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PhD Defence Yan Zhao | Solid Viscoelasticity in Contact Mechanics and Elastohydrodynamic Lubrication

Solid Viscoelasticity in Contact Mechanics and Elastohydrodynamic Lubrication

The PhD defence of Yan Zhao will take place (partly) online and can be followed by a live stream.

Yan Zhao is a PhD student in the research group Engineering Fluid Dynamics (EFD). Supervisor is prof.dr.ir. C.H. Venner from the Faculty of Engineering Technology (ET), co-supervisors are dr.ir. C.W. Visser (ET) and dr. G.E. Morales Espejel from SKF-ERC, Nieuwegein.

Driven by the urgent need for transition to new sustainable technology in terms of use of resources and generated waste products, optimal efficiency is of crucial importance. Energy losses at any scale in machines of old (transitional) and new technology and processes need be minimized and performance and service life maximized. In many applications, the service life increasingly depends on the interfaces in which power and forces are transmitted, i.e. in machine parts in relative motion. Also reducing the energy losses (friction) in these contacts directly contributes to increased efficiency. Therefore, detailed understanding of the underlying mechanisms of contact and lubrication at rolling/sliding interfaces plays an essential role towards more sustainable technology.

Lubricated concentrated contacts are common in engineering applications, mostly between "hard" materials (steel) as in rolling bearings, cam-followers, and gears, but also between a hard and a soft part as in the case of a steel shaft and an elastomeric seal. In recent decades, soft materials are increasingly used in mechanical engineering, in aviation industry, and in various processes in physics, and bio-(fluid) mechanics, due to advantages such as light weight, good recyclability, and controllable design-performance capability. For "hard" materials, the deformation of the contacting elements is predominantly elastic. The understanding of the physical mechanisms governing the lubrication film formation and the associated pressure generation has reached a very high level of predictability. However, soft materials exhibit viscoelastic behavior in contacts, and, in spite of the excellent work that has been done regarding the elastomeric seal contacts, the current understanding of the effect of solid viscoelasticity on the lubrication of soft contacts, particularly for point contacts, is less advanced than the understanding of the purely elastic contacts. This limits the design and performance optimization of the related tribological contacts and systems.

In this thesis, a detailed description and a systematic numerical simulation for three viscoelastic contact problems, viscoelastic dry contact (VED), viscoelastic lubricated contact (VEHL) and viscoelastic layered dry contact, are provided. The results presented provide a good framework for the understanding and interpretation of viscoelastic solid effects in highly deformed (soft) dry and lubricated contacts. The methodology proposed in this work may have spin-off to polymer elastic layer lubrication, to soft meta-material design for lubricated contacts, tomography based computational diagnostics, and to biomedical applications.