Volumetric assessment of fatigue damage in a SiCf/SiC ceramic matrix composite via in situ X-ray computed tomography

To enhance the understanding of matrix cracking and damage progression on the macroscopic scale, within a 0/90° fibre reinforced SiCf/SiC ceramic matrix composite (CMC), X-ray computed tomography (XCT) imaging and analysis have been performed in conjunction with a commercially available in-situ mechanical loading device. CMC test coupons were subjected to tensile cyclic loads and inspected using XCT without removal from the tensile loading device. Attempts to measure and quantify the resulting damage using volumetric image analysis techniques are presented, by characterising the crack network from XCT images acquired at both the maximum and minimum load condition during selected fatigue cycles. The XCT detection of significant crack development within the first loading half-cycle shows good agreement with cumulative acoustic emission energy data recorded under similar test conditions. The results are seen as an important step towards correlating the damage behaviour detected via different NDE and health monitoring techniques

The Postantibiotic Effect in the Treatment of Experimental Meningitis Caused by Streptococcus pneumoniae in Rabbits

The relevance of a postantibiotic effect in the treatment of pneumococcal meningitis was evaluated in a rabbit model. After administration of a single intravenous bolus of ampicillin at various dosages, such an effect was observed in all animals. The duration of this effect in vivo (2.5-18 hr) was consistently longer than that in vitro (1-4.3 hr); however, in rabbits the postantibiotic effect was eliminated by the administration of intravenous plus intracisternal β-lactamase. In an assessment of the potential therapeutic benefit of the postantibiotic effect, the efficacy of two regimens of treatment with different intervals between doses was compared. One group of animals received ampicillin every 4 hr and another every 12 hr. With sufficiently high doses, drug concentrations in cerebrospinal fluid exceeded the minimal bactericidal concentration for most of the 4-hr interval but for only about one-third of the 12-hr interval. The rate of cure was similar for the two regimens and approximated 100% when peak drug concentrations in cerebrospinal fluid exceeded the minimal bactericidal concentration by at least 10-fol

Measurements of residual stress and microstructural evolution in electron beam welded Ti-6Al-4V using multiple techniques

The evolution of residual stress and microstructure has been investigated in electron-beam welded Ti-6Al-4V alloy rings in order to develop an understanding of how the distribution of through-thickness residual stress correlates with microstructural evolution. A multiple technique approach to residual stress measurement was employed using a combination of different measurement techniques including X-ray diffraction (XRD), hole drilling method based on electronic speckle pattern interferometry (ESPI), and the contour method. It was found that there is a strong correlation between the change in residual stress and alpha phase morphology across the weld. The fusion zone exhibited highly tensile residual stress which was typified by an entirely acicular α′ microstructure formed by a displacive transformation within prior β grains on cooling. The tensile residual stress in the centre of the weld reduced towards the heat affected zone, transitioning to a compressive residual stress upon increasing distance from the weld centre. The transition from tensile to compressive residual stress correlates with a significant decrease in the volume fraction of α′ and an increase in the bimodal morphology of equiaxed primary alpha in a diffusional transformed beta matrix leading to elongated alpha in the base material

Winter 1975

Tall and Hard Fescues: Variety Trial Summary 1974-1975 (page 3) Red Fescue Variety Trial (4) Kentucky Bluegrass Variety Trial 1974 (4) Perennial Ryegrass Variety Trial (5) Kentucky Bluegrass Variety Trial (7) Lawn Specialists Find Liquids Fit the Bill for Urban Customers (8) Back and Beyond (10) Impact of Chemical and Mechanical Site Preparation on Wildlife Habitat (14) UMass Turfgrass Research Fund (20

ac-Field-Controlled Anderson Localization in Disordered Semiconductor Superlattices

An ac field, tuned exactly to resonance with the Stark ladder in an ideal tight binding lattice under strong dc bias, counteracts Wannier-Stark localization and leads to the emergence of extended Floquet states. If there is random disorder, these states localize. The localization lengths depend non-monotonically on the ac field amplitude and become essentially zero at certain parameters. This effect is of possible relevance for characterizing the quality of superlattice samples, and for performing experiments on Anderson localization in systems with well-defined disorder.Comment: 10 pages, Latex; figures available on request from [email protected]

Light scattering from disordered overlayers of metallic nanoparticles

We develop a theory for light scattering from a disordered layer of metal nanoparticles resting on a sample. Averaging over different disorder realizations is done by a coherent potential approximation. The calculational scheme takes into account effects of retardation, multipole excitations, and interactions with the sample. We apply the theory to a system similar to the one studied experimentally by Stuart and Hall [Phys. Rev. Lett. {\bf 80}, 5663 (1998)] who used a layered Si/SiO$_2$/Si sample. The calculated results agree rather well with the experimental ones. In particular we find conspicuous maxima in the scattering intensity at long wavelengths (much longer than those corresponding to plasmon resonances in the particles). We show that these maxima have their origin in interference phenomena in the layered sample.Comment: 19 pages, 12 figure

Insights into the behaviour of systems biology models from dynamic sensitivity and identifiability analysis: a case study of an NF-kB signaling pathway

Mathematical modelling offers a variety of useful techniques to help in understanding the intrinsic behaviour of complex signal transduction networks. From the system engineering point of view, the dynamics of metabolic and signal transduction models can always be described by nonlinear ordinary differential equations (ODEs) following mass balance principles. Based on the state-space formulation, many methods from the area of automatic control can conveniently be applied to the modelling, analysis and design of cell networks. In the present study, dynamic sensitivity analysis is performed on a model of the IB-NF-B signal pathway system. Univariate analysis of the Euclidean-form overall sensitivities shows that only 8 out of the 64 parameters in the model have major influence on the nuclear NF-B oscillations. The sensitivity matrix is then used to address correlation analysis, identifiability assessment and measurement set selection within the framework of least squares estimation and multivariate analysis. It is shown that certain pairs of parameters are exactly or highly correlated to each other in terms of their effects on the measured variables. The experimental design strategy provides guidance on which proteins should best be considered for measurement such that the unknown parameters can be estimated with the best statistical precision. The whole analysis scheme we describe provides efficient parameter estimation techniques for complex cell networks

Analyse mathématique et modélisation

Henri Berestycki, directeur d’études Méthodes d’EDP et numériques en finance de marchés (avec Olivier Pironneau, professeur à l’Université Paris-VI) Différents aspects de la modélisation mathématique des marchés financiers ont été présentés. On s’est attaché en particulier à la formulation en termes d’équations aux dérivées partielles (EDP) des problèmes d’évaluation de produits dérivés (options, contrats futurs, etc.) et de techniques de gestion du risque. Les outils d’analyse mathématique d..