An observation on the experimental measurement of dislocation density

The common practice of ignoring the elastic strain gradient in measurements of geometrically necessary dislocation (GND) density is critically examined. It is concluded that the practice may result in substantial errors. Our analysis points to the importance of spatial variations of the elastic strain field in relation to its magnitude in inferring estimates of dislocation density from measurements

Roughening and preroughening in the six vertex model with an extended range of interaction

We study the phase diagram of the BCSOS model with an extended interaction range using transfer matrix techniques, pertaining to the (100) surface of single component fcc and bcc crystals. The model shows a 2x2 reconstructed phase and a disordered flat phase. The deconstruction transition between these phases merges with a Kosterlitz-Thouless line, showing an interplay of Ising and Gaussian degrees of freedom. As in studies of the fully frustrated XY model, exponents deviating from Ising are found. We conjecture that tri-critical Ising behavior may be a possible explanation for the non-Ising exponents found in those models.Comment: 25 pages in RevTeX 3.0, seven uuencoded postscript figures, REPLACED because of submission error (figures were not included

Is surface melting a surface phase transition?

Monte Carlo or Molecular Dynamics calculations of surfaces of Lennard-Jones systems often indicate, apart from a gradual disordering of the surface called surface melting, the presence of a phase transition at the surface, but cannot determine the nature of the transition. In the present paper, we provide for a link between the continuous Lennard-Jones system and a lattice model. We apply the method for the (001) surface of a Lennard-Jones fcc structure pertaining to Argon. The corresponding lattice model is a Body Centered Solid on Solid model with an extended range of interaction, showing in principle rough, flat and disordered flat phases. We observe that entropy effects considerably lower the strength of the effective couplings between the atoms. The Argon (001) face is shown to exhibit a phase transition at T=70.5 +- 0.5 K, and we identify this transition as roughening. The roughening temperature is in good correspondence with experimental results for Argon.Comment: 17 pages REVTeX, 14 uuencoded postscript figures appende

Eléments pour une Politique du Volontariat

This report describes the voluntary sector in Belgium and abroad. It describes improvements that could be made to the juridical situation that governs the third sector. Additionally, it acknowledges the societal contribution of volunteers and the non-profit organisations for which they work

Treatment of Pelvic Ring Fractures with Pelvic Circumferential Compression Divices

__Abstract__ High energy pelvic fractures are life-threatening injuries and are among the most challenging injuries to treat. Complete evaluation of the patient with a high energy pelvic fracture is essential because this is rarely an isolated injury. Most deaths in patients with pelvic fractures are not caused by the pelvic fracture itself but are linked to associated injuries. The same forces that lead to disruption of the pelvic ring are frequently associated with abdominal, head, and thoracic injury. Bleeding remains the leading cause of death in patients with pelvic fractures but is rarely the only cause of blood loss in the patient with multiple injuries. In addition to bleeding from the fracture surfaces (i.e., cancellous bone) bleeding from the venous plexus and arterial lesions in a patient with a pelvic ring fracture potentially causes serious complications. These anatomical structures that are at risk are discussed into more detail in the pelvic anatomy section below

A neuro-mechanical model for the switching of stepping direction and transitions between walking gaits in the stick insect

In this study, a mathematical model for the locomotion of the stick insect is developed. This model takes physiological conditions into account and it is capable of mimicking biological relevant features. The model is predicated on the crucial role, that sensory feedback plays in the coordination of limbs during walking. Central Pattern Generators (CPGs), which produce the rhythm of locomotion, are affected by sensory influences between the segments. The activities of the CPGs are transferred by the motoneurons to the muscles. Starting with existing neuron models and neuronal network models, a neuro-mechanical model is developed that includes the coupling of segments inside of a leg as well as the coupling of multiple legs. Firstly, mechanical models concerning the motion of the three isolated main joints are derived. These mechanical models are fused with the neuronal one. Thus, they represent neuro-mechanical models for the single joints that are coupled via sensory feedback. By means of the introduction of a switching mechanism the model is able to produce forward, backward and sideward stepping of a middle leg. Through the junction of two stepping middle legs to the body of the modeled stick insect, curve walking sequences with different curvatures can be produced. By extending the model to the front and the hind leg, the structure of intersegmental connection between the legs during the tripod and tetrapod gait can be generated. The change of stepping direction can be brought about by changing one single central command. If the middle leg is stepping backwards, the curvature during turning is smaller than in the case of sideward stepping. Weakly inhibitory intersegmental connections show the most accommodating leg coordination during both the tetrapod and the tripod gait

Dynamical transitions in incommensurate systems

In the dynamics of the undamped Frenkel-Kontorova model with kinetic terms, we find a transition between two regimes, a floating incommensurate and a pinned incommensurate phase. This behavior is compared to the static version of the model. A remarkable difference is that, while in the static case the two regimes are separated by a single transition (the Aubry transition), in the dynamical case the transition is characterized by a critical region, in which different phenomena take place at different times. In this paper, the generalized angular momentum we have previously introduced, and the dynamical modulation function are used to begin a characterization of this critical region. We further elucidate the relation between these two quantities, and present preliminary results about the order of the dynamical transition.Comment: 7 pages, 6 figures, file 'epl.cls' necessary for compilation provided; subm. to Europhysics Letter