Low temperature fluorescence spectra of atropisomeric perylene dyes

The site selection spectra of highly photostable perylene fluorescent dyes at 4.2 K in a glassy matrix of three different organic solvents (ethanol, methyl tetrahydrofuran, n-butyl acetate) are presented. The spectra ofthe two isomers are identical for a given solvent and only very minute differences are detectable between different solvents. We conclude that the influence of the matrix on the fluorescence spectra is of only minor importance. The influence of the orientation in the matrix is negligible

Mikrostrukturelle Einflüsse auf das R-Kurvenverhalten bei Siliciumnitridkeramiken

In dieser Arbeit wurden Risswiderstandskurven (R-Kurven) für vier verschiedene Siliciumnitridkeramiken gemessen. Die R-Kurven zeigen einen sehr steilen Anstieg und erreichen nahezu den Plateauwert in 10 bis 20µm Rissverlängerung. Aus den R-Kurven wurden mit Hilfe der Gewichtsfunktion die jeweiligen Brückenspannungsverteilungen berechnet. Des Weiteren wurde ein neuer Ansatz zur Beschreibung der Verstärkungsmechanismen in Siliciumnitridkeramiken herausgearbeitet

Modeling the track geometry variability

International audienceAt its building, the theoretical new railway line is supposed to be made of perfect straight lines and curves. This track geometry is however gradually damaged and regularly subjected to maintenance operations. The analysis of these track irregularities is a key issue as the dynamic behaviour of the trains is mainly induced by the track geometry. In this context, this work is devoted to the development of a stochastic modeling of the track geometry and its identification with experimental measurements. Based on a spatial and statistical decomposition, this model allows the spatial and statistical variability and dependency of the track geometry to be taken into account. Moreover, it allows the generation of realistic track geometries that are representative of a whole railway network. These tracks can be used in any deterministic railway dynamic software to characterize the dynamic behavior of the train

Statistical inverse problems for non-Gaussian vector valued random fields with a set of experimental realizations

International audienceThe railway track irregularities, which is a four dimensions vector-valued random field, are the main source of excitation of the train. At first, using a revisited Karhunen-Loève expansion, the considered random field is approximated by its truncated projection on a particularly well adapted orthogonal basis. Then, the distribution of the random vector that gathers the projection coefficients of the random field on this spatial basis is characterized using a polynomial chaos expansion. The dimension of this random vector being very high (around five hundred), advanced identification techniques are introduced to allow performing relevant convergence analysis and identification. Based on the stochastic modeling of the non- Gaussian non-stationary vector-valued track geometry random field, realistic track geometries, which are representative of the experimental measurements and representative of the whole railway network, can be generated. These tracks can then be introduced as an input of any railway software to characterize the stochastic behavior of any normalized train

Karhunen-Loève based sensitivity analysis

International audienceThe identification of the most dangerous combinations of excitations that a non-linear mechanical system can be confronted to is not an easy task. Indeed, in such cases, the link between the maximal values of the inputs and of the outputs is not direct, as the system can be more sensitive to a problematic succession of excitations of low amplitudes than to high amplitudes for each kind of excitations. This work presents therefore an innovative method to identify the combined shapes of excitations that are the most correlated to problematic responses of the studied mechanical system

Influence of the track geometry variability on the train behavior

International audienceThis paper is devoted to the development of a stochastic modeling of the track geometry and its identiication with experimental measurements. This modeleing, which has to integrate the statistical and spatial variabilities and dependencies , is a keyu issue when using simulation for conception, maintenance or certification purposes

Karhunen-Loeve expansion revisited for vector-valued random fields: scaling, errors and optimal basis

International audienceDue to scaling effects, when dealing with vector-valued random fields, the classical Karhunen-Loève expansion, which is optimal with respect to the total mean square error, tends to favorize the components of the random field that have the highest signal energy. When these random fields are to be used in mechanical systems, this phenomenon can introduce undesired biases for the results. This paper presents therefore an adaptation of the Karhunen- Loève expansion that allows us to control these biases and to minimize them. This original decomposition is first analyzed from a theoretical point of view, and is then illustrated on a numerical example

Acutely damaged axons are remyelinated in multiple sclerosis and experimental models of demyelination

Remyelination is in the center of new therapies for the treatment of multiple sclerosis to resolve and improve disease symptoms and protect axons from further damage. Although remyelination is considered beneficial in the long term, it is not known, whether this is also the case early in lesion formation. Additionally, the precise timing of acute axonal damage and remyelination has not been assessed so far. To shed light onto the interrelation between axons and the myelin sheath during de- and remyelination, we employed cuprizone- and focal lysolecithin-induced demyelination and performed time course experiments assessing the evolution of early and late stage remyelination and axonal damage. We observed damaged axons with signs of remyelination after cuprizone diet cessation and lysolecithin injection. Similar observations were made in early multiple sclerosis lesions. To assess the correlation of remyelination and axonal damage in multiple sclerosis lesions, we took advantage of a cohort of patients with early and late stage remyelinated lesions and assessed the number of APP- and SMI32- positive damaged axons and the density of SMI31-positive and silver impregnated preserved axons. Early de- and remyelinating lesions did not differ with respect to axonal density and axonal damage, but we observed a lower axonal density in late stage demyelinated multiple sclerosis lesions than in remyelinated multiple sclerosis lesions. Our findings suggest that remyelination may not only be protective over a long period of time, but may play an important role in the immediate axonal recuperation after a demyelinating insult