Research Papers: Applications

J. Tribol. 2011;133(3):031101-031101-12. doi:10.1115/1.4003998.

The background of the present study is the rolling contact fatigue (RCF) in a brittle polymer disk. The disk has been tested on a two disk machine, under controlled normal and tangential loads, with no global slip. After several million cycles and under different operating conditions, it has been observed that (1) the tangential load highly influences the RCF phenomenon, (2) a network of regularly spaced cracks appears, and (3) in the driving position, the RCF phenomenon develops faster. To explain these observations, a numerical model based on the finite element method (FEM) has been built: the cracks have been quite simply modeled, stick-slip has been chosen as the friction model, and the disk-on-disk contact has been replaced by a disk-on-plane contact. To study the influence of some of the operating conditions, the design of experiments (DOE) techniques has been used. The statistical postprocessing associated to DOE has confirmed the experimental observations with a good reliability. In addition, with some mechanical considerations, scenarios of what experimentally happens are proposed. The association FEM/DOE is an original and efficient way to explain phenomena in the field of RCF: the accuracy of the FEM coupled with DOE statistical treatments make it possible to have a good predictability despite some uncontrolled parameters.

Commentary by Dr. Valentin Fuster

Research Papers: Coatings & Solid Lubricants

J. Tribol. 2011;133(3):031301-031301-10. doi:10.1115/1.4003997.

Hydrogenated diamondlike carbon (H-DLC) coatings provide excellent wear resistance and low friction for bearing applications. However, the use of such coatings with aqueous lubricants could pose some difficulties due to the hydrophobic nature of the surface. A thrust bearing tribometer was used to compare performance of hydrophilic and hydrophobic surfaces in hydrodynamic lubrication with a mixture of water and glycerol as the lubricant. Hydrophobic surfaces on both runner and bearing were achieved with the deposition of H-DLC films on titanium alloy surfaces. Hydrophilic surfaces were created through modification of H-DLC surface with covalently bonded heparin. Several possible combinations of hydrophobic and hydrophilic surface conditions were used on the bearing and runner surfaces to provide full-wetting, partial-wetting, and half-wetting conditions. The experimental results confirmed that load support is still possible, when the bearing is half-wetted or partially wetted. However, the full-wetted bearing combination (i.e., Reynolds no-slip boundary condition) provided the highest load support. Introduction of slip at the surface resulted in a lower measured torque. Heparin treatment resulted in a lower than expected static friction and friction in full lubrication regime. The durability of coated surfaces was evaluated in a series of start–stop tests and in impact tests. The results confirmed that the coatings are stable and survive the test regiment that exceeded 50 test cycles; whereas the uncoated titanium alloy bearing surfaces were damaged after ten test cycles.

Commentary by Dr. Valentin Fuster
J. Tribol. 2011;133(3):031302-031302-13. doi:10.1115/1.4004103.

Granular flows continue to be a complex problem in nature and industrial sectors where solid particles exhibit solid, liquid, and gaseous behavior, in a manner which is often unpredictable locally or globally. In tribology, they have also been proposed as lubricants because of their liquid-like behavior in sliding contacts and due to their ability to carry loads and accommodate surface velocities. The present work attempts to model a granular Couette flow using a lattice-based cellular automata computational modeling approach. Cellular automata (CA) is a modeling platform for obtaining fast first-order approximations of the properties of many physical systems. The CA framework has the flexibility to employ rule-based mathematics, first-principle physics, or both to rapidly model physical processes, such as granular flows. The model developed in this work incorporates dissipative effects due to friction between particles and between particles and boundaries, in addition to the derivative effects of friction, namely particle spin. This new model also includes a rigorous and physically relevant treatment of boundary–particle interactions. The current work compares this new friction and spin inclusive CA model and the author’s previous frictionless CA model against experimental results for an annular shear cell. The effects of granular collision properties were also examined through parametric studies on particle–particle coefficient of restitution (COR) and coefficient of friction (COF), which is a unique and added capability of the friction inclusive model.

Commentary by Dr. Valentin Fuster

Research Papers: Contact Mechanics

J. Tribol. 2011;133(3):031401-031401-8. doi:10.1115/1.4004104.

A numerical approach for solving the fretting contact on coated or layered materials, with consideration of loading history, is presented in the paper. The fretting problem was solved by using a semi-analytical method (SAM), in which analytical relations between a unit stress and corresponding displacements or stresses were obtained through the use of the Papkovich–Neuber potentials. Conjugate gradient method (CGM) and fast Fourier transform (FFT) technique were employed to increase the solution speed. The algorithm was very effective since the meshes applied to the positions were just in the contact areas of interest, which saves the computing time. The fretting contact of coated materials was studied and the effects of stick-slip behaviors were analyzed. Results show that the coupled effects between the shear tractions and the pressure make the contact behaviors quite different with the solutions from same materials. The solutions depend on the path or history of the loading process when the ball is under dynamic loads, and the contact behaviors rely on the degree of dissimilarity of material properties.

Commentary by Dr. Valentin Fuster
J. Tribol. 2011;133(3):031402-031402-10. doi:10.1115/1.4003766.

Applying the Hertz theory to some non-Hertzian contact problems can produce acceptable results. Nevertheless, including the influence of free surfaces requires numerical methods, many of which are based on the Boussinesq–Cerruti solution. This paper presents a new approach, which is better capable of releasing quarter-space free surfaces from shear and normal internal stresses without engendering any increase in calculation times. The mirrored pressure for shear correction is multiplied by a correction factor (ψ), which accounts for the normal load. The expression ψ is derived from the Hetényi correction process, and the resulting displacements show an enhanced correspondence with validation finite element method models; with an imposed fluctuating pressure, the maximum edge displacement error was −21.90% for a shear load correction (Poisson coefficient ν=0.3), and introducing the ψ factor reduced the deviation to −9.55%, while for ν of 0.15, the maximum error was −11.30%, which was reduced to +0.60% with the ψ factor. This study introduces the factor ψ in a 3D elastic contact algorithm. The resulting calculation scheme is then able to simulate any point or line contact problems. Compared with coincident ends and sharp edge contact validation values, the model shows high conformity levels.

Commentary by Dr. Valentin Fuster
J. Tribol. 2011;133(3):031403-031403-5. doi:10.1115/1.4003859.

Graded materials with high surface hardness and ductile cores are popularly used in high performance bearing applications to resist surface wear and fatigue damage. The gradient in hardness with depth is commonly determined using micro-indentation on the cross section of the material which contains the gradation in microstructure or composition. In the current study, a novel method is proposed to predict the hardness gradient profile using solely surface indentations at a range of loads. The method does not require the graded material to be sectioned, and has practical utility in the surface treatment industry. Two case hardened steels, M-50 NiL and Pyrowear® 675, and a through-hardened M50 steel, are used as model materials to illustrate the concepts. For a material with a decreasing gradient in hardness, higher indent loads result in a lower measured hardness due to the influence of the softer subsurface layers. A power-law model is presented which relates the measured surface indentation hardness under increasing load to the subsurface gradient in hardness. It is shown that the response of the material is not influenced greatly by the absolute surface hardness value, but instead sensitive to the sharpness of the gradient in subsurface hardness beneath the indented region. The proposed approach is not specific to case hardened steels and can be used to determine the subsurface hardness gradient for any plastically graded material (PGM).

Topics: Stress , Steel
Commentary by Dr. Valentin Fuster
J. Tribol. 2011;133(3):031404-031404-9. doi:10.1115/1.4003996.

Contact between a slider and a magnetic recording disk is modeled as transient contact of a sphere on a moving flat. The sphere is assumed to be rigid, and the flat is treated as an elastic-plastic body with isotropic hardening. Heat generation is related to friction at the contact interface. Dimensionless solutions are obtained for maximum temperature rise, maximum contact force, maximum contact area, and maximum penetration as a function of dimensionless vertical initial velocity of the sphere. It is observed that transient thermomechanical contact with elastic-plastic deformation deviates from “classical theories” for dynamic elastic and quasi-static elastic-plastic contacts as the dimensionless vertical initial velocity of the sphere increases. The results are applied to optimize the slider-disk interface in a hard disk drive with respect to slider-disk contacts.

Commentary by Dr. Valentin Fuster
J. Tribol. 2011;133(3):031405-031405-9. doi:10.1115/1.4004346.

Solution of a contact problem for a rough elastic half-plane is considered. Surface roughness is assumed to be small and stochastic. A perturbation solution of the problem for relatively small roughness with singly connected contact region is proposed and is conveniently expressed in terms of Chebyshev polynomials. Mean distribution of pressure and mean size of the contact are obtained analytically. A pitting model for rough surfaces is considered based on a generalization of an earlier proposed contact model with some stochastic parameters. An analytical formula relating subsurface originated fatigue is considered and fatigue life of rough and smooth surfaces is obtained which shows that fatigue life of rough solids is slightly shorter than of the smooth ones. In the general case of a contact region of rough surfaces with multiple connectivity subsurface originated fatigue possesses properties similar to the case of singly connected contact region. Surface roughness may have a significant effect only on surface and near surface originated fatigue such as wear, micropitting, and shallow flaking.

Commentary by Dr. Valentin Fuster
J. Tribol. 2011;133(3):031406-031406-5. doi:10.1115/1.4004343.

A finite element model of a layered hemisphere contacting a rigid flat, which includes the effect of adhesion, is developed. In this analysis elastic-plastic material properties were used for each of the materials comprising the layered hemisphere. The inclusion of the effect of adhesion, which was accomplished with the Lennard-Jones potential, required a special procedure. This configuration is of general theoretical interest in the understanding of adhesion. It has also been suggested as a possible design for a microswitch contact because, with an appropriate choice of metals, it has the potential to achieve low adhesion, low contact resistance, and high durability. The effect of the layer thickness on the adhesive contact was investigated. In particular the influences of layer thickness on the pull-off force, maximum contact radius, and contact resistance were determined. The results are presented as load versus interference and contact radius versus interference for loading and unloading from different values of the maximum interference.

Commentary by Dr. Valentin Fuster
J. Tribol. 2011;133(3):031407-031407-12. doi:10.1115/1.4004302.

The objectives of this paper are to develop a means to estimate the real area of contact in sliding systems using thermal measurements and to provide experimental design guidance for optimal sensor locations. The methods used are a modified cellular automata technique for the direct model and a Levenberg–Marquardt parameter estimation technique to stabilize inverse solutions. The modified cellular automata technique enables each piece of physics to be solved independently over a short time step, thus reducing a complicated model to a sequence of simpler problems. Overall, the method proved successful. The major results indicate that appropriately selected measurement locations can determine the contact distribution accurately. The best measurement location is found to be just downstream of the nominal contact zone in the moving body. This is significant since direct access to the contact zone is usually impossible. Results show that it is best to locate a sensor in the moving body. However, placing the sensor in the static body can also provide a reasonable image of the contact distribution. This is useful because the static body is easier to instrument than a moving body. Finally, the estimation method worked well for the most complex model utilized, even in a suboptimal measurement location

Commentary by Dr. Valentin Fuster
J. Tribol. 2011;133(3):031408-031408-7. doi:10.1115/1.4004338.

A new experimental apparatus is used to measure the static friction between tin surfaces under various loads. After the data is collected it is then compared to an existing theoretical model. The experiment uses the classical physics technique of increasing the incline of a plane and block until the block slides. The angle at the initiation of sliding is used to find the static friction coefficient. The experiment utilizes an automated apparatus to minimize human error. The finite element based statistical rough surface contact model for static friction under full stick by Li, Etsion, and Talke (2010, “Contact Area and Static Friction of Rough Surfaces with High Plasticity Index,” ASME Journal of Tribology, 132 (3), p. 031401) is used to make predictions of the friction coefficient using surface profile data from the experiment. Comparison of the computational and experimental methods shows similar qualitative trends, and even some quantitative agreement. After adjusting the results for the possible effect of the native tin oxide film, the theoretical and experimental results can be brought into reasonable qualitative and quantitative agreement.

Commentary by Dr. Valentin Fuster

Research Papers: Elastohydrodynamic Lubrication

J. Tribol. 2011;133(3):031501-031501-11. doi:10.1115/1.4004105.

The reflection of ultrasound can be used to determine oil film thickness in elastohydrodynamic lubricated (EHL) contacts if the opposing surfaces are fully separated by the liquid layer. The proportion of the wave amplitude reflected depends on the stiffness of the liquid layer, which is a function of its bulk modulus and thickness. However, in many practical applications, boundary or mixed film lubrication is a common occurrence as the nominal thickness of the separating film is of a similar order to the height of the surface asperities. The reflection is then dependent on both the liquid contact and solid contact parts and the total interfacial stiffness is the controlling parameter. In this paper an investigation was carried to study the reflection of ultrasonic waves from the lubricated contact between a sliding steel ball and a flat steel disc when substantial solid contact occurs. To interpret the ultrasonic reflection results, a mixed regime model for a circular point contact was established. The liquid film stiffness was calculated by using a predicted film thickness and a bulk modulus estimated from published rheological models of lubricants under high pressure. Solid contact stiffness was predicted using a statistical rough surface contact model. Under all operating conditions, the prediction of fluid stiffness was found to be much greater than the solid contact stiffness. The total stiffness predicted by the model showed good agreement with experimental measurements for kinematic cases. The model was used to separate the stiffness contributions from the asperity contact part and lubricant layer part from the experimental data. For contact pressures ranging from 0.42 to 0.84 GPa and sliding speed from zero to 2 m/s, the film thickness was found to vary from 0.01 to 0.8 μm, and the proportion of the load supported by asperity contact varied from 50% to 0%.

Commentary by Dr. Valentin Fuster
J. Tribol. 2011;133(3):031502-031502-10. doi:10.1115/1.4004100.

Elastohydrodynamic lubrication (EHL) is one of the most common types of lubrication, which widely exists in many machine elements such as gears, rolling bearings, cams and followers, metal rolling tools, and continuous variable transmissions. These components often transmit substantial power under heavy loading conditions that may possibly induce plastic deformation of contacting surfaces. Moreover, the roughness of machined surfaces is usually of the same order of magnitude as, or greater than, the average EHL film thickness. Consequently, most components operate in mixed lubrication with considerable asperity contacts, which may result in localized pressure peaks much higher than the Hertzian pressure, causing subsurface stress concentrations possibly exceeding the material yield limit. Plastic deformation, therefore, often takes place, which not only permanently changes the surface profiles and contact geometry, but alters material properties through work-hardening as well. Available mixed EHL models, however, do not consider plastic deformation, often yielding unrealistically high pressure spikes and subsurface stresses around asperity contact locations. Recently, a three-dimensional (3D) plasto-elastohydrodynamic lubrication (PEHL) model has been developed for investigating the effects of plastic deformation and material work-hardening on the EHL characteristics and subsurface stress/strain fields. The present paper is a continuation of the previous work done by Ren (2010, “PEHL in point contacts,” ASME J. Tribol., 132 (3), pp. 031501) that focused on model development and validation, as well as investigation of fundamental PEHL mechanisms in smooth surface contacts. This part of the study is mainly on the PEHL behavior involving simple surface irregularities, such as a single asperity or dent, which can be considered as basic elements of more complicated surface roughness. It is found that considerable plastic deformation may occur due to the pressure peaks caused by the surface irregularity, even though sometimes external loading is not heavy and the irregularity is concave. The plastic deformation may significantly affect contact and lubrication characteristics, resulting in considerable reductions in peak pressure and maximum subsurface stresses.

Commentary by Dr. Valentin Fuster
J. Tribol. 2011;133(3):031503-031503-9. doi:10.1115/1.4004312.

The influences of a single bump (dent) or regularly arranged bumps (dents) on the film thickness in impact circular elasthydrodynamic lubrication (EHL) contacts lubricated with a Newtonian lubricant are investigated numerically. It has been found that the deformation of the bump or the dent at the contact center depends mainly on the macroscopic pressure distribution produced between the smooth surfaces by impact. The macroscopic pressure distribution is influenced by the initial impact gap, the loading speed, and the mass of the moving body. The central bump or dent hardly deforms when the initial impact gap or the base radius of the bump or dent is small. When the initial impact gap is large and the radius of the base of the central bump is not so small, the oil is entrapped in the bump, and micro-dimple is formed. The deformation of noncentral bumps or dents is mainly influenced by the film profile under conditions of smooth surfaces.

Commentary by Dr. Valentin Fuster
J. Tribol. 2011;133(3):031504-031504-8. doi:10.1115/1.4004345.

This paper presents a model study of inductive, capacitive, and piezoelectric effects on the accuracy of pressure measurements in an EHD contact. Circuit and mathematical models of a thin-layer sensor and a measurement system were developed. It has been assumed that isolation layers of the sensor, deposited as SiOx (1 ≤ x ≤ 2) layers, have piezoelectric properties. The circuit model of the sensor contains a resistance, an electric capacitance, an inductance of a sensor’s circuit, and an ideal current source representing piezoelectric properties of isolating layers of the sensor. The circuit model of the measurement system forms a full measuring bridge with the thin-layer sensor in one of its branches. A derived equation for output voltage of the measurement bridge was used as a mathematical model of the measurement system. The investigations show that at inappropriate electric parameters of the measurement system, inappropriate shape of the sensor’s transducer and short transition time of the sensor through the contact zone, the capacitive, and piezoelectric effects have a significant impact on the accuracy of pressure measurement in the EHD contact. The transducer with an active part located along its connection edges (asymmetric transducer) and a transducer with the active part located in the middle of connections width (symmetric transducer) was tested. It was shown that in the case of the symmetric transducer, the pressure measurement signal change caused by the capacitive and piezoelectric effects, is much smaller than in the case of the asymmetric transducer.

Commentary by Dr. Valentin Fuster

Research Papers: Friction & Wear

J. Tribol. 2011;133(3):031601-031601-6. doi:10.1115/1.4004301.

Modeling of the complex tribological behavior of the elastomer parts is required when designing sliding seal applications. Friction, wear, and lubrication mechanisms of rubber-like materials differ from those in case of metals, ceramics, and rigid polymers; therefore, their modeling also requires other techniques. Tribological behavior of a sliding seal was investigated both experimentally and numerically. In the experimental setup, the counterpart of the seal was pressed and rubbed against the section of the seal in various lubrication conditions. The worn surface of the seal was inspected using white light profilometry. The test configuration was modeled by FEA. A wear algorithm (based on the linear wear theory) with an attached damage analysis was applied to the frictional contact simulation. The nonlinear and time dependent material behavior of the seal was also taken into account. The results of the tribological simulation (in which the internal friction and the effects of damage by rupture of the rubber material were considered) are in good agreement with the results of the surface inspections done on the worn seal specimens. The presented wear simulation technique of deactivating elements is suitable for modeling wear that is larger than the size of the elements in the FE mesh.

Topics: Wear , Simulation
Commentary by Dr. Valentin Fuster
J. Tribol. 2011;133(3):031602-031602-6. doi:10.1115/1.4004102.

Effects of heat treatments and applied loads on the tribological behavior of Ti-56.5 wt. % Ni and Ti-57.5 wt. % Ni alloys were investigated. Wear tests were performed on a pin-on-disk tribometer under normal loads of 20 N and 60 N at a sliding speed of 0.3 m/s. The results indicated that the alloys aged at 700 °C showed lower hardness comparing to the alloys aged at 400 °C. Under an applied load of 20 N, the samples aged at 700 °C showed better wear behavior in comparison with the samples aged at 400 °C with a higher hardness. This could be attributed to higher toughness of the samples aged at 700 °C. The results also showed that the wear of the samples aged at 400 °C decreased with the increase in normal load. However, there was an increase in the wear of the samples aged at 700 °C with the increase of load. Formation and stability of tribological layers on the contacting surfaces could be the main reason for the reduction of the wear of the samples aged at 400 °C with the increase in normal load.

Commentary by Dr. Valentin Fuster
J. Tribol. 2011;133(3):031603-031603-10. doi:10.1115/1.4004342.

Material loss due to erosion is a serious problem associated with the flow of solid-liquid mixtures. In the present work, erosion wear tests have been carried out in a slurry pot tester for seven different ductile type materials namely aluminum alloy (AA6063), copper, brass, mild steel, AISI 304L stainless steel, AISI 316L stainless steel, and turbine blade grade steel using three different erodents namely, quartz, alumina, and silicon carbide. Experiments have been performed at different orientation angles of target material at the velocities of 3, 6, and 8.33 m/s for solid concentrations of 10%, 20%, and 30% (by weight) and particle sizes of 363, 550, and 655 μm. The contribution of cutting wear in the total wear of ductile material at various orientation angles has been determined. It is observed that the maximum cutting wear angle for the ductile material depends on its hardness and a correlation is developed for its prediction. Also a methodology is proposed for estimation of the total erosion wear rate as a contribution of cutting and deformation wear rates. It is seen that this procedure results in an error of ±18% in estimation of erosion wear rate for the present experimental data.

Commentary by Dr. Valentin Fuster

Research Papers: Hydrodynamic Lubrication

J. Tribol. 2011;133(3):031701-031701-7. doi:10.1115/1.4004303.

The aeration of an oil film flowing between the faces of two closely spaced circular plates (one stationary, and one rotating) is examined experimentally, numerically, and with an improved lubrication model. The gap between the plates is small compared to their radii, making lubrication theory appropriate for modeling the flow. However, standard lubrication boundary conditions suggested by Reynolds (1886, "On the Theory of Lubrication and its Application to Mr. Beauchamp Tower’s Experiments, Including an Experimental Determination of the Viscosity of Olive Oil," Philos. Trans. R. Soc. London, 177 , pp. 157-234) of p = 0 and pn  = 0 (Dirichlet and Neumann conditions on pressure) at the gas-liquid interface do not allow for the inclusion of a contact line model, a phenomenon that is important in the inception of aeration. Hence, the standard theory does not adequately predict the experimentally observed onset of aeration. In the present work, we modify the Neumann boundary condition to include both interfacial tension effects and the dynamics of the interface contact angle. The resulting one-dimensional Cartesian two-phase model is formulated to incorporate the prescribed contact line condition and tracks the interface shape and its motion. This model is then implemented in an axisymmetric, two-dimensional model of the rotating disk flow and used to predict the onset of aeration for varying surface tension and static contact angles. The results of the modified lubrication model are compared with experimental observations and with a numerical computation of the aerating flow using a volume of fluid method.

Commentary by Dr. Valentin Fuster
J. Tribol. 2011;133(3):031702-031702-5. doi:10.1115/1.4004078.

Different averaging techniques have proved to be useful for analyzing the effects of surface roughness in hydrodynamic lubrication. This paper compares two of these averaging techniques, namely the flow factor method by Patir and Cheng (P&C) and homogenization. It has been rigorously proved by many authors that the homogenization method provides a correct solution for arbitrary roughness. In this work it is shown that the two methods coincide if and only if the roughness exhibits certain symmetries. Hence, homogenization is always the preferred method.

Commentary by Dr. Valentin Fuster
J. Tribol. 2011;133(3):031703-031703-9. doi:10.1115/1.4004456.

The present work aims to analytically study the performance of misaligned four-pocket, membrane compensated, hybrid journal bearing system operating with micropolar lubricant. In the present study, the flow characteristic of the lubricating oil containing additives and contaminants has been modeled using Erigen’s micropolar theory. The journal misalignment which may occur as a result of noncentral loading, improper assembly, shaft deflection due to elasticity and thermal distortions, etc. has been accounted for in the present study by defining a pair of misalignment parameters in vertical and horizontal directions (i.e., δ and σ).The modified Reynolds equation governing the flow of micropolar lubricant in the clearance space of a misaligned bearing has been solved using FEM and Newton Raphson method along with the appropriate boundary conditions. The numerically simulated results suggest that the effect of journal misalignment is to cause degradation in bearing performance, whereas the influence of micropolar effect of lubricant is to enhance the bearing performance. Therefore, it is imperative to account for the effect of misalignment and lubricant behavior during the design process in order to generate accurate bearing characteristics data.

Commentary by Dr. Valentin Fuster

Research Papers: Lubricants

J. Tribol. 2011;133(3):031801-031801-6. doi:10.1115/1.4004106.

Recently, research on the application of magnetorheological (MR) fluid in mechanical engineering has been widely expanded, while the performance of MR fluid in tribology has also been investigated. In this study, a modification of MR fluid, which is modified by adding certain additives, is attempted to improve tribological performance. After modification, wear and friction are measured by four-ball wear tester and linear-oscillation (SRV) tester under no magnetic fields. Subsequently, the oxidation induction time (OIT) is evaluated by pressure differential scanning calorimetry (PDSC). Then, to observe the tribological performance under magnetic fields, pin-on-disk test is conducted to confirm the effects of the commercial MR fluid and modified MR fluid on friction and wear in different operating and magnetic conditions. Also, the modification effects on surface roughness are investigated by using a profilometer. Moreover, the microscopic changes of surfaces and MR particles are investigated by using scanning electron microscopy (SEM).

Commentary by Dr. Valentin Fuster
J. Tribol. 2011;133(3):031802-031802-13. doi:10.1115/1.4003860.

In this paper, an asymptotic expansion is used to derive a description of Phan–Tien– Tanner (PTT)/Oldroyd-B flows in the thin film situation without the classical “upper convective maxwell”(UCM) assumption. We begin with a short presentation of the Phan–Thien–Tanner/Oldroyd-B models, which introduce viscoelastic effects in a solute–solvent mixture. The three-dimensional flow is described using five parameters, namely the Deborah number (De) (or the relaxation parameter λ), the viscosity ratio r, the bulk fluid viscosity η, the material slip parameter a related to the “convected derivative” and an elongation number κ. Then we focus on the thin film assumption and the related asymptotic analysis that allows us to derive a reduced model. A perturbation procedure for “not too small” values of κ allows us to obtain further results such as an asymptotic “effective viscosity/ shear rate” law, which appears to be a perturbation of the double Rabinowisch model, whose parameters are completely defined by those of the original three-dimensional model. And last a numerical procedure is proposed based on a penalized Uzawa method, to compute the corresponding solution. This algorithm can also be used for any generalized double Newtonian shear thinning Carreau law.

Commentary by Dr. Valentin Fuster
J. Tribol. 2011;133(3):031803-031803-8. doi:10.1115/1.4004339.

Minimum quantity lubrication (MQL) has been used as an alternative solution for flood cooling as well as dry machining. However, the benefit of MQL is only realized in mild machining conditions as the heat generation during more aggressive machining conditions cannot be effectively eliminated by the small amount of oil mist being applied during MQL process. To extend the applicability of MQL to more aggressive machining conditions, we have developed a potential additive to MQL lubricant. After the preliminary wetting angle measurement of the various lubricants, one commercially available MQL vegetable oil was chosen, which is then mixed in a high-speed mixer with exfoliated nanographene particles. The resulting nanoenhanced MQL lubricant was evaluated for its tribological and machining behaviors together with the suspension stability of the mixture. Friction coefficients of new nanoenhanced MQL oil were also measured in terms of loads, speeds and lubricants. Finally, MQL-ball milling tests with nanographene enhanced lubricant were performed to show a remarkable performance improvement in reducing both central wear and flank wear as well as edge chipping at cutting edge.

Commentary by Dr. Valentin Fuster

Research Papers: Magnetic Storage

J. Tribol. 2011;133(3):031901-031901-9. doi:10.1115/1.4004098.

The transient dynamics of a sub-10 nanometer air-bearing slider (pico glide head) are experimentally investigated using piezoelectric transducer (PZT)signal. Empirical mode decomposition is used to resolve the nonstationary and nonlinear response of the slider under short contacts at high temporal resolution at high frequencies. The results indicate that the empirical mode decomposition can clearly distinguish weak impacts between slider and defects on disk. By using the combination of the power spectrum density of the decomposed signal and Hilbert spectrum, multiple consecutive events during contact are effectively resolved, and the nonstationary and nonlinear spectrum signature of slider contact response are characterized.

Commentary by Dr. Valentin Fuster

Research Papers: Other (Seals, Manufacturing)

J. Tribol. 2011;133(3):032201-032201-7. doi:10.1115/1.4003858.

Metal-based microchannel heat exchangers (MHEs) may offer a solution to thermal management required in a unlubricated bearing with ceramic or coated rolling elements. In this paper, we report results of tribological testing on steel coupons cooled by Cu-based MHEs. Low-profile Cu-based MHEs were fabricated and bonded to steel coupons, whose front faces were made to contact a crowned rotating steel ring, with and without oil lubrication. A series of tribological tests were conducted under different conditions with and without cooling. The tests collectively show that low-profile Cu-based MHEs placed in proximity of a tribological contact can be effective in removing heat generated at the contact interface, decrease the contact interfacial friction, and mitigate the amount of interfacial wear in sliding contacts, with and without oil lubrication. The present results suggest that cooling of tribological contact zones by embedding MHEs in proximity can be beneficial to tribological performance and deserves further investigations for tribological contact applications.

Commentary by Dr. Valentin Fuster

Technical Briefs

J. Tribol. 2011;133(3):034501-034501-5. doi:10.1115/1.4002875.

This chapter deals with a hydrodynamic journal bearing provided with hollow bushings, capable of supporting very high loads, even at low speeds, at which separation between the shaft and the housing’s bushing is induced by a specific self-regulating hydrostatic system. The same amount of lubricant can be used for both hydrostatic and hydrodynamic lubrication, thereby facilitating transition between the two. This hybrid bearing can be used in several applications.

Commentary by Dr. Valentin Fuster
J. Tribol. 2011;133(3):034502-034502-3. doi:10.1115/1.4004304.

The evolution of static friction and tangential stiffness in presliding of an elastic-plastic sphere in contact with a rigid flat, under full stick contact condition, is analyzed. Empirical dimensionless equations are developed for these parameters.

Commentary by Dr. Valentin Fuster
J. Tribol. 2011;133(3):034503-034503-2. doi:10.1115/1.4002334.

The current paper analyses the complexity and precision of FEM calculations in 2 dimensional solid mechanics. The work shows that both grid refinement and domain enlargement are required to increase precision.

Topics: Errors
Commentary by Dr. Valentin Fuster

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