Grain boundary relaxation in yellow gold bi-crystals

The mechanical loss spectrum of a yellow gold bi-crystal is presented and analyzed in detail. The relaxation strength is monitored as a function of several geometrical parameters such as sample width, length and thickness. It is found that the relaxation strength is proportional to the GB density (the inverse width), whereas it depends linearly on the sample thickness. The experimental findings are compared to finite elements (FE) simulations, where the material can glide frictionless along the grain boundary. The simulations show the same dependencies as the mechanical loss measurements. The relaxation peak in the loss spectrum can be interpreted as due to GB sliding accommodated by the elastic deformation of the grains. (C) 2015 Elsevier B.V. All rights reserved.

Theoretical treatment and numerical simulation of potentiometric and amperometric enzyme electrodes and of enzyme reactors. Part 1: Steady-state concentration profiles, fluxes, and responses

Nico de Rooij, Peter Van der Wal

A unified and comparative treatment of traditional potentiometric and amperometric enzyme electrodes and of enzyme reactors is presented. Exact theoretical descriptions are given for limiting cases, and relatively simple approaches for general cases. The latter approximations are obtained from the respective solutions for a two-segmented membrane model. They allow an explicit description and a full characterization of the response curves by only two kinetic parameters, which are the ratio of overall reaction and diffusion rates and the Michaelis constant. Numerical simulations based on a finite-difference procedure are developed for the functional enzyme membrane being represented by a sufficiently large number of elements (slices). These simulations are applied as virtual experiments for the evaluation of concentration profiles and fluxes of substrate and product species. The response characteristics of potentiometric and amperometric sensor systems, as well as the product release from enzyme reactors, are analyzed, and the influence of the relevant parameters on the steady-state response is demonstrated and discussed. Potentiometric and amperometric enzyme electrodes of identical kinetic properties are shown to exhibit the same response patterns for the actually sensed versus the nominal substrate concentrations. The simulated results are in excellent agreement with the appropriate theories. For all examples studied in this work the standard deviation between computer-simulated and theoretically approximated response curves is in the range of only 0.1-4% with respect to the sensed concentration. (C) 2011 Elsevier B.V. All rights reserved.

2011

Three-dimensional granular model of semi-solid metallic alloys undergoing solidification: Fluid flow and localization of feeding

Jean-Marie Drezet, André Phillion, Michel Rappaz, Meisam Sistaninia

A three-dimensional (3-D) granular model which simulates fluid flow within solidifying alloys with a globular microstructure, such as that found in grain refined Al alloys, is presented. The model geometry within a representative volume element (RVE) consists of a set of prismatic triangular elements representing the intergranular liquid channels. The pressure field within the liquid channels is calculated using a finite elements (FEs) method assuming a Poiseuille flow within each channel and flow conservation at triple lines. The fluid flow is induced by solidification shrinkage and openings at grain boundaries due to deformation of the coherent solid. The granular model predictions are validated against bulk data calculated with averaging techniques. The results show that a fluid flow simulation of globular semi-solid materials is able to reproduce both a map of the 3-D intergranular pressure and the localization of feeding within the mushy zone. A new hot cracking sensitivity coefficient is then proposed. Based on a mass balance performed over a solidifying isothermal volume element, this coefficient accounts for tensile deformation of the semi-solid domain and for the induced intergranular liquid feeding. The fluid flow model is then used to calculate the pressure drop in the mushy zone during the direct chill casting of aluminum alloy billets. The predicted pressure demonstrates that deep in the mushy zone where the permeability is low the local pressure can be significantly lower than the pressure predicted by averaging techniques. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Elsevier2012

A reduced basis method for electromagnetic scattering by multiple particles in three dimensions

Jan Sickmann Hesthaven, Bernhard Stamm

We consider the development of efficient and fast computational methods for parametrized electromagnetic scattering problems involving many scattering three dimensional bodies. The parametrization may describe the location, orientation, size, shape and number of scattering bodies as well as properties of the source field such as frequency, polarization and incident direction. The emphasis is on problems that need to be solved rapidly to accurately simulate the interaction of scattered fields under parametric variation, e. g., for design, detection, or uncertainty quantification. For such problems, the use of a brute force approach is often ruled out due to the computational cost associated with solving the problem for each parameter value. In this work, we propose an iterative reduced basis method based on a boundary element discretization of few reference scatterers to resolve the computationally challenging large scale problem. The approach includes (i) a computationally intensive offline procedure to create a selection of a set of snapshot parameters and the construction of an associated reduced basis for each reference scatterer and (ii) an inexpensive online algorithm to generate the surface current and scattered field of the parametrized configuration, for any choice of parameters within the parameter domains used in the offline procedure. Comparison of our numerical results with directly measured results for some benchmark configurations demonstrate the power of our method to rapidly simulate the interacting electromagnetic fields under parametric variation of the overall multiple particle configuration. (C) 2012 Elsevier Inc. All rights reserved.

Elsevier2012