Sliding without slipping under Coulomb friction: opening waves and inversion of frictional force

An elastic layer slides on a rigid flat governed by Coulomb's friction law. We demonstrate that if the coefficient of friction is high enough, the sliding localizes within stick-slip pulses, which transform into opening waves propagating at intersonic speed in the direction of sliding or, for high Poisson's ratios, at supersonic speed in the opposite direction. This sliding mode, characterized by marginal frictional dissipation, and similar to carpet fold propagation, may result in inversion of the frictional force direction; at longer time intervals the system demonstrates stick-slip behavior. The mechanism is described in detail and a parametric study is presented.Comment: 17 pages, 4 figures, 1 tabl

Dynamic Authorization Specification for RBAC at CERN

Role-based access control (RBAC) project at CERN was designed to protect from accidental and unauthorized access to the LHC and injector equipment. Our model of RBAC introduces concept of dynamic authorization. Dynamic authorization is the authorization algorithm which takes into account not only defined permissions, but also the internal state of each device server, called checking policy. This paper describes motivation of this algorithm and gives detailed explanation for each checking policy

The role of phase interface energy in martensitic transformations: a lattice Monte-Carlo simulation

To study martensitic phase transformation we use a micromechanical model based on statistical mechanics. Employing lattice Monte-Carlo simulations and realistic material properties for shape-memory alloys (SMA), we investigate the combined influence of the external stress, temperature, and interface energy between the austenitic and martensitic phase on the transformation kinetics and the effective material compliance. The one-dimensional model predicts well many features of the martensitic transformation that are observed experimentally. Particularly, we study the influence of the interface energy on the transformation width and the effective compliance. In perspective, the obtained results might be helpful for the design of new SMAs for more sensitive smart structures and more efficient damping systems.Comment: 10 pages, 3 figures, 22 reference

The existence of a critical length scale in regularised friction

We study a regularisation of Coulomb's friction law on the propagation of local slip at an interface between a deformable and a rigid solid. This regularisation, which was proposed based on experimental observations, smooths the effect of a sudden jump in the contact pressure over a characteristic length scale. We apply it in numerical simulations in order to analyse its influence on the behaviour of local slip. We first show that mesh convergence in dynamic simulations is achieved without any numerical damping in the bulk and draw a convergence map with respect to the characteristic length of the friction regularisation. By varying this length scale on the example of a given slip event, we observe that there is a critical length below which the friction regularisation does not affect anymore the propagation of the interface rupture. A spectral analysis of the regularisation on a periodic variation of Coulomb's friction is conducted to confirm the existence of this critical length. The results indicate that if the characteristic length of the friction regularisation is smaller than the critical length, a slip event behaves as if it was governed by Coulomb's law. We therefore propose that there is a domain of influence of the friction regularisation depending on its characteristic length and on the frequency content of the local slip event. A byproduct of the analysis is related to the existence of a physical length scale characterising a given frictional interface. We establish that the experimental determination of this interface property may be achieved by experimentally monitoring slip pulses whose frequency content is rich enough.Comment: 21 pages, 7 figure

Electrical and Thermal Conductivity of Complex-Shaped Contact Spots

This paper explores the electrical and thermal conductivity of complex contact spots on the surface of a half-space. Employing an in-house Fast Boundary Element Method implementation, various complex geometries were studied. Our investigation begins with annulus contact spots to assess the impact of connectedness. We then study shape effects on "multi-petal" spots exhibiting dihedral symmetry, resembling flowers, stars, and gears. The analysis culminates with self-affine shapes, representing a multi-scale generalization of the multi-petal forms. In each case, we introduce appropriate normalizations and develop phenomenological models. For multi-petal shapes, our model relies on a single geometric parameter: the normalized number of "petals". This approach inspired the form of the phenomenological model for self-affine spots, which maintains physical consistency and relies on four geometric characteristics: standard deviation, second spectral moment, Nayak parameter, and Hurst exponent. As a by product, these models enabled us to suggest flux estimations for an infinite number of petals and the fractal limit. This study represents an initial step into understanding the conductivity of complex contact interfaces, which commonly occur in the contact of rough surfaces.Comment: 40 pages, 28 figure

Securing Controls Middleware of the Large Hadron Collider

The distributed control system of the Large Hadron Collider (LHC) presents many challenges due to its inherent heterogeneity and highly dynamic nature. One critical challenge is providing access control guarantees within the middleware. Role-based access control (RBAC) is a good candidate to provide access control. However, in an equipment control system transactions are often dependent on user context and device context. Unfortunately, classic RBAC cannot be used to handle the above requirements. In this paper we present an extended role-based access control model called CMW-RBAC. This new model incorporates the advantages of role-based permission administration together with a fine-grained control of dynamic context attributes. We also propose a new technique called dynamic authorization that allows phased introduction of access control in large distributed systems. This paper also describes motivation of the project, requirements, and overview of its main components: authentication and authorization

Discovering Real (Homogenous) Social Groups in the Russian Society: Methods and Results

The article focuses on the problem of identifying real social groups in the contemporary Russian society. The data from all-Russian monitoring surveys are used to compare two social structure models obtained by alternative methods. One of the models is similar to that of the European sociological tradition based on a socio-professional classification. The other one has been obtained by applying the cluster analysis after having ranked the stratification criteria derived from the entropy analysis.stratification; social structure; social inequality; occupational classification; entropy analysis; cluster analysis; real social groups