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Digitale Medien

Scaling of rock friction constitutive parameters: The effects of surface roughness and cumulative offset on friction of gabbro (1994)

Marone, Chris ; Cox, S. J. D.

Springer

Pure and applied geophysics 143 (1994), S. 359-385

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ISSN: 1420-9136

Schlagwort(e): Rock friction ; constitutive laws ; scaling ; characteristic friction distance

Quelle: Springer Online Journal Archives 1860-2000

Thema: Geologie und Paläontologie , Physik

Notizen: Abstract We describe experiments in which large (14×40 cm nominal contact area) blocks of gabbro were sheared in a direct shear apparatus at room temperature, 5 MPa normal stress, and slip velocities from 0.1 to 10 μm/s. The apparatus was servocontrolled using a displacement feedback measurement made directly between the gabbro blocks. Two surface roughnesses were studied (rough, produced by sandblasting, and smooth, produced by lapping with #60 grit) and accumulated displacements reached 60 mm. Measurements of surface topography were used to characterize roughness and asperity dimensions. Step changes in loading velocity were used to interrogate friction constitutive properties. Both rough and smooth surfaces showed appreciable displacement hardening. The coefficient of friction μ for rough surfaces was about 0.45 for initial slip and 0.7 after sliding 50 mm. Smooth surfaces exhibited higher μ and a greater tendency for unstable slip. The velocity dependence of frictiona−b and the characteristic friction distanceD c show systematic variations with accumulated displacement. For rough surfacesa−b started out positive and became negative after about 50 mm displacement andD c increased from 1 to 4 μm over the same interval. For smooth surfaces,a−b began negative and decreased slightly with displacement andD c was about 2 μm, independent of displacement. For displacements 〈30 mm, rough surfaces exhibit a second state variable with characteristic distance about 20 μm. The decrease ina−b with displacement is associated with disappearance of the second state variable. Our data indicate thatD c is controlled by surface roughness in a complex way, including but not limited to the effect of roughness on contact junction dimensions for bare rock surfaces. The data show that simple descriptions of roughness, such as rms and peak-to-trough, are not sufficient to inferD c . Our observations are consistent with a model in whichD c scales with gouge thickness.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1007/BF00874335

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2

Digitale Medien

Circular symmetrization and extremal Robin conditions (1999)

Cox, S. J. ; Kawohl, B.

Springer

Zeitschrift für angewandte Mathematik und Physik 50 (1999), S. 301-311

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ISSN: 0044-2275

Schlagwort(e): Key words. Average temperature, third type, dual formulation.

Quelle: Springer Online Journal Archives 1860-2000

Thema: Mathematik , Physik

Notizen: Abstract. We study the average temperature in a homogeneous disk subject to uniform heating in its interior and Newton's law of cooling, $hv+\partial v/\partial n=0$ , on its boundary. More precisely, among those h taking values in a prescribed interval, and of prescribed mean, we identify the minimizer and a maximizer of the average temperature. The latter characterization makes use of circular symmetrization. We include an elementary proof of the fact that such symmetrization does not increase the Dirichlet energy of any H 1 function on the disk.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1007/s000330050152

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3

Digitale Medien

Designing for optimal energy absorption, III: numerical minimization of the spectral abscissa (1997)

Cox, S. J.

Springer

Structural and multidisciplinary optimization 13 (1997), S. 17-22

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ISSN: 1615-1488

Quelle: Springer Online Journal Archives 1860-2000

Thema: Maschinenbau

Notizen: Abstract We address the optimal design problem associated with introducing passive damping devices into a structure in such a way thatall vibration frequencies are moved as far as possible into the left halfplane. As this natural objective function lacks a bounded derivative we are forced to adopt a derivative-free direct search method. We report on our application of parallel direct search to the optimal design of both shear buildings and strings.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1007/BF01198371

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4

Digitale Medien

On the Optimal Insulation of Conductors (1999)

Cox, S. J. ; Kawohl, B. ; Uhlig, P. X.

Springer

Journal of optimization theory and applications 100 (1999), S. 253-263

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ISSN: 1573-2878

Schlagwort(e): Two-phase conductors ; eigenvalues ; reinforcements ; boundary conditions of the third type

Quelle: Springer Online Journal Archives 1860-2000

Thema: Mathematik

Notizen: Abstract We coat a conductor with an insulator and equate the effectiveness of this procedure with the rate at which the body dissipates heat when immersed in an ice bath. In the limit, as the thickness and conductivity of the insulator approach zero, the dissipation rate approaches the first eigenvalue of a Robin problem with a coefficient determined by the shape of the insulator. Fixing the mean of the shape function, we search for the shape with the least associated Robin eigenvalue. We offer exact solutions for balls; for general domains, we establish existence and necessary conditions and report on the results of a numerical method.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1023/A:1021773901158

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Fingerprinting mine water in the eastern sector of the South Wales Coalfield (2002)

Brown, M. M. E. ; Jones, A. L. ; Leighfield, K. G. ; [weitere]

In: Geological Society Special Publication 198: 275-286.

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Publikationsdatum: 2002-01-01

Beschreibung: Coal has been mined in the South Wales Coalfield for centuries. Substantial dewatering operations have attended these activities, particularly with the advent of steam and electric pumps that facilitated deeper mining operations. However, underground coal mining in South Wales has been in decline since 1913. The closure of the last deep mine in the eastern part of the coalfield in 1992 entailed a cessation of pumping over a large geographical area, with a resultant recovery of the groundwater. A consequence of this recovery is that emissions of mine water are occurring at the surface. Surface water emissions of known provenance from the Upper Coal Measures were sampled in 2000 and characterized in terms of their major ion composition. Also, known emissions of mine water within the catchments of the Rivers Ebbw, Ebbw Fach, Rhymney and Taff Bargoed were sampled, and the major ion data plotted on trilinear Piper diagrams. The results indicate that two distinctive hydrochemical facies could be identified from these plots. By combining knowledge of the underground mine workings and lithology, and by comparison with the Upper Coal Measures fingerprint', it is possible to divide the mine water emissions into those sourced from the Upper and Middle and Lower Coal Measures, and at times to infer a degree of mixing between the two. The conclusion is that in the eastern valleys of the South Wales Coalfield, the Upper Coal Measures water has a calcium-sulphate facies and the Middle and Lower Coal Measures water has a sodium-sulphate to sodium-bicarbonate facies. This information can be added to the available knowledge to aid in assessing the nature and the progress of groundwater recovery within the coalfield. This paper refers to preliminary work that has been carried out in the South Wales Coalfield to attempt to fingerprint' mine water emissions to identify their underground source. This, in turn, provides valuable information on the hydrogeological regime and, coupled with the geological and mining setting, may further assist in identifying underground flow paths. This information helps to fill in some of the knowledge gaps' and provide a tool for use in the prediction of future surface mine water emissions and in the formulation of treatment-mitigation strategies. The current work uses observations and interpretations of the hydrochemistry of the coal Measures to develop a robust and simple to use method of characterizing and classifying mine water emissions within the South Wales Coalfield. It is anticipated that a similar interpretive process could be applied to mine water emissions in other coalfields, although it should be stressed that this cannot be considered as a stand alone' process and that the initial characterization of the mine water needs to be carried out with reference to the geological and mining setting for any particular area under consideration.

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