Showing papers by "ParisTech published in 2006"

Residual and Internal Stress States in Duplex Steel with TWIP Effect

Abstract: A new variety of duplex steels with superior mechanical properties has been studied. They exhibit a very interesting combination of strength (tensile strength of 680 MPa) and ductility values (more than 45% total elongation) due to the competition between different plasticity mechanisms. These steels contain two phases: austenite and ferrite and are characterized by low stacking fault energy at room temperature. In this work, four duplex steels with different chemical composition and phase volume fraction are studied. Residual and internal stresses in each phase were determined using the classical X-ray diffraction sin²ψ method. In the as-received state, both longitudinal and transverse residual stresses are in compression (until -350 MPa) for the ferrite and in tension (until +410 MPa) for the austenite. However, residual stresses in the austenitic phase decrease when its volume fraction increases. Moreover, internal stress distribution in one alloy was determined by X-ray diffraction during an in situ tensile test. The austenitic phase stress along the loading direction is higher than the macroscopic applied one, which is higher than the ferritic stress state, verifying a mixture rule and consistent with the initial residual stresses. For an applied macroscopic strain of about 1%, the austenite phase is subjected to a stress of about 600 MPa whereas the stress in the ferritic phase is about 300 MPa. It was also observed that as macroscopic strain increases, stress difference between the austenite and the ferrite decreases.

A Fast Adaptive Method for Subspace Based Blind Channel Estimation

Abstract: In this paper, a new fast adaptive blind channel estimation method is proposed using the subspace information from the correlation matrix. The algorithm is fully adaptive in the sense that both the subspace information and the optimization which leads to the channel estimation are computed adaptively. It is based on the recently proposed YAST subspace tracker which has been shown to outperform other methods both in terms of speed of convergence and computational complexity. A discussion on the convergence properties of the proposed algorithm is presented. We also propose a hybrid method which makes use of the YAST subspace tracker for initial fast convergence and the subspace information is then updated using the numerically stable OPAST subspace tracker.

Thixoforming of Steel – Influence of Thermal Parameters

Abstract: This paper presents experimental results on steel thixoforming. The influence of thermal effects on the semi-solid response is analysed through the influence of thermal exchanges with tools and environment. Several rheological experiments such as compression, extrusion or radial filling test were developed to understand the semi-solid steel behaviour and determine the parameters that have a major influence on thixoforming. Actually, in our experiments, the temperature of the slug and consequently the solid fraction were found first order parameters while; the morphology of the solid phase plays a minor role.

Evaluation by Synchrotron Radiation of Shape Factor Effects on Residual Stress in Nitrided Layers

Abstract: 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.

An extension to the Wiener space of the arbitrary functions principle

Abstract: The arbitrary functions principle says that the fractional part of $nX$ converges stably to an independent random variable uniformly distributed on the unit interval, as soon as the random variable $X$ possesses a density or a characteristic function vanishing at infinity. We prove a similar property for random variables defined on the Wiener space when the stochastic measure $dB\_s$ is crumpled on itself.

Characterisation of Residual Stresses by X-Ray Diffraction of Laser Welded AZ91 Magnesium Alloy

Abstract: This study looks at the effect of laser welding on residual stresses in sheets of moulded magnesium alloy (AZ91). The modifications are significant even though they are localised along the weld line. The welding process produces a gradient of residual stresses from the weld line to the base metal and throughout the thickness of the weld zone. The distribution of these residual stresses has been qualitatively explained by the kinetics of cooling and by the nature of the multiphase material. Finally, it has been demonstrated that a pronounced crystallographic texture modifies local distribution of residual stresses. These results show that there is a complex coupling between the metallurgical, thermal and mechanical aspects generated by high power welding procedures.

Wetting of Stressed Grain Boundaries in Polycrystals and Rheological Behaviour of Resulting Materials

Abstract: Orientational patterns of wetted grain boundaries (GB's) in stressed NaCl polycrystals have been used for describing the GB penetration process on the basis of combined interfacial and mechanical energy balance considerations. The response of the internally wetted polycrystals to static loading has been shown to obey “pressure solution” constitutive laws involving dissolution at stressed surfaces, diffusive transport of dissolved matter and precipitation at less stressed surfaces. Direct experimental evidence for pressure solution mechanism is presented.

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

Abstract: 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.