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An investigation into the mechanisms of fretting fatigue

Beard, J


J Beard


This thesis describes the experimental work carried
out to further the understanding of the mechanisms
involved in the process of fretting fatigue.
The design and construction of a rig to enable a
controlled fretting action to be applied to a push-pull
type of fatigue specimen is discussed. Special
consideration is given to the form of the fretting contact
geometry with regard to the generated stress field, and
the reasons behind the final choice of a circular Hertzian
contact are given.
The investigation of the effects of various
parameters on the fatigue life of the specimen are
reported. The parameters chosen for the investigation
were the slip amplitude, the bulk stress and the normal
Examination of the developing fretting damage was
carried out ty interrupting the tests at prescribed
intervals. The subsequent observations made of the
surface and sue-surface damage are illustrated by optical
and scanning electron micrographs. Two distinct forms of
damage are proposed, these have been termed type I and
type II fretting fatigue damage.
Type I damage is considered to be by crack nucleation
from the conjoint action of the bulk and surface stress
fields. These cracks were found to nucleate in the
direction of maximum shear at the edge of the fretting
contact, and at the position where the alternating tensile
stress was largest. The subsequent direction of
propagation of these cracks, determines whether or not
fatigue failure of the specimen will occur. The type I
fretting damage process, is noted to have been responsible
for every case of catastrophic fatigue failure of the test
Type II fretting damage was found at the centre of
the fretting contact area, where the hydrostatic stress
and surface shear traction are maximum. The material in
this region was found to have undergone extensive
microstructural alteration. White etching layers with a
hardness in excess of 1300Hv (more than four times the
original hardness) are reported. An extensive literature survey is presented which shows no previous evidence of white etching layers forming under clean fretting conditions.
The white layers are shown to have important
consequences on the rate of wear, thus it is postulated
that under certain conditions fretting wear and fretting
fatigue may be linked ty the same mechanism, i.e. white
etching layer formation. The white etching layers are
also discussed in terms of their potential for nucleating
fatigue cracks, this phenomenon is illustrated by a
service failure from a diesel engine.


Beard, J. An investigation into the mechanisms of fretting fatigue. (Thesis). University of Salford

Thesis Type Thesis
Deposit Date Jul 13, 2017
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