The production and evaluation of squeeze cast A1-alloy matrix-short ceramic fibre composites

Research work on metal matrix-fibre composites has concentrated in the past on aligned fibre composites. The poor transverse strength of these composites is seen as a major hindrance to their practical use in the majority of engineering applications because stresses exist in more than one direction. Materials with isotropic properties are preferred and consequently reinforcement of composites in three dimensions will be necessary. With this objective, an investigation was conducted to assess the method of fabrication and properties of A1-a110y reinforced with short fibres (SiC and A1 203) randomly oriented in three dimensions. Two composite systems were examined: Al-4.S Cu alloy reinforced with SiC fibre; and Al-3.7S Mg alloy reinforced with A1 203 fibre. The general approach was to establish a satisfactory manufacturing method for the composites before evaluating their mechanical properties. The vortex technique was used to introduce the fibres into the molten alloy. Pre-treatment of the fibres, to induce wetting, and the use of a specially designed device for fibre separation and introduction to the molten A1-ai10ys was found to be necessary so that a uniform distribution of fibres oriented in three dimensions could be achieved. The composites were squeeze cast, under conditions which were experimentally determined, to ensure the production of pore-free castings with fine equiaxed structures. The improvement in tensile strength and ductility of the cast metal, provided by squeeze casting, would be beneficial to composite properties. Composite castings, with up to 10% volume fibre, were produced with a sound structure and with fibres that were uniformly distributed and randomly oriented in three dimensions. It was found that the reaction between the fibres and the respective molten alloy must be closely controlled so that fibre reinforcement can be realised. In this respect the optimum time of contact between the fibres and the molten alloy was experimentally defined for both composite systems. The tensile properties (UTS, 0.1% proof stress, and ductility) of the fibre-free alloys were substantially improved by squeeze casting. The addition of fibre produced further substantial improvement in the tensile properties of the squeeze cast composites, in particular elastic modulus and 0.1% proof stress. Furthermore, composite properties were isotropic. The improvement in the tensile properties of composite castings (as a result of the addition of fibre) was maintained at elevated temperatures. At 250oC, castings of both composite systems with 10% (volume) fibre had 0.1% proof stress and elastic modulus values similar to those for the fibre-free castings at room temperature .. The tensile properties of the composite castings were not affected by thermal cycling (at experimental conditions). The fatigue life of the squeeze cast composite was substantially improved over and above the initial improvement in fatigue life of the fibre-free castings produced by squeeze casting. Wear of cutting tools was adversely affected by the presence of fibres

Transition from Passive Learner to Critical Evaluator through Peer-Testing of Programming Artefacts

Offering timely feedback on programming while encouraging learners to engage in critical evaluation of programs are the objectives of peer-testing. We report on a peer-testing experiment with students on distant campuses using a Web platform. The experiment shows the potential that peer-testing has to help students transition from passive learners to critical evaluators.  Keywords: Computer science education, peer testing, peer feedback, software testing, student transition

Nonlocal correlations in iron pnictides and chalcogenides

Deviations of low-energy electronic structurse of iron-based superconductors from density-functional-theory predictions have been parametrized in terms of band- and orbital-dependent mass renormalizations and energy shifts. The former have typically been described in terms of a local self-energy within the framework of dynamical mean field theory, while the latter appears to require nonlocal effects due to interband scattering. By calculating the renormalized band structure in both random phase approximation (RPA) and the two-particle self-consistent approximation (TPSC), we show that correlations in pnictide systems like LaFeAsO and LiFeAs can be described rather well by a nonlocal self-energy. In particular, Fermi pocket shrinkage as seen in experiments occurs due to repulsive interband finite-energy scattering. For the canonical iron chalcogenide system FeSe in its bulk tetragonal phase, the situation is, however, more complex since even including momentum-dependent band renormalizations cannot explain experimental findings. We propose that the nearest-neighbor Coulomb interaction may play an important role in band-structure renormalization in FeSe. We further compare our evaluations of nonlocal quasiparticle scattering lifetime within RPA and TPSC with experimental data for LiFeAs

The state of peer-to-peer network simulators

Networking research often relies on simulation in order to test and evaluate new ideas. An important requirement of this process is that results must be reproducible so that other researchers can replicate, validate and extend existing work. We look at the landscape of simulators for research in peer-to-peer (P2P) networks by conducting a survey of a combined total of over 280 papers from before and after 2007 (the year of the last survey in this area), and comment on the large quantity of research using bespoke, closed-source simulators. We propose a set of criteria that P2P simulators should meet, and poll the P2P research community for their agreement. We aim to drive the community towards performing their experiments on simulators that allow for others to validate their results

The production and evaluation of squeeze cast Al-alloy matrix-short cermaic fibre composites

SIGLEAvailable from British Library Document Supply Centre- DSC:DX97443 / BLDSC - British Library Document Supply CentreGBUnited Kingdo

Causality and a firm-level innovation scoreboard

In this article, we construct an innovation scoreboard based on a conceptually acceptable endogenous indicator which is shown to be the output caused by the innovation generating process. A balanced panel of data on 552 UK firms over the period 1994-2005 is used to test for causality using general methods of moments (GMM) estimation and other techniques. With innovation defined as the successful exploitation of new ideas, it is proposed that exploitation be measured by total factor productivity; success be measured by the return on capital employed and their multiple be taken as the indicator of innovation. Results for the sample firms are presented and discussed.innovation, scoreboards, exploitation,

New product innovations, information signalling and industry competition

This paper examines the impact of new product innovations on the market values of industry rivals. The evidence indicates that, on average, firms introducing new products experience a significantly positive valuation effect at announcement, while portfolios of industry rivals experience a significant negative valuation effect. This result is consistent with the hypothesis that signals of adverse changes in the competitive position of rivals dominate expected benefits from an innovation spillover. Crosssectional analysis of the announcement period returns reveals that the competitive effects are more pronounced in industries with less concentration and high leverage. Additionally, we find that industry rivals perform as well as the new product firms during the three years following the innovations. We conclude that over a longer period, rival firms are able to respond to the competitive disadvantage of the new product by some alternative innovation or an imitation.