Caractérisation et optimisation de l'environnement mécanique tridimensionnel des cellules souches au sein des bioréacteurs d'ingénierie tissulaire osseuse

Les cellules souches mésenchymateuses utilisées en ingénierie tissulaire osseuse sont sensibles aux contraintes mécaniques, particulièrement au cisaillement. Il est donc possible d'optimiser leurs conditions de culture (géométrie des scaffolds, flux) pour stimuler les cellules au sein des bioréacteurs. Pour cela, une étude numérique de dynamique des fluides a été réalisée sur différents types de scaffolds granulaires. Elle a montré que ces scaffolds ont une efficacité équivalente à ceux de la littérature pour convertir le flux de milieu de culture en stimulation mécanique. Ces résultats numériques ont ensuite été mis en ?uvre expérimentalement. Les expériences ont montré le rôle fondamental de la nature du biomatériau utilisé comme scaffold ainsi que de l'environnement tridimensionnel

Particle shape dependence in 2D granular media

Particle shape is a key to the space-filling and strength properties of granular matter. We consider a shape parameter $\eta$ describing the degree of distortion from a perfectly spherical shape. Encompassing most specific shape characteristics such as elongation, angularity and nonconvexity, $\eta$ is a low-order but generic parameter that we used in a numerical benchmark test for a systematic investigation of shape-dependence in sheared granular packings composed of particles of different shapes. We find that the shear strength is an increasing function of $\eta$ with nearly the same trend for all shapes, the differences appearing thus to be of second order compared to $\eta$. We also observe a nontrivial behavior of packing fraction which, for all our simulated shapes, increases with $\eta$ from the random close packing fraction for disks, reaches a peak considerably higher than that for disks, and subsequently declines as $\eta$ is further increased. These findings suggest that a low-order description of particle shape accounts for the principal trends of packing fraction and shear strength. Hence, the effect of second-order shape parameters may be investigated by considering different shapes at the same level of $\eta$.Comment: 5 pages, 8 figure

A benchmark for particle shape dependence

International audienceParticle shape is a major parameter for the space-filling and strength properties of granular materials. For a systematic investigation of shape effect, a numerical benchmark test was set up within a collaborative group using different numerical methods and particles of various shape characteristics such as elongation, angularity and nonconvexity. Extensive 2D shear simulations were performed in this framework and the shear strength and packing fraction were compared for different shapes.We show that the results may be analyzed in terms of a low-order shape parameter h describing the degree of distortion from a perfectly circular shape. In particular, the shear strength is an increasing function of h with nearly the same trend for all shapes, the differences being of second order compared to h. We also observe a nontrivial behavior of packing fraction which, for all our simulated shapes, increases with h from the random close packing fraction for disks, reaches a peak considerably higher than that for disks, and subsequently declines as h is further increased. Finally, the analysis of contact forces for the same value of h leads to very similar statistics regardless of our specific particle shapes

Effect of Grain Size and Shape on Undrained Behaviour of Sands

The stress–strain and stress path characteristics of sands are influenced by their grain size, shape, and packing. Morphological characteristics and size of particles play important role on the undrained shear strength of sands. Often, effects of these parameters are complex and cannot be easily distinguished. This study advances the knowledge of the role of particle size and shape on the undrained shear strength of sands. To eliminate the consequence of morphological characteristics, two sands with different particle sizes but similar angularity, and another sand with different roundness were selected for the study. These morphological characteristics for all three sands were determined from the analysis of scanning electron microscope images. F131 sand with higher median grain size and lower shape factors (rᵣ and rₛ) had highest undrained peak shear strength and phase transformation value. Undrained strength (qₚₜ) and effective principal stress (P′ₚₜ) in phase transformation point had direct relationship with grain median grain size (D₅₀) and inversely effect of shape factor (rᵣ and rₛ). F131 and F161 sands represented highest peak and ultimate steady-state strengths, respectively. Flow potential appeared to be directly proportional with (rᵣ and rₛ) and inversely with D₅₀. The peak index decreased with increasing shape factors (rᵣ and rₛ)

Évaluation de la qualité du grenaillage ultrasonore à l'aide d'une analyse combinée DEM-FEM

International audienceLe grenaillage ultrasonore un traitement mécanique de surface qui a pour objectif d'allonger la durée de vie des équipements. Ce procédé génère des contraintes résiduelles de compression à l'origine de l'augmentation de la tenue en fatigue du matériau traité. Grâce à une analyse combinant les résultats de simulations de la dynamique des billes par éléments discrets à des simulations mono-impact par éléments finis nous avons montré que, dans notre configuration, l'augmentation du nombre de billes permet de concentrer les contraintes de compression en surface du matériau traité

Modélisation du microbillage par ultrasons

National audienceSee http://hal.archives-ouvertes.fr/docs/00/59/29/12/ANNEX/r_GB8V2906.pd

Experimental study of tensile strength of pharmaceutical tablets: effect of the diluent nature and compression pressure

In the pharmaceutical field, tablets are the most common dosage form for oral administration in the world. Among different manufacturing processes, direct compression is widely used because of its economics interest and it is a process which avoids the steps of wet granulation and drying processes. Tablets are composed of at least two ingredients: an active pharmaceutical ingredient (API) which is mixed with a diluent. The nature of the powders and the processing conditions are crucial for the properties of the blend and, consequently, strongly influence the mechanical characteristics of tablets. Moreover, tablets have to present a suitable mechanical strength to avoid crumbling or breaking when handling, while ensuring an appropriate disintegration after administration. Accordingly, this mechanical property is an essential parameter to consider. Experimental results showed that proportion of the diluent, fragmentary (DCPA) or plastic (MCC), had a large influence on the tensile strength evolution with API content as well as the compression load applied during tableting process. From these results a model was developed in order to predict the tensile strength of binary tablets by knowing the compression pressure. The validity of this model was demonstrated for the two studied systems and a comparison was made with two existing models