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Review Article

Appl. Mech. Rev. 2017;69(6):060801-060801-13. doi:10.1115/1.4037216.

The Morton effect (ME) is a thermally induced instability problem that most commonly appears in rotating shafts with large overhung masses and supported by fluid-film bearings. The time-varying thermal bow, due to the asymmetric journal temperature distribution, may cause intolerable synchronous vibrations that exhibit a hysteresis behavior with respect to rotor speed. First discovered by Morton in the 1970s and theoretically analyzed by Keogh and Morton in the 1990s, the ME is still not fully understood by industry and academia experts. Traditional rotordynamic analysis generally fails to predict the potential existence of ME-induced instability in the design stage or troubleshooting process, and the induced excessive rotor vibrations cannot be effectively suppressed through conventional balancing, due to the continuous fluctuation of vibration amplitude and phase angle. In recent years, a fast growing number of case studies of ME have sparked academic interest in analyzing the causes and solutions of ME, and engineers have moved from an initial trial and error approach to more research inspired modification of the rotor and bearing. To facilitate the understanding of ME, the current review is intended to give the most comprehensive summary of ME in terms of symptoms, causes, prediction theories, and solutions. Published case studies in the past are also analyzed for ME diagnosis based on both the conventional view of critical speed, separation margin (SM), and the more recent view of the rotor thermal bow and instability speed band shifting. Although no universal solutions of ME are reported academically and industrially, recommendations to help avoid the ME are proposed based on both theoretical predictions and case studies.

Commentary by Dr. Valentin Fuster
Appl. Mech. Rev. 2017;69(6):060802-060802-18. doi:10.1115/1.4038130.

The size of the mechanical contact between nanoscale bodies that are pressed together under load has implications for adhesion, friction, and electrical and thermal transport at small scales. Yet, because the contact is buried between the two bodies, it is challenging to accurately measure the true contact area and to understand its dependence on load and material properties. Recent advancements in both experimental techniques and simulation methodologies have provided unprecedented insights into nanoscale contacts. This review provides a detailed look at the current understanding of nanocontacts. Experimental methods for determining contact area are discussed, including direct measurements using in situ electron microscopy, as well as indirect methods based on measurements of contact resistance, contact stiffness, lateral forces, and topography. Simulation techniques are also discussed, including the types of nanocontact modeling that have been performed and the various methods for extracting the magnitude of the contact area from a simulation. To describe and predict contact area, three different theories of nanoscale contact are reviewed: single-contact continuum mechanics, multiple-contact continuum mechanics, and atomistic accounting. Representative results from nanoscale experimental and simulation investigations are presented in the context of these theories. Finally, the critical challenges are described, as well as the opportunities, on the path to establishing a fundamental and actionable understanding of what it means to be “in contact” at the nanoscale.

Commentary by Dr. Valentin Fuster
Appl. Mech. Rev. 2017;69(6):060803-060803-10. doi:10.1115/1.4038229.

The tiny contact zone (approximately 1 cm2) where steel wheel meets steel rail is fundamental to rail transport. This work is a comprehensive presentation of recent research in wheel–rail contact tribology. It stresses that, unlike gears or rolling bearings which are sealed contacts with reduced exposure to the surrounding environment, a wheel–rail contact is an open system that is exposed to dirt and particles as well as to applied and natural lubrication (the latter category includes rain, dew, and biological materials such as leaves). As an open system contact, it also radiates sound and airborne wear particles. These characteristics of an open system underscore the need for special studies of open system tribology. Areas requiring study include airborne particle emissions and the environmental effects of applied lubrication and friction modification. Given that adhesion, wear, and sound and particle emission are closely related in an open system, these should be studied together rather than independently.

Topics: Friction , Wear , Rails , Wheels , Adhesion
Commentary by Dr. Valentin Fuster
Appl. Mech. Rev. 2017;69(6):060804-060804-30. doi:10.1115/1.4038187.

In typical metallic contacts, stresses are very high and result in yielding of the material. Therefore, the study of contacts which include simultaneous elastic and plastic deformation is of critical importance. This work reviews the current state-of-the-art in the modeling of single asperity elastic–plastic contact and, in some instances, makes comparisons to original findings of the authors. Several different geometries are considered, including cylindrical, spherical, sinusoidal or wavy, and axisymmetric sinusoidal. As evidenced by the reviewed literature, it is clear that the average pressure during heavily loaded elastic–plastic contact is not governed by the conventional hardness to yield strength ratio of approximately three, but rather varies according to the boundary conditions and deformed geometry. For spherical contact, the differences between flattening and indentation contacts are also reviewed. In addition, this paper summarizes work on tangentially loaded contacts up to the initiation of sliding. As discussed briefly, the single asperity contact models can be incorporated into existing rough surface contact model frameworks. Depending on the size of a contact, the material properties can also effectively change, and this topic is introduced as well. In the concluding discussion, an argument is made for the value of studying hardening and other failure mechanisms, such as fracture as well as the influence of adhesion on elastic–plastic contact.

Commentary by Dr. Valentin Fuster

Discussion

Appl. Mech. Rev. 2017;69(6):065502-065502-3. doi:10.1115/1.4038188.

Jacobs and Martini (JM) give a nice review of direct measurement methods (in situ electron microscopy), as well as indirect methods (which are based on contact resistance, contact stiffness, lateral forces, and topography) for measurement of the contact area, mostly at nanoscale. They also discuss simulation techniques and theories from single-contact continuum mechanics, to multicontact continuum mechanics and atomistic accounting. As they recognize, even at very small scales, “multiple-contacts” case occurs, and a returning problem is that the “real contact area” is often an ill-defined, “magnification” dependent quantity. The problem remains to introduce a truncation to the fractal roughness process, what was called in the 1970s “functional filtering.” The truncation can be “atomic roughness” or can be due to adhesion, or could be the resolution of the measuring instrument. Obviously, this also means that the strength (hardness) at the nanoscale is ill-defined. Of course, it is perfectly reasonable to fix the magnification and observe the dependence of contact area, and strength, on any other variable (speed, temperature, time, etc.).

Commentary by Dr. Valentin Fuster

Closure

Appl. Mech. Rev. 2017;69(6):066001-066001-2. doi:10.1115/1.4037364.

The authors express their gratitude to Professor Keogh for contributing his keen insights and perspectives and illuminating analysis on the Morton effect (ME) and on general thermally induced, near synchronous, rotordynamic instability problems [1]. Keogh and Morton's [2,3] landmark papers on the theoretical analysis of the ME launched an evolution of the understanding of the ME, which was stimulated by actual industrial experience and improved theoretical models. These models were developed to more accurately predict the ME for design and troubleshooting. This is a timely accomplishment since as recognized by Professor Keogh, modern rotordynamic systems continually push the boundaries in complexity, speed, loads, performance, efficiency, and structural lightness, which amplify higher order vibrational effects that disrupt machinery operation.

Commentary by Dr. Valentin Fuster
Appl. Mech. Rev. 2017;69(6):066002-066002-2. doi:10.1115/1.4038230.

We thank Prof. Ciavarella and Dr. Papangelo for the supplemental discussion on the challenge of describing contact area and contact parameters between surfaces with fractal roughness. Their commentary brings up many points of debate in the literature on contact of surfaces with multiscale roughness, some of which were omitted from the original review because of its focus on nanoscale contacts.

Commentary by Dr. Valentin Fuster

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