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Research Papers: Contact Mechanics

J. Tribol. 2018;140(5):051401-051401-7. doi:10.1115/1.4039530.
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This study proposed a physics-based heuristic modeling for the nonlinear constitutive relation of bolted joints based on the Iwan model accompanying with the rough surface contact theory. The approach led to an Iwan distribution function which possesses the tribology-related features of the contact interface. In particular, the break-free force distribution function of the Jenkins elements could be expressed in terms of height distribution of surface asperities. The model considered the contribution of elastically, elasto-plastically as well as plastically deformed asperities to the total tangential loads. Following this, constitutive relations for lap-type bolted joints and the corresponding backbone curves, hysteresis loops, and energy dissipation per cycle were obtained. A model application was implemented and the results were compared with the published experimental results. The proposed model agrees very well with the experimental results when the contact parameters met the actual contact situation. The obtained results indicated that the model can be used to study the tangential behaviors of rough surfaces.

Commentary by Dr. Valentin Fuster

Research Papers: Elastohydrodynamic Lubrication

J. Tribol. 2018;140(5):051501-051501-8. doi:10.1115/1.4039529.

The present paper deals with an investigation of film formation in compliant lubricated contact. Despite these contacts can be found in many applications of daily life including both biological and technical fields, so far little is known about the lubrication mechanisms inside the contacts. The main attention is paid to the effect of kinematic conditions on central film thickness. For this purpose, fluorescent microscopy method was employed. Experiments were realized in ball-on-disk configuration, while the ball was made from rubber and the disk was from optical glass. The contact was lubricated by glycerol and polyglycol to examine the effect of fluid viscosity. The measurements were conducted under pure rolling and rolling/sliding conditions. The entrainment speed varied from 10 to 400 mm/s and constant load of 0.2 N was applied. Experimental results were compared with two theoretical predictions derived for isoviscous-elastohydrodynamic lubrication (I-EHL) regime. It was found that the thickness of lubricating film gradually increases with increasing entrainment speed, which corresponds to theoretical assumptions. Against expectations, evident influence of slide-to-roll ratio (SRR) on film formation was observed. In the last part of the paper, some limitations of this study are discussed and several recommendations for further methodology improvement are suggested.

Commentary by Dr. Valentin Fuster

Research Papers: Friction and Wear

J. Tribol. 2018;140(5):051601-051601-9. doi:10.1115/1.4039412.

The steady-state described by running-in attractor in the perspective of nonlinearity, is closely dependent on the running-in parameters. To study the dependence of running-in attractor on system parameters, pin-on-disk friction tests were performed. A suitable contact between pin and disk was ensured by a self-adaptive pin holder, standard block, and self-adapting amendment with sandpaper. Range analysis of correlation dimension, predictability, and entropy shows that running-in attractor is system dependent, which is manifested by the dependence of nonlinear parameters of the attractor on the running-in parameters. Further results indicate that the correlation dimension and entropy increase with load and velocity, but decrease along with initial roughness of a harder counterface, and predictability shows an inverse variation tendency with correlation dimension and entropy.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;140(5):051602-051602-23. doi:10.1115/1.4039410.

Tribological properties, i.e., wear rate, coefficient of friction (COF), and roughness of worn surfaces of an Al-Mg-Zn-Cu alloy and its composite reinforced with 20 wt % Al2O3 particles developed by stir-casting method have been studied and compared under two-body abrasion considering four independent control factors, i.e., load, abrasive grit size, sliding distance, and velocity each at three different levels. Design of test conditions and analyses of output responses have been performed employing standard Taguchi L27 orthogonal array, signal-to-noise ratio, analysis of variance technique, and regression method. Irrespective of wear conditions, composite exhibits lower wear rate and reduced COF with reference to base alloy owing to the load bearing ability and better wear resistance capability of Al2O3 particles. Roughness of worn surfaces of composite is, however, found to be higher over base alloy due to nonuniform abrasion in case of composite that generates the protruded Al2O3 particles on contact surfaces as the surrounding soft matrix is easily removed. For all three tribo-responses of both materials, the most influential factor is identified as grit size followed by load and then, grit size-load interaction except for the roughness of worn surfaces where the influence of sliding distance is also considerable. Linear regression models with excellent predictability have been developed for all tribo-characteristics separately for base alloy and composite. The predominant mechanisms of abrasion are identified as plowing and microcutting for base alloy, but delamination for composite.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;140(5):051603-051603-7. doi:10.1115/1.4039411.

(Ti,Mo)C, TiB2, and Mo2B particles reinforced Fe-based composite coatings were fabricated by laser cladding process. The effects of Molybdenum (Mo) on the microstructure and wear properties of the coatings were investigated. The results show that block-like or cuboidal TiB2, Mo2B and flower-like (Ti, Mo)C ceramics reinforcements were formed in the coatings. The size of reinforcements reduced with the increasing of FeMo70. However, cracks were found in the coating, while the addition of FeMo70 exceeded 9 wt %. The laser cladding coating presented a good wear resistance with a 9 wt % addition of FeMo70. With the increasing of FeMo70, the coatings enhanced the capability of resisting microcutting, microplowing, and surface plastic deformation.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;140(5):051604-051604-8. doi:10.1115/1.4039525.

Wear experiments are performed to explore dynamic states changes of friction noise signals. A new characteristic parameter, moving cut data-approximate entropy (MC-ApEn), is adopted to quantitatively recognize dynamic states. Additionally, determinism (DET), one key parameter of recurrence quantification analysis, is applied to verify the reliability of recognition results of MC-ApEn. Results illustrate that MC-ApEn of friction noise has distinct changes in different wear processes, and it can accurately detect abrupt change points of dynamic states for friction noise. Furthermore, DET of friction noise rapidly declines first, then fluctuates around a small value, and finally increases sharply, which just corresponds to the evolution process of MC-ApEn. So, the reliability of wear state recognition on the basis of MC-ApEn can be confirmed. It makes it possible to accurately and reliably recognize wear states of friction pairs based on MC-ApEn.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;140(5):051605-051605-8. doi:10.1115/1.4039528.

The current design of materials against wear considers hardness as the sole material property. As a result, the brittleness associated with increased hardness leads to severe damage. The purpose of this research is to understand the nature of conflicts between hardness and toughness of a new alloy composite. First, we designed Al-Cu-Fe alloys containing crystal structures of λ, β, and quasi-crystalline i-phase. These and their combination with others lead to a set of alloys with various hardness and fracture toughness. Experimental study was carried out using a noble and hard tungsten carbide (WC) ball against sample disks. The WC ball did not produce any wear. The wear rate of those alloys was found to be dependent not only on their hardness, but also the toughness, an alternative to the well-accepted Archard-based equations.

Commentary by Dr. Valentin Fuster

Research Papers: Hydrodynamic Lubrication

J. Tribol. 2018;140(5):051701-051701-8. doi:10.1115/1.4039408.

Reduced oil supply flow rates in fluid film bearings can cause cavitation, or lack of a fully developed hydrodynamic film layer, at the leading edge of the bearing pads. Reduced oil flow has the well-documented effects of higher bearing operating temperatures and decreased power losses and is commonly referred to as starvation. This study looks at the effects of oil supply flow rate on steady-state bearing performance and provides increased experimental data for comparison to computational predictions. Tests are conducted on a five-pad tilting-pad bearing positioned in a vintage, flooded housing with oil supply nozzles. Pad temperatures, sump temperature, journal operating position, and motor input power are measured at various operating speeds ranging from 2000 to 12,000 rpm and various oil supply flow rates. Predicted results are obtained from bearing modeling software based on thermoelastohydrodynamic (TEHD) lubrication theory. A starved flow model was previously developed as an improvement over the original flooded flow model to more accurately capture bearing behavior under reduced flow conditions. Experimental results are compared to both flow models. The starved bearing model predicts significantly higher journal operating positions than the flooded model and shows good correlation with the experimental data. Predicted pressure profiles from the starved bearing model show cavitation of the upper unloaded pads that increase in severity with increasing speed and decreasing oil supply flow rate. The progressive unloading of these top pads explains the rise in shaft centerline position and helps further validate the starvation model.

Commentary by Dr. Valentin Fuster

Research Papers: Other (Seals, Manufacturing)

J. Tribol. 2018;140(5):052201-052201-17. doi:10.1115/1.4039428.

Numerical and experimental analyses were carried out to investigate the dynamic characteristics of liquid annular seals with helical grooves in the seal stator. In the numerical analysis, the governing equations were the momentum equations with turbulent coefficients and the continuity equation, all averaged across the film thickness and expressed using an oblique coordinate system in which the directions of coordinate axes coincided with the circumferential direction and the direction along the helical grooves. These governing equations were solved numerically to obtain the dynamic characteristics, such as the dynamic fluid-film forces, dynamic coefficients, and whirl-frequency ratio (WFR). The numerical analysis included the effect of both fluid inertia and energy loss at the steps between the helical groove and the land sections. In the experiments, the dynamic fluid-film pressure distributions, which were induced by a small whirling motion of the rotor about the seal center, were measured to obtain the dynamic characteristics. The equivalent numerical results reasonably agree with the experimental results, demonstrating the validity of the numerical analysis. The value of the tangential dynamic fluid force induced by the rotor whirling motion decreased with increasing the helix angle γ. Consequently, the values of the cross-coupled stiffness coefficient and WFR decreased with increasing γ and became negative for large γ. In general, pump rotors rotate with a forward whirling motion under normal operating conditions. Hence, the negative value of WFR for helically grooved seals contributes to rotor stability by suppressing the forward whirling motion of the rotor.

Commentary by Dr. Valentin Fuster

Research Papers: Tribochemistry and Tribofilms

J. Tribol. 2018;140(5):052301-052301-12. doi:10.1115/1.4039527.

An attempt has been made to develop and study the properties and behavior of structure-based aluminum composite. Aluminum (AA6063)-based composites were fabricated by stir casting technique (also known as liquid metallurgy route) by varying weight percentage of metallic-based copper nitrate Cu (NO3)2 with fixed proposition of ceramic-based silicon nitride (Si3N4) reinforcement. The mechanical and corrosion properties and tribological behavior of composite were studied. Further, the sample microstructure and characterizations were investigated by scanning electron microscope (SEM) and X-ray diffraction (XRD) technique. The composite with fixed weight proportion of ceramic and higher metallic reinforced samples shows higher tensile strength, improved corrosion resistance, and higher hardness behavior. Due to higher hardness nature, the tribological properties of composite such as wear rate and coefficient of friction have been reduced. Moreover, the impact strength of composite decreased due to combination of ceramic and metallic reinforcement. In addition to the above study, design of experiment (DOE) was adopted to optimize the major wear test parameters such as percentage of reinforcement, applied load, sliding distance, and sliding speed. Finally, analysis of variance (ANOVA) was carried out to identify the most significant test parameter and its interaction affecting wear behavior and its coefficient of friction of composite sample.

Commentary by Dr. Valentin Fuster

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