Detection of Change--Points in the Spectral Density. With Applications to ECG Data

We propose a new method for estimating the change-points of heart rate in the orthosympathetic and parasympathetic bands, based on the wavelet transform in the complex domain and the study of the change-points in the moments of the modulus of these wavelet transforms. We observe change-points in the distribution for both bands.Comment: proceeding of the workshop 'Fouille de donn\'ees temporelles et analyse de flux de donn\'ees' EGC'2009, january 27, Strasbourg, Franc

Emploi pratique des analyses fourragères pour l'appréciation des pâturages tropicaux

Les taux de matières sèches, de matières azotées brutes, de cellulose et de cendres constituent les éléments de calcul de la valeur bromatologique des fourrages. A partir de résultats expérimentaux, les tables hollandaises donnent directement la valeur bromatologique d'un kg de matières sèches de fourrage en fonction du taux de cellulose et de cendres pour la valeur énergétique et en fonction du taux de matières azotées brutes pour les matières azotées digestibles. Par analogie à l'unité gros bétail (U.G.B.) des pays tempérés, un animal de référence de 250 kg pour les pays tropicaux peut être adopté comme Unité-Bétail tropical (U.B.T.). La consommation journalière théorique étant évaluée à 2,5 kg de matière sèche pour 100 kg de poids vif, les besoins de l'U.B.T. sont rapportés au kg de matière sèche ingérée sous le nom d'Equivalent-Rati on exprimé en unité fourragère et matière azotée digestible. Le dépouillement de plus de 500 analyses bromatologiques de plantes fourragères tropicales a mis en évidence: l'importance des pâturages aériens comme complément de la ration en matières azotées digestibles au cours de la saison sèche tant sur steppes que sur savanes; l'importance périodique des espèces d'appoint pour assurer la nourriture du bétail sur steppes; - le nombre limité d'espèces assurant une ration d'entretien pendant une longue période sur steppes; - l'importance du temps de croissance pour les espèces vivaces de savanes, la valeur bromatologique des repousses dépendant plus de ce facteur que de la date de récolt

Crack Front Waves and the dynamics of a rapidly moving crack

Crack front waves are localized waves that propagate along the leading edge of a crack. They are generated by the interaction of a crack with a localized material inhomogeneity. We show that front waves are nonlinear entities that transport energy, generate surface structure and lead to localized velocity fluctuations. Their existence locally imparts inertia, which is not incorporated in current theories of fracture, to initially "massless" cracks. This, coupled to crack instabilities, yields both inhomogeneity and scaling behavior within fracture surface structure.Comment: Embedded Latex file including 4 figure

Continuum field description of crack propagation

We develop continuum field model for crack propagation in brittle amorphous solids. The model is represented by equations for elastic displacements combined with the order parameter equation which accounts for the dynamics of defects. This model captures all important phenomenology of crack propagation: crack initiation, propagation, dynamic fracture instability, sound emission, crack branching and fragmentation.Comment: 4 pages, 5 figures, submitted to Phys. Rev. Lett. Additional information can be obtained from http://gershwin.msd.anl.gov/theor

Theory of dynamic crack branching in brittle materials

The problem of dynamic symmetric branching of an initial single brittle crack propagating at a given speed under plane loading conditions is studied within a continuum mechanics approach. Griffith's energy criterion and the principle of local symmetry are used to determine the cracks paths. The bifurcation is predicted at a given critical speed and at a specific branching angle: both correlated very well with experiments. The curvature of the subsequent branches is also studied: the sign of $T$, with $T$ being the non singular stress at the initial crack tip, separates branches paths that diverge from or converge to the initial path, a feature that may be tested in future experiments. The model rests on a scenario of crack branching with some reasonable assumptions based on general considerations and in exact dynamic results for anti-plane branching. It is argued that it is possible to use a static analysis of the crack bifurcation for plane loading as a good approximation to the dynamical case. The results are interesting since they explain within a continuum mechanics approach the main features of the branching instabilities of fast cracks in brittle materials, i.e. critical speeds, branching angle and the geometry of subsequent branches paths.Comment: 41 pages, 15 figures. Accepted to International Journal of Fractur

Unified force law for granular impact cratering

Experiments on the low-speed impact of solid objects into granular media have been used both to mimic geophysical events and to probe the unusual nature of the granular state of matter. Observations have been interpreted in terms of conflicting stopping forces: product of powers of projectile depth and speed; linear in speed; constant, proportional to the initial impact speed; and proportional to depth. This is reminiscent of high-speed ballistics impact in the 19th and 20th centuries, when a plethora of empirical rules were proposed. To make progress, we developed a means to measure projectile dynamics with 100 nm and 20 us precision. For a 1-inch diameter steel sphere dropped from a wide range of heights into non-cohesive glass beads, we reproduce prior observations either as reasonable approximations or as limiting behaviours. Furthermore, we demonstrate that the interaction between projectile and medium can be decomposed into the sum of velocity-dependent inertial drag plus depth-dependent friction. Thus we achieve a unified description of low-speed impact phenomena and show that the complex response of granular materials to impact, while fundamentally different from that of liquids and solids, can be simply understood