Applied Mechanics Reviews (AMR) is an international review journal that serves as a premier venue for state-of-the-art and retrospective survey articles and reviews of research areas and curricular developments across all subdisciplines of applied mechanics and engineering science, including fluid and solid mechanics, heat transfer, dynamics and vibration, and applications. AMR works closely with other ASME Technical Journals in serving the broad ASME community with unique content of high quality and long shelf life. Collaborative special issues between ASME and other ASME Technical Journals collect contributions of all forms described above for a specific discipline within a single issue.
Since its first publication in 1967—then known as the Journal of Lubrication Technology—the ASME Journal of Tribology (JoT) is globally known as a leading international resource for authoritative, original peer-reviewed research that advances the science and technology of friction, lubrication, and wear. Its main goal is to present multidisciplinary R&D studies of important scientific merit that deal with interfacial phenomena in natural and man-made tribocomponents. The journal sets the standard for innovations in the design and development of mechanical systems that include gears, bearings, seals, and clutches, and provides an influential forum for the timely exchange of information for scientists and engineers.
The goal of this collaborative issue is to equally serve the readerships of AMR and JoT by focusing on topics of importance to the applied mechanics and tribology communities alike. The issue collects four in-depth, state-of-the-art reviews. Independent expert discussions and author responses accompany two of the reviews and highlight complementary viewpoints and challenges for future research. Together, the papers in this issue provide authoritative commentary on the existing literature on fundamental theories, experimental analyses, and empirical observations of contact phenomena involving friction, adhesion, wear, or thermal effects. They further discuss the implications of such phenomena to large-scale applications, including traction and breaking performance of railway vehicles and dynamic instability of rotating shafts supported by fluid-film journal bearings.
As described in the review by Jacobs and Martini, an improved understanding of the nature of nanoscale contact—particularly the ability to control the effective size of nanocontacts—presents opportunities for achieving finer resolution in material characterization and nanomanufacturing, as well as enhanced performance of nanoscale electromechanical devices. To this end, Jacobs and Martini review indirect experimental techniques for inferring the size of the contact area, for example, through its effect on electrical and thermal contact resistance, as well as direct imaging methods using transmission electron microscopy. They expertly show how atomistic simulations, for example, of the nanocontact between the apex of an SPM tip and a substrate, can be used to deduce the dependence of the contact area on load conditions, given a criterion for determining contact at the atomic scale. Finally, they examine theoretical models of single-asperity, rough-surface, and noncontinuum atomistic contact with emphasis on the underlying assumptions, potential shortcomings in the context of nanocontacts, and the availability of supporting experimental evidence. The in-depth commentary by Ciavarella and Papangelo introduces a complementary discussion of the mathematical challenges associated with describing the behavior of rough contacts using continuum mechanics, specifically the fractal nature of the contact. In their response, Jacobs and Martini reflect on the possibility that observations of nanoscale contacts may generate insights that generalize to larger contacts and shed light on the relationship between different functional properties of a nanocontact and the scale at which the contact must be resolved.
At the macroscale, the development of accurate and reliable predictive tools that can realistically characterize the behavior of vital mechanical components—e.g., ball and rolling element bearings, cam-followers, gears, face-seals, electrical contacts and switches—calls for a detailed description of the load response of surface asperity contacts as they undergo elastic, elasto-plastic, and plastic deformation. The review of this topic by Ghaednia, Wang, Saha, Xu, Sharma, and Jackson leverages the senior author's well over two decades of intimate engagement with the modeling and simulation of asperity contact to construct a thorough review of some 286 papers with a very useful summary of model formulations that can be readily put to use. The paper systematically steps through different contact geometries with particular emphasis on the phenomenological relationship between contact hardness—the average pressure during fully plastic contact—and yield strength, demonstrating significant variations relative to a conventionally used constant ratio of 2.84. The results of original finite-element simulations are reported and compared against several competing empirical models in the literature. Finally, the authors discuss the effects of adhesion and implications of single-asperity contact models to the description of rough surface contacts.
The review by Olofsson and Lyu is concerned with the difficult problem of wheel-rail tribology and the implications for safe operation of railway vehicles in a variety of environmental conditions. This is yet another interfacial contact problem, but with the added complication of exposure to a wide variety of contaminants such as dirt and biological debris, as well as environmental factors such as precipitation, condensation, and temperature variation. These elements aggressively affect the behavior of the contact and cause degradation ranging from abrasive and adhesive wear to corrosive wear. The authors are particularly concerned with the effects of friction modifiers used to increase or decrease the friction coefficient to a desired level along distinct lengths of track, and note the possibility of unintended consequences of the application of such modifiers on the wear rate. In a call for continued research, they argue for the need to consider the interplay of adhesion, wear, and sound and particle emissions on the wheel-rail contact.
In fully lubricated fluid-film bearings, the journal and bushing surfaces are completely separated by a layer of oil to protect from contact. As reviewed by Tong, Palazzolo, and Suh, synchronous orbital motion of the journal inside the bearing, for example, associated with mechanical imbalance or large overhung rotor masses, is known to trigger a thermally induced rotordynamic instability as a consequence of circumferentially nonuniform viscous shearing of the lubricant film. Research into this so-called Morton effect, named for the author of a pioneering technical report from 1975, picked up significantly in the past 20 years, following the first theoretical treatment of the phenomenon from the early 1990s. In the present review, the authors demonstrate that analysis of this effect requires careful attention to the formulation of the energy equation and appropriate boundary conditions. They provide a summary of techniques intended to mitigate the impact of the Morton instability, including changes to bearing geometries, the installation of thermal isolation components, or redesign of the rotor structure, as well as prescriptions for recovering stability by quickly raising the rotor speed. In an insightful commentary, Keogh provides examples of other bearing types that may support asymmetric heating and trigger rotor instability, and cites additional literature that the interested reader will find useful, for example, on the topic of relying on an “inverse” Morton effect for vibration control. In their response, Tong, Palazzolo, and Suh comment on the applicability of their models to other bearing designs, for example, gas bearing systems, where they again find evidence of thermally induced rotor instabilities.
We hope that this collaborative issue of AMR and JoT provides service to the applied mechanics and tribology communities, enlightens the readers, and stimulates further research of benefit to society.