Oscillatory behavior of hollow grid cathode discharges

Multiple complex space-charge structures in unmagnetized low-temperature plasmas arise from ionization phenomena near additional negatively or positively biased electrodes or due to local constraints. Because of their usually spherical form, such structures are called fireballs. If they appear inside hollow grids, they are called inverted fireballs or plasma bubbles. The temporal evolution of such structures is often accompanied by strong plasma instabilities. The dynamics of complex space-charge structures have been investigated by using single spherical grid cathode with an orifice. Langmuir probe and optical emission spectroscopy were used to diagnose the structures. Measurements delivered the axial profiles of the plasma potential, electron temperature and density, and the densities of excited atoms and ions, that confirmed the formation of a fireball in the region near the orifice (also evidenced by visual observation). Inside the grid, a plasma bubble has developed, with a high ion density inside due to the hollow cathode effect. Information on the nonlinear dynamics of the complex space charge structures was obtained from the analysis of the oscillations of the discharge current

A study of the Yld2004 yield function and one extension in polynomial form: A new implementation algorithm, modeling range, and earing predictions for aluminum alloy sheets

As shown recently in (Soare and Barlat, 2010. Convex polynomial yield functions. J. Mech., Phys. Solids, 58, 1804-1818), the principal values based yield function Yld2004, proposed in (Barlat et al., 2005. Linear transformation based anisotropic yield function. Int. J. Plast., 21, 1009-1039), is polynomial for integer exponents. Based on this observation, a new algorithm is proposed for implementing symmetric yield functions formulated in terms of principal values. The algorithm is tested here by simulating with a commercial FE code the cylindrical deep drawing of two aluminum sheets. It is found that the classical description of the in-plane directional properties of the sheet (uniaxial r-values and yield stresses), even if modeled correctly by the yield function, is not sufficient for a unique characterization of the predicted earing profile. For certain combinations of the directional properties the r-value in biaxial stressing has to be considered for a correct calibration of the material model. This in turn requires a finer detail in yield surface modeling and, to achieve it, an ad-hoc extension of Yld2004 is constructed. In combination with the proposed implementation algorithm, the extension is shown to be a useful research tool, having some interesting modeling capabilities and satisfactory FE runtime. (C) 2011 Elsevier Masson SAS. All rights reserved.X1156sciescopu

On the use of homogeneous polynomials to develop anisotropic yield functions with applications to sheet forming

This paper investigates the capabilities of several non-quadratic polynomial yield functions to model the plastic anisotropy of orthotropic sheet metal (plane stress). Fourth, sixth and eighth-order homogeneous polynomials are considered. For the computation of the coefficients of the fourth-order polynomial an improved set of analytic formulas is proposed. For sixth and eighth-order polynomials the identification uses optimization. Simple constraints on the optimization process are shown to lead to real-valued convex functions. A general method to extend the above plane stress criteria to full 3D stress states is also suggested. Besides their simplicity in formulation, it is found that polynomial yield functions are capable to model a wide range of anisotropic plastic properties (e.g., the Numisheet’93 mild steel, AA2008-T4, AA2090-T3). The yield functions have then been implemented into a commercial finite element code as constitutive subroutines. The deep drawing of square (Numisheet’93) and cylindrical (AA2090-T3) cups have been simulated. In both cases excellent agreement with experimental data is obtained. In particular, it is shown that non-quadratic polynomial yield functions can simulate cylindrical cups with six or eight ears. We close with a discussion on earing and further examples

Approximate yield criteria for anisotropic porous metals with non spherical voids

International audienc

quasar - A Generic Framework for Rapid Development of OPC UA Servers

This paper describes a new approach for generic design and efficient development of OPC Unified Architecture (UA) servers. Development starts with creation of a design XML file, describing an OO information model of the target system or device. Using this model, the framework generates an executable OPC UA server exposing the per-design address space without writing a single line of code while supporting standalone or embedded platforms. Further, the framework generates skeleton code for the interface logic of the target system or device. This approach allows both novice and expert developers to create servers for the systems they are experts in while greatly reducing design and development effort as compared to developments based on COTS OPC UA toolkits. Higher level software such as SCADA systems may benefit from using the design description to generate client connectivity configuration and data representation as well as validation tools. In this contribution, the concept and implementation of this framework is detailed along with examples of actual production-level usage in the detector control system of the ATLAS Experiment at CERN and beyond