Improved preventive maintenance in the framework of forecasting problem under subcontractor constraint

The paper presents a new improved preventive maintenance strategy for a forecasting problem of production and maintenance optimisation under subcontractor constraint. In order to satisfy the customer, the manufacturing system consists of a principal machine M₁ and called upon subcontractor machine M_s. Knowing that both machines are subjected to random failures, failure rate of main machine increases with time and according to production rates. An improved preventive maintenance strategy is used for control of the machine M₁, whereas subcontractor machine M_s is uncontrollable from preventive maintenance point of view. An analytic formulation of problem has been proposed in order to determine the economical production plans for M₁ and M_s. An improved maintenance strategy (IMS) is developed in order to minimise the total production loss, when the subcontractor machine is unavailable. It consists of determining the best time to perform preventive maintenance actions taking into account production rates, history of M₁ and the state of subcontractor machine M_s. Numerical results and sensitivity analysis are presented to highlight the performance measure and the usefulness of the IMS.     

Elasticity of hexagonal liquid crystals

An investigation of the elastic properties of hexagonal phases of typical liquid crystal samples on an ordered hexagonal discotic hexa‐n‐octyloxytriphenylene (C₈HET), a disordered one a triphenylene hexa‐n‐dodecanoate (C₁₁HAT) and a hexacatenar phasmid was carried out. Characterization of their mechanical stability and analysis of light scattering were used; the same results were obtained for the three samples in spite of the differences in their molecular shapes and molecular orders in columns. From the instability of undulating columns submitted to a dilative strain normal to columns, we deduced an anomalously large value of the correlation length m (m ≈ 0.1 µm), compared to that of the smectic liquid crystals (m ≈ 20 Å). Results from depolarized Rayleigh scattering showed that, although consisting of one‐dimensional liquid‐like structures, the studied samples in their hexagonal phases behaved elastically like three‐dimensional solids. Considering the differences in molecular shapes and orders, this elastic behaviour was attributed to regular defects. Therefore, one can probably generalize this elastic behaviour as being due to the columnar hexagonal structure. © 2001 Society of Chemical Industry     

Characterization of a New TiO<Subscript>2</Subscript> Nanoflower Prepared by the Sol–Gel Process in a Reverse Microemulsion

TiO₂ nanoflowers were synthesized for the first time via the sol–gel method inside a reverse microemulsion. The characterization of the products by Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) show that this material is constituted of belt-shaped particles that change into a nanoflower morphology after heat treatment at 600 °C. A proposed model could explain the growth process from nanobelt to nanoflower.     

Study of structure and scaling behavior of chemically unfolded β‐casein by means of small‐angle neutron scattering

Structures of β‐casein formed in water with various concentrations of guanidine hydrochloride (GdmCl) were studied by small‐angle neutron scattering (SANS). A transition in structure and properties of the protein seems to occur around a GdmCl concentration of 1.2 mol L^?1. The data are interpreted assuming that the protein molecules behave like multi‐block copolymers with alternating hydrophilic and hydrophobic sequences. Below the transition, the protein structure is still native and has a fractal dimension larger than that beyond the transition where the β‐casein has the feature of a three‐dimensional excluded‐volume coil. A possible interpretation of these results is that the presence of salt increases the value of the critical micelle concentration. This allows a comparison of the structure of β‐casein and synthetic macromolecules. We find that, although the protein is short, excluded‐volume interactions between monomers are present, and that the structure of β‐casein is very similar to what was found in dilute solutions of linear polymers in a good solvent. Thus, it is reasonable to assume that above the critical salt concentration, the protein is completely isolated, whereas below, some structure remains present. Copyright © 2006 Society of Chemical Industry     

Availability modelling and analysis of multi-product flexible transfer lines subject to random failures

Product requirements have changed dramatically in today’s industries forcing companies to implement flexible transfer lines manufacturing diverse product types. However, most of the researches dealing with transfer lines are so far limited to the mono-product and homogeneous lines (i.e., perfectly balanced). This paper addresses the problem of estimating the performance of multi-product, flexible transfer lines. Such lines are composed of several machines in series and can be used to manufacture a variety of products in a batch production environment. The machines are subject to random operation-dependent failures, and there are no intermediate buffers between adjacent machines. This paper aims to propose an analytical model for assessing the steady-state availability of such lines. A simulation model is also developed to mimic the real dynamic behaviour of flexible transfer lines. Thousands of production line configurations are tested in order to compare analytical results with simulation results. We used statistical tests to prove that the proposed formulae are exact and robust to model the steady-state availability of multi-product, unbuffered, flexible transfer lines subject to random failures.     

PIXE analysis of medieval silver coins

We applied the proton-induced X-ray emission (PIXE) analytical technique to twenty-eight medieval silver coins, selected from the Tunisian treasury. The purpose is to study the fineness evolution from the beginning of the 7th to the 15th centuries AD. Each silver coin was cleaned with a diluted acid solution and then exposed to a 3 MeV proton beam from a 1.7 MV tandem accelerator. To allow the simultaneous detection of light and heavy elements, a funny aluminum filter was positioned in front of the Si(Li) detector entrance which is placed at 135° to the beam direction. The elements Cu, Pb, and Au were observed in the studied coins along with the major component silver. The concentration of Ag, presumably the main constituent of the coins, varies from 55% to 99%. This significant variation in the concentration of the major constituent reveals the economical difficulties encountered by each dynasty. It could be also attributed to differences in the composition of the silver mines used to strike the coins in different locations. That fineness evolution also reflects the poor quality of the control practices during this medieval period. In order to verify the ability of PIXE analytical method to distinguish between apparently similar coins, we applied hierarchical cluster analysis to our results to classify them into different subgroups of similar elemental composition.     

Hydrothermal synthesis of mesoporous VO2·½(H2O) nanosheets and study of their electrical properties

Layered sheet-like nanocrystalline VO₂·½(H₂O) has been synthesized by hydrothermal process using V₂O₅ as vanadium source and 2-phenylethylamine as a reducing agent and a structure-directing template. Techniques X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR) and nitrogen adsorption/desorption isotherms have been used to characterize the structure, morphology and composition of the materials. Electrical conductivity measurements showed that the as synthesized VO₂·½(H₂O) nanosheets has a conductivity value which goes from 75 × 10^?6 Ω^?1 cm^?1 at 298 K, to 68 10^?5 Ω^?1 cm^?1 at 386 K with activation energy of 0.24 eV.     

Application of an expert system to optimize reservoir performance

The main challenge of oil displacement by an injected fluid, such as in Enhanced Oil Recovery (EOR) processes, is to reduce the cost and improve reservoir performance. An optimization methodology, combined with an economic model, is implemented into an expert system to optimize the net present value of full field development with an EOR process. The approach is automated and combines an economic package and existing numerical reservoir simulators to optimize the design of a selected EOR process using sensitivity analysis. The EOR expert system includes three stages of consultations: (1) select an appropriate EOR process on the basis of the reservoir characteristics, (2) prepare appropriate input data sets to design the selected EOR process using existing numerical simulators, and (3) apply the discounted-cash-flow methods to the optimization of the selected EOR process to find out under what conditions at current oil prices this EOR process might be profitable. The project profitability measures were used as the decision-making variables in an iterative approach to optimize the design of the EOR process. The economic analysis is based on the estimated recovery, residual oil in-place, oil price, and operating costs. Two case studies are presented for two reservoirs that have already been produced to their economic limits and are potential candidates for surfactant/polymer flooding, and carbon-dioxide flooding, respectively, or otherwise subject to abandonment. The effect of several design parameters on the project profitability of these EOR processes was investigated.     

Investigation of effect of fibres modulus on pilling of acrylic fabrics

Pills are formed by entangling fibres into discrete balls on the surface of fabrics which are usually the result of wear, abrasion, washing or a combination of all three. In the present study, the effect of initial modulus of fibres on the pilling of woven fabrics was investigated. For this purpose, non-shrinkable (regular) and shrinkable (high bulk) acrylic fibres which have different initial modulus, were used. By using different percentages of them, five kinds of yarn were produced. Twill woven fabrics (T 1/2 z¹) produced from these yarn were tested on ICI pilling box tester.

Results of pilling test performed on the sample showed that by increasing the percentage of non-shrinkable (regular) fibres with high initial modulus, the tendency of fibres to leave the structure of yarn increases and they will present on the surface of fabrics as pills.