Publications

The current paper presents a two scale Finite Element approach (FE 2 ), adopting the periodic homogenization method, for fully coupled thermo-mechanical processes. The aim of this work is to predict the overall response of rate-dependent, non-linear, thermo-mechanically coupled problems of 3D periodic composite structures. The material constituents implicated in the analyses obey generalized standard materials laws, while the characteristic equations of the problem (balance law, first law of thermodynamics) are expressed and satisfied in both microscopic and macroscopic scales. For the numerical implementation in both scales, the finite element commercial software ABAQUS is utilized in the framework of small strains and rotations. A set of dedicated scripts and a specially designed Meta-UMAT subroutine allow the connection between the macroscopic structure and the microscopic unit cells attached to every macroscopic integration point. The two-scale finite element framework is applied to simulate thermoelastic-viscoplastic materials of complex 3D composite structures, and its capabilities are demonstrated with proper numerical examples. It is worth mentioning that the proposed computational strategy can be applied for any kind of 3D periodic microstructure and non-linear constitutive law.

This paper deals with mean field multiscale approaches for coated fiber- or particle-reinforced composites under nonlinear strain. The current work attempts to extend Dvorak’s well-known transformation field analysis for mean field approaches, in which the composite’s constitutive law is split into an elastic and an inelastic part. The classical Eshelby’s inhomogeneity problem considering eigenstrains is revisited in order to address the presence of a coating layer. For this scope, three different methodologies are employed, one for general ellipsoidal inhomogeneities, a modified composite cylinder method for long cylindrical fibers and a modified composite sphere method for spherical particles. After identifying proper interaction tensors for the inhomogeneity and its coating layer, the composite’s overall response is evaluated by extending classical mean field techniques, such as the Mori–Tanaka and the self-consistent methods. Numerical examples illustrate the differences in macroscopic and microscopic predictions between the general approach and the modified composite cylinder and sphere Assemblages.

Cycle de vie des produits, procédés et services. Considération des nanos dans les calculs d'Analyse de Cycle de Vie: Limites et opportunités

Additive manufacturing offers new available categories of geometries to be built. Among those categories, one can find the well developing field of lattice structures. Attention has been paid on lattice structures for their lightweight and mechanical efficiency ratio, thus leading to more optimized mechanical parts for systems. However this lightness only holds true from a mass related point of view. The files sent to additive manufacturing machines are quite large and can go up to such sizes that machines can freeze and get into malfunction. This is directly related to the lattice structures tendency to be of a high geometric complexity. a large amount of vertices and triangles is necessary to describe them geometrically, thus leading to larger file sizes. With the increasing use of lattice structures, the need for their files to be lighter is also rising. This paper aims at proposing a method for tessellating a certain category of such structures, using topologic and geometric criteria to generate as few as possible triangles, thus leading to lightweight files. The triangulation technique is driven by a chordal error that control the deviation between the exact and tessellated structures. It uses interpolation, boolean as well as triangulation operators. The method is illustrated and discussed through examples from our prototype software.

The conservation and restoration of historical monuments require a diagnostic analysis carried out by amultidisciplinary team. The results of the diagnosis include data produced by different techniques andprotocols, which are used by conservation scientists to assess the built heritage. Nowadays, together withthe aforementioned data, a great deal of heterogeneous information is also available, including descriptiveand contextual information, as well as 2D/3D geometrical restitution of the studied object. However, theintegration of these diverse data into a unique information model capable of fully describing the buildingconservation state, as well as integrating future data, is still an open issue within the Cultural Heritagecommunity. It is of paramount importance to correlate these data and spatialize them in order to providescientists in charge of our heritage with a practical and easy means to explore the information usedduring their assessment, as well as a way to record their scientific observation and share them withintheir community of practice. In order to resolve this issue, we developed a correlation pipeline for theintegration of the semantic, spatial and morphological dimension of a built heritage. The pipeline uses anontological model for recording and integrating multidisciplinary observations of the conservation stateinto structural data spatialized into a semantic-aware 3D representation. The pipeline was successfullytested on the Saint Maurice church of Caromb in the south of France, integrating into a unique spatialrepresentation information about material and alteration phenomena, providing users with a means tocorrelate, and more importantly retrieve several types of information.

Autonomous vehicles are expected to start reaching the market within the next years. However in practical applications, navigation inside dynamic environments has to take many factors such as speed control, safety and comfort into consideration, which is more paramount for both passengers and pedestrians. In this paper, a novel speed profile planner based on an optimal control approach considering passenger comfort is proposed. The approach is accomplished by minimizing jerk under certain comfort constraints, which inherently gives a speed profile for the central nervous system to follow naturally. Imposed with the same conditions, the widely used Jerk Limitation method is interpreted as an equivalent of the minimum time control method, the latter being used to verify that our method can ensure better continuity and smoothness of the speed profiles. A validation test was specifically designed and performed in order to show the feasibility of our method.

In this paper, we present the special solution of the two-dimensional problem of a continuously graded composite made of thermovisco- rigid plastic materials under combined biaxial quasistatic compression and tension (or compression) and shear, that preserves prescribed uniform normal strains. The related reference initial-boundary value problem is fully defined and the corresponding solution is analyzed and computed numerically. In the context of a linearized instability analysis, critical conditions, as the critical shear banding angle, in terms of the loading level and material heterogeneities, are presented. In the context of non-linear instability, these results are examined and explained. Additionally, in the same context of non-linear analysis, the destabilizing mechanism, the onset of instability and the critical time for prescribed lower and upper bounds of equivalent strain-rate and upper bound of equivalent strain are defined and related to the lateral normal stress. The limitations of linearized analysis results are also revealed. Moreover, a semi-analytical homogenization scheme for a periodically-graded plate is presented. The related results are used for the derivation of a homogenized problem of a multilayered composite with continuously graded interlayers.

We investigate perturbations that maximize the gain of disturbance energy in a two-dimensional isolated vortex and a counter-rotating vortex pair. The optimization is carried out using the method of Lagrange multipliers. For low initial energy of the perturbation ( ), the nonlinear optimal perturbation/gain is found to be the same as the linear optimal perturbation/gain. Beyond a certain threshold , the optimal perturbation/gain obtained from linear and nonlinear computations are different. There exists a range of for which the nonlinear optimal gain is higher than the linear optimal gain. For an isolated vortex, the higher value of nonlinear optimal gain is attributed to interaction among different azimuthal components, which is otherwise absent in a linearized system. Spiral dislocations are found in the nonlinear optimal perturbation at the radial location where the most dominant wavenumber changes. Long-time nonlinear evolution of linear and nonlinear optimal perturbations is studied. The evolution shows that, after the initial increment of perturbation energy, the vortex attains a quasi-steady state where the mean perturbation energy decreases on a slow time scale. The quasi-steady vortex state is non-axisymmetric and its shape depends on the initial perturbation. It is observed that the lifetime of a quasi-steady vortex state obtained using the nonlinear optimal perturbation is longer than that obtained using the linear optimal perturbation. For a counter-rotating vortex pair, the mechanism that maximizes the energy gain is found to be similar to that of the isolated vortex. Within the linear framework, the optimal perturbation for a vortex pair can be either symmetric or antisymmetric, whereas the structure of the nonlinear optimal perturbation, beyond the threshold, is always asymmetric. No quasi-steady state for a counter-rotating vortex pair is observed.

A fully three-dimensional linear stability analysis is carried out to investigate the unstable bifurcations of a compressible viscous fluid past a sphere. A time-stepper technique is used to compute both equilibrium states and leading eigenmodes. In agreement with previous studies, the numerical results reveal a regular bifurcation under the action of a steady mode and a supercritical Hopf bifurcation that causes the onset of unsteadiness but also illustrate the limitations of previous linear approaches, based on parallel and axisymmetric base flow assumptions, or weakly nonlinear theories. The evolution of the unstable bifurcations is investigated up to low-supersonic speeds. For increasing Mach numbers, the thresholds move towards higher Reynolds numbers. The unsteady fluctuations are weakened and an axisymmetrization of the base flow occurs. For a sufficiently high Reynolds number, the regular bifurcation disappears and the flow directly passes from an unsteady planar-symmetric solution to a stationary axisymmetric stable one when the Mach number is increased. A stability map is drawn by tracking the bifurcation boundaries for different Reynolds and Mach numbers. When supersonic conditions are reached, the flow becomes globally stable and switches to a noise-amplifier system. A continuous Gaussian white noise forcing is applied in front of the shock to examine the convective nature of the flow. A Fourier analysis and a dynamic mode decomposition show a modal response that recalls that of the incompressible unsteady cases. Although transition in the wake does not occur for the chosen Reynolds number and forcing amplitude, this suggests a link between subsonic and supersonic dynamics.

Nowadays, the environmental impact of a product is a major factor for the design team and for the end customer. To decrease the environmental impact of a product during its whole lifecycle, many design methods are available focusing on the multitude of data from the product once it is fully designed (Lifecycle Analysis etc ...). However, the design choices made in the early design stages have a crucial importance on the environmental impact of the chosen solution. It is therefore necessary to propose to the multidisciplinary design team a tool to help them choosing the best concept and the best way to produce it as early as possible. The emphasis in this paper is on Additive Manufacturing technologies, which are widely used in concept development. A design tool prototype is presented and evaluated in order to foster early eco-additive manufacturing of concepts.