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Research Papers: Applications

J. Tribol. 2017;140(2):021101-021101-10. doi:10.1115/1.4037463.

Effects of flexible body and clearance spherical joint on the dynamic performance of 4-SPS/CU parallel mechanism are analyzed. The flexible moving platform is treated as thin plate based on absolute nodal coordinate formulation (ANCF). In order to formulate the parallel mechanism's constraint equations between the flexible body and the rigid body, the tangent frame is introduced to define the joint coordinate. One of the spherical joints between moving platform and kinematic chains is introduced into clearance. The normal and tangential contact forces are calculated based on Flores contact force model and modified Coulomb friction model. The dynamics model of parallel mechanism with clearance spherical joint and flexible moving platform is formulated based on equation of motion. Simulations show that the dynamic performance of parallel mechanism is not sensitive to the flexible body because of the inherent property of moving platform; however, when the clearance spherical joint is considered into the parallel mechanism with flexible body, the flexible moving platform exhibits cushioning effect to absorb the energy caused by clearance joint.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;140(2):021102-021102-13. doi:10.1115/1.4037464.

Hydraulic cylinders are generally used as power take-off (PTO) mechanisms in wave energy converters (WECs). The dynamic behavior of its PTO force, which integrates friction and pressure forces, is a difficult constraint to include in an analytical or in a numerical model. In this paper, the PTO force characteristics of a hydraulic cylinder are experimentally and numerically investigated under different magnitudes of controlled excitation force. In order to characterize the dynamic behaviors of PTO force, the displacement, acceleration, and pressure in the cylinder chamber for given excitation forces are measured. The pressure force is calculated using the measured value of the pressure, and the friction force is calculated based on the equation of motion using measured values of the pressure, excitation force, and acceleration of the piston. Experimental results show clearly a strong nonlinear force–velocity characteristics, including stochastic and hysteretic behaviors. To model the hysteretic behavior, the modified LuGre model is used for the friction force and a new approach is proposed for the pressure force. To model the stochastic behavior of the friction and pressure forces, the spectral representation method is used. The systematically comparison between measured and simulated results shows that the numerical model captures most of dynamic behaviors of PTO force.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;140(2):021103-021103-10. doi:10.1115/1.4037539.

In a discharge reed valve for compressors, the oil stiction by the oil film between the reed and the valve seat is investigated experimentally, and a simulation model is developed. Through a model experiment, the initial oil film thickness is measured by an interferometry method, and the valve displacement and the bore pressure are measured from the stiction to the valve opening. The opening delay time together with the initial oil film thickness is measured while changing the contact area and the oil species. In the simulation model, the deformation of the reed and the pressure of the oil film as a result of cavitation are coupled. In order to take into account the tensile stress in the oil film, a cavitation model directly simulating the expansion of cavitation bubbles is developed (herein, dynamic cavitation model). In the experiment, a smaller contact area, a larger initial film thickness, and a smaller oil viscosity yield a shorter delay. In the simulation, the dynamic cavitation model is advantageous in representing the experimental delay time. In particular, with respect to the relationship between the initial film thickness and the delay time, the dynamic cavitation model with an initial bubble radius that depends on the oil film thickness yields results similar to the experimental results.

Commentary by Dr. Valentin Fuster

Research Papers: Coatings and Solid Lubricants

J. Tribol. 2017;140(2):021301-021301-11. doi:10.1115/1.4037354.

Strict environmental laws enforced on manufacturing industries resulted in the development of alternative techniques to reduce or eliminate the use of lubricants during sliding contact as well as machining. Tribology plays a very important role for tool life in machining. To improve the life of cutting tool, cutting fluids are used. However, cutting fluids only penetrate into the region of sliding contact. In this study, the effect of surface texturing on plasma nitrided high-speed steel (HSS) pins during dry sliding test is investigated for understanding the performance of textured HSS tools in machining. Microtextures were fabricated using Vickers hardness tester on the surface of HSS pins. Tribological tests of molybdenum disulphide (MoS2) filled as well as unfilled microtextured HSS with area density of textures varying from 2% to 14% were performed with the aid of pin-on-disk tribometer against an abrasive sheet. Friction and wear performance were assessed in terms of the pin surface temperature, coefficient of friction (COF), wear, weight loss of the pin and wear rate. Worn-out test surfaces were observed under scanning electron microscope to understand the wear mechanism. The best results were obtained with MoS2-filled microtextures having 10% texture area density. Tool–chip interface temperature, cutting force, feed force, and centerline average (CLA) surface roughness were also assessed during machining test with 10% area density of textured cutting tools.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;140(2):021302-021302-5. doi:10.1115/1.4037953.

The paper presents the results of an X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) study of chemical and phase composition of the friction surfaces Fe–Mn–C–B–Si–Ni–Cr hardfacing coatings. The alloy was used as a core mixture to produce flux-cored wire of 2.4 mm in diameter. The coating was deposited by gas metal arc welding using CO2 as a shielding gas. The tribological examination was conducted in a ball on disk system with a load of 20 N under dry friction conditions. A XPS were used to examine the structures on the friction surface and depend on depth 5, 10, 15, 20, 50, 100, 200, and 6000 nm. The segregation of C, B, and Si atoms was observed in the process of the friction. The presence of compounds such as oxides (B2O3, SiO2, Cr2O3), carbides (Fe3C, Cr7C3), and borides (FeB, Fe2B) was detected on the surface and in the subsurface layer of the Fe–Mn–C–B–Si–Ni–Cr coating. The formation of these structures increases the wear resistance of composite coatings.

Commentary by Dr. Valentin Fuster

Research Papers: Contact Mechanics

J. Tribol. 2017;140(2):021401-021401-11. doi:10.1115/1.4037359.

The analytical expressions currently available for Hertzian contact stresses are applicable only for homogeneous materials and not for case-hardened bearing steels, which have inhomogeneous microstructure and graded elastic properties in the subsurface region. Therefore, this article attempts to determine subsurface stress fields in ball bearings for graded materials with different ball and raceway geometries in contact. Finite element models were developed to simulate ball-on-raceway elliptical contact and ball-on-plate axisymmetric contact, to study the effects of elastic modulus variation with depth due to case hardening. Ball bearings with low, moderate, and heavy load conditions are considered. The peak contact pressure for case-hardened steel is always more than that of through-hardened steel under identical geometry and loading conditions. Using equivalent contact pressure approach, effective elastic modulus is determined for case-carburized steels, which will enable the use of Hertz equations for different gradations in elastic modulus of raceway material. Nonlinear regression tools are used to predict effective elastic modulus as a weighted sum of surface and core elastic moduli of raceway material and design parameters of ball–raceway contact area. Mesh convergence study and validation of equivalent contact pressure approach are also provided. Implications of subsurface stress variation due to case hardening on bearing fatigue life are discussed.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;140(2):021402-021402-10. doi:10.1115/1.4037356.

Measuring and verification of contact force in a rolling element bearing is a big problem. In this study, a new measuring method for contact force in a large-scale ball bearing is developed. The idea is to measure the deformation under the ball–race contact by displacement sensor at first, and the displacement of the end face of load bearing ring is also measured to determine the contact angle of ball–race contact. Then, the corresponding theory is developed to calculate the contact angle of ball–race contact by the displacement of the end face of load bearing ring. At last, the ball–race contact force is determined by accurately calculating through finite element method (FEM). Results show that the relation between contact force and deformation of measuring surface which is under ball–race contact is linear. The position of ball greatly affects the contact angle of ball–race contact. The contact angle of the ball which is near the arm of force is larger than that of the ball which is far from the arm of force. On the contrary, the measuring deformation of ball–race contact that is near the arm of force is less than that of ball–race contact that is far from the arm of force. The method developed here is only suitable for large-scale rolling element bearing because of the size constraint of the sensor.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;140(2):021403-021403-10. doi:10.1115/1.4037793.

The simulated rough surface with desired parameters is widely used as an input for the numerical simulation of tribological behavior such as the asperity contact, lubrication, and wear. In this study, a simulation method for generating non-Gaussian rough surfaces with desired autocorrelation function (ACF) and spatial statistical parameters, including skewness (Ssk) and kurtosis (Sku), was developed by combining the fast Fourier transform (FFT), translation process theory, and Johnson translator system. The proposed method was verified by several numerical examples and proved to be faster and more accurate than the previous methods used for the simulation of non-Gaussian rough surfaces. It is convenient to simulate the non-Gaussian rough surfaces with various types of ACFs and large autocorrelation lengths. The significance of this study is to provide an efficient and accurate method of non-Gaussian rough surfaces generation to numerically simulate the tribological behavior with desired rough surface parameters.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;140(2):021404-021404-12. doi:10.1115/1.4037954.

A mechanical interface behaves as the stiffness and damping when the interface is bearing a static normal force and a sine normal exciting force. For the interfacial normal damping, a calculating model was proposed. This proposed model studied the lateral contact (shoulder–shoulder contact) between upper and lower asperities in the elastic and elastic-perfectly plastic stages, which is neglected by other classical models. The normal force can be divided into a normal component and a tangential component when two asperities are contacting in dislocation. The relation between the loading–unloading normal component forces and deformation can be calculated, and then the strain energy dissipation between asperities can be gotten by integral. The friction energy dissipation also can be calculated based on the relation between loading–unloading tangential component forces and the slippage. Furthermore, the total interfacial energy dissipation can be obtained according to the statistical theory. Finally, the equivalent viscous damping is estimated using the vibration theory. The proposed model and classical models are compared by simulation and experiment, and it was found that the interfacial damping of the proposed model is more than the damping of the classical models. Moreover, the proposed model is consistent with the experimental results.

Commentary by Dr. Valentin Fuster

Research Papers: Friction and Wear

J. Tribol. 2017;140(2):021601-021601-16. doi:10.1115/1.4037335.

This research work presents a modified mathematical method to estimate the specific wear rate of spur gears for specified service conditions by calculating the coordinates of the point on the involute of gear tooth profile. This work stands apart in a way that an entirely novel manufacturing process developed in-house is used to fabricate functionally graded materials (FGMs) based thermoplastic gears, which have never been explored before, and the specific wear rate of manufactured gears is estimated using the proposed method. FGM and homogeneous gears are manufactured by means of an especially designed mold and a punch. Polyamide 66 (PA66) filled with 15 wt. % and 30 wt. % glass fibers is used to fabricate FGM and homogeneous gears. Neat PA66 gear is also fabricated for comparative study. Gradation in FGM gears is verified by scanning electron microscope (SEM) analysis and hardness measurements. Thermal and wear tests of the gears are conducted over a range of rotational speed (500–1700 rpm) and torque (0.8–3.2 N·m). Thermal and wear behavior of developed gears is successfully analyzed using Taguchi methodology and analysis of variance (ANOVA). The service life of FGM gears is found to be superior as compared to unfilled and homogeneous gear. FGM gear filled with 30 wt. % glass fiber exhibited minimum gear tooth surface temperature and specific wear rate among all the fabricated gears.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;140(2):021602-021602-5. doi:10.1115/1.4037465.

For the first time, formation of cubic boron nitride (c-BN), besides Fe2B and iron nitrides, was detected in boronitride coating, which was applied via a duplex diffusional surface treatment. Boronitride coating was applied on AISI 1045 steel substrates via duplex treatment of pack boriding and plasma nitriding (B-PN). Scanning electron microscope (SEM), X-ray diffractometer (XRD), and pin-on-disk wear test were utilized to evaluate microstructures, phases, and wear properties of the coatings. The surface of the as-coated sample exhibited a pebblelike and compact structure, and the cross-sectional morphology of the coatings showed a saw-tooth interface with substrate. Duplex treated sample revealed superior wear resistance in comparison with the borided one.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;140(2):021603-021603-9. doi:10.1115/1.4037697.

AA2218–Al2O3(TiO2) composites are synthesized by stirring 2, 5, and 7 wt % of 1:2 mixture of Al2O3:TiO2 powders in molten AA2218 alloy. T61 heat-treated composites characterized for microstructure and hardness. Dry sliding wear tests conducted on pin-on-disk setup at available loads 4.91–13.24 N, sliding speed of 1.26 m/s up to sliding distance of 3770 m. Stir cast AA2218 alloy (unreinforced, 0 wt % composite) wears quickly by adhesion, following Archard's law. Aged alloy exhibits lesser wear rate than unaged (solutionized). Mathematical relationship between wear rate and load proposed for solutionized and peak aged alloy. Volume loss in wear increases linearly with sliding distance but drops with the increase in particle wt % at a given load, attributed to the increase in hardness due to matrix reinforcement. Minimum wear rate is recorded in 5 wt % composite due to increased particles retention, lesser porosity, and uniform particle distribution. In composites, wear phenomenon is complex, combination of adhesive and abrasive wear which includes the effect of shear rate, due to sliding action in composite, and abrasive effect (three body wear) of particles. General mathematical relationship for wear rate of T61 aged composite as a function of particle wt % load is suggested. Fe content on worn surface increases with the increase in particle content and counterface temperature increases with the increase in load. Coefficient of friction decreases with particle addition but increases in 7 wt % composite due to change in microstructure.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;140(2):021604-021604-10. doi:10.1115/1.4037728.

Three-dimensional (3D) woven fabrics have been considered by biomedical researchers to be used as load-bearing surfaces in joint and ligament replacements. In this regard, wear is a crucial phenomenon that determines material failure as well as biological response of body to wear debris. The current study evaluates various microscale screening methods with the aid of atomic force microscopy (AFM) for biocompatible polymer fibers that are used in 3D woven fabrics. Fibers in mono- and multi-filament forms were subjected to indentation, scratching, and line wear testing in dry and soaked conditions, and the effect of key parameters such as applied normal load, sliding velocity, and number of wear cycles was investigated. The area of worn material was determined by geometric approximation superimposed on the measured residual scratch of line wear. Moisture was found to lower the indentation hardness of some fibers while increasing the hardness of others. Line wear results clearly suggest ultrahigh molecular weight polyethylene (UHMWPE) to be the primary material for further investigation and that monofilament fibers should be avoided.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;140(2):021605-021605-12. doi:10.1115/1.4037729.

The friction and wear properties of in situ Al-matrix composites prepared by selective laser melting (SLM) were evaluated on a ball-on-disk tribometer by sliding against GCr15 steel at room temperature. The influence of the applied load, sliding speed, and long-time continuous friction on the friction and wear properties of Al-matrix composites was systematically investigated. It showed that the wear rate and coefficient of friction (COF) increased when the applied load increased, due to the higher contact stress and larger extent of particle fracturing. As the sliding speed increased, the elevated rate of the formation of Al-oxide layer and the transfer of Fe-oxide layer from the counterface to the worn surface led to a significant reduction in wear rate and COF. As the sliding distance prolonged, the worn surface successively experienced the adhesive wear, the abrasive wear, the particle fracturing and crack nucleation, and the delaminated wear. The above processes were repeated on each exposed fresh surface, resulting in the fluctuation of COF. In the later stage of wear process, a large amount of oxides were produced on the worn surface, caused by the long-time accumulated frictional heat, which reduced the fluctuation of COF. The wear mechanisms of SLM-processed Al-matrix composite parts under various loads were dominated by abrasive wear and oxidation wear, whereas the predominant wear mechanisms were oxidation wear and delamination wear at different sliding speeds. For the long-time friction, all of these wear mechanisms were operational.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;140(2):021606-021606-7. doi:10.1115/1.4037767.

A functionally graded Cu–10Sn–5Ni metal matrix composite (MMC) reinforced with 10 wt % of Al2O3 particles was fabricated using the centrifugal casting process with dimension Φout100 × Φin85 × 100 mm. The mechanical and wear resistance of the composite has been enhanced through heat treatment. Samples from of the inner zone (9–15 mm) were considered for heat treatment, as this zone has higher concentration of less dense hard reinforcement particles. The samples were solutionized (620 °C/60 min) and water quenched followed by aging at different temperatures (400, 450, and 550 °C) and time (1–3 h). Optimum parametric combination (450 °C, 3 h) with maximum hardness (269 HV) was considered for further analysis. Dry sliding wear experiments were conducted based on Taguchi's L27 array using parameters such as applied loads (10, 20, and 30 N), sliding distances (500, 1000, and 1500 m), and sliding velocities (1, 2, and 3 m/s). Results revealed that the wear rate increased with load and distance whereas it decreased initially and then increased with velocity. Optimum condition for maximum wear resistance was determined using signal-to-noise (S/N) ratio. Analysis of variance (ANOVA) predicted the major influential parameter as load, followed by velocity and distance. Scanning electron microscope (SEM) analysis of worn surfaces predicted the wear mechanism, observing more delamination due to increase in contact patch when applied load increased. Results infer 8% increase in hardness after heat treatment, making it suitable for load bearing applications.

Commentary by Dr. Valentin Fuster

Research Papers: Hydrodynamic Lubrication

J. Tribol. 2017;140(2):021701-021701-12. doi:10.1115/1.4037699.

This paper presents analytical bifurcations analysis of a “Jeffcott” type rigid rotor supported by five-pad tilting pad journal bearings (TPJBs). Numerical techniques such as nonautonomous shooting/arc-length continuation, Floquet theory, and Lyapunov exponents are employed along with direct numerical integration (NI) to analyze nonlinear characteristics of the TPJB-rotor system. A rocker pivot type five-pad TPJB is modeled with finite elements to evaluate the fluid pressure distribution on the pads, and the integrated fluid reaction force and moment are utilized to determine coexistent periodic solutions and bifurcations scenarios. The numerical shooting/continuation algorithms demand significant computational workload when applied to a rotor supported by a finite element bearing model. This bearing model may be significantly more accurate than the simplified infinitely short-/long-bearing approximations. Consequently, the use of efficient computation techniques such as deflation and parallel computing methods is applied to reduce the execution time. Loci of bifurcations of the TPJB-rigid rotor are determined with extensive numerical simulations with respect to both rotor spin speed and unbalance force magnitude. The results show that heavily loaded bearings and/or high unbalance force may induce consecutive transference of response in forms of synchronous to subsynchronous, quasi-periodic responses, and chaotic motions. It is revealed that the coexistent responses and their solution manifolds are obtainable and stretch out with selections of pad preload, pivot offset, and lubricant viscosity so that the periodic doubling bifurcations, saddle node bifurcations, and corresponding local stability are reliably determined by searching parameter sets. In case the system undergoes an aperiodic state, the rate of divergence/convergence of the attractor is examined quantitatively by using the maximum Lyapunov exponent (MLE).

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;140(2):021702-021702-10. doi:10.1115/1.4037355.

This paper presents a general formulation of the Reynolds equation for gas and liquid lubricants, including cavitation. A finite element solution of this equation is also given. The model is compared to those obtained in the previous literature on liquid and gas lubrication. One of the advantages of the model is the continuous description of cavitation in liquid lubrication. It is possible to deal with all lubricants by adjusting the amount of gas in the fluid.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;140(2):021703-021703-11. doi:10.1115/1.4037360.

Different profile structures were designed for a high-power engine piston, and engine tests were carried out to analyze and compare the influences of the widest point position and contraction rate on the skirt wear property. The results show that the lower position of the widest point will cause poor guidance, and at the same time the rapid radial reduction in both the upper and lower parts will increase the swing angles and the kinetic energy; the uniformity of wear loads can be improved effectively by increasing the height of the widest point and the width of the maximum diameter region; the degree of wear of the skirt can be considered through a comparison of the outer diameter variation.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;140(2):021704-021704-15. doi:10.1115/1.4037730.

To determine the bifurcation types in a rotor-bearing system, it is required to find higher order derivatives of the bearing forces with respect to journal velocity and position. As closed-form expressions for journal bearing force are not generally available, Hopf bifurcation studies of rotor-bearing systems have been limited to simple geometries and cavitation models. To solve this problem, an alternative nonlinear coefficient-based method for representing the bearing force is presented in this study. A flexible rotor-bearing system is presented for which bearing force is modeled with linear and nonlinear dynamic coefficients. The proposed nonlinear coefficient-based model was found to be successful in predicting the bifurcation types of the system as well as predicting the system dynamics and trajectories at spin speeds below and above the threshold speed of instability.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;140(2):021705-021705-19. doi:10.1115/1.4037731.

The nonlinear stability of a flexible rotor-bearing system supported on finite length journal bearings is addressed. A perturbation method of the Reynolds lubrication equation is presented to calculate the bearing nonlinear dynamic coefficients, a treatment that is suitable to any bearing geometry. A mathematical model, nonlinear coefficient-based model, is proposed for the flexible rotor-bearing system for which the journal forces are represented through linear and nonlinear dynamic coefficients. The proposed model is then used for nonlinear stability analysis in the system. A shooting method is implemented to find the periodic solutions due to Hopf bifurcations. Monodromy matrix associated to the periodic solution is found at any operating point as a by-product of the shooting method. The eigenvalue analysis of the Monodromy matrix is then carried out to assess the bifurcation types and directions due to Hopf bifurcation in the system for speeds beyond the threshold speed of instability. Results show that models with finite coefficients have remarkably better agreement with experiments in identifying the boundary between bifurcation regions. Unbalance trajectories of the nonlinear system are shown to be capable of capturing sub- and super-harmonics which are absent in the linear model trajectories.

Commentary by Dr. Valentin Fuster

Research Papers: Magnetic Storage

J. Tribol. 2017;140(2):021901-021901-9. doi:10.1115/1.4037847.

Passive magnetic bearing's (PMB) adaptability for both lower and higher speed applications demands detailed and critical analysis of design, performance optimization, and manufacturability. Optimization techniques for stacked PMB published in recent past are less accurate with respect to complete optimum solution. In this context, the present work deals with a pragmatic optimization of axially stacked PMBs for the maximum radial load using three-dimensional (3D) equations. Optimization for three different PMB configurations, monolithic, conventional, and rotational magnetized direction (RMD), is presented based on the constraints, constants, and bounds of the dimensions obtained from published literature. Further, to assist the designers, equations to estimate the mean radius and clearance being crucial parameters are provided for the given axial length and outer radius of the stator with the objective of achieving maximum load-carrying capacity. A comparison of the load-carrying capacity of conventional stacked PMB using the proposed equation with the equation provided in literature is compared. Finally, effectiveness of the proposed pragmatic optimization technique is demonstrated by analyzing three examples with reference to available literature.

Commentary by Dr. Valentin Fuster

Research Papers: Micro-Nano Tribology

J. Tribol. 2017;140(2):022001-022001-7. doi:10.1115/1.4037698.

In this study, the effects of addition of boron nitride nanoplatelets (BNNPs) upon friction and wear behavior of epoxy resin have been investigated by using pin-on-disk test. It has been reported in the literature that certain amounts of BNNP addition can be useful for enhancement of mechanical properties. Therefore, it is very important to obtain the effect of such addition upon friction and wear performance of epoxy resin. BNNPs have been incorporated at 0.3–0.5–0.7–1 wt %. It is shown that BNNP addition results in decrease in friction coefficient and wear. It is also shown that the best results are obtained with 0.5% nanoplatelet addition. It is also observed that heat conduction of epoxy resin is enhanced by the nanoplatelet addition.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;140(2):022002-022002-6. doi:10.1115/1.4037357.

In the present work, composites were developed with novel combination of particular fillers and fibers for an automotive brake system. The influence of short carbon fiber (SCF) on wear rate, coefficient of friction (CoF), modulus, compressive strength, hardness, and surface morphology of worn surface were examined. This investigation confirmed that 0.1% multiwalled carbon nanotubes (MWCNTs) reduced wear rate, CoF for all combinations of composite with carbon fiber. Results indicate that 0.1% (MWCNTs) and 10% SCF-filled composite had superior properties. This performance may be attributed to the uniform dispersion of fiber and the synergistic effect of SCF and MWCNTs, acting in concert that formed a more stable structure resulting in a high strength, stiffness, tougher, and high-heat absorption. Scanning electron microscopy (SEM) microstructure subsequently performed show change in structural development with a corresponding increase of the incorporation of SCF and MWCNTs, which eventually explained the improved properties of composite.

Commentary by Dr. Valentin Fuster

Research Papers: Other (Seals, Manufacturing)

J. Tribol. 2017;140(2):022201-022201-8. doi:10.1115/1.4037361.

A magnetorheological fluid (MRF) is one of many smart materials that can be changed their rheological properties. The stiffness and damping characteristics of MRF can be changed when a magnetic field is applied. This technology has been successfully employed in various low and high volume applications, such as dampers, clutches, and active bearings, which are already in the market or are approaching production. As a result, the sealing performance of MRF has become increasingly important. In this study, the wear properties of seals with MRFs were evaluated by a rotary-type lip seal wear tester. The test was performed with and without a magnetic field. The leakage time was monitored during the tests in typical engine oil conditions. The results showed that the wear resistance of the seal with MRF was decreased under the magnetic field.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Tribol. 2017;140(2):024501-024501-4. doi:10.1115/1.4037358.

This paper presents research activities regarding the systematic error of the pressure measurement film when measuring the area of the wheel–rail contact. In particular, an explanation for the different error values shown by the different film types was sought. A finite element model was created based on the assumption that not only the film, but also the microcapsules on top of it alter the results. The performance of the existing film models was enhanced by defining microcapsules with element failure and deletion behaviors. The new model was capable of reproducing the trend shown by the systematic error in the experiments. The simulation results confirmed that the measurement error of a certain film type is not only caused by the film itself, but also depends on the failure pressure and especially the diameter of the capsules.

Commentary by Dr. Valentin Fuster

Discussion

J. Tribol. 2017;140(2):025501-025501-1. doi:10.1115/1.4037466.

The theoretical premise of the discussed work [1] is generalization of “stationary nonequilibrium thermodynamic states” to tribological systems. Such a state characterizes a “discontinuous” thermodynamic system with minimum entropy production. It is satisfied, mathematically at least, when the phenomenological thermodynamic coefficients being “identically” constant [2]. The discusser argues that for such a condition to take place, within the context of the tribosystem considered, the thermal storage term expressed in Eq. (6) of the discussed paper has to be stated in terms of the constant volume specific heat Cv, (instead of the specific heat at constant pressure Cp used). Thus, the first entropy contribution is rewritten as

$Sfo 1=(1Tflash−1Tbulk) Qo = mo ∫TflashTbulkCvT dT$

Commentary by Dr. Valentin Fuster

Closure

J. Tribol. 2017;140(2):026001-026001-1. doi:10.1115/1.4037467.

We thank Dr. Abdel-aal for his comments and discussion. First, we would like to emphasize that the main purpose of our work was to include abrasion, besides the typically studied dissipation mechanisms in sliding pairs, i.e., thermal gradient and heat conduction. A second premise was to consider the friction and wear processes from the point of view of the systems theory. To do so, we defined a control volume corresponding to a severely deformed portion of material, or as Dr. Abdel-aal refers to, the mechanically affected zone. Such volume of material is exposed to an amount of thermal energy sufficient to initiate tribofailure, so this energy level is directly associated with the systems damage. The control volume is, as a whole, affected by the thermal gradient, however, only a small part of it contains a density of energy high enough to cause failure, so we are assuming that the majority of the volume will experience plastic deformation instead. In other words, in our model the control volume (or the mechanically affected zone) is larger than the volume removed by wear processes [1].

Topics: Wear , Entropy
Commentary by Dr. Valentin Fuster

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