Agnès Fabre

Evaluation by synchrotron radiation of shape factor effects on residual stress in nitrided layers

Nitriding is a thermo-chemical treatment to improve fatigue life of steel parts what are exposed to high cyclic loading on and close to the surface like gears for example. During the nitrogen diffusion, the precipitation of nitrides and carbides generates residual stresses and increases the superficial hardness. These residual stresses are function of the conditions of the nitriding process but also critically depend on the geometry of the steel components. Indeed, the result of the diffusion process is different for a plane geometry or a curvilinear one. In this present work, the sample is a part of a gear, composed by two teeth. Between two teeth of gear, the determination of in-depth stress gradients by classical X-rays diffraction cannot be done with a great accuracy: the spatial resolution is not sufficient because the irradiated area has the same dimension than the surface curvature of the component. Furthermore, it is very difficult to take into account the removal of matter that is required to determine in-depth stress profile because of the particular geometry. The synchrotron diffraction technique is a well-adapted method to determine such stress gradients in strongly absorbing materials due to the capability of penetration power of high energy X-rays. The removal of matter is not required and it is possible to determine an in-depth map of the stress tensor. After measurements on the ESRF ID15 beam line, stress profile has been calculated without the σ33 equal to zero hypothesis. The results are as following: compressive residual stresses were found close to the surface, and the calculated σ33 component of stress tensor is really not equal to zero. This important result seems to show the geometrical effect on stress state near non-plane surface.

Neutron Determination of Residual Stress in a Nitrided Notched Part

Nitriding is an hardening thermomechanical treatment generally used to improve fatigue life of steel parts, like gear for example. Another effect of this treatment is generating superficial stress, influenced by nitriding conditions, composition of steel and geometry of the part. This work deals with the effect of shape on the residual stress profile obtain after nitriding on a gear tooth. The residual stress profile was determined using neutrons diffraction technique.

Microgeometrical influences on micropitting fatigue damage: multi-scale analysis:

Effective prediction methods are needed by engineers to design parts used in fatigue conditions. In gears, surface fatigue damage is observed at two scales: pitting and micropitting. These two phenomena occur in response to the variation of the local loading on the contact zone between teeth. The loading is influenced by the shape of the teeth and their surface roughness, which induces stress raisers. The pertinent geometrical parameters that determine fatigue lifetime need to be identified, and in this situation improved knowledge is still required. In the current study, seven parameters are used to define a generic tooth roughness profile. The transient mechanical loading acting on this rough tooth in rolling/sliding contact with a smooth tooth is obtained using elastohydrodynamic code. The stress time history is determined by integration so that pitting and micropitting can be predicted using the Crossland fatigue criterion. The ana- lyses were applied to results obtained for 32CrMoV13 quenched and tempered steel for about 50 variations of the generic roughness profile parameters. It was found that pitting (at the position of the maximum Hertzian shear stress) is influenced by four of the parameters, the extent to which the von Mises equivalent stress exceeds the material yield stress in the zone where micropitting occurs is influenced by two parameters, and the fatigue lifetime is influenced by three parameters.

Variation of Residual Stresses in Drawn Copper Tubes

Seamless tubes are used for many applications, e.g. in heating, transport gases and fluids, evaporators as well as medical use and as intermediate products for hydroforming and various mechanical applications, where the final dimensions normally are given by some cold drawing steps. The first process step – piercing of the billet, for example by extrusion or 3-roll-milling - typically results in ovality and eccentricity in the tube causing non-symmetric material flow during the cold drawing process, i.e. inhomogeneous deformation. Because of this non-axisymmetric deformation and of deviations over tube length caused by moving tools, this process step generates residual stresses. To understand the interconnections between the geometrical changes in the tubes and the residual stresses, the residual strains in a copper tube had been measured by neutron diffraction.

A Perspective of Pulsed Laser Deposition (PLD) in Surface Engineering: Alumina Coatings and Substrates

In this article, two original studies of the alumina as porous substrate and PLD (pulsed laser deposition) thin films in view of its biomedical and tribological applications are presented. The first biomedical study aimed to evaluate the role of Al2O3 on thin deposited nanostructures. For this purpose, cerium stabilized zirconia doped hydroxyapatite thin films were deposited by PLD onto high purity, high density alumina substrates with different low porosities. For deposition, an UV KrF* (λ=248 nm, τ ~ 25 ns) excimer laser was used for the multi-pulse irradiation of the targets. The nanostructured surface morphologies of the thin films with micro droplets were evidenced by atomic force microscopy and scanning electron microscopy and the compositions with a Ca/P ratio of 1.7 by energy dispersive spectroscopy. The films were seeded with mesenchymal stem cells for in vitro tests. The cells showed good attachment and spread and covered uniformly the surface of the samples. Different functions of substrate porosities are observed in the efficiency of developing long filopodia and of obtaining the optimal intracellular organization. The second study aimed to understand the influence of micro-structural and mechanical characteristics on the tribological behaviour of stainless steel samples with PLD alumina coatings produced using an UV KrF* (λ=248 nm, τ ~ 20 ns) excimer laser and a sintered alumina target. Various microscopic observation techniques were used in order to connect the tribological response to the amorphous microstructure of the coatings. The results correspond to the determination of the mechanical characteristics by nanoindentation tests, scratch tests, and a tribological behaviour analysis of the treated steel against 100Cr6. The films were stoichiometric, partially crystallized with an amorphous matrix and their surfaces had few particulates deposited on. The obtained values of hardness and elastic modulus of the films were in good agreements with literature data.

Neutron Evaluation of Stress in Industrial Screws

Neutron diffraction measurements were used in this study in order to determine the axial stress state in loaded screw from a specific assembly. The knowing of stress gradient is need to qualify a standard gauge used to calibrate the response of in-situ measurements using ultrasonic nondestructive technique. US is well adapted to perform measurements of the evolution of stress state on industrial screws during service life of the bolded assemblies.

A Thermodynamic and Experimental Study of Low-Alloy Steels After Carbonitriding in a Low-Pressure Atmosphere

The effect of the composition of two steels (B and 6MnCr5) on precipitation of undesirable phases (carbides, nitrides and carbonitrides) under thermochemical treatment (low-pressure or vacuum carbonitriding) is investigated. Metallographic and x-ray diffraction studies and thermodynamic computations are performed.

Forging and Shot-Peening Contribution on Residual Stresses for a Textured Biphasic Titanium Alloy

Residual stresses are to be determined to predict mechanical behaviour of vital parts in aircraft. This part constituted by Ti-10V-2Fe-3Al is analysed using x-ray diffraction (XRD). However, the broadening and sometime the disappearance of peaks involve a major difficulty to quantify the strains and the stresses which are close to the surface. In the current study, some investigations are conducted in order to understand the difficulties to evaluate stresses by XRD. The microstructure of samples is studied by taking into account the history of surface generation: after the forging and after the shot-peening process. Shot-peening induces a distortion of the primary phase α which is close to the surface. Different deformation rates introduced by forging process have generated a different organisation in the second phase constituted by α secondary and β. To complete the investigation, crystallographic textures are performed on α phase. The main difficulty in analysing when using XRD is due to the peak broadening. The conclusion of this study identifies three separate zones beneath the surface: where the stresses can be determined, it is possible to analyze it using different method of analysis with different levels of accuracy.

Representative simulations of the shot peening process

This study presents 3D finite element (FE) simulations of the representative geometry of the shot peening process. As a preamble, a single shot simulation of a spherical particle is extensively analyzed in order to capture the effect of process parameters (velocity, size, shooting angle) on the plastic strain, residual stress profile and surface topology. In a second part, representative multiple shot simulations are performed to analyse the effect of the coverage ratio.

Comparative Studies of Textured Pulsed Laser Deposition and Sol-Gel Growth of Thin Hydroxyapatite Layers on Titanium Substrates

The aim of this paper is the residual stress evaluation in dental implants and the improvement of adherence at the metal-ceramic interface. This study is focused on the development of a multi-layer system model of Hydroxyapatite/TiO2/Ti components. Our aim is to validate new methods of laser ablation deposition and sol-gel, by controlling the residual stresses and actual adherence to titanium substrates. We present a report of the growth of hydroxyapatite layers by PLD (pulsed laser deposition) and sol-gel deposition, and the measurement of their residual stresses.