Virtual Internal Pair-Bond Model for Quasi-Brittle Materials

The present multiscale investigation employs the initial and total fracture energy through a virtual internal pair-bond (VIPB) model. The proposed VIPB model is an extension of the traditional virtual internal bond (VIB) model. Two different types of potentials, a steep short-range potential and a shallow long-range potential, are employed to describe the initial and the total fracture energies, respectively. The Morse potential function is modified for the virtual bond potential so that it is independent of specific length scales associated with the lattice geometry. This feature is incorporated in the VIPB model, which uses both fracture energies and cohesive strength. With respect to the discretization by finite elements, we address the element size dependence in conjunction with the J integral. Parameters in the VIPB model are evaluated by numerical simulations of a pure tension test in conjunction with measured fracture parameters. We also validate the VIPB model by predicting load versus crack mouth opening displacement curves for geometrically similar specimens, and the measured size effect. Finally, we provide an example involving fiber-reinforced concrete, which demonstrates the advantage of the VIPB model over the usual VIB model.     

Field calibration studies for ionisation chambers in mixed high-energy radiation fields

The monitoring of ambient doses at work places around high-energy accelerators is a challenging task due the complexity of the mixed stray radiation fields encountered. At CERN, mainly Centronics IG5 high-pressure ionisation chambers are used to monitor radiation exposure in mixed fields. The monitors are calibrated in the operational quantity ambient dose equivalent H*(10) using standard, source-generated photon- and neutron fields. However, the relationship between ionisation chamber reading and ambient dose equivalent in a mixed high-energy radiation field can only be assessed if the spectral response to every component and the field composition is known. Therefore, comprehensive studies were performed at the CERN-EU high-energy reference field facility where the spectral fluence for each particle type has been assessed with Monte Carlo simulations. Moreover, studies have been performed in an accessible controlled radiation area in the vicinity of a beam loss point of CERN's proton synchrotron. The comparison of measurements and calculations has shown reasonable agreement for most exposure conditions. The results indicate that conventionally calibrated ionisation chambers can give satisfactory response in terms of ambient dose equivalent in stray radiation fields at high-energy accelerators in many cases. These studies are one step towards establishing a method of 'field calibration' of radiation protection instruments in which Monte Carlo simulations will be used to establish a correct correlation between the response of specific detectors to a given high-energy radiation field.     

Determination of the kink point in the bilinear softening model for concrete

The characterization of the softening curve from experimental results is essential for predicting the fracture behavior of quasi-brittle materials like concrete. Among various shapes (e.g. linear, exponential) to describe the softening behavior of concrete, the bilinear softening relationship has been extensively used and is the model of choice in this work. Currently, there is no consensus about the location of the kink point in the bilinear softening curve. In this study, the location of the kink point is proposed to be the stress at the critical crack tip opening displacement. Experimentally, the fracture parameters required to describe the bilinear softening curve can be determined with the “two-parameter fracture model” and the total work of fracture method based on a single concrete fracture test. The proposed location of the kink point compares well with the range of kink point locations reported in the literature, and is verified by plotting stress profiles along the expected fracture line obtained from numerical simulations with the cohesive zone model. Finally, prediction of experimental load versus crack mouth opening displacement curves validate the proposed location of the kink point for different concrete mixtures and also for geometrically similar specimens with the same concrete mixture. The experiments were performed on three-point bending specimens with concrete mixtures containing virgin coarse aggregate, recycled concrete coarse aggregate (RCA), and a 50-50 blend of RCA and virgin coarse aggregate. The verification and validation studies support the hypothesis of the kink point occurring at the critical crack tip opening displacement.     

Benchmark studies of induced radioactivity produced in LHC materials, Part II: Remanent dose rates

A new method to estimate remanent dose rates, to be used with the Monte Carlo code FLUKA, was benchmarked against measurements from an experiment that was performed at the CERN-EU high-energy reference field facility. An extensive collection of samples of different materials were placed downstream of, and laterally to, a copper target, intercepting a positively charged mixed hadron beam with a momentum of 120 GeV c(-1). Emphasis was put on the reduction of uncertainties by taking measures such as careful monitoring of the irradiation parameters, using different instruments to measure dose rates, adopting detailed elemental analyses of the irradiated materials and making detailed simulations of the irradiation experiment. The measured and calculated dose rates are in good agreement.     

Myocardial [18F]FDG tomography using a conventional gamma camera and a seven pinhole collimator

Interventional radiology is a rapidly expanding subspecialty in radiology in which the imaging specialist can diagnostically and therapeutically access many organ systems percutaneously, simplifying the treatment of many conditions previously managed surgically. It minimises patient discomfort, renders general anaesthesia unnecessary, reduces morbidity and mortality and decreases the length and cost of hospitalisation. It can also play a role in the management of inoperable cases. Radiologists today are not just "shadow gazers" but can actively participate in patient care and management. In a modern teaching hospital, interventional radiology has an increasingly important role in patient management and is changing the practice of clinical medicine. With advances in imaging, bio-technology and innovation, there has been an explosive development in interventional radiology for the past 15 years and it is hard to keep abreast of what is happening in this field. This article is a brief summary of the state of the art in interventional radiology in 1993 and our experience in Westmead, Sydney, Australia, in some of the many procedures that have recently evolved.     

Rezensionen

Ohne Zusammenfassung     

Involvement of hippocampal NMDA receptors in retention of shuttle avoidance conditioning in rats

The purpose of this research was to evaluate the role of hippocampal N-methyl-D-aspartate (NMDA) receptors in acquisition and consolidation of memory during shuttle avoidance conditioning in rats. Adult male Wistar rats were surgically implanted with cannulae aimed at the CA1 area of the dorsal hippocampus. After recovery from surgery, animals were trained and tested in a shuttle avoidance apparatus (30 trials, 0.5-mA footshock, 24-h training-test interval). Immediately before or immediately after training, animals received a bilateral intrahippocampal 0.5-microliter infusion containing 5.0 microgram of the NMDA competitive receptor antagonist aminophosphonopentanoic acid (AP5) or vehicle (phosphate-buffered saline, pH 7.4). Infusion duration was 2 min per side. Pre-training infusion of AP5 impaired retention test performance (mean +/- SEM number of conditioned responses (CRs) during retention test session was 16.47 +/- 1.78 in the vehicle group and 9.93 +/- 1.59 in the AP5 group; P < 0.05). Post-training infusion of AP5 did not affect retention (mean +/- SEM number of conditioned responses during retention test session was 18.46 +/- 1.94 in the vehicle group and 20.42 +/- 2.38 in the AP5 group; P > 0.10). This impairment could not be attributed to an effect on acquisition, motor activity or footshock sensitivity since AP5 affected neither training session performance measured by the number of CRs nor the number of intertrial crossings during the training session. These data suggest that NMDA receptors in the hippocampus are critical for retention of shuttle avoidance conditioning, in agreement with previous evidence showing a role of NMDA receptors in fear memory.     

Automated parallel polyurethane dispersion synthesis and characterization

The synthesis of waterborne polyurethane dispersions (PUDs) using an automated parallel reactor system was explored. Waterborne PUDs are an important class of polymer dispersion that can be used in many applications such as coatings for wood finishing, glass fiber sizing, adhesives, automotive topcoats, and other applications. Herein, we present the synthesis of aqueous PUDs using a Chemspeed Autoplant A100™ automated parallel reactor system. This is the first time a PUD has been synthesized using an automated parallel reactor system. The synthesis involves the formation of an isocyanate-terminated prepolymer followed by neutralization, dispersion in water, and chain extension. Details of the methodology are discussed with respect to the process of writing the program for the synthesis to synthesizing the PUD itself with the Chemspeed. It is demonstrated that an aqueous PUD can be synthesized with an automated parallel process and the unit-to-unit results are similar. Process variables such as agitator design, rate of neutralization, and rate of water dispersion are varied as these are the three major factors which lead to the desired end product property. The controlled addition of neutralizer, water, and chain extender is an added advantage with this automated technique and gave consistent results in all the units. The PUDs were characterized for their particle size, viscosity, and percent solids.     

Involvement of mechanisms dependent on NMDA receptors, nitric oxide and protein kinase A in the hippocampus but not in the caudate nucleus in memory

The effects of the NMDA receptor antagonist AP5, the nitric oxide synthase (NO) inhibitor NO-arg or the protein kinase A (PKA) inhibitor KT5720 on memory were evaluated. Rats bilaterally implanted in the CA1 region of the dorsal hippocampus were trained and tested in a step-down inhibitory avoidance task, and rats unilaterally implanted in the left posteroventral region of the caudate nucleus were trained and tested in a cued water maze task. Previous findings from this and other laboratories had found that lesions or pharmacological treatments of these sites significantly altered memory of these two tasks. Immediately after training, animals received intrahippocampal or intracaudate 0.5 microliter microinfusions of saline, AP5, NO-arg or KT5720. All three drugs impaired retention of inhibitory avoidance, but did not affect retention of the cued water maze. The findings suggest that NMDA receptor-, NO- and PKA-mediated processes in the dorsal hippocampus, but not in the caudate nucleus, are involved in memory.     

Experience using Web-based shotgun measures for large-systemcharacterization and improvement

This article discusses our experience in using a World Wide Web-based shotgun measurement approach for mining and characterizing large software systems. The approach recognizes that measurement information is essentially management information, that different levels and functions of the organizational hierarchy require different information to make decisions, and that a measurement program is typically a discovery process about an organization's current modes of operations. What we found was the usefulness of a measurement program that also allows managers to dynamically formulate new goals and get answers to questions not specifically related to original goals but raised nonetheless by metric data. We describe three specific cases of decisions that were made using this approach and data collected from one large system and accessed using the company's intranet over the past two years