Asperity Spacing Research Articles

Examining theoretical applicability of displacement discontinuity model to wave propagation across rock discontinuities

Rock discontinuities such as joints widely exist in natural rock masses, and wave attenuation through rock masses is mainly caused by discontinuities. The displacement discontinuity model (DDM) has been widely used in theoretical and numerical analysis of wave propagation across rock discontinuity. However, the circumstance under which the DDM is applicable to predict wave propagation across rock discontinuity remains poorly understood. In this study, theoretical analysis and ultrasonic laboratory tests were carried out to examine the theoretical applicability of the DDM for wave propagation, where specimens with rough joints comprising regular rectangular asperities of different spacings and heights were prepared by 3D printing technology. It is found that the theoretical applicability of the DDM to predict wave propagation across rock discontinuity is determined by three joint parameters, i.e. the dimensionless asperity spacing (L), the dimensionless asperity height (H) and the groove density (D). Through theoretical analysis and laboratory tests, the conditions under which the DDM is applicable are derived as follows: and , . With increase in the groove density, the thresholds of the dimensionless asperity spacing and the dimensionless asperity height show a decreasing trend. In addition, the transmission coefficient in the frequency domain decreases with increasing groove density, dimensionless asperity spacing or dimensionless asperity height. The findings can facilitate our understanding of DDM for predicting wave propagation across rock discontinuity.

動脈弁の弁葉に与えた構造的工夫が弁機能におよぼす影響

Recently novel artificial heart valves, which are made by using tissue regeneration techniques or the latest biocompatible materials, are actively developing. The purpose of this study is to clarify the effects of mechanical anisotropy of valve leaflet on valve functions because the new materials and human native heart valves are anisotropic. We fabricated the human aortic valve model made of polyurethane with mechanical anisotropy in valve leaflet by adding spatially periodical surface asperities. Furthermore, to determine the optimal anisotropic property of the valve leaflet, the amount of sheet thickness and the spacing of surface asperities were varied as experimental parameters. Using our fabricated in-vitro simulator, experiments were conducted under the physiological conditions of healthy adults to compare valve function between isotropic and anisotropic models. The valve function was evaluated on basis of the ISO 5840-1 and ISO 5840-2 international standards for artificial valves. The obtained results indicate that anisotropic models with 3 mm spacing were found to have the best valve function, and isotropic models and anisotropic models with 1 mm spacing were found to have similar performance. Furthermore, comparison with data from clinically used prosthetic valves showed that among anisotropic models with 3 mm spacing, the valve leaflet thickness of 280 μm was superior in both opening and closing performance.

Design of bidirectional frictional behaviour for tactile contact using ellipsoidal asperity micro-textures

Tactile perception and friction can be modified by producing a deterministic surface topography. Change of surface feature arrangement and texture symmetry can produce an anisotropic frictional behaviour. It is generally achieved through skin hysteresis by promoting its deformation. This work investigates whether a bidirectional friction can be created with microscale ellipsoidal asperity textures, thus relying on the adhesive component of friction. For this purpose, four textured samples with various asperity dimensions were moulded with a silicone rubber having an elastic modulus comparable to that of the skin. Coefficient of friction measurements were conducted in-vivo in two sliding directions with a range of normal loads up to 4 N. Finite element method (FEM) was used to study elastic deformation effects, explain the observed friction difference, and predict surface material influence. Measurements performed perpendicular to the asperity major radii showed consistently higher friction coefficients than that during parallel sliding. For the larger asperity dimensions, a change of the sliding direction increased friction up to a factor of 2. The numerical analysis showed that this effect is mostly related to elastic asperity deflection. Bidirectional friction differences can be further controlled by asperity dimensions, spacing, and material properties.

Decadal Seismicity Prior to Great Earthquakes at Subduction Zones: Roles of Major Asperities and Low-Coupling Zones

Decadal forerunning seismic activity of magnitude Mw ≥ 5.0 is mapped for all 45 mainshocks of Mw 7.7 to 9.1 at subduction zones of the world from 1993 to mid 2020. The zones of high slip in nearly all great earthquakes were nearly quiescent beforehand and are identified as the sites of great asperities and zones of strong seismic coupling. Much forerunning activity occurred at smaller asperities along the peripheries of the rupture zones of many great and giant mainshocks. Those sizes of great asperities as ascertained from forerunning activity generally agree with the areas of high seismic slip as determined by others from geodetic and tide-gauge data and finite-source seismic modeling. Asperities are strong, well-coupled portions of plate interfaces. Different patterns of forerunning activity on time scales of about 5 to 45 years are attributed to either the sizes and spacing of asperities (or lack of). This permits many great asperities to be mapped decades before they rupture in great and giant shocks. Several poorly coupled subduction zones such as Java, Lesser Sunda, Marianas, Tonga and Kermadec are characterized by few great thrust earthquakes and little, in any forerunning activity. Rupture zones of many great and giant earthquakes are bordered either along strike, updip, or downdip by zones of lower plate coupling. Several bordering regions were sites of forerunning activity, aftershocks, and slow-slip events. The detection of forerunning and precursory activities of various kinds should be sought on the peripheries of great asperities as well as within zones of high co-seismic slip.

Rapid prototyping of geosynthetic interfaces: Investigation of peak strength using direct shear tests

Rapid prototyping offers a platform technology for investigations within the geosynthetics research and manufacturing sectors. This paper considers the application of rapid prototyping for the development of geosynthetic interfaces. The benefits and challenges of three rapid prototyping techniques (fused filament fabrication, selective laser sintering and laser thermal ablation) are considered and comparisons are presented between the three technologies. The paper then compares prototyped models of geomembrane texturing to those of a factory sourced reference geomembrane, leading on to a systematic geometric assessment using laser sintered model geomembranes. The geometric assessment highlights the benefits of hooked geomembrane asperities to interact with geotextiles in low normal stress applications, with a 69% increase in peak shear strength reported for hooked asperities, compared to the factory reference geometry. Asperity spacing is shown to influence the measured shear strength, with an increase for a geomembrane geotextile interfaces with closer asperities and an optimum spacing observed for geomembrane clay interfaces, below which the failure plane slides over the top of the texturing. Increases in asperity height correlated to smaller than expected increases in shear stresses for both geomembrane-geotextile and geomembrane clay interfaces.Whilst current rapid manufacturing techniques are shown to offer the ability to test the influence of variables on the performance characteristics of geosynthetic materials, the limitations of each technique, polymer utilised and resulting chemical and physical behaviour of the sample must be understood to allow these techniques to be successfully deployed.

Effect of particle size of sand and surface asperities of reinforcement on their interface shear behaviour

This paper investigates the effect of particle size of sand and the surface asperities of reinforcing material on their interlocking mechanism and its influence on the interfacial shear strength under direct sliding condition. Three sands of different sizes with similar morphological characteristics and four different types of reinforcing materials with different surface features were used in this study. Interface direct shear tests on these materials were performed in a specially developed symmetric loading interface direct shear test setup. Morphological characteristics of sand particles were determined from digital image analysis and the surface roughness of the reinforcing materials was measured using an analytical expression developed for this purpose. Interface direct shear tests at three different normal stresses were carried out by shearing the sand on the reinforcing material fixed to a smooth surface. Test results revealed that the peak interfacial friction and dilation angles are hugely dependent upon the interlocking between the sand particles and the asperities of reinforcing material, which in turn depends on the relative size of sand particles and asperities. Asperity ratio (AS/D50) of interlocking materials, which is defined as the ratio of asperity spacing of the reinforcing material and the mean particle size of sand was found to govern the interfacial shear strength with highest interfacial strength measured when the asperity ratio was equal to one, which represents the closest fitting of sand particles into the asperities. It was also understood that the surface roughness of the reinforcing material influences the shear strength to an extent, the influence being more pronounced in coarser particles. Shear bands in the interface shear tests were analysed through image segmentation technique and it was observed that the ratio of shear band thickness (t) to the median particle size (D50) was maximum when the AS/D50 was equal to one.

Effect of Plastic Flattening on the Shearing Response of Metal Asperities: A Dislocation Dynamics Analysis

Discrete dislocation (DD) plasticity simulations are carried out to investigate the effect of flattening and shearing of surface asperities. The asperities are chosen to have a rectangular shape to keep the contact area constant. Plasticity is simulated by nucleation, motion, and annihilation of edge dislocations. The results show that plastic flattening of large asperities facilitates subsequent plastic shearing, since it provides dislocations available to glide at lower shear stress than the nucleation strength. The effect of plastic flattening disappears for small asperities, which are harder to be sheared than the large ones, independently of preloading. An effect of asperity spacing is observed with closely spaced asperities being easier to plastically shear than isolated asperities. This effect fades when asperities are very protruding, and therefore plasticity is confined inside the asperities.

Friction Characteristics of Polymeric Nanofiber Arrays against Substrates with Tailored Geometry

We present a study on macroscale friction of polyethylene nanofibrillar arrays against patterned rough surfaces with various asperity heights, spacings, and area fractions. These surfaces are prepared by utilizing colloidal lithography and silica evaporation, which allows the independent control of geometric parameters. While the nanofiber arrays exhibit high friction on a smooth surface, much lower friction is observed when the asperity height becomes larger than can be compensated by fiber compliance, or when the asperity spacing becomes small enough to prevent fiber penetration for contact. The observed behavior is discussed with simple mechanical models and summarized to provide some criteria to maintain high friction against rough surfaces.

An Improved Model of Asperity Interaction in Normal Contact of Rough Surfaces

Normal contact of nominally flat rough surfaces is studied with an improved analytical model of asperity interaction. A finite element model of a representative model rough surface is created, and normal contact of this model is simulated to provide a basis of validation for the developed model. The developed model is also compared to an existing model in the literature and shown to correlate better with the finite element model. Furthermore, asperity spacing (density) is shown to be an important roughness parameter in determining the effects of asperity interaction in the load-interference response of the rough contact.

Analytical and Numerical Models for Tangential Stiffness of Rough Elastic Contacts

The paper considers elastic contact of rough surfaces and develops a simple analytical expression for the stiffness of the contact under tangential loading, which predicts that the contact stiffness is proportional to normal load and independent of Young’s Modulus. The predictions of this model are compared to a full numerical analysis of a rough elastic contact of finite size. The two approaches are found to be in good agreement at low loads, when the asperity spacing is large, but the numerical approach predicts much lower stiffnesses at medium and high loads. It is shown that the overall stiffness cannot exceed that of the equivalent smooth contact, and a simple means of modifying the analytical approach is proposed and validated.

Multiscale analysis on two-dimensional nanoscale adhesive contacts

Nanoscale adhesive contacts play a key role in micro/nano-electro-mechanical systems as the dimension of the components come to nanometer. Experimental studies on nanoscale adhesive contacts are limited by some uncertain factors and the cost of experiments is too high. Besides, nanoscale textured surfaces are difficult to process and nanoscale adhesive contacts of textured surfaces are still lack of investigation. By using multiscale method, which couples molecular dynamics simulation and finite element method, two-dimensional nanoscale adhesive contacts between a rigid cylindrical tip and an elastic substrate are investigated. For the contacts between the rigid cylindrical tip and smooth surface, Von Mises stress distributions, the maximum Von Mises stresses, and contact forces are compared for different radii to show the size effects and adhesive effects. The phenomena of hysteresis are observed and more obvious as the radii of the tip increase. The influences of indentation depth and indentation speed are also discussed. Then two series of textured surfaces are employed, and the influences of the texture asperity shape, asperity height, and asperity spacing on contact forces are studied. The contact forces comparisons show that textured surfaces can reduce contact forces effectively in the range of the two series. Contact forces of textured surfaces increase as the asperity heights increase, and textured surfaces with smaller asperity spacing will get higher contact forces. Contact forces may be controlled through textured surfaces in the future. The obtained results will help to improve contact condition and provide theory basis for texture design.

Numerical Analysis of Contact Mechanics between a Spherical Slider and a Flat Disk with Low Roughness Considering Lennard–Jones Surface Forces

Although analytical and numerical analyses of the contact mechanics of a completely smooth sphere–flat contact have been done, the analysis of a realistic sphere–flat contact with a surface roughness whose mean height planes have a spacing greater than the atomic equilibrium distance has not been done thoroughly. This paper is a fundamental study of the elastic contact mechanics due to Lennard–Jones (LJ) intermolecular surface forces between a spherical slider and a flat disk with low roughness whose height is larger than equilibrium distance z 0. First, neglecting the effect of the attractive force at contacting asperities, adhesion contact characteristics of a 2-mm-radius glass slider with a magnetic disk are presented in relation to the asperity spacing σ between mean height planes. Results showed that the contact behavior at a small asperity spacing of ∼0.5 nm cannot be predicted either by the Johnson–Kendall–Roberts or Derjaguin–Muller– Toporov theories. Second, contact characteristics of a 1-μm-radius sphere on a flat disk are presented to examine how LJ attractive force at contacting asperities can be evaluated. It was found that the adhesion force of contacting asperity is a function of separation in general, but it becomes almost constant when σ = ∼z 0. A simple equation to evaluate the LJ attractive pressure of contacting asperities is presented for the rough contact analysis. Third, numerical calculation methods for a sphere–flat contact including LJ attractive forces between the mating mean height planes and contacting asperities are presented. Then, adhesion characteristics of a 2-mm-radius glass slider and magnetic disk are calculated and compared with the previous experimental results of dynamic contact test. It is shown that the calculated LJ adhesion force is much smaller than the experimental adhesion force, justifying that the adhesion force observed at the separation of contact is caused by meniscus force rather than by vdW force.

Prediction of Contact Temperature Distribution during Fretting Fatigue in Titanium Alloys

Fretting is a critical problem to the manufacturers and operators of turbine engines, reducing the useful life of fan, compressor, and turbine blade components. The fretting action between the two components in contact leads to temperature rise, which in most cases, may be significant enough to cause a number of adverse effects. The knowledge of the temperature profiles inside the material is very important because it enables the prediction of possible oxide formation and the development of thermal stresses. To this end, the temperature profile for Ti-6Al-4V alloys has been predicted in this work utilizing a heat flow channel (HFC) model. The model requires as input the material constants (thermal conductivity, thermal diffusivity, and flow stress), the material surface characteristics (roughness and asperity spacing), the frictional conditions between the two surfaces, as well as the external parameters (frequency, amplitude of oscillation, magnitude of contact area). The analysis enabled the determination of the temperature field as a function of time and the three spatial coordinates. The model showed that the temperature increases quite rapidly at the beginning of the process and reaches an essentially constant value after approximately 1000 cycles. The predictions revealed a rapid decay of temperature with the thickness of the specimen, while the temperature drop was found to be much smoother in the other two specimen directions. Additionally, a first-order validation of the model was achieved by determining the oxygen concentration in fretting-fatigued Ti-6Al-4V specimens, where it was found that an increased oxygen concentration existed in the fretting area compared to the non-contact and stick areas.

Pore evolution during high pressure atomic vapor deposition

The development of physical vapor deposition systems that employ inert gas jets to entrain and deposit atomic and molecular fluxes have created an interest in the atomic assembly of thin films under high pressure (10–500 Pa) deposition conditions. Thin films grown under elevated pressure and low surface mobility conditions can contain a higher volume fraction of porosity and a different pore morphology to coatings created by conventional, low pressure (<10−4 Pa) deposition processes. A recent direct simulation Monte Carlo simulation analysis of binary vapor–gas jet atom interactions has shown that the incident angle distribution (IAD) for vapor atom impacts with a substrate is strongly effected by the background pressure. Here, these results are combined with a kinetic Monte Carlo technique to simulate the high pressure growth of vapor deposited nickel films and identify the mechanisms of pore formation. We find that when the surface atom mobility is low, shadowing of oblique angle arrivals by features on the substrate result in the incorporation of porosity with a hierarchical size distribution that includes elongated, inter-columnar pores and finer scale intra-columnar pores. The nucleation of the inter-columnar pores is related not only to the IAD, but also to the height and spacing of the initial asperities on the substrate and to those that subsequently evolve during deposition. The volume fraction of the inter-columnar pores is found to increase as both the fraction of oblique atom arrivals and the height of the asperities increase. For each prescribed IAD and asperity height, an asperity spacing is found that maximizes the inter-columnar pore fraction. By varying the IAD for a given substrate surface topology, in conjunction with intermittent observations of the coating structure during the growth process, the flux shadowing mechanisms that govern the inter-columnar pore nucleation have been determined.

Examination of road roughness parameters for abating tire vibration and radiated noise

Several studies demonstrated that road roughness plays an important role in the generation of tire vibration and radiated noise. One of these studies theoretically predicted the relationship between road roughness parameters and tire vibration and radiated noise. In this paper, the relationship is discussed in order to propose a method of designing a quiet pavement. The prediction is examined by two sets of measurements on actual road surfaces. First, the tire tread vibration was measured on three types of road surface. Second, tire/road noise was measured on 13 samples of road surface. The results show a tendency that supports the prediction which indicates that the spacing and height unevenness of pavement asperities should be small to abate tire vibration and radiated noise.

Contact simulation for predicting surface topography in metal forming

A surface contact model that takes account of flattening, roughening and tool elastic microwedge effects on workpiece surface is developed. The model can be implemented in FEM codes to predict the final surface qualities of the products in metal forming. As an example, the proposed model has been combined with a membrane finite element code of sheet metal forming process to predict the contact area ratio, surface roughness and mean asperity spacing. Numerical results showed good agreement with the experiments.

Definition of road roughness parameters for tire vibration noise control

Road roughness plays an important role in the generation of tire vibration noise. However, it is unclear which kinds of road roughness parameters should be controlled to reduce the noise. In this paper, we define the essential road roughness parameters that govern tire tread vibration and provide information on tire/road noise abatement. The detailed effects of road roughness parameters on tire tread vibration are estimated using a tire/road contact model. The results reveal that pavement asperity height itself is not an essential parameter, but asperity height unevenness, asperity radius, and asperity spacing are important for the abatement of tire vibration noise.

Characterization of fretting fatigue crack initiation processes in CR Ti–6Al–4V

A study was conducted to quantify fretting fatigue damage and to evaluate the residual fatigue strength of specimens subjected to a range of fretting fatigue test conditions. Flat Ti–6Al–4V specimens were tested against flat Ti–6Al–4V fretting pads with blending radii at the edges of contact. Fretting fatigue damage for two combinations of static average clamping stress and applied axial stress was investigated for two percentages of total life. Accumulated damage was characterized using full field surface roughness evaluation and scanning electron microscopy (SEM). The effect of fretting fatigue on uniaxial fatigue strength was quantified by interrupting fretting fatigue tests, and conducting uniaxial residual fatigue strength tests at R=0.5 at 300 Hz. Results from the residual fatigue strength tests were correlated with characterization results. While surface roughness measurements, evaluated in terms of asperity height and asperity spacing, reflected changes in the specimen surfaces as a result of fretting fatigue cycling, those changes did not correspond to decreases in residual fatigue strength. Neither means of evaluating surface roughness was able to identify cracks observed during SEM characterization. Residual fatigue strength decreased only in the presence of fretting fatigue cracks with surface lengths of 150 μm or greater, regardless of contact condition or number of applied fretting fatigue cycles. No cracks were observed on specimens tested at the lower stress condition. Threshold stress intensity factors were calculated for cracks identified during SEM characterization. The resulting values were consistent with the threshold identified for naturally initiated cracks that were stress relieved to remove load history effects.

Behavior of Dilative Sand Interfaces in a Geotribology Framework

Frictional resistance along the exterior of an embedded structure or structural element develops through relative displacement at the interface. An understanding of how surface topography influences interface strength and deformation behavior is required to develop comprehensive interface models for soil-structure analyses, to develop interface design methods and for producing enhanced construction materials. This paper presents the results of an investigation to quantify the influence of surface topography on shear stress and volume change behavior of dilative granular material interface systems. The root spacing, asperity spacing, asperity height, and asperity angle of machined, idealized surfaces are systematically varied. Direct interface shear test results using Ottawa 20/30 sand and glass microbeads show that maximum interface efficiency for these materials is achieved for a asperity spacing to median grain diameter ratio between 1.0 and 3.0, and an asperity height to median grain diameter ratio greater than 0.9. An asperity angle of 50 degrees or greater yields maximum efficiency for any given asperity spacing or height. The results suggest that interface behavior is governed by predictable geometric and mechanical relationships that are applicable to more complex manufactured surfaces.

Transition from the displacement discontinuity limit to the resonant scattering regime for fracture interface waves

Abstract The validity of the displacement discontinuity model for elastic wave propagation across and along fractures is related to the spacing of asperities in fractures. In this paper, the transition from the displacement-discontinuity regime to the resonant scattering of Rayleigh waves is explicitly observed for waves propagating along synthetic fractures. The fractures are engineered to have increasing asperity separation with fixed aperture orientation. The seismic signals of elastic interface waves for all three polarizations S v , S h and P propagating along the synthetic fractures are recorded, and the waveforms are analyzed using the new Nolte–Hilbert wavelet that balances time-frequency localization without violating the wavelet admissibility condition which impedes the use of the Morlet wavelet transform. The wavelet spectra of the elastic waves are measured in response to the changing asperity separation for waves propagating parallel and perpendicular to the asperities. Clear evidence for both Rayleigh-mode and P-mode fracture interface waves, as well as resonantly scattered Rayleigh waves, are observed in the wavelet transforms.