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  • 1
    Electronic Resource
    Electronic Resource
    AN IMPROVED A.C. POTENTIAL DROP METHOD FOR DETECTING SURFACE MICROCRACKS DURING FATIGUE TESTS OF UNNOTCHED SPECIMENS (1980)
    Oxford, UK : Blackwell Publishing Ltd
    Fatigue & fracture of engineering materials & structures 3 (1980), S. 0 
    Details
    ISSN: 1460-2695
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract—The detection and measurement of surface microcracks in unnotched specimens are becoming more important, both from the point of view of description of crack growth by linear elastic fracture mechanics and from the classical endurance limit approach. Theoretical analyses of the current distribution in a cylindrical test specimen show promise that the a.c. potential drop method will become more sensitive for surface microcracks when higher current frequencies are applied. This effect was experimentally affirmed during fatigue tests on unnotched cylindrical specimens. With a 40 kHz current frequency semicircular surface microcracks with an area of 0.0066 mm2 (0.05% of the specimen cross-section) were detected. For accurate and reproducible crack growth measurements, a 5 kHz current frequency is preferable.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1460-2695.1980.tb01360.x
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    Paper (German National Licenses)
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  • 2
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    Structure finding in cosmological simulations: the state of affairs (2013)
    Oxford University Press
    Details
    Publication Date: 2013-09-26
    Description: The ever increasing size and complexity of data coming from simulations of cosmic structure formation demand equally sophisticated tools for their analysis. During the past decade, the art of object finding in these simulations has hence developed into an important discipline itself. A multitude of codes based upon a huge variety of methods and techniques have been spawned yet the question remained as to whether or not they will provide the same (physical) information about the structures of interest. Here we summarize and extent previous work of the ‘halo finder comparison project’: we investigate in detail the (possible) origin of any deviations across finders. To this extent, we decipher and discuss differences in halo-finding methods, clearly separating them from the disparity in definitions of halo properties. We observe that different codes not only find different numbers of objects leading to a scatter of up to 20 per cent in the halo mass and V max function, but also that the particulars of those objects that are identified by all finders differ. The strength of the variation, however, depends on the property studied, e.g. the scatter in position, bulk velocity, mass and the peak value of the rotation curve is practically below a few per cent, whereas derived quantities such as spin and shape show larger deviations. Our study indicates that the prime contribution to differences in halo properties across codes stems from the distinct particle collection methods and – to a minor extent – the particular aspects of how the procedure for removing unbound particles is implemented. We close with a discussion of the relevance and implications of the scatter across different codes for other fields such as semi-analytical galaxy formation models, gravitational lensing and observables in general.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 3
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    The persistent percolation of single-stream voids (2015)
    Oxford University Press
    Details
    Publication Date: 2015-05-14
    Description: We study the nature of voids defined as single-stream regions that have not undergone shell-crossing. We use origami to determine the cosmic web morphology of each dark matter particle in a suite of cosmological N -body simulations, which explicitly calculates whether a particle has crossed paths with others along multiple sets of axes and does not depend on a parameter or smoothing scale. The theoretical picture of voids is that of expanding underdensities with borders defined by shell-crossing. We find instead that locally underdense single-stream regions are not bounded on all sides by multi-stream regions, thus they percolate, filling the simulation volume; we show that the set of multi-stream particles also percolates. This percolation persists to high resolution, where the mass fraction of single-stream voids is low, because the volume fraction remains high; we speculate on the fraction of collapsed mass in the continuum limit of infinite resolution. By introducing a volume threshold parameter to define underdense void ‘cores’, we create a catalogue of origami voids which consist entirely of single-stream particles and measure their percolation properties, volume functions, and average densities.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 4
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    Truthing the stretch: non-perturbative cosmological realizations with multiscale spherical collapse (2015)
    Oxford University Press
    Details
    Publication Date: 2015-10-22
    Description: Here we present a simple, parameter-free, non-perturbative algorithm that gives low-redshift cosmological particle realizations accurate to few-Megaparsec scales, called muscle (MUltiscale Spherical-ColLapse Evolution). It has virtually the same cost as producing N -body-simulation initial conditions, since it works with the ‘stretch’ parameter , the Lagrangian divergence of the displacement field. It promises to be useful in quickly producing mock catalogues, and to simplify computationally intensive reconstructions of galaxy surveys. muscle applies a spherical-collapse prescription on multiple Gaussian-smoothed scales. It achieves higher accuracy than perturbative schemes (Zel'dovich and second-order Lagrangian perturbation theory – 2LPT), and, by including the void-in-cloud process (voids in large-scale collapsing regions), solves problems with a single-scale spherical-collapse scheme. Slight further improvement is possible by mixing in the 2LPT estimate on large scales. Additionally, we show the behaviour of for different morphologies (voids, walls, filaments, and haloes). A python code to produce these realizations is available at http://skysrv.pha.jhu.edu/~neyrinck/muscle.html .
    Print ISSN: 1745-3925
    Electronic ISSN: 1745-3933
    Topics: Physics
    Published by Oxford University Press
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  • 5
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    Tetrahedral collapse: a rotational toy model of simultaneous dark-matter halo, filament and wall formation (2016)
    Oxford University Press
    Details
    Publication Date: 2016-06-02
    Description: We discuss an idealized model of halo formation, in which a collapsing halo node is tetrahedral, with a filament extruding from each of its four faces, and with a wall connecting each pair of filaments. In the model, filaments generally spin when they form, and the halo spins if and only if there is some rotation in filaments. This is the simplest possible fully three-dimensional halo collapse in the ‘origami approximation’, in which voids are irrotational, and the dark-matter sheet out of which dark-matter structures form is allowed to fold in position–velocity phase space, but not stretch (i.e. it cannot vary in density along a stream). Up to an overall scaling, the four filament directions, and only three other quantities, such as filament spins, suffice to determine all of the collapse's properties: the shape, mass, and spin of the halo; the densities per unit length and spins of all filaments; and masses per unit area of the walls. If the filaments are arranged regular-tetrahedrally, filament properties obey simple laws, reminiscent of angular-momentum conservation. The model may be most useful in understanding spin correlations between neighbouring galaxies joined by filaments; these correlations would give intrinsic alignments between galaxies, essential to understand for accurate cosmological weak-lensing measurements.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 6
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    Kolmogorov complexity in the Milky Way and its reduction with warm dark matter (2015)
    Oxford University Press
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    Publication Date: 2015-07-03
    Description: We discuss the Kolmogorov complexity of primordial patches that collapse to form galaxies like the Milky Way; this complexity quantifies the amount of initial data available to form the structure. We also speculate on how the quantity changes with time. Because of dark-matter and baryonic collapse processes, it likely decreases with time, i.e. information sinks dominate sources. But sources of new random information do exist; e.g. a central black hole with an accretion disc and jets could in principle broadcast small-scale quantum fluctuations over a substantial portion of a galaxy. A speculative example of how this concept might be useful is in differentiating between warm (WDM) and cold (CDM) dark matter. With WDM, the initial patch that formed the Milky Way would have had few features, making the present high degree of structure a curiosity. The primordial patch would have had only several billion independent information-carrying ‘pixels’ if the WDM particle had a mass of 1 keV. This number of ‘pixels’ is much less than even the number of stars in the Milky Way. If the dark matter is proven to be warm, the high degree of structure in the Milky Way could have arisen in two ways: (i) from a high sensitivity to initial conditions, like an intricate fractal arising from a relatively simple computer code; or (ii) from random information generated after the Galaxy formed, i.e. not entirely deterministically from the initial conditions.
    Print ISSN: 1745-3925
    Electronic ISSN: 1745-3933
    Topics: Physics
    Published by Oxford University Press
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  • 7
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    Recovering dark-matter clustering from galaxies with Gaussianization (2016)
    Oxford University Press
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    Publication Date: 2016-02-28
    Description: The Gaussianization transform has been proposed as a method to remove the issues of scale-dependent galaxy bias and non-linearity from galaxy clustering statistics, but these benefits have yet to be thoroughly tested for realistic galaxy samples. In this paper, we test the effectiveness of the Gaussianization transform for different galaxy types by applying it to realistic simulated blue and red galaxy samples. We show that in real space, the shapes of the Gaussianized power spectra of both red and blue galaxies agree with that of the underlying dark matter, with the initial power spectrum, and with each other to smaller scales than do the statistics of the usual (untransformed) density field. However, we find that the agreement in the Gaussianized statistics breaks down in redshift space. We attribute this to the fact that red and blue galaxies exhibit very different fingers of god in redshift space. After applying a finger-of-god compression, the agreement on small scales between the Gaussianized power spectra is restored. We also compare the Gaussianization transform to the clipped galaxy density field and find that while both methods are effective in real space, they have more complicated behaviour in redshift space. Overall, we find that Gaussianization can be useful in recovering the shape of the underlying dark-matter power spectrum to k ~ 0.5 h  Mpc –1 and of the initial power spectrum to k ~ 0.4 h  Mpc –1 in certain cases at z = 0.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 8
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    The clustering of galaxies in the SDSS-III Baryon Oscillation Spectroscopic Survey: mock galaxy catalogues for the BOSS Final Data Release (2016)
    Oxford University Press
    Details
    Publication Date: 2016-01-16
    Description: We reproduce the galaxy clustering catalogue from the SDSS-III Baryon Oscillation Spectroscopic Survey Final Data Release (BOSS DR11&DR12) with high fidelity on all relevant scales in order to allow a robust analysis of baryon acoustic oscillations and redshift space distortions. We have generated (6000) 12 288 MultiDark patchy BOSS (DR11) DR12 light cones corresponding to an effective volume of ~192 000 [ h –1 Gpc] 3 (the largest ever simulated volume), including cosmic evolution in the redshift range from 0.15 to 0.75. The mocks have been calibrated using a reference galaxy catalogue based on the halo abundance matching modelling of the BOSS DR11&DR12 galaxy clustering data and on the data themselves. The production follows three steps. First, we apply the patchy code to generate a dark matter field and an object distribution including non-linear stochastic galaxy bias. Secondly, we run the halo/stellar distribution reconstruction hadron code to assign masses to the various objects. This step uses the mass distribution as a function of local density and non-local indicators (i.e. tidal field tensor eigenvalues and relative halo exclusion separation for massive objects) from the reference simulation applied to the corresponding patchy dark matter and galaxy distribution. Finally, we apply the sugar code to build the light cones. The resulting MultiDark patchy mock light cones reproduce the number density, selection function, survey geometry, and in general within 1, for arbitrary stellar mass bins, the power spectrum up to k  = 0.3 h Mpc –1 , the two-point correlation functions down to a few Mpc scales, and the three-point statistics of the BOSS DR11&DR12 galaxy samples.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 9
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    The integrated Sachs-Wolfe effect in f(R) gravity (2014)
    Oxford University Press
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    Publication Date: 2014-03-21
    Description: We study the late-time integrated Sachs–Wolfe (ISW) effect in f ( R ) gravity using N -body simulations. In the f ( R ) model under study, the linear growth rate is larger than that in general relativity (GR). This slows down the decay of the cosmic potential and induces a smaller ISW effect on large scales. Therefore, the $\dot{\Phi }$ (time derivative of the potential) power spectrum at k  〈 0.1 h  Mpc –1 is suppressed relative to that in GR. In the non-linear regime, relatively rapid structure formation in f ( R ) gravity boosts the non-linear ISW effect relative to GR, and the $\dot{\Phi }$ power spectrum at k  〉 0.1 h  Mpc –1 is increased (100 per cent greater on small scales at z  = 0). We explore the detectability of the ISW signal via stacking supercluster and supervoids. The differences in the corresponding ISW cold- or hotspots are ~20 per cent for structures of ~100 Mpc  h –1 . Such differences are greater for smaller structures, but the amplitude of the signal is lower. The high amplitude of ISW signal detected by Granett et al. cannot be explained in the f ( R ) model. We find relatively big differences between f ( R ) and GR in the transverse bulk motion of matter, and discuss its detectability via the relative frequency shifts of photons from multiple lensed images.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 10
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    A halo bias function measured deeply into voids without stochasticity (2014)
    Oxford University Press
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    Publication Date: 2014-05-04
    Description: We study the relationship between dark-matter haloes and matter in the MIP (multum in parvo) N -body simulation ensemble, which allows precision measurements of this relationship, even deeply into voids. What enables this is a lack of discreteness, stochasticity, and exclusion, achieved by averaging over hundreds of possible sets of initial small-scale modes, while holding fixed large-scale modes that give the cosmic web. We find (i) that dark-matter-halo formation is greatly suppressed in voids; there is an exponential downturn at low densities in the otherwise power-law matter-to-halo density bias function. Thus, the rarity of haloes in voids is akin to the rarity of the largest clusters, and their abundance is quite sensitive to cosmological parameters. The exponential downturn appears both in an excursion-set model, and in a model in which fluctuations evolve in voids as in an open universe with an effective m proportional to a large-scale density. We also find that (ii) haloes typically populate the average halo-density field in a super-Poisson way, i.e. with a variance exceeding the mean; and (iii) the rank-order-Gaussianized halo and dark-matter fields are impressively similar in Fourier space. We compare both their power spectra and cross-correlation, supporting the conclusion that one is roughly a strictly increasing mapping of the other. The MIP ensemble especially reveals how halo abundance varies with ‘environmental’ quantities beyond the local matter density; (iv) we find a visual suggestion that at fixed matter density, filaments are more populated by haloes than clusters.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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