Fracture of notched and perforated polymeric bars produced by longitudinal impact

An experimental investigation was executed to ascertain the stress intensity factors at the tip of stationary or moving cracks and the stress distribution at central holes in thin rectangular specimens of polymethyl methacrylate (PMMA) and polycarbonate of bisphenol (Lexan) as the result of longitudinal impact by 10 mm dia. steel spheres at velocities ranging from 15 to 130 m/s and by the hammers of standard Charpy impact testers. The initial and rebound velocities of the spheres as well as the strain histories in the specimens were measured. Crack kinematics, the stress intensity factor at the tip and the nominal stress at central holes were ascertained by means of a shadowgraphic technique using a 24-spark Cranz-Schardin camera.Stepwide propagation in the PMMA specimens, frequently found for low sphere-impact velocities, occurred at crack speeds of 250–340 m/s, while catastrophic failure in a single pass of the crack involved tip speeds from 625 to 700 m/s. For the Lexan samples, the velocities of all cracks whether moving intermittently or continuously, were observed to be between 435 and 510 m/s; thus, the average of the fast crack speeds corresponds to about 31% of the rod wave velocity for both materials. Dynamic stress intensity factors were found to be equal to or larger than corresponding static values of fracture toughness, and their variation with nominal stress in the bar followed different mechanical-optical paths during loading and unloading. Stress histories at holes in PMMA determined from strain gage data indicated a stress concentration when compared to those obtained from the shadowgraphs. Current analyses of the process based on static stress distributions in an infinite elastic plate under constant load require modification to provide an explanation of some of the phenomena observed. Such dynamic stress conditions in the vicinity of a crack or hole will also require an alteration of the photomechanical relations currently employed to ascertain crack tip intensity factors and stress conditions at circular holes determined from the geometry of shadow-graphic patterns.     

Photoelectrocatalytic inactivation of E. coli XL‐1 blue colonies in water

BACKGROUND: The aim of the present work was the preparation of TiO₂ P‐25 working electrodes on Ti substrates (TiO₂/Ti), their characterization and the study of their photoelectrocatalytic activity towards the inactivation of E. coli XL‐1blue (E. Coli) colonies, used as model pathogenic bacteria, in a novel batch photoelectrochemical reactor. RESULTS: After annealing of the TiO₂/Ti specimens at 500 °C, the surface morphology and crystal structure of the TiO₂ film electrodes were examined by scanning electronic microscopy (SEM) and X‐ray diffraction (XRD), while from differential capacitance measurements the flat band potential was calculated (V_fb = ? 0.54 V versus Ag/AgCl). The results of photoelectrocatalytic (PEC) experiments concerning the disinfection of E. coli colonies were compared with those of electrochemical (EC) and photocatalytic (PC) inactivation of the pathogen and showed a significant synergy effect in the case of PEC disinfection, leading, at + 1.0 V vs Ag/AgCl cell voltage, to a 100% increase of the apparent rate constant, k_o, in comparison with the simple photocatalytic process. Reuse experiments showed that the working electrode retains its effectiveness after, at least, 15 times of reuse. CONCLUSIONS: The photoelectrocatalytic inactivation of E. coli colonies has been studied under artificial illumination in a novel photoelectrocatalytic reactor. The inactivation of 10³ CFU mL^?1 E. coli colonies followed first‐order kinetics, while parameters such as type of semiconductor and concentration of the microorganisms play an important role affecting the reaction rate constant. Copyright © 2010 Society of Chemical Industry     

Estimation of Power Spectra of Acoustic-Doppler Velocimetry Data Contaminated with Intermittent Spikes

Spectral analysis of velocity signals recorded by acoustic Doppler velocimetry (ADV) and contaminated with intermittent spikes remains a challenging task. In this paper, we propose a new method for reconstructing contaminated time series which integrates two previously developed techniques for detecting and replacing spurious spikes. The spikes are first detected using a modified version of the universal phase-space-thresholding technique and subsequently replaced by the last valid data points. The accuracy of the new approach is evaluated by applying it to identify and remove spikes and reconstruct the spectra of two clean data sets which are artificially contaminated with random spikes: (1) high-quality hot-wire measurement and (2) numerically simulated velocity time series with bimodal probability density distribution. The technique is also applied to reconstruct the spectra obtained from intentionally contaminated ADV measurements and compare them with ADV spectra at the same point in the flow obtained using proper ADV settings. Special emphasis is placed on testing the ability of the technique to reproduce realistic power spectra in flows with rich coherent dynamics. The results show that the power spectra of the reconstructed time series contain a filtered white noise caused by the steps in the reconstruction technique using the last valid data point. We show that even for a severely contaminated time series, the proposed method can accurately recover the power spectra up to the frequency corresponding to the half the mean sampling rate of the valid data points.     

Creating personalized assessments based on learner knowledge and objectives in a hypermedia Web testing application

This paper presents an adaptive Web testing system where the adaptation of the testing procedure relies on the performance, the prior knowledge and the goals and preferences of the test participants. The main aim of the paper is to discuss the utility of the adaptive assessment tool. Therefore, the paper presents the authoring process of a number of adaptive assessments, where the educators utilized different adaptive rules to create summative and formative assessments. The paper presents the employed adaptive rules and summarizes the key points of the pilot study.     

Photoelectrochemical characterisation of thermal and particulate titanium dioxide electrodes

The photoelectrocatalytic performance of thermal and particulate TiO₂ films on Ti electrode substrates has been studied by photovoltammetry and bulk photoelectrolysis. The thermal TiO₂ film electrodes were prepared by Ti annealing in air at 700 °C and 500 °C while the particulate electrodes were prepared from dispersions of Degussa P-25 TiO₂ deposited onto Ti substrates and subsequently sintered at 700 °C and 500 °C. The photocurrent in the absence and presence of the model organic species of oxalate as well as its photooxidation rate depends on coating surface area, type (thermal or particulate) and crystallographic form (anatase or rutile). A method is proposed to account for surface area variations by normalising the data with respect to the electroactive surface area of the TiO₂ electrodes, as estimated by their surface electrochemistry in the dark. The thermal electrodes show high photocurrents both in the absence and the presence of oxalate whereas the performance of particulate electrodes is significantly improved upon oxalate addition. Nevertheless, the efficiency of thermal 700 °C TiO₂ for oxalate photooxidation during bulk photoelectrolysis is comparable to that of Degussa P-25 TiO₂-coated electrodes.     

Electronic structure modifications and HER of annealed electrodeposited Ni overlayers on Mo polycrystalline surface

The nature, the chemical state and electrocatalytic activity of Ni overlayers electrochemically prepared on a polycrystalline Mo surface were studied by means of X-ray and UV photoelectron spectroscopies (XPS, UPS) and steady state potentiostatic polarization measurements for the H₂ evolution reaction in 1 M NaOH solution. Upon cathodic polarization, a Ni(OH)₂-α phase adlayer is formed by precipitation which is gradually reduced by annealing under UHV (ultra-high vacuum) conditions forming a new Ni-Mo intermetallic phase at temperatures as low as 670 K. This was confirmed both by following the Ni 2p core-level shift and the appearance of new features in the UV spectrum thus denoting the modification of the electronic structure of Ni due to the formation of a common strongly hybridised Ni-Mo electronic state. The steady state polarization measurements revealed that sub-monolayer deposition of Ni on the Mo surface (0.8 ML) with subsequent heating at 870 K results in the threefold enhancement of its electrocatalytic activity for the hydrogen evolution reaction (HER) with respect to the Mo polycrystalline foil and the corresponding increase in its desorption kinetic constants in comparison to both Ni and Mo polycrystalline surfaces. As has been recorded for the first time by UPS, in the case of Ni-Mo, this can be attributed to the downshift of the d-band center of the Ni-Mo intermetallic phase, thus resulting in a weaker bonding strength of H_ad with respect to its adsorption on the Ni foil.     

Artisanal alcohol production in Mayan Guatemala: Chemical safety evaluation with special regard to acetaldehyde contamination

There is a lack of knowledge regarding the composition, production, distribution, and consumption of artisanal alcohol, particularly in the developing world. In Nahualá, an indigenous Mayan municipality located in highland Guatemala, heavy alcohol consumption appears to have had a significant negative impact on health, a major role in cases of violence and domestic abuse, and a link to street habitation. Cuxa, an artisanally, as well as commercially produced sugarcane alcohol, is widely consumed by heavy drinkers in this community. Cuxa samples from all distribution points in the community were obtained and chemically analyzed for health-relevant constituents and contaminants including methanol, acetaldehyde, higher alcohols, and metals. From those, only acetaldehyde was confirmed to be present in unusually high levels (up to 126 g/hl of pure alcohol), particularly in samples that were produced clandestinely. Acetaldehyde has been evaluated as “possibly carcinogenic” and has also been identified as having significant human exposure in a recent risk assessment. This study explores the reasons for the elevated levels of acetaldehyde, through both sampling and analyses of raw and intermediary products of cuxa production, as well as interviews from producers of the clandestine alcohol. For further insight, we experimentally produced this alcohol in our laboratory, based on the directions provided by the producers, as well as materials from the town itself. Based on these data, the origin of the acetaldehyde contamination appears to be due to chemical changes induced during processing, with the major causative factors consisting of poor hygiene, aerobic working conditions, and inadequate yeast strains, compounded by flawed distillation methodology that neglects separation of the first fractions of the distillate. These results indicate a preventable public health concern for consumers, which can be overcome through education about good manufacturing practices, as well as financial incentives to separate the acetaldehyde-rich fractions during distillation.     

Bin weather data of Thessaloniki, Greece

The necessary analysis of the ambient dry bulb temperature and of the relative humidity for elaboration of bin weather data is discussed. Using weather data from Thessaloniki, Greece, the annual total bin data as well as monthly bin data in 4-h periods are calculated and presented in tabular form. The presented data serve for the estimation of the energy requirements, and fuel consumption of heating and air conditioning systems for either short or long time periods of operation.     

The elimination of the reverse circulation in thermosiphon solar water heaters

The causes of reverse circulation in thermosyphon solar water heaters, the geometry of the systems, and the affects of nocturnal radiative cooling of the collector are discussed. A means of interconnecting collector and tank to drastically reduce reverse circulation is presented.