Extension of fibre bundle models for creep rupture and interface failure

We present two extensions of the classical fibre bundle model to study the creep rupture of heterogeneous materials and the shear failure of glued interfaces of solid blocks. To model creep rupture, we assume that the fibres of a parallel bundle present time dependent behaviour under an external load and fail when the deformation exceeds their local breaking threshold. Assuming global load sharing among fibres, analytical and numerical calculations showed that there exists a critical load below which only partial failure occurs while above which the system fails globally after a finite time. Approaching the critical point from both sides the system exhibits scaling behaviour which implies that creep rupture is analogous to continuous phase transitions. To describe interfacial failure, we model the interface as an array of elastic beams which experience stretching and bending under shear load and break if the two deformation modes exceed randomly distributed breaking thresholds. The two breaking modes can be independent or combined in the form of a von Mises type breaking criterion. In the framework of global load sharing, we obtain analytically the macroscopic constitutive behaviour of the system and describe the microscopic process of the progressive failure of the interface.     

Supercritical carbon dioxide extraction of onion oleoresin

Supercritical carbon dioxide (SC-CO₂) extraction on a laboratory-scale, was applied to produce oleoresin from dried onion (Allium cepa L). The goal of this work was to determine the optimal conditions of extraction for producing oleoresin with the highest yield and the best quality. A polynomial model was developed on the data. The square of correlation coefficient was R²=0·9022 (at P⩽0·05). The statistical analysis showed that the extraction pressure, temperature, and time have significant influence on the yield of the oleoresin. Increasing these parameters resulted in an increase in the yield. Maximum yield was obtained when the extraction pressure and temperature were above 350 bar and 57°C, respectively. At the lowest pressure (150 bar) applied the highest concentration of sulphur was produced in the oleoresin. The concentration of sulphur increased as a function of temperature when the pressure was maintained at 300 and 400 bar. The composition of volatile compounds of oleoresin differed from that of distilled onion oil. © 1998 SCI.     

Free radicals in gamma-irradiated polyamide 6 from the point of view of chemical ESR-dating

Polyamide 6 was irradiated with gamma rays in air at room temperature with a radiation dose of 17 kGy. The arising composite spectrum indicates the presence of two types of free radicals. The ratio of spectral lines was measured for the time period of 45 days. As the rate constants of decay of individual types of free radicals are different, the intensity ratio of lines of the composite spectrum significantly changes in the course of decay. It has been pointed out that this temporal dependence of the line ratio can be used for determining the time interval between the radiation impact and the measurement of ESR spectrum. The possibilities of using this information for the purpose of retrospective chronodosimetry are discussed.     

Simultaneous ultrasound and MRI system for breast biopsy: compatibility assessment and demonstration in a dual modality phantom

Simultaneous capturing of ultrasound (US) and magnetic resonance (MR) images allows fusion of information obtained from both modalities. We propose an MR-compatible US system where MR images are acquired in a known orientation with respect to the US imaging plane and concurrent real-time imaging can be achieved. Compatibility of the two imaging devices is a major issue in the physical setup. Tests were performed to quantify the radio frequency (RF) noise introduced in MR and US images, with the US system used in conjunction with MRI scanner of different field strengths (0.5 T and 3 T). Furthermore, simultaneous imaging was performed on a dual modality breast phantom in the 0.5 T open bore and 3 T close bore MRI systems to aid needle-guided breast biopsy. Fiducial based passive tracking and electromagnetic based active tracking were used in 3 T and 0.5 T, respectively, to establish the location and orientation of the US probe inside the magnet bore. Our results indicate that simultaneous US and MR imaging are feasible with properly-designed shielding, resulting in negligible broadband noise and minimal periodic RF noise in both modalities. US can be used for real time display of the needle trajectory, while MRI can be used to confirm needle placement.     

A Detailed Experimental Study of the LEO Satellite to Indoor Channel Characteristics

General investigations and results from ratherextensive measurement campaigns are presented tocharacterize the propagation channel of SatellitePersonal Communication Systems; in the measurements the satellite was simulated by a helicopter andmeasurements at S band and L band were made. A newconceptual partitioning of the full link is introduced;the measrement equipment and the environment aredescribed. The major part of the paper deals withpresentation of the measurement results and theiranalyses. Measured and analyzed are average lossincluding satellite elevation and azimuth positiondependence, first-order statistics, Doppler characteristics, andwide-band characteristics. Finally, some rather generalconclusions are drawn.     

Rankings,research styles,and publication cultures: a study of American sociology departments

Rankings have become a major form of quality assessment in higher education over the past few decades. Most rankings rely, to varying extent, on bibliometric indicators intended to capture the quantity and quality of the scientific output of institutions. The growing popularity of this practice has raised a number of concerns, one of the most important being whether evaluations of this sort treat different work styles and publication habits in an unbiased manner and, consequently, whether the resulting rankings properly respect the particular modes of research characteristic of various disciplines and subdisciplines. The research reported in this paper looked at this issue, using data on more than one hundred US sociology departments. Our results showed that institutions that are more quantitative in character are more likely to favor journals over books as the dominant form of scientific communication and fare, in general, considerably better on the National Research Council’s assessment than their qualitative equivalents. After controlling for differences in publication practices, the impact of research style declined but remained statistically significant. It thus seems that the greater preference of qualitative departments for books over articles as publication outlets puts them at a disadvantage as far as quality assessments are concerned, although their lagging behind their quantitative counterparts cannot fully be explained by this factor alone.     

A study of transverse ply cracking using a discrete element method

We study the transverse cracking of the 90° ply in [0/90]_S cross-ply laminates by means of a discrete element method. To model the 90° ply a two-dimensional triangular lattice of springs is constructed where nodes of the lattice model fibers, and springs with random breaking thresholds represent the disordered matrix material in between. The spring-lattice is coupled by interface springs to two rigid bars which represent the two 0° plies in the model, which could be sublaminate as well. Molecular dynamics simulation is used to follow the time evolution of the model system. It was found that under gradual loading of the specimen, after some distributed cracking, segmentation cracks occur in the 90° ply which then develop into a saturated state where the ply cannot support additional load. The stress distribution between two neighboring segmentation cracks was determined, furthermore, the dependence of the microstructure of damage on the ply thickness was also studied. To give a quantitative characterization of stiffness degradation, the Young modulus of the system is monitored as a function of the density of segmentation cracks. The results of the simulations are in satisfactory agreement with experimental findings and with the results of analytic calculations.     

Rotor-stator molecular crystals of fullerenes with cubane

Cubane (C8H8) and fullerene (C60) are famous cage molecules with shapes of platonic or archimedean solids. Their remarkable chemical and solid-state properties have induced great scientific interest. Both materials form polymorphic crystals of molecules with variable orientational ordering. The idea of intercalating fullerene with cubane was raised several years ago but no attempts at preparation have been reported. Here we show that C60 and similarly C70 form high-symmetry molecular crystals with cubane owing to topological molecular recognition between the convex surface of fullerenes and the concave cubane. Static cubane occupies the octahedral voids of the face-centred-cubic structures and acts as a bearing between the rotating fullerene molecules. The smooth contact of the rotor and stator molecules decreases significantly the temperature of orientational ordering. These materials have great topochemical importance: at elevated temperatures they transform to high-stability covalent derivatives although preserving their crystalline appearance. The size-dependent molecular recognition promises selective formation of related structures with higher fullerenes and/or substituted cubanes.     

Simulation of uplooking and downlooking high-resolution radiance spectra with two different radiative transfer models

Measurements of up-looking spectral radiances measured during the Convection and Moisture Experiment and down-looking spectral radiances measured at one of the Atmospheric Radiation Measurement sites are compared with simulations with use of two different line-by-line models. Simulations are performed in tightly controlled conditions to verify the behavior of the models. Spectra computed at higher samplings are used to study the spectral structure of the differences between simulations and measurements. A revised list of water vapor spectroscopic parameters is used to test the impact of improved spectroscopic data on the accuracy of the line-by-line calculations. The sensitivity of the results to errors that result from uncertainties in the input atmospheric temperature and humidity profiles is also investigated.