Failure time and critical behaviour of fracture precursors in heterogeneous materials

The acoustic emission of fracture precursors, and the failure time of samples of heterogeneous materials (wood, fiberglass) are studied as a function of the load features and geometry. It is shown that in these materials the failure time is predicted with a good accuracy by a model of microcrack nucleation proposed by Pomeau. We find that the time interval $$ between events (precursors) and the energy $\varepsilon$ are power law distributed and that the exponents of these power laws depend on the load history and on the material. In contrast, the cumulated acoustic energy $E$ presents a critical divergency near the breaking time $\tau$ which is % E\sim \left( \frac{\tau -t}\tau \right) ^{-\gamma }. The positive exponent $$ is independent, within error bars, on all the experimental parameters.Comment: to be published on European Physical Journa

The cooperative effect of load and disorder in thermally activated rupture of a two-dimensional random fuse network

A random fuse network, or equivalently a two-dimensional spring network with quenched disorder, is subjected to a constant load and thermal noise, and studied by means of numerical simulations. Rupture is thermally activated and the lifetime follows an Arrhenius law where the energy barrier is reduced by disorder. Due to the non-homogeneous distribution of forces from the stress concentration at microcrack tips, spatial correlations between rupture events appear, but they do not affect the energy barrier's dependence on the disorder; they affect only the coupling between the disorder and the applied load

Thermal activation of rupture and slow crack growth in a model of homogenous brittle materials

Slow crack growth in a model of homogenous brittle elastic material is described as a thermal activation process where stress fluctuations allow to overcome a breaking threshold through a series of irreversible steps. We study the case of a single crack in a flat sheet for which analytical predictions can be made, and compare them with results from the equivalent problem of a 2D spring network. Good statistical agreement is obtained for the crack growth profile and final rupture time. The specific scaling of the energy barrier with stress intensity factor appears as a consequence of irreversibility. In addition, the model brings out a characteristic growth length whose physical meaning could be tested experimentally.Comment: To be published in : Europhysics Letter

Travailleurs et champs électromagnétiques industriels: Exposition des travailleurs aux champs électromagnétiques industriels

National audiencePour protéger les travailleurs contre les risques dus aux champs électromagnétiques, la directive européenne 2013/35/UE a défini des valeurs limites d’exposition et des valeurs déclenchant l’action pour les différentes bandes de fréquences.Dans le cas des machines industrielles utilisant généralement des radiofréquences, il est parfois délicat de calculer les valeurs d’exposition des travailleurs dans l’environnement de travail.Le projet de recherche EXTI vise à étudier l’exposition des travailleurs aux champs électromagnétiques RF en milieu industriel et à mettre en place une nouvelle méthode de caractérisation de l’exposition par dosimétrie numérique

Failure time in the fiber-bundle model with thermal noise and disorder

The average time for the onset of macroscopic fractures is analytically and numerically investigated in the fiber-bundle model with quenched disorder and thermal noise under a constant load. We find an implicit exact expression for the failure time in the low-temperature limit that is accurately confirmed by direct simulations. The effect of the disorder is to lower the energy barrier.Comment: 11 pages, 6 figures; accepted for publication in Phys. Rev.

Krylov-based algebraic multigrid for edge elements

International audienceThis work tackles the evaluation of a multigrid cycling strategy using inner flexible Krylov subspace iterations. It provides a valuable improvement to the Reitzinger and Sch¨oberl algebraic multigrid method for systems coming from edge element discretization

Omicidio e lesioni personali da incidenti della strada in stato di ebbrezza e di alterazione psico-fisica. Criticit\ue0 e problematiche. Considerazioni medico-legali

Road traffic crashes are a main cause of physical harm and mortality. That means a heavy burden in terms of short, medium and long-term disability and public costs for society. Among the most relevant cases of severe accidents is driving under the influence of alcohol and psychoactive drugs. That\u2019s the reason of the international strong effort to combating the phenomenon. In Italy the draft law concerning the criminal offence of vehicular homicide and road traffic personal injuries is currently being discussed. Previously the law n. 120/2010 has already changed the regulation in matter of drink-driving and driving under the influence of controlled substances with stiffer penalties for transgressors and (at least some) guidance as to assess the infringement. The draft law suggests that clinical and toxicological evidences of actual driving impairment has to be proved to reach significance in court, nevertheless no standard and effective procedure has being established for detection, at least for what concerns controlled substances. The purpose of this article is to put in evidence, in accordance with a medico-legal perspective, some of the issues that emerge from this complex matter (i.e. the need of a list of the impairing substance under detection, of well-defined legal cut-off values, of a better definition of the problems following the therapeutic use of drugs of abuse and other concerns), issues, those, that are now more troubling whereas the introduction of new crimes and punishments is becoming more concrete

The effect of disorder on the fracture nucleation process

The statistical properties of failure are studied in a fiber bundle model with thermal noise. We show that the macroscopic failure is produced by a thermal activation of microcracks. Most importantly the effective temperature of the system is amplified by the spatial disorder (heterogeneity) of the fiber bundle. The case of a time dependent force and the validity of the Kaiser effects are also discussed. These results can give more insight to the recent experimental observations on thermally activated crack and can be useful to study the failure of electrical networks.Comment: 22 pages, 11 fgure