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Zhu, Tengtuo (2022) A model for the role of debris ejection in development of fretting wear. PhD thesis, University of Nottingham.

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Abstract

The paper ‘The third-body approach: a mechanical view of wear’ by Maurice Godet (Wear, 100 (1984), pp 437–452) was perhaps the first to articulate clearly the key role of the rate of debris expulsion from a fretting contact in controlling the overall rate of wear; the framework of the third body approach was further developed by the concept of tribology circuit by Berthier. Whilst subsequent research over the past four decades has acknowledged this, the role of debris ejection in fretting has been generally addressed qualitatively rather than quantitatively. Moreover, calculation of wear rates in fretting have continued to employ Archard wear equation (or approaches directly derived from it), despite this approach assuming that the rate of wear is controlled by the rate of generation of wear debris (as opposed to the rate of its ejection from the contact).

In this thesis, it is proposed that there are a number of processes which need to take place for fretting wear to proceed. These can be grouped into (i) debris formation within the contact and (ii) debris ejection from the contact. Moreover, it is proposed for the first time that wear can only proceed at the rate of the slower of these two processes – that process being termed the rate-determining process. Furthermore, a physically based relationship between the debris-expulsion limited wear rate and the contact size is proposed and demonstrated, namely that the instantaneous rate of wear is inversely proportional to a characteristic dimension of the wear scar. This is a key finding of this thesis which has significant implications for all fretting research and development, whether addressing laboratory testing or service environments.

A key issue resulting from this is that it is recognised that during fretting of contacts with non-conforming geometries (such as cylinder-on-flat or sphere-on-flat) – either in service or in a test environment, the wear scar size increases as wear proceeds, and thus the instantaneous rate of wear decreases. It has been shown in this work for the first time that the amount of wear in a non-conforming contact geometry can be well approximated from the lateral size of the scar through a simple geometric relationship. This then laid the foundation for the derivation of wear equations for three commonly employed non-conforming pair specimen geometries (cylinder-on-flat, sphere-on-flat and crossed-cylinders); those wear equations all take the form V_w=KR^(n-1) E_d^n (V_w is the wear scar volume, R is the radius of the non-plane specimen(s) in the pair and E_d is the frictional energy dissipated) where n varies between 0.67 and 0.8 depending upon the geometry and assumptions made regarding the governing equation. One key assumption is that debris is ejected from the contact only in the direction of the fretting motion, i.e. that side leakage can be ignored. Consideration of experimental data related to cylinder-on-flat contacts (generated within this work) and sphere-on-flat contacts (from the literature) indicated that the validity of this assumption was strongest when the length of the contact was large compared to its width in the fretting direction.

The long-debated role of slip amplitude in fretting has been investigated in terms of its role in debris ejection. Tests were conducted across a range of test durations up to 107 cycles (such extended test durations are rarely seen in literature), and these extended tests provide new insight into the effects of slip amplitude in the evolution of wear scar. There were two main observations: (i) the change of the wear scar profile (U-shaped or W-shaped) is associated with both the slip amplitude and test duration; (ii) the incubation period can be significantly extended when the slip amplitude is small, suggesting the necessity of long duration to fully reveal the evolution of wear.

Item Type:	Thesis (University of Nottingham only) (PhD)
Supervisors:	Shipway, Philip Sun, Wei
Keywords:	Fretting wear, third-body approach, wear rate, debris ejection, analytical modelling
Subjects:	T Technology > TA Engineering (General). Civil engineering (General)
Faculties/Schools:	UK Campuses > Faculty of Engineering
Item ID:	67336
Depositing User:	ZHU, Tengtuo
Date Deposited:	31 Jul 2022 04:40
Last Modified:	31 Jul 2022 04:40
URI:	https://eprints.nottingham.ac.uk/id/eprint/67336

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