as compressive residual stresses, are in that way studied for a better understanding of their influence on the global mechanical response of the nanostructured material. In regards of this, a simple way to increase the ductility of such a nanostructured material is also presented.
Authors: Laurent Waltz, Delphine Retraint, Arjen Roos, Patrick Olier, Jian Lu
Abstract: . In the present study, a method is presented combining surface nanocrystalline treatment (SMAT) and the co-rolling process. The aim of this duplex treatment is the development of a 316L stainless steel semi-massive multilayered bulk structure with improved yield and ultimate tensile strengths, while conserving an acceptable elongation to failure by optimizing the volume fraction and distribution of the nano-grains in the laminate. To characterize this composite structure, tensile tests as well as sharp nanoindentation tests were carried out to follow the local hardness evolution through the cross-section of the laminate. Furthermore, transmission electron microscope (TEM) observations were carried out to determine the correlation between the microstructure, the local hardness and the mechanical response of the structure.
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