Influence of Heat Treatment on the Bond Coat Cyclic Oxidation Behaviour in an Air-plasma-sprayed Thermal Barrier Coating System

THE METALLIC BOND COAT is an importantconstituent in a TBC system.It enhances the adhesionof the ceramic thermal barrier layer(the topcoat)to thesubstrate and also provides oxidation protection to thesubstrate metal.The composition of the bond coat,generalized as M-Cr-Al-Y,where M represents Ni,Coand/or Fe,generally allows a layer of alumina(A12O3)to form during high temperature exposure.If acontinuous scale of A12O3forms along the interfacebetween the bond coat and the ceramic to…     

ANALYSIS OF THE EFFECT OF FLOW INTERRUPTIONS ON FED-BATCH FERMENTATION FOR PHB PRODUCTION BY RALSTONIA EUTROPHA IN FINITELY DISPERSED BIOREACTORS

Unlike laboratory-scale bioreactors, large reactors have finite dispersion and are affected by unexpected interruptions in the feed streams. For the optimum level of dispersion determined earlier, the effects of interruptions in the inflow of the carbon (glucose) and nitrogen (NH₄Cl) substrates have been studied for fed-batch fermentation by Ralstonia eutropha to produce poly-β-hydroxybutyrate (PHB). Optimum uninterrupted operation by neural control at dispersion corresponding to Pe = 20 was used as the reference. While glucose stoppage mainly affected cell growth, nitrogen flow suppressed polymer synthesis more strongly. The effects depended also on the time at which the interruptions occurred. These results are interpreted in a metabolic context, emphasizing the complexity of the system and the importance of ‘intelligent’ monitoring and control under disturbed conditions.     

The Quasi-static Deformation, Failure, and Fracture Behavior of Titanium Alloy Gusset Plates Containing Bolt Holes

In this article, the influence of bolt holes, specifically their number and layout on strength, deformation, and final fracture behavior of titanium alloy gusset plates under the influence of an external load is presented and discussed. Several plates having differences in both the number and layout of the bolt holes were precision machined and then deformed under quasi-static loading. The specific influence of number of bolt holes and their layout on maximum load-carrying capability and even fracture load was determined. The conjoint influence of bolt number, bolt layout pattern, nature of loading, contribution from local stress concentration, and intrinsic microstructural effects in governing the macroscopic fracture mode and intrinsic microscopic mechanisms is presented and discussed.     

Hybrid composite friction materials reinforced with combination of potassium titanate whiskers and aramid fibre: Assessment of fade and recovery performance

Composite friction materials based on synergistic ternary combination of potassium titanate whiskers, aramid fibre and graphite have been characterized for friction braking performance on Krauss friction tester. The dynamics of friction build-up and friction-decay as a function of number of braking instances and modes of braking cycles have been found to be more consistent in the composites with ≥7.5 wt% of aramid fibres whereas the absolute friction effectiveness remained higher in the composites with ≥25 wt% of potassium titanate whiskers. Wear surface morphology has revealed topographical variations and their underlying role in controlling the friction and wear performance.     

Interferon-inducer polyriboinosinic-polyribocytidylic acid: a potent anti-gastric ulcer agent and inhibitor of the gastric proton pump in rats

Obesity was defined by a body mass index more than 30 kg/m2. Many risks were related to this pathology, and sometimes, menstrual disorders or infertility. In order to obtain an adequate response to ovarian stimulation during IVF cycles, higher doses of menotropins are necessary in the group of obese patients. The mechanism of this phenomenon is still unclear. Leptin is one of the main hypothesis, and could act on obesity and reproductive system simultaneously. The likelihood to have an ongoing pregnancy after IVF treatment is less in the group of obese patients because of the increased risk of miscarriage and obstetrical complications. Weight loss prior IVF remains the main advice in order to decrease the risks of the procedure and to treat successfully these patients.     

Micromixing and the steady-state performance of bioreactors using recombinant bacteria--analysis through a reversed two-environment model

A reversed two-environment model has been used to study micromixing in continuous fermentation for growth and product formation by recombinant bacteria. As an example, an Escherichia coli M72 strain harbouring the plasmid pPLc23trpA1 and producing tryptophan synthetase is considered. With excess substrate, 10% plasmid-free bacteria in the initial broth do not affect the steady-state concentrations of plasmid-containing bacteria but their mass fractions decrease significantly. With a smaller substrate concentration, the mass fractions decrease sharply as the dilution rate increases if micromixing is good; but concentrations still remain high. There is also a clear demarcation between regions of good and poor micromixing. These results are explained and an optimal combination of micromixing and dilution rate is suggested to maximise productivity.     

PROTECTIVE COATINGS FOR AERO ENGINE HOT SECTION COMPONENTS

Aluminide coatings have been widely used in the aircraft industries for the protection of gas turbine engine hot section components against oxidation and/or hot corrosion. This paper considers modes of coating degradation under conditions of cyclic oxidation, hot corrosion and corrosion-erosion interactions during service, as well as the effects of interdiffusion between coating and substrate alloys either during service or coating application. It also discusses means of improving existing coatings as well as advanced coating systems currently under development. In assessing coating performance, consideration must be given to the influence that coatings may have on substrate properties such as mechanical strength, resistance to creep and resistance to mechanical and thermal fatigue. Finally, it is stressed that proven performance for a given coating/substrate combination is no guarantee that no deleterious reaction will occur when the same coating is used with a different substrate alloy. Therefore, coating substitution requires requalification.     

Towards formal specification of a distributed computing system

Onboard spacecraft computing system is a case of a functionally distributed system that requires continuous interaction among the nodes to control the operations at different nodes. A simple and reliable protocol is desired for such an application. This paper discusses a formal approach to specify the computing system with respect to some important issues encountered in the design and development of a protocol for the onboard distributed system. The issues considered in this paper are concurrency, exclusiveness and sequencing relationships among the various processes at different nodes. A 6-tuple model is developed for the precise specification of the system. The model also enables us to check the consistency of specification and deadlock caused due to improper specification. An example is given to illustrate the use of the proposed methodology for a typical spacecraft configuration. Although the theory is motivated by a specific application the same may be applied to other distributed computing system such as those encountered in process control industries, power plant control and other similar environments.     

Singularity in structural optimization

Singularity conditions that arise during structural optimization can seriously degrade the performance of the optimizer. The singularities are intrinsic to the formulation of the structural optimization problem and are not associated with the method of analysis. Certain conditions that give rise to singularities in linear elastic structures have been identified in earlier papers, along with a proposition to alleviate the consequences of their presence.^1-3 These singularities were global in nature, encompassing the entire structure. Further examination revealed more complex sets of conditions in which singularities occur. Some of these singularities are local in nature, being associated with only a segment of the structure. Moreover, the likelihood that one of these local singularities may arise during an optimization procedure can be much greater than that of the global singularity identified earlier. This paper provides examples of these additional forms of singularities. It gives a framework in which these singularities can be recognized. In particular, the singularities can be identified by examination of the stress-displacement relations along with the compatibility conditions and/or the displacement-stress relations derived in the integrated force method of structural analysis. Methods for the elimination of the effects of these singularities are suggested and numerical illustrations are given.