Research Papers: Applications

J. Tribol. 2014;136(2):021101-021101-11. doi:10.1115/1.4026078.

Diamondlike-carbon (DLC) coatings have received a lot of research attention by physicists and engineers, especially in the past 25 years. Attempts to use such materials in tribological applications have achieved variable success. The rationale for this work was to investigate the wear durability of three types of DLC coatings applied to hardened and tempered bearing steel and subject them to realistic high pressure cyclic loading under oil lubricated conditions for long duration. A thrust bearing design was deployed for this purpose. The wear and friction behavior of the DLC coated materials relative to uncoated materials was compared when using base (additive free) oils and typical autoengine formulated oils. The type of oil used made no difference to the dynamic friction and oil temperature for all the material and oil combinations used. Durability of the coated and uncoated roller bearings was determined by the type of material. For the uncoated bearings, life was limited after very many test cycles (approaching a billion) via classical rolling contact fatigue pitting. For all the DLC coated rollers life was governed by wear of their coatings. In the case of the tungsten doped DLCs (a-C:H:W), these were worn progressively and uniformly via microabrasion, whereas the nondoped ta-C and a-C DLC coatings were principally worn via delamination and tearing. The latter effect was relatively rapid and was considered to be initiated by blistering of the coating, a process that was probably driven by the high elastic energy/internal stress within the nondoped coating materials. The durability to delamination and tearing of the ta-C coatings was slightly lowered in formulated oil compared to tests made in base oil. Overall, for the test conditions used here, there was no apparent benefit in using DLC coatings.

Commentary by Dr. Valentin Fuster
J. Tribol. 2014;136(2):021102-021102-5. doi:10.1115/1.4026348.

The formation of a ring area around the cavitation erosion pit on carbon steel is due to the cavitation erosion–corrosion. For the ultrasonic irradiation in water, both the ultrasonic cavitation and the acoustic streaming are generated. Due to the oscillation of the horn, the acoustic streaming flows from the tip of the horn to the bulk water. So far, the acoustic streaming has not been considered in previous studies on the ultrasonic cavitation erosion–corrosion. This study first reveals that the acoustic streaming noticeably affects the evolution of the nascent ring area. The mean velocity of acoustic streaming is inversely proportional to the gap (H) between the tip of the horn and the surface of the specimen. When H is 65 mm and 40 mm, the corresponding ring area is mainly composed of vertical sandwich sheets. In contrast, when H is 17 mm, the corresponding ring area is mainly composed of horizontal thin sheets. This study provides a complete picture to understand the ultrasonic cavitation erosion–corrosion on carbon steel.

Commentary by Dr. Valentin Fuster
J. Tribol. 2014;136(2):021103-021103-7. doi:10.1115/1.4026442.

Extraction and testing of miniature compression specimens from localized regions of components affected by rolling contact fatigue loading can provide significant insight into material degradation. Current ASTM standards for compression testing of cylindrical specimens become too stringent and difficult to achieve when specimen size is reduced to around 1 mm in diameter. The tolerances for surface flatness, parallelism of the loading surfaces, and the perpendicularity between the axis and the loading surfaces play crucial roles in the resulting stress-strain curves under uniaxial compression loading. In this manuscript, a systematic study is performed to quantify the influence of the above geometric parameters on the stress-strain response. Based on the analysis, the allowable geometric tolerances of miniature cylindrical specimens for a valid compression tests are recommended. The analysis results are validated and the usefulness of the method is demonstrated on miniature specimens extracted from the rolling contact fatigue affected regions of high strength M50 bearing balls. The yield stress within the rolling contact fatigue affected region is shown to increase by over 12%.

Commentary by Dr. Valentin Fuster
J. Tribol. 2014;136(2):021104-021104-9. doi:10.1115/1.4026062.

The present paper proposes to analyze relations between the behavior of two bodies in contact (local stress and vibration modes) and the rheology of third-body particles. Experiments are performed on a system composed of a polycarbonate disk in contact with a steel cylinder, where birefringent property of polycarbonate allows us to observe shear-stress isovalues. Multiscale numerical simulations involve the coupling between finite elements and discrete elements to model simultaneously nonhomogeneous third-body flows within a confined contact and dynamical behavior of the bodies in contact. Comparisons between experiments and simulations are performed on the dynamic response of the system, the stress distribution, as well as the evolution of third-body particles within the contact. Such comparisons exhibit not only qualitative results but also quantitative ones and suggest a new approach to study in deeper third-body rheology.

Commentary by Dr. Valentin Fuster
J. Tribol. 2014;136(2):021105-021105-11. doi:10.1115/1.4026502.

A new fluid dynamics model is proposed to predict the power losses due to pocketing of air, oil, or an air-oil mixture in the helical gear meshes. The proposed computational procedure treats a helical gear pair as a combination of a number of narrow face width spur gear segments staggered according to the helix angle and forms a discrete fluid dynamics model of the medium being pocketed in the gear mesh. Continuity and conservation of momentum equations are applied to each coupled control volume filled with a compressible fluid mixture to predict fluid pressure and velocity distributions from which the instantaneous pocketing power loss is calculated. The proposed model is exercised in order to investigate the fluid pressure and velocity distributions in time along with the pocketing power loss as a function of the speed, helix angle, and oil-to-air ratio.

Commentary by Dr. Valentin Fuster

Research Papers: Elastohydrodynamic Lubrication

J. Tribol. 2013;136(2):021501-021501-10. doi:10.1115/1.4025939.

Solid materials forming the boundaries of a lubrication interface may be elastoplastic, heat treated, coated with multilayers, or functionally graded. They may also be composites reinforced by particles or have impurities and defects. Presented in this paper is a model for elastohydrodynamic lubrication interfaces formed with these realistic materials. This model considers the surface deformation and subsurface stresses influenced by material inhomogeneities, where the inhomogeneities are replaced by inclusions with properly determined eigenstrains by means of the equivalent inclusion method. The surface displacement or deformation caused by inhomogeneities is introduced to the film thickness equation. The stresses are the sum of those caused by the fluid pressure and the eigenstrains. The lubrication of a material with a single inhomogeneity, multiple inhomogeneities, and functionally graded coatings are analyzed to reveal the influence of inhomogeneities on film thickness, pressure distribution, and subsurface stresses.

Commentary by Dr. Valentin Fuster
J. Tribol. 2014;136(2):021502-021502-9. doi:10.1115/1.4026073.

A thermal elastohydrodynamic lubrication (TEHL) finite line contact model is developed for a helical gear pair lubricated with an Eyring fluid or a power-law fluid in order to investigate the effects of the working conditions. A lubrication analysis within a meshing period shows that the differences between the Eyring and Newtonian solutions mainly lie in the film temperature and the shear stress. For the power-law fluid, the power index n has a significant effect on the film thickness. The effects of load and speed on lubrication performance along the line of action are discussed.

Commentary by Dr. Valentin Fuster
J. Tribol. 2014;136(2):021503-021503-8. doi:10.1115/1.4026111.

This paper addresses a largely ignored aspect pertaining to the elastohydrodynamic lubrication (EHL) traction behavior of fragile lubricants which undergo transition to glassy state at typical EHL contact zone pressures. For such lubricants, a conventional EHL model predicts extremely high and unrealistic values of traction coefficient, especially under near pure rolling conditions where thermal effect is negligible. Therefore, an EHL model incorporating the effect of limiting shear stress and the associated wall slip phenomenon is presented herein. Unlike the other such investigations involving limiting shear stress behavior, the present model employs Carreau-type power-law based models to describe the rheology of lubricants below the limiting shear stress along with realistic pressure-viscosity relationships (WLF and Doolittle-Tait). The use of Carreau-type shear-thinning model in this analysis allows the simultaneous prediction of minimum film thickness and traction coefficient for lubricants which shear-thin in the inlet zone and exhibit limiting shear stress behavior in the contact zone, a feature absent in the existing EHL models utilizing ideal visco-plastic or some other unrealistic rheological model. Using published experimental data pertaining to the shear-thinning and pressure-viscosity response of two fragile lubricants (L100 and LVI260), it has been demonstrated that the present model can explain the appearance of plateau in the experimental traction curve. Also, the influence of shear-thinning parameters and the pressure-viscosity coefficient on the predicted limiting shear stress zone has been studied.

Commentary by Dr. Valentin Fuster
J. Tribol. 2014;136(2):021504-021504-13. doi:10.1115/1.4026347.

An integrated model is proposed for involute gear pair combining the mixed elastodhydrodynamic lubrication (EHL) theory for finite line contact with surface temperature rise equations considering tribo-dynamic loading behaviors. The film stiffness and viscous damping as well as the friction force are taken into account. The surface topography of tooth flank measured by 3D surface profiler is also included to solve the local temperature and pressure distribution in the contact area. The results show that the temperature distributions in different meshing positions along the line of action exhibit dissimilar characteristics due to the varying of dynamic load and the changing slip-to-roll ratio, which denotes the relationship between sliding velocity and rolling velocity on the tooth flank. Besides, the maximum of temperature is likely to appear at different sides of the gear tooth width as the gear pair meshes along the line of action. Moreover, with the increasing surface roughness, the ratio of asperity contacts becomes larger, so more heat generates from the contact area and leads to higher temperature rise.

Commentary by Dr. Valentin Fuster
J. Tribol. 2014;136(2):021505-021505-6. doi:10.1115/1.4026591.

Shear-dependent viscosities have been measured over a range of temperature and pressure for seven engine oils blended to have kinematic viscosity of 14 mm2/s at 100 °C with two base oils and four viscosity modifiers. Elevated pressure measurements were performed with a pressurized thin-film Couette viscometer and ambient pressure measurements were done with a PCS USV viscometer. These measurements were fitted to a generalized Newtonian model with the effective shear modulus specified by an empirical power-law shifting rule. The use of PAO-40 as a thickener delayed the shear-thinning to very high stress as compared with the polymers. The rate sensitivity of the oils thickened with nondispersant polymers was similar. Like the Tannas TBS viscometer, the PCS Instruments USV viscometer provides shear-dependent viscosity measurements, which can be essential for the most accurate time-temperature-pressure shifting. Viscosities measured at high viscous power in the ambient pressure viscometer, however, tend to be influenced by thermal softening and at high stress by shear cavitation.

Commentary by Dr. Valentin Fuster

Research Papers: Friction & Wear

J. Tribol. 2013;136(2):021601-021601-5. doi:10.1115/1.4025655.

The tribological properties of polytetrafluoroethylene (PTFE)/expanded graphite (EG) nanocomposites were evaluated on a pin-on-disk wear tester under dry conditions for the first time. Scanning electron microscopy (SEM) was used to examine worn surfaces and debris of the worn samples. The wear rate and coefficient of friction of 2 wt. % EG filled nanocomposite were reduced approximately 162 times and 38%, respectively. The significant decrease in wear rate was attributed to a thin and tenacious transfer film on the counter-surface. However, when 5 wt. % EG was filled into the PTFE/EG nanocomposite, the wear rate did not decrease further.

Commentary by Dr. Valentin Fuster
J. Tribol. 2013;136(2):021602-021602-12. doi:10.1115/1.4026174.

In the present work, nanoindentation experiments were carried out to characterize the localized transfer film layer (TFL) on a steel disk, which resulted from a sliding contact of the latter against a polymer composite pin. It was found that the hybrid nanocomposites filled with both nanoparticles and traditional tribo-fillers were more effective to form durable TFLs on the steel counterpart, associated with desirable tribological properties of the sliding system, i.e., a low friction coefficient and a low wear rate. By studying the load-displacement behavior of polymeric TFLs on metallic substrates, the thickness of TFLs could be estimated, thus, allowing the comparison of TFLs formed under different sliding conditions in a quantitative way. Based on the experimental data, the effects of TFLs on the tribological performance of polymer composites were further discussed in terms of a “transfer film efficiency factor” λ, which was calculated by the ratio of the average thickness of the TFL to the surface roughness of the steel counterpart. The factor mainly considered the relative contributions of the TFL and the metallic counterface to the wear process of the polymer-on-metal system. Accordingly, the wear rate and the friction coefficient of the sliding system could be analyzed as a function of the transfer film efficiency factor, resulting in a Stribeck type diagram. The analyses provided new insight into the role of TFLs in polymer tribology.

Commentary by Dr. Valentin Fuster
J. Tribol. 2014;136(2):021603-021603-11. doi:10.1115/1.4026053.

A proper understanding of the processes of friction and wear can only be reached through a detailed study of the contact interface. Empirical laws, such as Archard's, are often used in numerical models. They give good results over a limited range of conditions when their coefficients are correctly set, but they cannot be predicted: any significant change of conditions requires a new set of experimental coefficients. In this paper, a new method, the use of discrete element models (DEMs), is proposed in order to tend to predictable models. As an example, a generic biphasic friction material is modeled, of the type used in aeronautical or automotive brake systems. Micro-scale models are built in order to study material damage and wear under tribological stress. The models show what could be achieved by these numerical methods in tribological studies and how they can reproduce the behavior and mechanisms seen with real-life friction materials without any empirical law or parameter.

Commentary by Dr. Valentin Fuster
J. Tribol. 2014;136(2):021604-021604-7. doi:10.1115/1.4026079.

In order to investigate the vacuum tribological properties of a Ti-46Al-2Cr-2Nb alloy, dry-sliding tribological tests of the alloy against AISI 52,100 steel ball under different sliding speeds and loads were performed at a high vacuum of 4.0 × 10−4 Pa by ball-on-disk rotating configuration, and the same tests were done in air for comparative purposes. It is an important finding that the TiAl intermetallics have good wear resistance in vacuum, not like that in air. The wear rate of the Ti-46Al-2Cr-2Nb alloy in vacuum is almost lower by an order of magnitude than that in air.

Commentary by Dr. Valentin Fuster
J. Tribol. 2014;136(2):021605-021605-11. doi:10.1115/1.4026063.

In this paper, a compensation method of nonlinear friction using on-line input estimation (IE) method is developed. To illustrate the validity and performance of the proposed algorithm applied to positioning system, comparisons with the results using the Gomonwattanapanich method and robustness analysis are performed. The simulation result shows that the estimated friction torque does not need any assumption in the pattern of friction model in advance, the proposed algorithm has consistent robustness to diverse friction characteristics, and the method can significantly improve the performance of a control system.

Topics: Friction , Algorithms , Torque
Commentary by Dr. Valentin Fuster
J. Tribol. 2014;136(2):021606-021606-7. doi:10.1115/1.4026346.

The application of advanced high-strength steels (AHSS) generally makes it necessary to use higher tool-sheet contact pressures compared with those used for forming low-strength steel, and it leads to significant changes in frictional behavior, which in turn change the final product characteristics. In order to understand frictional behaviors between steel sheets and tool materials under high contact stresses present in real stamping conditions, a novel friction tester was conceived, fabricated, and used. This tester can generate high normal loads, as high as 625 MPa, whereas traditional friction testers were limited to 10 MPa or less. A mild steel and a TRIP780 steel were paired with Cr-coated D2 tool steel, and friction behaviors were observed under various conditions, including the use of two lubricants, wide ranges of sliding speeds, and normal contact stresses. The coefficient of friction (COF) decreased at a low contact pressure as the sliding velocity increased. The contact pressure had a significant effect, albeit too complex to be explained by simple models. It was also evident that lubricant effects must be studied coupled with the contact pressure and sliding speed. In a nonlubricated condition at normal stresses roughly half of the steel’s yield strength, the friction event caused plastic deformation that reached up to 0.2 mm from the surface. In this deformed region, the amount of retained austenite in the TRIP steel decreased substantially, and significant residual compressive stress, reaching 350 MPa, also developed in the ferrite phase (plus a minor amount of martensite, which is undistinguishable from ferrite by the X-ray diffraction method used herein). The magnitude of change of friction constant due to changes in contact conditions was enough to significantly affect springback of automotive body panels.

Commentary by Dr. Valentin Fuster
J. Tribol. 2014;136(2):021607-021607-6. doi:10.1115/1.4026421.

A very useful model for predicting abrasive wear is the linear wear law based on the Rabinowicz's equation. This equation assumes that the removed volume of the abraded material is inversely proportional to its hardness. This paper focuses on the stochastic modeling of the abrasive wear process, taking into account the experimental uncertainties in the identification process of the worn material hardness. The description of hardness is performed by means of the maximum entropy principle (MEP) using only the information available. Propagation of the uncertainties from the data to the volume of wear produced is analyzed. Moreover, comparisons and discussions with other probabilistic models for worn material hardness usually proposed in the literature are done.

Commentary by Dr. Valentin Fuster
J. Tribol. 2014;136(2):021608-021608-9. doi:10.1115/1.4026420.

In the present work, stepwise erosion technique was carried out to investigate in detail the influence of impact angle on the erosion process of AISI 5117 steel. The number of impact sites and their morphologies at different impact angles were investigated using scanning electron microscope (SEM) examination and image analysis. The tests were carried out with particle concentration of 1 wt. %, and the impact velocity of slurry stream was 15 m/s. Silica sand—which has a nominal size range of 250–355 μm—was used as an erodent, using whirling-arm test rig. The results have shown that the number of craters, as expected, increases with the increase in the mass of erodent for all impact angles and this number decreases with the increase of the impact angle. In addition, the counted number of craters is larger than the calculated number of particles at any stage for all impact angles. This may be explained by the effect of the rebound effect of particles, the irregular shape for these particles, and particle fragmentation. The effect of impact angle based on the impact crater shape can be divided into two regions; the first region for θ ≤ 60 deg and the second region for θ ≥ 75 deg. The shape of the craters is related to the dominant erosion mechanisms of plowing and microcutting in the first region and indentation and lip extrusion in the second region. In the first region, the length of the tracks decreases with the increase of impact angle. The calculated size ranges are from few micrometers to 100 μm for the first region and to 50 μm in the second region. Chipping of the former impact sites by subsequent impact particles plays an important role in developing erosion.

Commentary by Dr. Valentin Fuster

Research Papers: Hydrodynamic Lubrication

J. Tribol. 2013;136(2):021701-021701-7. doi:10.1115/1.4026061.

The performance of an aerostatic bearing with a pocketed orifice-type restrictor is affected by the bearing size, pocket size, orifice design, supply pressure, and bearing load. This study proposes a modified particle swarm optimization (MPSO) algorithm to optimize a double-pad aerostatic bearing. In bearing optimization, the upper and lower bearing designs are independent and several design variables that affect bearing performance must be considered. This study also applies the concept of mutation from a genetic algorithm. The results show that the MPSO algorithm has a global search capability and high efficiency to optimize a problem with several design variables and that the mutation can provide an avenue for particles to escape from a local optimal value.

Commentary by Dr. Valentin Fuster
J. Tribol. 2014;136(2):021702-021702-8. doi:10.1115/1.4026060.

A numerical analysis on the factors affecting the hydrodynamic performance for parallel surfaces with microtextures is presented in this paper. The semianalytical method and fast Fourier transform technique are implemented in the analysis. The numerical procedure is validated by comparing the results from the present model with the analytical solutions for the lubrication problem in an infinitewide sliding bearing with step-shaped textures. The numerical results show that the hydrodynamic performance can be greatly affected by the factors, such as the boundary conditions, cavitation pressure, microtextures, surface deformation, etc. This study can be of a great help for better understanding the mechanism of hydrodynamic pressure generated between parallel surfaces and realistically evaluating the improvement of tribological performance caused by textures.

Commentary by Dr. Valentin Fuster
J. Tribol. 2014;136(2):021703-021703-11. doi:10.1115/1.4026080.

An experimental study is presented with the main objective of understanding the hydrodynamic behavior of a tapered-land thrust bearing with fixed geometry. The experimental results were obtained using an original test rig designed at the “Institut Pprime.” Extensive instrumentation applied to the thrust bearing allows a precise evaluation of various characteristics such as the temperature, the film thickness and the friction torque. The results are in good agreement with the findings of other surveys in the literature. However, large differences between the measured parameters were observed from one pad to another. The authors demonstrate that this is due to the imperfections on the active surface, produced during machining. For a better understanding of the influence of irregularities in the flatness, the test was repeated with a thrust bearing manufactured using a high-precision surface polishing process. Experimental results with respect to the real geometry of the bearings were presented with both processes being compared. Interesting features, such as hot spots and a pressure peak, were identified on the pad at different supply temperatures and inlet pressures. This experimental study significantly advances the comprehension of the hydrodynamic behavior of tapered-land thrust bearings.

Commentary by Dr. Valentin Fuster
J. Tribol. 2014;136(2):021704-021704-8. doi:10.1115/1.4026501.

In this paper, a nonisothermal fluid-structure interaction mathematical model for the piston/cylinder interface leakage is presented. Full account is taken of the piston eccentricity, elastic deformations of the piston pair, the nonisothermal flow in the interface, and the physical properties of the fluid such as the pressure-viscosity and temperature-viscosity effects. The numerical method for the solution of the model is given, which can simultaneously solve for the fluid pressure distribution and leakage rate in the interface. The model is validated by comparing the calculated leakage rates with the measurements. Results show the good accuracy of the model. The impacts of parameters such as the piston diameter, the initial clearance between the piston pair, and the piston velocity on the leakage rate are discussed. Some of the conclusions provide good guidance for the design of high-pressure fuel pumps.

Commentary by Dr. Valentin Fuster
J. Tribol. 2014;136(2):021705-021705-8. doi:10.1115/1.4026588.

This work investigates the effect of various assumptions proposed by the classical Reynolds lubrication equation. In particular, a microplate oscillating at high frequencies (beyond cutoff) and high velocities leading to appreciable displacement within the film gap is studied. An analytical model is derived with special emphasis on the fluid's inertia effect on the fluid/solid interface. By implementing the direct simulation Monte Carlo (DSMC) method, a numerical method for modeling rarefied gas flow, the analytically based model is adjusted for the force exerted by the gas on the oscillating micro-structure to capture various significant effects related to the fluid's inertia, compressibility, stiffness, and damping.

Commentary by Dr. Valentin Fuster

Research Papers: Lubricants

J. Tribol. 2014;136(2):021801-021801-10. doi:10.1115/1.4025992.

In dipped (splash) lubrication, a rotating component, such as a gear, is partly submerged in a reservoir of liquid lubricant and acts to distribute it within the lubricated machine. Dipped lubrication is widely used for low to medium speed applications in the industrial and automotive sectors and there is a significant interest in the associated energy loss (the “churning” loss) because of its influence on efficiency and fuel consumption. In this study, a simple test rig consisting of a spur gear rotating in a cylindrical enclosure, partly filled with a liquid, was used to study the effect of fluid properties on the churning loss. The inertia rundown method was used to determine the power losses. Lubricating oils, water and aqueous glycerol solutions were among the fluids used. Correlations with Froude and Reynolds and Bond numbers are presented. It was found that the churning losses were significantly affected by the fluid disposition within the housing. In turn this was affected by the ratio of inertial forces to gravity (Froude number) and by air pressure. The influence of the pressure of the air within the enclosure was also investigated. When the air was evacuated from the enclosure, the churning losses increased, by a factor of up to 4.5 times. This can be explained by the effect of air (windage and aeration) on the liquid disposition, factors neglected in most previous work.

Commentary by Dr. Valentin Fuster

Research Papers: Tribochemistry & Tribofilms

J. Tribol. 2014;136(2):022301-022301-9. doi:10.1115/1.4026589.

A mathematical model is presented in this paper for rolling-sliding contacts operating in a mixed regime of elastohydrodynamic lubrication and boundary lubrication. The model is based on the framework of Johnson et al. (1972, “A Simple Theory of Asperity Contacts in Elastohydrodynamic Lubrication,” Wear, 19, pp. 91–108). It incorporates into this framework a number of important asperity-level variables including asperity friction, friction-induced plastic flow, flash temperature, and boundary-film tribo-chemistry. The model yields a number of variables useful for the assessment of the state of the mixed lubrication. They include the load sharing between fluid and asperities, area of asperity contacts, and fraction area of asperity contacts undergoing plastic flow along with experimentally measurable variables such as the traction coefficient, friction power intensity, and temperature of the overall contact. The model is limited to mixed-lubrication problems in which the load is mainly carried by the fluid pressure and the total area of asperity contacts is a small percentage of the Hertz area. Further development is possible to formulate a model into a wider mixed-lubrication regime using some modeling concepts developed in this paper in conjunction with other modeling techniques.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Tribol. 2013;136(2):024501-024501-8. doi:10.1115/1.4025760.

Mobility and impedance methods provide extremely efficient and robust journal orbit/trajectory calculation through use of bearing characteristics stored (at least conceptually) in the form of widely-available “maps.” These inversely-related methods are widely used for design optimization and for dynamic system analysis, both of which are often computationally intensive. All concepts and data sources necessary for straightforward application to both rotating and reciprocating machinery are identified and presented in some detail in this brief note.

Commentary by Dr. Valentin Fuster


J. Tribol. 2014;136(2):027001-027001-1. doi:10.1115/1.4026593.

Typographical Correction

Commentary by Dr. Valentin Fuster

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