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Understanding and modelling wear rates and mechanisms in fretting via the concept of rate-determining processes - Contact oxygenation, debris formation and debris ejection
Wear ( IF 5 ) Pub Date : 2021-08-17 , DOI: 10.1016/j.wear.2021.204066
P.H. Shipway 1 , A.M. Kirk 1 , C.J. Bennett 1 , T. Zhu 1
Affiliation  

A new framework which describes the role of three key processes in fretting wear of metals is proposed, with these three processes being: (i) oxygen transport into the contact; (ii) formation of oxide-based wear debris in the contact and (iii) ejection of the wear debris from the contact. Based upon a physical understanding, rate equations for the three key processes are proposed, which provide a basis for the influence of test parameters (such as contact geometry, applied load, slip amplitude, fretting frequency etc) on the three rates to be understood. To maintain system equilibrium in steady-state fretting, the three processes must operate at the same rate as each other (debris cannot be ejected from the contact faster than it is formed, and debris cannot be formed faster than it is ejected). Accordingly, the observed wear rate is the rate of the process with the lowest rate of the three processes, with this process being termed the rate-determining process. The effect of test parameters on the three key processes differs, and thus the effect of changes in any test parameter on the observed rate of wear will itself be dependent upon which of the three processes is rate-determining.

A number of assumptions have been made in deriving the equations which describe the key processes and it is recognised that these equations themselves may be refined in light of future research; however, any such revised equations can simply replace those proposed as part of the rate-determining process framework.

The framework can be applied to both conforming and non-conforming contact geometries. In tests with non-conforming contact geometries, the contact size increases with wear; since it is proposed that the rates of two of the three processes (oxygen transport and debris ejection) are dependent upon the size of the contact, the rate determining process can change during such a test and the rate of wear will continually fall when either of these processes are rate-determining. This complexity means that the evolution of the wear volume can only be evaluated numerically through the use of a time-marching analysis in such cases. In contrast, in tests with conforming contact geometries, the contact size does not change as wear occurs, and so there will be no changes in the rate-determining process as wear proceeds.

It is recognised that this framework addresses wear under steady-state conditions and does not consider the initial period of exposure where steady-state conditions are being developed. In the case of conforming contacts where the contact size is large, the duration of the initial transient period may be substantial and may form a significant proportion of the test duration or component lifetime. This needs to be recognised both in the design of tests and in the application of this framework.



中文翻译:

通过速率确定过程的概念了解和模拟磨损率和微动机制 - 接触氧化、碎屑形成和碎屑喷射

提出了一个新的框架,描述了金属微动磨损中三个关键过程的作用,这三个过程是:(i)氧传输到接触中;(ii) 在触点中形成基于氧化物的磨屑,以及 (iii) 磨屑从触点中排出。基于物理理解,提出了三个关键过程的速率方程,为测试参数(如接触几何形状、施加载荷、滑动幅度、微动频率等)对三个速率的影响提供了基础。为了在稳态微动中保持系统平衡,三个过程必须以彼此相同的速率运行(碎片不能比形成的速度更快地从触点中排出,并且碎片不能比排出的速度更快)。因此,观察到的磨损率是三个过程中速率最低的过程的速率,这个过程被称为速率决定过程。测试参数对三个关键过程的影响不同,因此任何测试参数的变化对观察到的磨损率的影响本身将取决于三个过程中的哪一个是决定速率的过程。

在推导描述关键过程的方程时已经做出了一些假设,并且认识到这些方程本身可以根据未来的研究进行改进;然而,任何此类修改后的方程式都可以简单地取代作为速率确定过程框架一部分而提出的方程式。

该框架可应用于符合和非符合接触几何。在不符合接触几何形状的测试中,接触尺寸随着磨损而增加;由于建议三个过程中的两个过程(氧气传输和碎片喷射)的速率取决于接触的大小,因此在此类测试期间,速率确定过程可能会发生变化,并且磨损率将在以下任一情况下持续下降这些过程是决定速率的。这种复杂性意味着在这种情况下只能通过使用时间推进分析以数值方式评估磨损量的演变。相比之下,在具有一致接触几何形状的测试中,接触尺寸不会随着磨损发生变化,因此随着磨损的进行,速率确定过程不会发生变化。

人们认识到,该框架解决了稳态条件下的磨损,并且没有考虑稳态条件正在形成的初始暴露期。在触点尺寸较大的符合触点的情况下,初始瞬变期的持续时间可能很长,并且可能占测试持续时间或组件寿命的很大一部分。这需要在测试设计和该框架的应用中得到承认。

更新日期:2021-08-27
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