Guillaume Puel

Improved calibration of simulation models in railway dynamics: application of a parameter identification process to the multi-body model of a TGV train

This paper aims at estimating the vehicle suspension parameters of a TGV (Train à Grande Vitesse) train from measurement data. A better knowledge of these parameters is required for virtual certification or condition monitoring applications. The estimation of the parameter values is performed by minimising a misfit function describing the distance between the measured and the simulated vehicle response. Due to the unsteady excitation from the real track irregularities and nonlinear effects in the vehicle behaviour, the misfit function is defined in the time domain using a least squares estimation. Then an optimisation algorithm is applied in order to find the best parameter values within the defined constraints. The complexity of the solution surface with many local minima requires the use of global optimisation methods. The results show that the model can be improved by this approach providing a response of the simulation model closer to the measurements.

Two-timescale homogenization method for the modeling of material fatigue

When dealing with Combined Cycle Fatigue (CCF), characterized by two simultaneous periodic loads with different frequencies, what appears to be a more robust and coherent approach than classical methods is to use a two-timescale homogenization strategy. First, two different time scales are identified: a fast one and a slow one associated with the high frequency and low frequency loadings respectively. It is then assumed that the ratio of the periods associated with the two time scales is small enough so that asymptotic developments of the mechanical quantities to be calculated can be written. The method was applied to the study of academic as well as actual specimens, considering a quasi-static behavior or a dynamic behavior. We conclude that separating the two time scales allows to compute the evolution at the slow time scale of homogenized variables instead of accurately computing the problem at the fast time scale. With such a method it is then possible to calculate the fatigue strength under CCF conditions without any further assumption than the asymptotic decomposition.

Identification of a Spatial Field of Material Properties With Adaptive Regularization and Meshes

It is well known that the solution of an inverse problem is unstable and not unique. In order to avoid these difficulties when solving such a problem, a Tikhonov’s regularization term is usually added to the norm quantifying the discrepancy between the model’s predictions and experimental data. However, this regularization term is often insufficient to deal with the identification of a spatially variable field of material properties. Here is discussed a general strategy using classical adaptive meshing methods in order to improve the regularization of the inverse problem in such cases. The first step consists in using two distinct meshes: one associated with the discretization of the sought spatial field, and one associated with the resolution of the mere mechanical problems. Then the introduction of usual local error estimators in a second step can drive the refinement of the coarse mesh associated with the sought parameters. This general strategy is then applied to a practical case of study: the detection of subterranean cavities using experimental data obtained by an interferometric device on a satellite.Copyright

Material fatigue simulation using a periodic time homogenization method

This paper deals with the numerical simulation of combined cycle fatigue, which is characterised by two periodic loads, whose frequencies are very different one from the other. Rather than using classical fatigue life estimations, a time transient evolution model is solved using a periodic time-homogenisation method. This latter is based on the assumption that the time scales associated with the two periodic loads are decoupled. Different results on academic as well as industrial examples are presented. An extension of the proposed method up to three time scales is eventually proposed in order to speed up the numerical simulations.

Study of the Effect of Mechanical Loading on Cell Cultures in Bone Tissue Engineering

Bone tissue engineering currently represents one of the most interesting alternatives to autologous transplants and their drawbacks in the treatment of large bone defects. Mesenchymal stem cells are used to build new bone in vitro in a bioreactor. Their stimulation and our understanding of the mechanisms of mechanotransduction need to be improved in order to optimize the design of bioreactors.In this study, several geometries of bioreactor were analyzed experimentally and biological results were linked with numerical simulations of the flow inside the bioreactor. These results will constitute a base for an improved design of the existing bioreactor.Copyright

Elastic electron scattering using the Finite Element Method: forward and inverse problems

We address here the case of electron-matter elastic interaction as it occurs in Transmission Electron Microscopy (TEM) experiments. In the forward problem, we show that it is possible to derive the scattered electron wave function as the solution of a Helmholtz equation. This equation depends on the spatial potential associated with the analyzed sample, and can be relevantly solved using the Finite Element Method (FEM). Then we present an inverse formulation dealing with the determination of the sample’s potential when the total wave function is measured at the exit plane of the sample.

Inverse elastic scattering with adaptive regularization and meshes

A simplified modelling of the inverse problem is proposed in the case of a scattering experiment such as in transmission electron microscopy (TEM). The two major tools are the use of the finite element method (FEM) for the wave equation, derived from a Helmholtz type equation, and a Tikhonov regularization for the misfit function. The equations for the wave function, the adjoint state and the optimal potential are established. The direct problem is illustrated with a 2D case of iron in the [011] zone axis using a Yukawa potential. The obtaining of the regularization by a coarse mesh rather than by the usual penalisation parameter is discussed.