Habibou Maitournam

Growth and cellular differentiation: a physico-biochemical conundrum? The example of the hand

Currently, the predominant hypothesis explains cellular differentiation as an essentially genetic intracellular process. The goal of this paper is to suggest that cell growth and differentiation may be, simply, the result of physical and chemical constraints. Bone growth occurs at the level of cartilage conjunction (growth plate) in a zone of lesser constrain. It appears that this growth also induces muscle, tendon, nerve and skin elongation. This cartilage growth by itself seems to explain the elongation of the hand. Growth stops at puberty likely because of feed-back from an increasing muscle load. The ossification (that is differentiation of cartilage into bone) appears to result from the shear stress induced. The study of bone age, obtained by X-ray picture of the hand, shows that ossification of epiphyses is very precise both in time and space. Computer modelization suggests that this ossification occurs where shear stress is greatest. The cartilage which does not ossify (joint, nose, larynx, ear, bronchus, etc.) is not exposed to high shear. Shear stress induces the secretion of extracellular matrix and a change of the biochemical environment of the cell. Precipitation of calcium phosphate, as in ossification, seems related to the alkalosis induced by shear stress. To speak in more general terms, loss of cellular differentiation, as occurs with cancer, can result from a change in the physical-chemical environments.

A Modeling Approach to Predict Fretting Fatigue on Highly Loaded Blade Roots

A lifing technique for predicting fretting fatigue on highly loaded blade-disk attachments has been developed and calibrated. The approach combines extensive testing on nickel and titanium based alloys using a specially devised multiaxial fretting test machine and an analytical lifing procedure, based on finite element contact calculations and multiaxial shakedown fatigue models. In order to reproduce realistic operational conditions and standardize testing conditions, a special fretting fatigue testing machine with high temperature testing capabilities was developed. The machine was employed to perform systematic testing under prescribed load and displacement conditions at representative temperatures. Making use of FEA, the rig test results were calculated to identify relevant parameters such as friction coefficient, slip conditions, and machine compliance. The computation procedure involves the calculation of several major loading cycles until a stabilized response of the structure is achieved. The material response is assumed to be elastoplastic, and a nonlinear friction law (space and time) was applied. From the computed mechanical variables, several life prediction models are benchmarked to establish their capabilities to predict fretting fatigue life. Finally, a most promising life estimation procedure was applied to predict life in a real compressor blade-disk attachment. Predicted failure location and number of cycles to failure are compared against engine test results. The experimental-analytical approach has the potential to predict fretting fatigue risk during the design phase on highly loaded joints, as well as estimating the preventive overhaul intervals for parts already in service.

Un élément fini de poutre fissurée application à la dynamique des arbres tournants

Dans ce travail on présente une méthode originale de construction d’un élément fini de poutre affectée de fissurations. La souplesse additionnelle due à la présence des fissures est identifiée à partir de calculs éléments finis tridimensionnels tenant compte des conditions de contact unilatéral entre les lèvres. Cette souplesse est répartie sur toute la longueur de l’élément dont on se propose de construire la matrice de rigidité. La démarche permet un gain considérable en temps de calcul par rapport à la représentation nodale de la section fissurée lors de l’intégration temporelle de systèmes différentiels en dynamique des structures.

Pressure rolling contact : Steady state flow analysis and comparison with experimental data

Abstract In the present work the pressure rolling treatment has been modelled by a direct stationary method which gives the stabilized mechanical state after complete rolling. One important input to the model is the pressure distribution at the contact it is found that the pressure distribution differs from the one calculated with the Hertz theory. The numerical outputs from the pressure rolling model have been compared to experimental data, i.e. the residual stresses measured by X-ray diffraction. A good agreement between experimental and numerical results is obtained for the penetration depth and the residual stress profile. It is found that friction has a strong influence on the surface residual stress.

Modélisation par équations intégrales du frottement sur un demi-espace élasto-plastique.

Abstract.This paper deals with a regularised integral equation formulation for contact problems on elastic-plastic bodies, under the plane strain assumption. The formulation has been implemented for indentation and rolling/sliding contact problems on isotropic homogeneous half planes. It constitutes a natural extension to non-linear problems of the classical analytical methods developed for linear elastic contact mechanics. Two versions of an implicit elastic-plastic constitutive integration algorithm, dealing with loads respectively fixed or moving at constant velocity, have been implemented in this integral equation framework. The steady-state version can be applied to multiple rolling/sliding loads and to the determination of the stabilised elastic-plastic state. Several numerical examples illustrate the various aspects of our formulation: fixed or moving load, contact pressure known a priori or iteratively computed, numerical classification of the stabilised elastic-plastic states reached as a functio...

On a new computational method for the simulation of periodic structures subjected to moving loads

The purpose of the paper is to present a new numerical method suitable for the computation of periodic structures subjected to repeated moving loads. It directly derives from the stationary methods proposed for cylindrical and axisymmetrical structures. Its mains features are the use of a calculation reference related to the moving loads and the periodic property of the thermomechanical response. These methods are developped by PSA and the Ecole Polytechnique, in order to design vented brake discs. In this paper, a brief description of the algorithm is first given and examples of numerical simulations of a vented brake disc are treated.

A Reduced Integration for Reissner-Mindlin Non-linear Shell Analysis Using T-Splines

We propose a reduced shell element for Reissner-Mindlin geometric non-linear analysis within the context of T-spline analysis. The shell formulation is based on the displacements and a first order kinematic in the thickness is used for the transverse shear strains. A total Lagrangian formulation is considered for the finite transformations. The update of the incremental rotations is made using the quaternion algebra. The standard two-dimensional reduced quadrature rules for structured B-spline and NURBS basis functions are extended to the more flexible T-meshes. The non-uniform Gauss-Legendre and patchwise reduced integrations for quadratic shape functions are both presented and compared to the standard full Gauss-Legendre scheme. The performance of the element is assessed with linear and geometric non-linear two-dimensional problems in structural analysis. The effects of mesh distortion and local refinement, using both full and reduced numerical quadratures, are evaluated.