Heterogeneous Mohr-Coulomb plastic material

We consider a heterogeneous medium which locally obeys a Mohr-Coulomb rigid-plastic behavior. We are interested in the macroscopic yield criterion based on an admissible stress formulation. We propose a two-dimensional analysis based on Airy potential formulation, using a first order perturbation. We first derive an upper bound for the macroscopic yield, and then construct explicitly an admissible stress field which reaches this bound. We finally show that within this framework, the macroscopic friction angle tends to the average of the local friction angle. Corrections to this value decay with the system size as [log(L)/L]^1/2, for all (narrow) distributions of local friction angle. Received: 26 June 1999     

A systemic approach for glass manufacturing process modeling

Due to the difficulty in defining accurate models to represent the physical phenomena in glass furnaces, a large number of studies consider the process as a black-box system. The systemic approach proposed in this paper relies on a decomposition of the furnace into two entities, i.e. the combustion chamber and the glass bath. Next, a reactor network structure is automatically designed to reproduce the hydrodynamic features in the considered zone. These computations are based on the minimization of the quadratic difference between the network output and Residence Time Distributions (RTDs) obtained through CFD simulations.The optimization results provide a network structure showing good agreement with the RTDs computed by CFD. In addition, they highlight, in both furnace zones, some useless volumes that do not participate to the global flow. These results stand as a good basis to extend the study to the computations of kinetics, energy or environmental criteria, which in turn may be optimized.     

Noise-robust stress intensity factor determination from kinematic field measurements

Stress intensity factors are often estimated numerically from a given displacement field through an interaction integral formalism. The latter method makes use of a weight, the virtual crack extension field, which is under-constrained by first principles. Requiring a least noise sensitivity allows one to compute the optimal virtual crack extension. Mode I and mode II specialized fields are obtained and particularized for a given displacement functional basis. The method is applied to an experimental case study of a crack in a silicon carbide sample, whose displacement field is obtained by a digital image correlation technique. The optimization leads to a very significant uncertainty reduction up to a factor 100 of the non-optimized formulation. The proposed scheme reveals additional performances with respect to the integral domain choice and assumed crack tip geometry, which are shown to have a reduced influence.     

Sintering and crystallisation of vanadium doped CaO–ZrO2–SiO2 glass-ceramics

The effect of addition of V₂O₅ on densification and crystallisation of glass powder compacts in the system CaO–ZrO₂–SiO₂ has been investigated. A well crystallized material of relatively high density (95% of theoretical density) was obtained after sintering at 900 °C for 1 h by adding 5 mol% V₂O₅. Crystalline phases identified were wollastonite, calcium zirconium silicate and calcium zirconium oxide. The addition of V₂O₅ was shown to reduce the sintering temperature and also to enhance the electrical conductivity of the base CaO–ZrO₂–SiO₂ glass-ceramic. Possible applications of the present vanadium containing glass-ceramics are the production of coloured glazes and in coatings and enamels with “anti-static” behaviour.     

Microstructure characterization of polyamide fibre/latex-filled plaster composites

We have investigated the microstructure of polyamide fibre/plaster and latex-filled plaster binary blends, and fibre/latex-filled plaster ternary blends. The influences of the latex (nature and concentration) and the fibres (concentration, length) were analysed in terms of the gypsum crystals morphology, the distribution of the polymer phase and the porosity of the materials. Addition of polyamide fibres and/or polymer latexes to plaster results in a coarser crystallisation, and disturbs the hydration of plaster; the porosity raises with the amount of fibres but decreases with the concentration of latex. It is shown that the presence of latex may noticeably modify the porous network of the plaster. These results are correlated to the setting of the materials.     

High-alumina porcelain with the addition of a Li2O-bearing fluxing agent

We have studied the influence of increasing the amount of β-spodumene (LiAlSi₂O₆), as a Li₂O-containing flux, on the phase composition, the microstructure evolution and the physical properties of high-alumina porcelain. Quartz reacts with β-spodumene in the temperature range 1150–1250 °C, forming lithium aluminium silicates with a larger amount of SiO₂. The presence of lithium minerals contributes to a lower CTE for the fired bodies. At 1300 °C an improved flexural strength is achieved with compositions containing 1.0 or 1.2 wt.% of Li₂O, as a result of a more uniform microstructure. With increasing amounts of Li₂O the overfiring effect is greatly enhanced. The most favourable characteristics from an industrial perspective with regard to flexural strength and deformation during firing were attained by using a high-alumina porcelain composition containing 1.0 wt.% Li₂O.     

Thermodynamic functions of the grain-like ZnO nanostructures

Grain-like ZnO nanostructures were prepared by a simple microemulsion-mediated hydrothermal route, and were characterized by X-ray powder diffraction (XRD) and field-emission scanning electron microscopy (FE-SEM). In order to associate thermodynamic functions of nano ZnO with bulk ZnO, a novel thermochemical cycle was designed. Combined with in situ microcalorimetry, the standard molar enthalpy of formation, standard molar Gibbs free energy of formation and standard molar entropy of the obtained products at 298.15 K were successfully acquired as (? 322.36 ± 0.42) kJ mol^? 1, (? 318.74 ± 0.03) kJ mol^? 1 and (44.61 ± 1.02) J mol^? 1 K^? 1, respectively.     

Selective transport of attached particles across the pulp–froth interface

A technique for determining the recovery of attached particles across the froth phase in flotation that relies on measuring the rate at which bubble–particle aggregates enter the froth is used to investigate the selectivity of attached particles across the froth phase. Combining these measurements with those of other techniques for determining the froth recovery of attached particles provides an insight into the different sub-processes of particle rejection in the froth phase. The results of experiments conducted in a 3 m³ Outokumpu tank cell show that the detachment of particles from aggregates in the froth phase occurs largely at the pulp–froth interface. In particular it is shown that the pulp–froth interface selectively detaches particles from aggregates according to their physical attributes.