Self-replicating cracks: a collaborative fracture mode in thin films

Straight cracks are observed in thin coatings under residual tensile stress, resulting into the classical network pattern observed in china crockery, old paintings or dry mud. Here, we present a novel fracture mechanism where delamination and propagation occur simultaneously, leading to the spontaneous self-replication of an initial template. Surprisingly, this mechanism is active below the standard critical tensile load for channel cracks and selects a robust interaction length scale on the order of 30 times the film thickness. Depending on triggering mechanisms, crescent alleys, spirals or long bands are generated over a wide range of experimental parameters. We describe with a simple physical model the selection of the fracture path and provide a configuration diagram displaying the different failure modes

Wrinkling hierarchy in constrained thin sheets from suspended graphene to curtains

We show that thin sheets under boundary confinement spontaneously generate a universal self-similar hierarchy of wrinkles. From simple geometry arguments and energy scalings, we develop a formalism based on wrinklons, the transition zone in the merging of two wrinkles, as building-blocks of the global pattern. Contrary to the case of crumple paper where elastic energy is focused, this transition is described as smooth in agreement with a recent numerical work. This formalism is validated from hundreds of nm for graphene sheets to meters for ordinary curtains, which shows the universality of our description. We finally describe the effect of an external tension to the distribution of the wrinkles.Comment: 7 pages, 4 figures, added references, submitted for publicatio

Integrated presentation of ecological risk from multiple stressors

Current environmental risk assessments (ERA) do not account explicitly for ecological factors (e.g. species composition, temperature or food availability) and multiple stressors. Assessing mixtures of chemical and ecological stressors is needed as well as accounting for variability in environmental conditions and uncertainty of data and models. Here we propose a novel probabilistic ERA framework to overcome these limitations, which focusses on visualising assessment outcomes by construct-ing and interpreting prevalence plots as a quantitative prediction of risk. Key components include environmental scenarios that integrate exposure and ecology, and ecological modelling of relevant endpoints to assess the effect of a combination of stressors. Our illustrative results demonstrate the importance of regional differences in environmental conditions and the confounding interactions of stressors. Using this framework and prevalence plots provides a risk-based approach that combines risk assessment and risk management in a meaningful way and presents a truly mechanistic alternative to the threshold approach. Even whilst research continues to improve the underlying models and data, regulators and decision makers can already use the framework and prevalence plots. The integration of multiple stressors, environmental conditions and variability makes ERA more relevant and realistic

Dual mode transducers based on cMUTs technology

International audienceMore and more medical ultrasonic applications are strongly interested by the development of dual acoustic sources enable to emit high frequency ultrasound (echographic imaging) and low frequency pressure field (therapeutic ultrasounds). The use of the piezoelectricity to fabricate such device requires overcoming strong technological bottlenecks. The objective of this paper is to demonstrate that the technology of capacitive micro-machined ultrasonic transducer is able to take up this challenge. To this end a demonstrator was designed and manufactured. The first part of the paper is devoted to the design of the low and high frequency cMUTs. For the low acoustic source, a dedicated time domain model was used, taking into account the nonlinearity of the cMUT. Several simulations were conducted to optimize the emitted pressure field intensity at 1 MHz for a set of diaphragm with different sizes and geometries. The high frequency source was designed on the help of a linear model, where output parameters were central frequency, bandwidth and collapse voltage. The second part of this paper reports a set of characterization results and performances of the fabricated device: electrical impedance, mechanical displacements performed in water and acoustic pressure fields

Thermal Jeans fragmentation within 1000 AU in OMC-1S

We present subarcsecond 1.3 mm continuum ALMA observations towards the Orion Molecular Cloud 1 South (OMC-1S) region, down to a spatial resolution of 74 AU, which reveal a total of 31 continuum sources. We also present subarcsecond 7 mm continuum VLA observations of the same region, which allow to further study fragmentation down to a spatial resolution of 40 AU. By applying a Mean Surface Density of Companions method we find a characteristic spatial scale at ~560 AU, and we use this spatial scale to define the boundary of 19 `cores' in OMC-1S as groupings of millimeter sources. We find an additional characteristic spatial scale at ~2900 AU, which is the typical scale of the filaments in OMC-1S, suggesting a two-level fragmentation process. We measured the fragmentation level within each core and find a higher fragmentation towards the southern filament. In addition, the cores of the southern filament are also the densest (within 1100 AU) cores in OMC-1S. This is fully consistent with previous studies of fragmentation at spatial scales one order of magnitude larger, and suggests that fragmentation down to 40 AU seems to be governed by thermal Jeans processes in OMC-1S.Comment: Accepted to Ap

Mechanics and Energetics of Electromembranes

The recent discovery of electroactive polymers has shown great promises in the field of soft robotics and was logically followed by experimental, numerical, and theoretical developments. Most of these studies were concerned with systems entirely covered by electrodes. However, there is a growing interest for partially active polymers, in which the electrode covers only one part of the membrane. Indeed, such actuation can trigger buckling instabilities and so represents a route toward the control of three-dimensional shapes. Here, we study theoretically the behavior of such partially active electroactive polymer. We address two problems: (1) the electrostatic elastica including geometric nonlinearities and partially electroactive strip using a variational approach. We propose a new interpretation of the equations of deformation, by drawing analogies with biological growth, in which the effect of the electric voltage is seen as a change in the reference stress-free state. (2) We explain the nature of the distribution of electrostatic forces on this simple system, which is not trivial. In particular, we find that edge effects are playing a major role in this problem. </p

Compaction de plaques minces : Instabilités et motifs

Lorsqu'une plaque mince est compactée entre deux calottes sphériques de même rayon de courbure, les contraintes compressives résultantes engendrent la formation de plis radiaux à partir d'une certaine distance du centre de la plaque. De plus, la force de compaction présente un maximum indépendant de l'épaisseur de la plaque. Contrairement au flambement d'Euler pour lequel l'amplitude des plis n'est pas limité, les plis sont dans cette situation contraints par l'espacement qui est imposé entre les deux sphères. Nous montrons à partir d'un modèle unidimensionnel comment la longueur d'onde de ces plis résulte de cette contrainte d'amplitude et tend vers une limite finie lorsque l'espacement devient de l'ordre de l'épaisseur de la plaque. Ce modèle permet également d'expliquer l'évolution de la force de compaction avec l'espacement imposé. Cette approche unidimensionnelle est ensuite étendue au cas axisymétrique initial

Complications Associated With Two Laparoscopic Procedures Used in the Management of Rectal Endometriosis

This study suggests that bladder and rectal dysfunction occur more frequently with colorectal resection in rectal endometriosis compared with excision of the nodules alone