The influence of consolidation on shear test results

Measurements of the yield behaviour of a cohesive bulk solid in the Jenike shear tester are normally done in two steps. First, pre-shearing to a steady state yielding, and secondly, shearing under a lower normal stress to establish the shear stress at incipient failure.\ud \ud Due to the limited strain of the sample in this apparatus, the material must be consolidated in an earlier stage. This is done by the twisting action of a loaded cover, at which the vertical load and the amount of twists are chosen such that a suitable stress—strain curve will be obtained in the pre-shearing stage. The ratio n between the vertical load during consolidation and pre-shearing, respectively, can have a strong impact on the results.\ud \ud In this article, results of measurements with various n-values are given, leading to a variation in shear stress levels for both steady state and incipient failure. This can be explained by variations in the real normal stresses on the shear plane, which can substantially deviate from the externally applied normal stresses, due to contact between ring and base. To establish the real normal stress on the plane of shear, a somewhat modified shear tester was used. Furthermore, measurements were performed where the contact between ring and base was avoided by a removeable pair of half-rings.\ud \ud The results of those measurements and the implications for the derived quantities are amply discussed, resulting in some recommendations for a more reliable testing procedure

Possible deviations in the determination of bulk solid characteristics, caused by the loading mechanism of the Jenike Shear cell

In the literature ample attention is given to the assumptions on which the interpretation of shear-test results from a Jenike shear tester are based. Very little is said, however, about the influence on the test data of the construction of the apparatus itself. In the work described here, the authors have investigated the influence of the loading mechanism on the results. The original Jenike-type tester and a slightly modified one are considered.\ud \ud From the equilibrium conditions, the real normal and shear stresses on the assumed shear plane were derived, which were found to deviate from the external applied stresses. The deviations in the primary results and derived data were quantified by introducing correction factors, the boundary values of which could be calculated.\ud \ud Experiments with three different bulk materials were performed to measure the relevant quantities and to verify the assumptions used in theoretical analysis. Furthermore, the real normal stress on the shear plane was measured directly. From these measurements the experimental correction factors could be derived, which appeared to lie within the boundaries as theoretically predicted.\ud \ud In general it may be concluded that the yield loci and their derived quantities as determined by a properly used Jenike shear tester are, as a rule, suitable for design purposes of e.g. silos. For comparison of results from different testing techniques, however, the deviations introduced by the loading mechanism cannot be neglected

Evidence for multiple steps in the pre-steady-state electron transfer reaction of nitrogenase from Azotobacter vinelandii

AbstractThe effect of the NaCl concentration and the reaction temperature on the MgATP-dependent pre-steady-state electron transfer reaction (from the Fe protein to the MoFe protein) of nitrogenase from Azotobacter vinelandii was studied by stopped-flow spectrophotometry and rapid-freeze EPR spectroscopy. Besides lowering the reaction temperature, also the addition of NaCl decreased the observed rate constant and the amplitude of the absorbance increase (at 430 nm) which accompanies pre-steady-state electron transfer. The diminished absorbance increase observed at 5°C (without NaCl) can be explained by assuming reversible electron transfer, which was revealed by rapid-freeze EPR experiments that indicated an incomplete reduction of the FeMo cofactor. This was not the case with the salt-induced decrease of the amplitude of the stopped-flow signal: the observed absorbance amplitude of the electron transfer reaction predicted only 35% reduction of the MoFe protein, whereas rapid-freeze EPR showed 80% reduction of the FeMo cofactor. In the presence of salt, the kinetics of the reduction of the FeMo cofactor showed a lag period which was not observed in the absorbance changes. It is proposed that the pre-steady-state electron transfer reaction is not a single reaction but consists of two steps: electron transfer from the Fe protein to a still unidentified site on the MoFe protein, followed by the reduction of the FeMo cofactor. The consequences of our finding that the pre-steady-state FeMo cofactor reduction does not correlate with the amplitude and kinetics of the pre-steady-state absorbance increase will be discussed with respect to the present model of the kinetic cycle of nitrogenase

Investigation of the influence of wall stiffness on the stress ratio in mammoth silos

To calculate the stresses on the walls of silos, it is necessary to have a good estimate of the ratio of horizontal to vertical stress. This ratio however is not known precisely, especially in cases of static stress as found in a mammoth silo. In this paper the influence of the wall stiffness on the ratio of horizontal to vertical stresses in situations with limit strain is investigated. A new test method is developed, with which the ratio can directly be measured. A key feature of this method is the comparability with the practice. The method uses reinforced rubber membranes which have the same order of radial stiffness as mammoth silos. Another objective was to determine the extent to which the ratio of horizontal to vertical stresses with limit strain is dependent on the internal angle of friction, ¿i. Special attention was paid to Jaky's formula, which is often used to calculate the neutral stress ratio

Particles in non-Abelian gauge potentials - Landau problem and insertion of non-Abelian flux

We study charged spin-1/2 particles in two dimensions, subject to a perpendicular non-Abelian magnetic field. Specializing to a choice of vector potential that is spatially constant but non-Abelian, we investigate the Landau level spectrum in planar and spherical geometry, paying particular attention to the role of the total angular momentum J = L +S. After this we show that the adiabatic insertion of non-Abelian flux in a spin-polarized quantum Hall state leads to the formation of charged spin-textures, which in the simplest cases can be identified with quantum Hall Skyrmions.Comment: 24 pages, 10 figures (with corrected legends

Habitat of endangered white abalone, Haliotis sorenseni.

Abstract Surveys with a submersible at offshore islands and banks in southern California found that white abalone were most abundant at depths between 43 and 60 m. This is deeper than estimates taken when white abalone were more abundant. Densities were highest at sites far from fishing ports. Controlling for depth and site found that white abalone were significantly more abundant in areas with Laminaria farlowii (an alga) but abalone were not associated with areas high in the cover of other algae (Pelagophycus porra or Eisenia arborea) or the amount of sand in the habitat (except that abalone always occurred on rock). Within an area with abalone, the particular rock they occurred on was significantly larger than unoccupied neighboring rocks. Occupied rocks were not significantly different in algal cover or in sea urchin density than unoccupied neighboring rocks. The position of abalone on a rock was nearer to the rock-sand interface than would be expected based on a random distribution. More white abalone were feeding when in association with red urchins, perhaps because both grazers capture drift algae to eat. These data may aid future efforts to locate white abalone brood stock and identify locations for outplanting.