Frictional Drop Research Articles

Lubricant Effects in the Transition from Boundary to Microelastohydrodynamic Lubrication

Lubricant effects in the friction transition from boundary to microelastohydrodynamic lubrication were investigated by using a ballon-flat tribotester at sliding speeds from 0.02 to 0.88 mm/sec. Three lubricants—cyclophosphazine (X-IP), poly-alpha-olefin (PAO) and Z-DOL—were used, in this investigation. When X-IP was used at room temperature, a drop in friction coefficient from 0.22 to 0.12 at sliding speeds ≥0.10 mm/sec. (an unusually low speed) was observed, accompanied by a rise in the contact electrical resistance across the ball-fiat interface. The friction drop did not occur at temperatures ≥100°C. The friction transition was achieved at lower speeds when sliding perpendicular to the surface roughness texture. No transition occurred when PAO and Z-DOL were substituted as the lubricant. The latter him lubricants were working in the boundary lubrication regime as indicated by the contact resistance measurement.

Model of earthquake recurrence: Role of elastic wave radiation, relaxation of friction, and inhomogeneity

We have constructed a dynamical model of seismicity, wherein a fault is embedded in an infinite, continuous, elastic medium. Therefore the influence of the energy dissipated in seismic wave radiation oil the sequential history of model earthquakes is fully taken into account. In this model, the drop in friction at rupture takes place gradually, thus introducing a relaxation dimension. As an example, we consider a finite homogeneous fault that is terminated by infinitely strong barriers at the ends. The features of the seismicity are dominated by the stresses that are reflected from the unbreakable barriers. There is a strong dependence of the patterns of seismicity on the ratio of the relaxation dimension to the distance between the barriers at the ends. For large values of this parameter, we find that periodicity begins after a short transient interval and a dominance of the statistical distributions by large events that break completely through the fault from end to end. For small values of this parameter, smaller‐scale seismicity is interspersed with the large events, and no periodicity is observed within the time spanned by the computations. We conclude that major unquenched heterogeneities, such as those found at barriers, which we suppose arise in nature due to the nonuniform geometry of faults, are vital ingredients for generating complex seismic histories as well as giving rise to the complex features of individual earthquake source‐time functions in models of seismicity as a dynamical process.

A study of rupture length vs. moment for synthetic earthquakes

From the events synthesized from the one-dimensional dynamical mass-spring model proposed byBurridge andKnopoff (1967), the relation between rupture length Δ and earthquake momentM is studied for various model parameters. The earthquake moment is defined to be the total displacement of a connected set of mass elements which slide during an event. A parameter stiffness ratios is defined as the ratio of the spring constant between the two mass elements to that between one mass element and the moving plate. The velocity-dependent friction law (including weakening and hardening processes) is taken to control the sliding of a mass element. The distribution of the breaking strengths over the system is considered to be a fractal function. The cases for severals values and different velocity-dependent friction laws with different decreasing ratesrw of the frictional force with sliding velocity are studied numerically. The weakening process of the frictional force from the static one to the dynamic one obviously affects theM−Δ relation. Meanwhile, a rapid weakening process rather than a slow weakening process can result in aM−Δ relation, which is comparable to the observed one. Although an increase in thes value can yield an increase in the upper bound of the Δ value and the number of events with largeM and Δ values, the scaling of theM−Δ relation is not affected by the change of thes value. For the cases in this study, the theoretical Δ−M relations for small events withM 1 have a scaling exponent less than but close to 1. In addition, the fractal dimension, the friction drop ratio and the roughness of the distribution of the breaking strengths over the fault surface are the minor parameters influencing the Δ−M relation. A comparison between the theoreticalM−Δ relation and the observed one for strike-slip earthquakes shows that for large events the theoreticalM−Δ relation is quite consistent with the observed one, while for small events there is a one-order difference in the two relations. For the one-dimensional model, the decreasing rate of the dynamic frictional force with velocity is the main factor in affecting the characteristic value of the earthquake moment, at which the scaling of theM−Δ relation changes.

Tribology at high temperatures

The paper reviews publications, primarily of the Institute of Machinery Science of the Russian Academy of Sciences. The techniques of investigations and the results are briefly outlined as to how various high-melting point materials withstand friction in vacuum and in gaseous atmospheres (within a wide temperature range) in like or unlike combinations, such as metals, metal-like compounds, oxides, graphite materials and molybdenum disulphide. The results of measurements of adhesion, including that between diamond and various metals, are given. The friction drop of graphite material on corundum ceramics caused abnormally by tribochemical interaction, is described. The ‘eutectic wear’ accompanied by a sharp friction reduction is based on an investigation of the phenomenon of eutectic melting of diamond and graphite with experimental validation. The results of studies of vacuum ion-plasma deposited CrO and MoS 2 coatings are demonstrated and a technique is suggested as to how to make the friction of MoS 2 very low.

Creep and fatigue interaction in a nickel-based alloy: Wang, X., Wang, D.N. and Kong, Q.P. Scr. Metall. Mater. (15 Feb. 1993) 28 (4), 401–404

An optimized low-temperature plasma carbonitriding process at 430 °C for 15 h was used to treat the surface of 316LVM medical grade austenitic stainless steel. By using a servo-hydraulic dynamic test machine, the fretting behaviour of both as-received and the surface-treated 316LVM plate samples against martensitic stainless steel balls was studied in the Ringer's solution at various displacement amplitudes. The experimental results demonstrate that the optimized low-temperature plasma surface treatment can produce a precipitate-free, C/N supersaturated S-phase layer. Such layer revealed a hardness of 5 times that of 316LVM substrate. The formation of S-phase can protect the 316LVM from corrosion in the Ringer's solution, and the hard surface-treated layer can significantly improve the fretting wear resistance of 316LVM material. When fretted in a mixed regime, a sharp drop of friction force was observed for untreated 316LVM after fretting for about 1000 cycles, which was mainly caused by the penetration of Ringer's solution into the contact area of the fretting pairs. As a comparison, no friction drop was observed through the fretting process on the surface-treated samples. Due to the high ratio of elastic modulus to hardness (E/H) of untreated sample, the main damage mechanism on untreated sample was adhesive and abrasive wear during the fretting process. However, because of the high hardness and low E/H ratio of the surface-treated sample, the main damage mechanism was mild abrasive wear. In conclusion, the low temperature plasma surface treatment can effectively improve the fretting wear properties of 316LVM stainless steel, which could pave the way to develop the high-performance, long-life body implants.

A Reynolds stress model for near-wall turbulence

A tensorially consistent near-wall second-order closure model is formulated. Redistributive terms in the Reynolds stress equations are modelled by an elliptic relaxation equation in order to represent strongly non-homogeneous effects produced by the presence of walls; this replaces the quasi-homogeneous algebraic models that are usually employed, and avoids the need for ad hoc damping functions. A quasi-homogeneous model appears as the source term in the elliptic relaxation equation-here we use the simple Rotta return to isotropy and isotropization of production formulae. The formulation of the model equations enables appropriate boundary conditions to be satisfied. The model is solved for channel flow and boundary layers with zero and adverse pressure gradients. Good predictions of Reynolds stress components, mean flow, skin friction and displacement thickness are obtained in various comparisons to experimental and direct numerical simulation data. The model is also applied to a boundary layer flowing along a wall with a 90°, constant-radius, convex bend. Because the model is of a general, tensorially invariant form, special modifications for curvature effects are not needed; the equations are simply transformed to curvilinear coordinates. The model predicts many important features of this flow. These include: the abrupt drop of skin friction and Stanton number at the start of the curve, and their more gradual recovery after the bend; the suppression of turbulent intensity in the outer part of the boundary layer; a region of negative (counter-gradient) Reynolds shear stress; and recovery from curvature in the form of a Reynolds stress ‘bore’ propagating out from the surface. A shortcoming of the present model is that it overpredicts the rate of this recovery. A heat flux model is developed. It is shown that curvature effects on heat transfer can also be accounted for automatically by a tensorially invariant formulation.

Post-seismic stress evolution in a boundary-zone model with depth-dependent rheology

Tectonic boundary zones are characterized by a shallow, brittle region (seismogenic region) where frictional slip occurs on fault planes, and by a lower, ductile and aseismic region. The nucleation depth and the extent of slip planes in great interplate earthquakes are determined by the relationships between frictional resistance, applied stress and stress drop. The post-seismic stress evolution on a fault plane is investigated by a model which reproduces a transcurrent boundary zone subject to a constant strain rate. Dislocation models with frictional stress threshold for slip arrest must be considered in order to study post-seismic stress evolution in the proximity of dislocation edges. It is found that dislocations which are nucleated in the seismogenic region can propagate downward into the aseismic region: in this case earthquakes are followed by relaxation of the stress concentration at the lower dislocation edge. This has effects on the space-time distribution of aftershocks.

An investigation of the complexity of the earthquake source time function using dynamic faulting models

By constructing more than 30 numerical models of spontaneous earthquake rupture on heterogeneous faults, the complexity of the far‐field pulse shapes (the source time function), that is, the pulse consisting of several distinct subpulses, is investigated. The influence of multiple strength barriers and/or dry friction on the far‐field waveforms has been studied using the simple model of a square fault with uniformly spaced square barriers. A central asperity is allowed to break to initiate the dynamic process. It is found that the pulse duration is shorter the greater the density of barriers (ratio of barrier size to barrier spacing). The presence of friction has been found to have three different consequences. First, the slipping area is reduced giving rise to a shorter pulse duration. Second, the rate of slip propagation is reduced which increases the pulse duration. Third, the frictional stress drop, if present, increases the amplitude of the pulse. The interaction of these three effects together with the effect of the barriers results in different pulses depending on the relative role of these effects. Breaking of multiple asperities with relatively short time delay between their fracturing produced a single pulse with ripples in its rising portion, the pulse duration depending on the total time required for the asperities to break. Breaking of, say, two asperities with sufficient time delay between them produced a double pulse which is in accord with the asperity model of complex earthquakes. In one case of a fault with barriers and friction, an additional peak was observed in some directions from the source, the peak form and amplitude being essentially the same as those of the peaks due to breaking of asperities. One may conclude that each subpulse observed on a complex far‐field pulse is not necessarily interpretable as being due to the failure of an asperity on the fault. Furthermore, for the subpulses which can be related to asperity fracture, the duration is not determined by the size of the corresponding asperity but rather by the physical conditions on the fault surrounding the asperity.

Detection of a Formation Fracture in a Waterflooding Experiment

Summary A procedure to detect and to evaluate fracturing during waterflooding is described. The approach requires (1) use of a radial-flow analysis to detect changes in fluid transmissibility, (2) determination of the in-situ stress changes, caused by pore pressure buildup and temperature decrease, and comparison of the modified stresses with the bottomhole pressure (BHP), and (3) modeling of the fracture by means of a pressure (BHP), and (3) modeling of the fracture by means of a three-dimensional(3D) hydraulic fracture simulator. This procedure is applied to 30-day waterflooding injection into a limestone oil reservoir located in an offshore well within the Idd el Shargi reservoir (Qatar) in which fracture occurrence was suspected. Both the radial-flow analysis and the quantification of stress changes indicated the occurrence of fracture. Finally, the resulting fracture geometry was delimited by simulation of the fracturing process. Introduction Injection of water or other fluids into a reservoir for relatively long periods of time can alter the in-situ state of stress by decreasing the formation temperature and increasing pore pressure. Depending on the injection history and on the injected fluid and reservoir temperature, these changes can have an effect on the in-situ state of stresses that is sufficient to give rise to fracture onset. Perkins and Gonzales and Marx and Langenheim used an energy balance method to determine the reservoir temperature profile and the elasticity theory to quantify the resulting in-situ stress changes. Similarly, the elasticity theory was used to quantify stress changes caused by the pore pressure buildup. With the assumption of a linear elastic model, the stress changes calculated can be superposed on the undisturbed state of stresses to determine the modified closure stresses. Comparison of these stresses with the bottomhole fluid pressures may be used to estimate fracture onset. Alternatively, a radial-flow analysis can evaluate changes in formation transmissibility, kh/mu, relative to the value at the initial stages of injection. Changes in kh/mu would indicate changes in reservoir conditions caused by formation fracturing or formation damage. This work describes the detection of a formation fracture in a 30-day waterflooding experiment. This approach (1) uses a radial-flow analysis to determine changes in fluid transmissibility, (2) determines the changes in insitu stresses caused by changes in pore pressure and temperature and compares the modified stresses with the bottomhole fluid pressures, and (3) models the fracture geometry resulting from the injection of fluid after fracture onset. Description of the Waterflooding Experiment. The waterflooding test consisted of a 30-day filtered seawater injection into two perforated formations defined as Zones A and B of an oil-bearing limestone reservoir located offshore near the west coast of the Persian Gulf (Fig. 1). Zone A is 103 ft [31.4 m] thick while Zone B is 63 ft [19.2 m] thick. The average porosities for both zones were approximately equal ( =27%), while the average permeability for Zone A (k=2.21 md) was higher than for permeability for Zone A (k=2.21 md) was higher than for Zone B (k=0.87 md). Before injection, in-situ stresses were measured at several depths along the wellbore, as shown in Figs. 1 and 2. The injection test was conducted at three levels of constant tubing-head pressures (THP). The initial THP of 990 psig [6826 kpa] was increased to an intermediate step of psig [6826 kpa] was increased to an intermediate step of 1,240 psig [8550 kPa], and finally to a pressure of 1,490 psig [10 273 kpa]. Plots of the recorded injection rates psig [10 273 kpa]. Plots of the recorded injection rates and THP vs. time are displayed in Fig. Recordings of BHP's indicated negligible friction drop. Except for some interruptions (i.e., acid wash and pump malfunctions), pumping was continuous. The injected seawater pumping was continuous. The injected seawater temperature (110 degrees F [43 degrees C]) was approximately 51 degrees F [11 degrees C] lower than reservoir temperature (161 degrees F [72 degrees C]). A comparison of injectivity indices calculated from spinner surveys before test completion with expected injectivity indices obtained from kh values indicated 70% greater-than-expected injectivity for Zone B and 15% smaller-than-expected injectivity for Zone A. These differences led us to suspect the occurrence of a fracture growing predominantly into Zone B and extending into Zone A where the smallest closure stresses were measured. Flow Rate Analysis. The system of concern is a single well with an inner moving seawater bank displacing an outer oil bank. In this system, a radial-flowrate analysis would provide an indication of changes in the kh/mu with changes in the reservoir conditions. JPT P. 1113

High-Speed High-Current Copper Finger Brushes for Pulsed Homopolar Generator Service

ln continuing research into higher energy density, higher current rated pulsed homopolar generators (HPG's), solid copper fingers are explored as sliding electrical contacts to slip speeds of 300 m/s and collector area current densities above 75 MA/m2. Achieving increased compactness in HPG's is easily accomplished by spinning the inertial energy storage flywheel to higher tip speeds but generally involves brush operation at higher Slip speeds. Sintered copper-graphite brushes presently limit slip velocity to 220 m/s and collector area current densities to 7-15 MA/m2, if a reasonable brush wear rate is expected. Development of copper finger brushes began in 1981 by Marshall who performed low speed (15 mm/s) testing at current densities above 450 MA/m2. Based off encouraging results obtained by Marshall, the present investigation focused on characterization of copper finger brushes in an actual HPG operating environment. Experimental data were collected using the Compact HPG System Tester, a working 5-MJ HPG which can deliver current pulses in excess of 1 MA into resistive loads for the evaluation of brush performance. The copper finger brushes tested successfully transferred 1.15 MA at a slip speed of 100 m/s and currents of 600 kA at 180 m/s. Experimental evaluation of effective contact resistance as functions of slip speed, Current transferred, and brush normal force was performed, and the results are discussed. Interfacial heat fluxes due to friction and contact voltage drop for the test conditions were also determined, and then extrapolated to operating levels of 300 m/s and 3 MA. These projections were used to determine the feasibility of rejecting the heat flux through direct injection (transpiration) cooling methods. Although problems with coolant delivery and hydrodynamic forces do exist, the approach holds promise in the continuing effort to extend pulsed HPG brush operating limits.

Temperature Effects on Drip Line Hydraulics

ABSTRACT SEVERAL theoretical equations were drived to describe the effect of temperature changes on the hydraulics of drip irrigation lateral lines. When the total discharge in a lateral line remains constant the temperature effect on total friction drop at the end of the line can reach as much as 10% for a temperature difference from 50 C to +50 C compared with a base condition, T = 20 C and no temperature difference along the length. The temperature effect on the shape of energy gradient was found to be insignificant. A temperature difference ranging from 20 C to 50 C along the line causes approximately 1 to 2% (of the total friction AHL) difference at the middle section of the lateral line.

Studies on friction materials of automobile. 1st report Squeal of disc brake pad.

A device in which the sliding interface between a friction material and a brake disc can be observed directly, was developed to investigate the phenomenon of brake squeal with reference to the composition and method of manufacturing the disc brake pad. The results of investigation show that, in a disc brake pad which is liable to squeal, scales of thermally-decomposed material are produced easily on the frictional surface at a low temperature and with a small compression force. It was also ascertained that, when the quantity of inorganic substances in a brake pad is increased, the brake pad becomes liable to squeal; and that, when the void content of the brake pad is increased, the rate of generation of thermally-decomposed material on its frictional surface drops and the brake pad is very unlikely to squeal. These results prove that the degree of brake squeal is governed by the rate of deposition of thermally-decomposed materials on the frictional surface.

Computation of transonic flow around airfoils with trailing edge andshock/boundary layer interactions

A viscous/inviscid interaction procedure is developed for computing steady transonic flows over single air foils It combines the method used in the GRUMFOIL code with a solution for transonic shock/boundary layer interactions For airfoil shock locations around midchord it was found that marching under the shock using boundary layer theory gives results which are similar to those obtained with a detailed interaction theory except for significant changes in the prediction of displacement thickness rise and skin friction drop downstream of the shock However the two procedures give significantly different results when the shock position is rearward (70% chord or more) Then the shock/boundary layer interaction module alters the flow with consequent global effects on shock location and lift

Design of drip irrigation lateral length on uniform slopes

When a lateral line is designed for a uniform slope situation, a dimensionless design parameter ΔH'/ΔH (a ratio of total energy gain by downslope ΔH' to friction drop ΔH at the end of line) can be used to determine the pressure profile and maximum pressure difference Δh,max along the line. Graphical solutions and mathematical equations were derived for determining the maximum pressure difference, Δhmax. A design length can be determined directly by using the developed designs charts. The developed design procedure can also be used for upslope (uniform) situations.

Sprinkler irrigation design for uniformity on slopes

ABSTRACT THE energy gradient line of a sprinkler irrigation lateral (or submain) can be expressed by a dimensionless energy gradient curve. The pressure variation along a lateral (or submain) can be determined by combining the friction drop and energy gain (or loss) by slopes. A design chart has been developed for uniform down slope situations. A simple table is presented for non-uniform slope situation calculations. The primary object is to include slope effects in the design of sprinkler systems.

Drip Irrigation Lateral Line Network Design

ABSTRACT Adimensionless design parameter, AH' /AH, the ratio of the total energy gain by slope to the friction drop at the end of a single inlet lateral line, is used not only to classify the shape of the pressure profile of a single inlet system but also for designing the double-inlet and the inflow-outflow systems. A set of simple design charts is developed for determining the inlet discharges and the pressure difference along the lateral line for both systems when the dimensionless design parameter AH'/AH is known. This can be expanded to the multiple-inlet system of lateral line design. The drip irrigation lateral line network system using this information can also be designed.

Influence of wall waviness on friction and pressure drop in channels

An attention has been given to investigate the flow behavior of an incompressible viscous fluid confined in horizontal wavy channels and set in motion due to the movement of the upper wall and the pressure differences. The governing equations have been solved analytically as well as numerically subject to the relevant boundary conditions by assuming that the solution consists of two parts: a mean part and a disturbance or perturbed part. For small and moderate Reynolds numbers, the analytical solution for the perturbed part has been found to be in good agreement with the numerical one. The effects of Reynolds number, the pressure gradient parameter, and the undulation wavenumber on friction and pressure drop are found to be quite significant. In addition to the flow behavior for both long and short waves and for large Reynolds numbers, the effect of the wall waviness on friction and pressure drop has been examined for any arbitrary amplitude of the wavy wall.

Drip Irrigation Design Equations

In a drip irrigation system, a major design is the minimization of the discharge (or emitter flow) variation along a drip irrigation line, either a lateral or a submain. When the kinetic energy is considered to be small and neglected in a drip irrigation line, the pressure variation will be simply a linear combination of the friction drop and energy gain or loss due to slopes. Five pressure profiles were presented and considered. Procedures were developed to identify pressure profiles by landslope and total friction drop at the end of the line. Equations for designing lateral length (or submain) based on a given criterion were derived for all five pressure profiles. These equations cannot be solved directly, but solutions can be obtained using a trial and error technique on a pocket calculator or by using Newton's method of approximation in a computer program. The developed mathematical equations will be useful in the future development of computerized drip irrigation system design.

Hydraulics of Microtube Emitters

Laboratory experiments show that the Darcy-Weisbach equation for hydraulically smooth pipe can be used to determine friction drop for microtubes with a diameter ranging from 0.8 mm to 4 mm. It can be concluded that the smooth pipe equation (using Blasius friction equation) can be used with reasonable accuracy for all sizes of drip irrigation tubes in turbulent flow conditions. A computer numerical solution was applied to separate minor loss from the total pressure drop; a minor loss equation was developed for microtube for combined flow, transition zone and turbulent conditions. The minor loss for laminar flow condition is small and can be neglected. For flow in the transition zone and turbulent flow the minor loss coefficient was determined to be 2.32 and 1.69 respectively. A better correlation was obtained between the friction coefficient f and Reynolds number when the minor loss was separated. It shows the microtube is slightly smoother than the hydraulically smooth tube as specified in the Moody diagram.

Effect of aqueous solutions of cuprammonium complexes on the machinability of brass

Thermodynamic analysis of the conditions under which the environment has a physicochemical effect on the mechanical properties of solids (the Rebinder effect) gives grounds for the view that fracture in solids may be facilitated by irreversible chemical reactions taking place on their surfaces [1, 2] as well as by the action of related (similar in molecular nature) liquid media. This permits examination from a common viewpoint of an extensive series of processes in which the environment affects the mechanical properties of solids: from purely physical adsorption to purely chemical reaction, for example, during the stresscorrosion cracking of metals and alloys. It is an essential condition for these processes that the energy of the solid-environmen t reaction should be commensurate with the surface energy [3], similar to the situation occurring when a solid is in contact with a related environment [4]. The purpose of our work was the experimental study of the effect of a chemically active environment on the mechanical properties of a solid material under conditions as favorable as possible for fracture to be facilitated as a result of surface reactions. Forced localized fracture (scratching with a diamond point) makes it possible to compare the results obtained with the facilitation of metal machining in the presence of metallic fusions previously studied in [5], a classic example of a reduction in strength resulting from a crop in surface energy; the system studied (brass solutions of ammonia and its derivatives) provides an opportunity for comparing the facilitation of fracture discovered with the patterns of stress corrosion. The investigation was carried out with a specially designed pendulum-type scratch sclerometer. The linear dimensions of the scratch (its width for the most part) were recorded at a constant level of dissipated energy (identical initial amplitude of pendulum swing), as was the speed of pendulum oscillation damping, which is proportional to the coefficient of friction (the linear reduction in amplitude with time characteristic of dry friction was established by preliminary experiments). The relative percentage increase in scratch width was taken as the measure of the effect of the environment on strength. During the experiments, the concentration of ammonia and of bivalent copper in the solution was varied over a wide range. The test agent was prepared by mixing solutions of ammonia and of copper salt of identical percentage concentrations in specified proportions. The liquid obtained was applied by pipette to the carefully polished surface of a brass plate on which a diamond cutting tool connected to the pendulum had made a scratch in about 100 consecutive passes; the cutting tool was pressed against the plate by a constant force. The test results showed that solutions of cuprammonium complexes in specified concentrations could greatly facilitate the fracture of alpha brass (Fig. 1). In these circumstances an increase in the coefficient of friction can be detected in this concentration region. Pure ammonia does not facilitate fracture; what is more, at high ammonia concentrations there is a considerable drop in friction and a reduction in the depth of the scratch as a result of tool slippage. It was necessary to study the connection between the data obtained by us and the composition of the solutions used in the experiments, because contradictory opinions are expressed in the literature on stress corrosion of alpha brass in cupramm0nium solutions as to precisely which ammonia complex is responsible for the fracture.