Showing papers on "Surface roughness published in 1979"

Modeling of rock friction: 1. Experimental results and constitutive equations

Abstract: Direct shear experiments on ground surfaces of a granodiorite from Raymond, California, at normal stresses of ∼6 MPa demonstrate that competing time, displacement, and velocity effects control rock friction. It is proposed that the strength of the population of points of contacts between sliding surfaces determines frictional strength and that the population of contacts changes continuously with displacements. Previous experiments demonstrate that the strength of the contacts increases with the age of the contacts. The present experiments establish that a characteristic displacement, proportional to surface roughness, is required to change the population of contacts. Hence during slip the average age of the points of contact and therefore frictional strength decrease as slip velocity increases. Displacement weakening and consequently the potential for unstable slip occur whenever displacement reduces the average age of the contacts. In addition to this velocity dependency, which arises from displacement dependency and time dependency, the experiments also show a competing but transient increase in friction whenever slip velocity increases. Creep of the sliding surface at stresses below that for steady state slip is also observed. Constitutive relationships are developed that permit quantitative simulation of the friction versus displacement data as a function of surface roughness and for different time and velocity histories. Unstable slip in experiments is controlled by these constitutive effects and by the stiffness of the experimental system. It is argued that analogous properties control earthquake instability.

Investigation of effective-medium models of microscopic surface roughness by spectroscopic ellipsometry

Abstract: Using measured dielectric function data from 2.1 to 5.5 eV for chemical-vapor-deposition---grown smooth amorphous ($a$-Si) and microscopically rough fine-grained polycrystalline ($p$-Si) films, we show that the dielectric properties of microscopically rough layers of thicknesses 100-500 \AA{}A are accurately modeled in the effective-medium approximation. These microscopically rough layers show essentially no macroscopic light scattering, and thus are inaccessible to measurement by usual scattering techniques. The unambiguous identification of microscopic roughness, as opposed to, e.g., an overlying oxide, is shown to require a spectroscopic capability. Statistical-analysis techniques are introduced to determine model parameters systematically and objectively, and also to establish correlations and confidence limits that show which parameters are defined by the data and which are statistically indeterminate. A best-fit five-parameter model for the sample with the thickest surface region shows that the density profile is characteristic of hemispherical, not pyramidal, irregularities. This indicates that surface roughness arises from a three-dimensional nucleation and growth process in these samples. In a comparison of the three one-parameter effective-medium models, Bruggeman and Maxwell Garnett(2) theories are found to adequately represent the data, while the Lorentz-Lorenz model, previously used exclusively to model roughness in single-wavelength applications, predicts only qualitatively the spectral dependence and gives poor results.

Effect of surface roughness on the microwave emission from soils

Abstract: The effect of surface roughness on the brightness temperature of a moist terrain was studied through the modification of Fresnel reflection coefficient and using the radiative transfer equation. The modification involves introduction of a single parameter to characterize the roughness. It is shown that this parameter depends on both the surface height variance and the horizontal scale of the roughness. Model calculations are in good quantitative agreement with the observed dependence of the brightness temperature on the moisture content in the surface layer. Data from truck mounted and airborne radiometers are presented for comparison. The results indicate that the roughness effects are greatest for wet soils where the difference between smooth and rough surfaces can be as great as 50K.

Relationship between Surface Scattering and Microtopographic Features

Abstract: In the smooth-surface limit, the angular distribution of the light scattered from a surface maps the power spectral density of its residual surface roughness. This result is essentially independent of the scattering theory used and the statistical properties of the surface roughness. The power spectral densities of engineering surfaces are generally broad and increase with increasing spatial wavelength. As a result, practical surface finish parameters are not intrinsic properties of the surface, but depend, with varying degrees of sensitivity, on the bandwidth limits inherent in their measurement or dictated by their application. These issues are discussed with reference to two classes of finish parameters: those related to the central moments of the scattering spectrum, and those related to the coefficients in the expansion of the shape of the spectrum in inverse powers of the scattering angle. The significance of "1/02" scattering in this context is emphasized. A shot model of surface roughness is then introduced to gain further insight into the relationship between scattering and surface features. In this model inverse power terms are related to "edge" scattering effects from critical points in various types of elemental microdefects. The relationship between this view and electronic noise is pointed out; in particular, the correspondence between "1/02" scattering and "1/f" or flicker-noise phenomena.

Relation between the angular dependence of scattering and the statistical properties of optical surfaces

Abstract: A relation from vector scattering theory has been used to predict the angular distribution of scattered light from optical surfaces as a function of the wavelength, optical constants of the material, and spectral density function. For calculations of one-dimensional (two-dimensional) scattering, the spectral density function of the surface roughness is obtained from the Fourier transform (Hankel transform) of the autocovariance function, which in turn is determined from surface-profile data. Measured statistics presented for various types of optical surfaces indicate that there are three basic components of surface structure: long-range waviness, short-range random roughness, and periodicity; one or more of which may be present on a given surface. Averaged and unaveraged surface-profile data for the same surface are shown to be consistent. Experimental data are presented that yield an exponential autocovariance function, and give a reasonably good fit to a Poisson distribution of zero crossings. Finally, angular scattering values calculated using measured surface statistics with vector scattering theory are compared to scattering values measured on the same surface. The shapes of the measured and calculated curves are similar, but the magnitudes are not. However, the rms surface roughnesses calculated from total integrated scattering measurements are in excellent agreement with values measured directly on these same surfaces.

A Model for the Microwave Emissivity of the Ocean's Surface as a Function of Wind Speed

Abstract: A quantitative model is presented, which describes the ocean surface as an ensemble of flat facets with a normal distribution of slopes. The variance of the slope distribution is linearly related to frequency up to 35 GHz and constant at higher frequencies. These facets are partially covered with an absorbing nonpolarized foam layer. Experimental evidence is presented for this model.

Scalar Scattering Theory for Multilayer Optical Coatings

Abstract: A scalar theory for scattering from multilayer coatings due to surface roughness is presented. A new model is considered which assumes that the roughness of the top surface of a given layer is due to the roughness introduced by the previously deposited layers and to the variations in thickness of the layer itself. The variations in the layer thickness are assumed to be uncorrelated from one layer to the next, but the roughnesses of the various surfaces are partially correlated. This model is compared to two existing models: one which assumes that the surfaces of the layers are completely uncorrelated and another which assumes that all of the surfaces are identical to that of the substrate. A fourth model is also introduced which assumes that the scattering is due to variations of refractive index within each layer. A matrix formulation for calculating the total integrated scattering and the change in specular reflectance and transmittance is presented. Predictions of the four different models are compared for several multilayer designs.

Vector Scattering Theory

Abstract: Vector theories dealing with the interaction of light with surface roughness are reviewed. Light scattering from microirregularities whose heights are much smaller and much larger than the wavelength of the incident light are discussed. The theories apply to metal-coated surfaces and, in addition, surfaces covered with multilayer dielectric stacks. Surface plasmon effects, which are also discussed, include the decrease in the specular reflectance caused by conversion of photon energy into surface plasmons (i.e., additional absorption due to surface plasmons), as well as the radiative decay of surface plasmons into photons (i.e., surface plasmon "scattering"). There is a brief discussion of how to use experimental surface height data in the theoretical formulae. Finally, published experimental data on scattering and surface plasmon effects are compared to theoretical predictions.

Comparison of 2.8‐ and 21‐cm microwave radiometer observations over soils with emission model calculations

Abstract: An airborne experiment was conducted under NASA auspices to test the feasibility of detecting soil moisture by microwave remote sensing techniques over agricultural fields near Phoenix, Arizona at midday of April 5, 1974 and at dawn of the following day Extensive ground data were obtained from 96 bare, sixteen hectare fields Observations made using a scanning (28 cm) and a nonscanning (21 cm) radiometer were compared with the predictions of a radiative transfer emission model It is shown that (1) the emitted intensity at both wavelengths correlates best with the near surface moisture, (2) surface roughness is found to more strongly affect the degree of polarization than the emitted intensity, (3) the slope of the intensity-moisture curves decreases in going from day to dawn, and (4) increased near surface moisture at dawn is characterized by increased polarization of emissions The results of the experiment indicate that microwave techniques can be used to observe the history of the near surface moisture The subsurface history must be inferred from soil physics models which use microwave results as boundary conditions

A Review of Electrical Breakdown in Mixtures of SF6 and Other Gases

Abstract: The outstanding electrical and arc-quenching characteristics of sulphur-hexafluoride (SF6) have already set new trends and standards for the construction of high voltage equipment. Intensive research is being carried out on the electrical properties of SF6, other electronegative gases and their mixtures with more common gases. The aims are to obtain an economical insulation with optimal electrical properties and to acquire a better understanding of their prebreakdown and breakdown behavior. Recently [1] we reviewed the breakdwon mechanisms operating in compressed gases and discussed the theoretical results in the light of experimental measurement in SF6. This paper deals with the breakdown and prebreakdown behavior of SF6 mixtures with other electron attaching and non-attaching gases. Uniform and non-uniform field breakdown under the applications of direct, alternating and impulse voltages is discussed. Improvement in properties such as higher breakdown voltages in non-uniform field gaps, less sensitivity to the electrode surface roughness and to the presence of contaminants and improved arc interruption characteristics of SF6-gas mixtures are discussed.

Correlation of nosetip boundary-layer transition data measured in ballistics-range experiments

Abstract: Preablated nosetips of various carbonaceous materials were tested in a ballistics range. Surface-temperature contours, measured with image-converter cameras, were used to define boundary-layer transition-front contours. Measurements of surface roughness, surface temperature, average transition-front location, and freestream environment were combined with calculations of nosetip flowfields, and with calculations of laminar boundary-layer development in these flowfields, to transform all data into various dimensionless parameters. These parameters were defined by previous attempts to correlate existing wind-tunnel data for transition on rough/blunt bodies. Of the available correlating techniques, only one, based on the concept of a constant (critical) roughness Reynolds number for transition, was found to successfully describe both the wind-tunnel and ballistics-range data, thereby validating the extrapolation of this concept to actual re-entry vehicle materials and environments.

Looking at Rubber Adhesion

Abstract: Rubber has been employed in contact with a variety of surfaces for decades, yet the adhesional mechanisms involved are not fully understood. This article describes fundamental investigations carried out over the last decade. Emphasis is placed upon the use of optical techniques for looking directly at the contact area rubber makes with another surface. The interpretation is based upon a rate-dependent surface energy approach. In this way it is possible to predict the level of peel adhesion between surfaces, for example, when a ball rolls on smooth rubber. Other examples treated in the same way are the time for detachment of a ball from a smooth rubber track under gravity, its resilience when bounced on the track, and its friction when slid over the track. The influence of surface roughness, electrostatic forces, surface bloom, and humidity are considered, together with rubber compound variables such as glass transition temperature, crosslink density, and fillers. The central theme is that the adh...

Modified power law equations for vertical wind profiles

Abstract: Equations are presented for calculating power law exponents from wind speed and surface roughness data. Results are evaluated by comparison with wind profile data measured at a variety of sites.

Friction and Wear Behavior of Single-Crystal Silicon Carbide in Sliding Contact with Various Metals

Abstract: Sliding friction experiments were conducted with single-crystal silicon carbide in contact with various metals. Results indicate the coefficient of friction is related to the relative chemical activity of the metals. The more active the metal, the higher the coefficient of friction. All the metals examined transferred to silicon carbide. The chemical activity of the metal and its shear modulus may play important roles in metal-transfer, the form of the wear debris and the surface roughness of the metal wear scar. The more active the metal, and the less resistance to shear, the greater the transfer to silicon carbide and the rougher the wear scar on the surface of the metal. Hexagon-shaped cracking and fracturing formed by cleavage of both prismatic and basal planes is observed on the silicon carbide surface. Presented at the 33rd Annual Meeting in Dearborn, Michigan, April 17–20, 1978

The effect of wind on the microwave emission from the ocean's surface at 37 GHz

Abstract: The microwave brightness temperature measurements from the Electrically Scanned Microwave Radiometer (frequency = 37 GHz) are compared with oceanic wind measurements from data buoys. It is shown that the brightness temperature can be manipulated to yield a measure of the surface roughening which can be very well accounted for by a simple geometric optics model. The data of 1.4, 8.36 and 19.34 GHz were similarly manipulated and shown to require a surface with less slope variance than predicted by optical measurements. It is also shown that the surface may be treated as isotropic to an accuracy equivalent to the roughening produced by a 2 m/s wind speed increment.

Experimental Attenuation of Rectangular Waveguides at Millimeter Wavelengths

Abstract: The experimental values of attenuation of commercially available rectangular waveguides were determined at frequencies between 25 and 200 GHz with emphasis on high accuracy. They were compared with the theoretical values computed from the dc conductivities, taking into consideration temperature effects, work hardening, size effects, surface roughness, and a room-temperature anomaly of the skin effect. A new way to express the excess attenuation due to these effects was fomulated. Excess ratios of attenuation of coin-silver waveguides were found to be well below the values used in engineering in the past. They can satisfactorily be explained by surface roughness. The normalized excess attenuations of copper guides are higher than those of guides made of silver but lower than cited in the literature.

Effects of surface condition on the infrared absorptivity of 304 stainless steel

Abstract: The absorptivity of 304 stainless steel at 10.6 μm has been obtained between room temperature and 1000 °C by laser calorimetry. Highly polished specimens have been compared with as‐received and rough‐polished specimens in order to determine the effects of surface condition on the absorptivity. When the surfaces are well annealed, the calorimetry data show that the absorptivities of the highly polished surface and the rougher surfaces are nearly the same over the entire range of temperature. No influence of grain size on the absorptivity has been observed, despite the substantial growth of the grain boundaries at elevated temperatures. When the surfaces are deformed by cold rolling or rough polishing, the absorptivity at room temperature increases by 22–27% due to increased scattering from nonplanar defects within the surface layer. The absorptivity data on damage‐free surfaces of stainless steel have been found to agree closely with the predictions of generalized Drude theory, applied to disordered alloys. The calorimetry data have also been used to calculate the hemispherical emissivity of 304 stainless steel between 400 and 1000 °C.

Role Of Surface Topography In X-Ray Scattering

Abstract: This paper discusses the relationship between x-ray mirror scattering and surface topography using vector electromagnetic scattering theory The results relate the angular distribution of the scattered intensity to various surface-finish parameters which can, in principle, be determined independently; for example, by x-ray scattering in other geometries, visible-light scattering, or stylus measurements The key role of spatial band-width limits in such parameters is emphasized General results are described and illustrated by a discussion of the scattering from isotropically rough surfaces, measured in both a point-detector and in a line-detector geometry Recent experimental results are then interpreted in terms of the surface-tension model of surface roughness, which predicts a hyperbolic scattering in the line-detector geometry The present results are offered as a subject for further experimental investigation, a mechanism for predicting the scattering from a given surface, and as a rationale for specifying surface finish in terms of system performance

Surface roughness measurements with an optical Fourier spectrum analyzer.

Abstract: The far-field pattern of light scattered by a small rough metallic surface area is obtained with a coherent optical system and used to quantify roughness measurements of machined surfaces. The distribution is usually anisotropic due to tooling marks. A cylinder lens forms a 1-D transform at favorable angular orientations about the optic axis to circumvent this problem and provides useful spectrum averaging. The integrated Gaussian curve fits to experimentally observed scattered light patterns from individual roughness samples yield an empirical linear relation between a Gaussian width parameter and average roughness height.