Bond rolling resistance and its effect on yielding of bonded granulates by DEM analyses

A discrete element modelling of bonded granulates and investigation on the bond effect on their behaviour are very important to geomechanics. This paper presents a two‐dimensional (2‐D) discrete element theory for bonded granulates with bond rolling resistance and provides a numerical investigation into the effect of bond rolling resistance on the yielding of bonded granulates. The model consists of mechanical contact models and equations governing the motion of bonded particles. The key point of the theory is that the assumption in the original bond contact model previously proposed by the authors (55th CSCE‐ASCE Conference, Hamilton, Ont., Canada, 2002; 313–320; J. Eng. Mech. (ASCE) 2005; 131 (11):1209–1213) that bonded particles are in contact at discrete points, is here replaced by a more reliable assumption that bonded particles are in contact over a width. By making the idealization that the bond contact width is continuously distributed with the normal/tangential basic elements (BE) (each BE is composed of spring, dashpot, bond, slider or divider), we establish a bond rolling contact model together with bond normal/tangential contact models, and also relate the governing equations to local equilibrium. Only one physical parameter β needs to be introduced in the theory in comparison to the original bond discrete element model. The model has been implemented into a 2‐D distinct element method code, NS2D. Using the NS2D, a total of 86 1‐D, constant stress ratio, and biaxial compressions tests have been carried out on the bonded granular samples of different densities, bonding strengths and rolling resistances. The numerical results show that: (i) the new theory predicts a larger internal friction angle, a larger yielding stress, more brittle behaviour and larger final broken contact ratio than the original bond model; (ii) the yielding stress increases nonlinearly with the increasing value of β, and (iii) the first‐yield curve (initiation of bond breakage), which define a zone of none bond breakage and which shape and size are affected by the material density, is amplified by the bond rolling resistance in analogous to that predicted by the original bond model. Copyright © 2006 John Wiley & Sons, Ltd.     

Force-free magnetic fields on curvilinear coordinate surfaces

To seek nonlinear solutions of force-free magnetic fields, some symmetries or approximations are usually invoked. We consider magnetic fields lying on coordinate surfaces of an orthogonal curvilinear coordinate system. We conclude that only fields on parallel planes or spherical shells can be expressed in the form provided by Low in 1980s. These force-free fields are stable against small perturbations with rigid boundaries. Fields on cylindrical shells are also considered.     

On the microwave millisecond spike emission and its associated phenomena during the impulsive phase of large solar flare

We propose a model of two acceleration regions, which can explain, on the basis of microwave maser caused by a “hollow-beam” distribution of electrons, the presence of millisecond spikes in the event of 1981 May 16 and their absence in the event of 1981 October 12, and the enhanced continuous emission in the latter. We have also uncovered relations among the features, e.g. the Type III_G, Type IV_DCIM and hard x-ray bursts, that accompany the microwave millisecond spikes during the impulsive phase of a large flare.     

Development and Extension of Seawater Desalination by Reverse Osmosis

Seawater desalination has been people‘s fond dream since ancient times,the dream is now becoming a reality.This paper presents a brief development history of reverse osmosis.Much attention was paid to innovative development in mebranes,modules.equipments and applied technology,including asymmetric and composite membranes,spiral-wound element and hollow fiber module,energy recovery equipments and different technological processes.The extension of reverse osmosis,such as desalination,pre-concentration,integrated processes and nanofiltration,is also briefly mentioned.     

Relationship between footwall composition,crustal contamination,and fluid–rock interaction in the Platreef,Bushveld Complex,South Africa

In the northern limb of the 2.06-Ga Bushveld Complex, the Platreef is a platinum group elements (PGE)-, Cu-, and Ni-mineralized zone of pyroxenite that developed at the intrusion margin. From north to south, the footwall rocks of the Platreef change from Archaean granite to dolomite, hornfels, and quartzite. Where the footwall is granite, the Sr-isotope system is more strongly perturbed than where the footwall is Sr-poor dolomite, in which samples show an approximate isochron relationship. The Nd-isotope system for samples of pyroxenite and hanging wall norite shows an approximate isochron relationship with an implied age of 2.17 ± 0.2 Ga and initial Nd-isotope ratio of 0.5095. Assuming an age of 2.06 Ga, the ɛNd values range from −6.2 to −9.6 (ave. −7.8, n = 17) and on average are slightly more negative than the Main Zone of the Bushveld. These data are consistent with local contamination of an already contaminated magma of Main Zone composition. The similarity in isotope composition between the Platreef pyroxenites and the hanging wall norites suggests a common origin. Where the country rock is dolomite, the Platreef has generally higher plagioclase and pyroxene δ ¹⁸O values, and this indicates assimilation of the immediate footwall. Throughout the Platreef, there is considerable petrographic evidence for sub-solidus interaction with fluids, and the Δ _{plagioclase–pyroxene} values range from −2 to +6, which indicates interaction at both high and low temperatures. Whole-rock and mineral δD values suggest that the Platreef interacted with both magmatic and meteoric water, and the lack of disturbance to the Sr-isotope system suggests that fluid–rock interaction took place soon after emplacement. Where the footwall is granite, less negative δD values suggest a greater involvement of meteoric water. Consistently higher values of Δ _{plagioclase–pyroxene} in the Platreef pyroxenites and hanging wall norites in contact with dolomite suggest prolonged interaction with CO₂-rich fluid derived from decarbonation of the footwall rocks. The overprint of post crystallization fluid–rock interaction is the probable cause of the previously documented lack of correlation between PGE and sulfide content on the small scale. The Platreef in contact with dolomite is the focus of the highest PGE grades, and this suggests that dolomite contamination played a role in PGE concentration and deposition, but the exact link remains obscure. It is a possibility that the CO₂ produced by decarbonation of assimilated dolomite enhanced the process of PGE scavenging by sulfide precipitation.     

Natural radioactivity in sediment of Wei River,China

The concentrations of natural radionuclides in sediment of Wei River of China were measured using γ-ray spectrometry with the aim of estimating the radiation hazard as establishing a database for radioactivity levels of river sediment of China. The activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K in sediment samples ranged from 10.4 to 39.9 Bq kg⁻¹, 15.3 to 54.8 Bq kg⁻¹ and 514.8 to 1,175.5 Bq kg⁻¹, respectively. The concentrations of these radionuclides were compared with the typical world values and the average activities of Chinese soil and Shaanxi soil. Radium equivalent activity (Ra_eq), external hazard index (H _ex) and representative level index (I _γr) were calculated for the samples to assess the radiation hazards arising due to the use of these sediment samples in the construction of dwellings. All the sediment samples have Ra_eq lower than the limit of 370 Bq kg⁻¹, H _ex less than unity and I _γr close to 1 Bq kg⁻¹. The overall mean outdoor terrestrial gamma dose rate is 64.8 nGy h⁻¹ and the corresponding outdoor annual effective dose is 0.079 mSv. None of the studied location is considered a radiological risk and sediment can be safely used in construction.     

Triassic Nb-enriched basalts, magnesian andesites, and adakites of the Qiangtang terrane (Central Tibet): evidence for metasomatism by slab-derived melts in the mantle wedge

New chronological, geochemical, and isotopic data are reported for Triassic (219–236 Ma) adakite-magnesian andesite-Nb-enriched basaltic rock associations from the Tuotuohe area, central Qiangtang terrane. The adakites and magnesian andesites are characterized by high Sr/Y (25–45), La/Yb (14–42) and Na₂O/K₂O (12–49) ratios, high Al₂O₃ (15.34–18.28 wt%) and moderate to high Sr concentrations (220–498 ppm) and ε_ND (t) (+0.86 to +1.21) values. Low enrichments of Th, Rb relative to Nb, and subequal normalized Nb and La contents, and enrichments of light rare earth elements combine to distinguish a group of Nb-enriched basaltic rocks (NEBs). They have positive ε_ND (t) (+2.57 to +5.16) values. Positive correlations between Th, La and Nb and an absence of negative Nb anomalies on mantle normalized plots indicate the NEBs are products of a mantle source metasomatized by a slab melt rather than by hydrous fluids. A continuous compositional variation between adakites and magnesian andesites confirms slab melt interaction with mantle peridotite. The spatial association of the NEBs with adakites and magnesian andesites define an “adakitic metasomatic volcanic series” recognized in many demonstrably subduction-related environments (e.g., Mindanao arc, Philippines; Kamchatka arc, Russia; and southern Baja California arc, Mexico). The age of the Touhuohe suite, and its correlation with Triassic NEB to the north indicates that volcanism derived from subduction-modified mantle was abundant prior to 220 Ma in the central Qiangtang terrane.     

Controls on porphyroblast size along a regional metamorphic field gradient

Garnet-bearing schists from the Waterville Formation of south-central Maine provide an opportunity to examine the factors governing porphyroblast size over a range of metamorphic grade. Three-dimensional sizes and locations for all garnet porphyroblasts were determined for three samples along the metamorphic field gradient spanning lowest garnet through sillimanite grade, using high-resolution X-ray computed tomography. Comparison of crystal size distributions to previous data sets obtained by stereological methods for the same samples reveals significant differences in mode, mean, and shape of the distributions. Quantitative textural analysis shows that the garnets in each rock crystallized in a diffusion-controlled nucleation and growth regime. In contrast to the typical observation of a correlation between porphyroblast size and position along a metamorphic field gradient, porphyroblast size of the lowest-grade specimen is intermediate between the high- and middle-grade specimens’ sizes. Mean porphyroblast size does not correlate with peak temperatures from garnet-biotite Fe-Mg exchange thermometry, nor is post-crystallization annealing (Ostwald Ripening) required to produce the observed textures, as was previously proposed for these rocks. Robust pseudosection calculations fail to reproduce the observed garnet core compositions for two specimens, suggesting that these calc-pelites experienced metasomatism. For each of these two specimens, Monte Carlo calculations suggest potential pre-metasomatism bulk compositions that replicate garnet core compositions. Pseudosection analyses allow the estimation of the critical temperatures for garnet growth: ∼481, ∼477, and ∼485°C for the lowest-garnet-zone, middle-garnet-zone, and sillimanite-zone specimens, respectively. Porphyroblast size appears to be determined in this case by a combination of the heating rate during garnet crystallization, the critical temperature for the garnet-forming reaction and the kinetics of nucleation. Numerical simulations of thermally accelerated, diffusion-controlled nucleation, and growth for the three samples closely match measured crystal size distributions. These observations and simulations suggest that previous hypotheses linking the garnet size primarily to the temperature at the onset of porphyroblast nucleation can only partially explain the observed textures. Also important in determining porphyroblast size are the heating rate and the distribution of favorable nucleation sites.     

In situ strength measurements on natural upper-mantle minerals

Using in situ strength measurements at pressures up to 10 GPa and at room temperature, 400, 600, and 700°C, we examined rheological properties of olivine, orthopyroxene, and chromian-spinel contained in a mantle-derived xenolith. Mineral strengths were estimated using widths of X-ray diffraction peaks as a function of pressure, temperature, and time. Differential stresses of all minerals increase with increasing pressure, but they decrease with increasing temperature because of elastic strain on compression and stress relaxation during heating. During compression at room temperature, all minerals deform plastically at differential stress of 4–6 GPa. During subsequent heating, thermally induced yielding is observed in olivine at 600°C. Neither orthopyroxene nor spinel shows complete stress relaxation, but both retain some stress even at 700°C. The strength of the minerals decreases in the order of chromian-spinel ≈ orthopyroxene > olivine for these conditions. This order of strength is consistent with the residual pressure of fluid inclusions in mantle xenoliths.     

The city-text in post-communist Budapest: street names,memorials, and the politics of commemoration

As in most parts of Central and Eastern Europe, there is a tradition in Hungary of changing street names and memorials in the wake of major political transitions. This article focuses on the change of street names and memorials, i.e. the city-text, in Hungary’s political capital, Budapest, between 1985 and 2001. The city-text in Budapest became a locus of dispute between different political authorities, including the nation state, the metropolitan municipality, and the district, each bearing different political ideals during and after the fall of communism. Discursive changes in the post-communist city-text emerged expressing specific conceptions of national sovereignty, but the direction of the changes were debated. Different levels of administration in Budapest and Hungary had divergent visions of what the new discourse on national sovereignty should be. The changes, therefore, did not express a simple transition to an agreed-upon post-communist value system, but were the result of a symbolic struggle between different levels of administration over what should be commemorated in the city-text.