dc.description.abstract |
Fretting fatigue is a major concern in the design of engineering components that will be
subjected to clamping pressures whilst operating in a vibrational environment. The type
of loading environment generated in these applications is generally of a variable
amplitude nature. Therefore it is important for engineers to understand how such
loading affects the severity of fretting fatigue. In the absence of this understanding,
engineering designers are left to apply simplistic plain fatigue life prediction rules, such
as Miners linear damage law, to try to estimate the damage generated from a particular
loading history.
In order to try to establish a baseline for the understanding of fretting fatigue under
variable amplitude loading, a structured series of CAL and V AL tests were designed
and carried out. The aim of the CAL tests was to characterise the fretting fatigue test rig
(designed by the author) so that an experimental foundation could be developed, upon
which the fretting fatigue study was based. A structured investigation into the effects on
fretting fatigue of applying various specifically designed loading histories was carried
out and some very distinct relationships between the applied loading spectra and fretting
fatigue life were found. For the application of overloads it was observed that overload
size, overload application interval and the number of overloads in each application all
had a sizeable effect on the severity of fretting. Two-level, three-level and five-level
block loading tests were also investigated and numerous loading history dependent
results were obtained.
The applicability of Miners law to fretting fatigue was assessed. Combining Miners law
with well-defined CAL stress-life results gave reasonable predictions for all of the V AL
fretting fatigue tests carried out, with damage summations at failure ranging from 0.64
(non-conservative) to 3.19 (conservative). However, a more detailed investigation into
the relationship between fretting fatigue lives and damage summations due to the
individual loading levels highlighted that the assumption of linear damage may not be
applicable to fretting fatigue, especially during crack initiation and early crack growth.
A hypothesis was proposed to explain the fretting fatigue life and Miner damage results
observed for the different loading spectra, and reasonable success was obtained for tests
where the Miners law proved conservative (in 27 of 36 tests), although the hypothesis
was not able to explain the non-conservative results obtained in the other nine tests. |
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