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 load. 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 specimens. 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.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:237768 |
| Date | January 1982 |
| Creators | Beard, J. |
| Publisher | University of Salford |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://usir.salford.ac.uk/42967/ |
Page generated in 0.0017 seconds