An investigation into the mechanisms of fretting fatigue
This thesis describes the experimental work carriedout to further the understanding of the mechanismsinvolved in the process of fretting fatigue.The design and construction of a rig to enable acontrolled fretting action to be applied to a push-pulltype of fatigue specimen is discussed. Specialconsideration is given to the form of the fretting contactgeometry with regard to the generated stress field, andthe reasons behind the final choice of a circular Hertziancontact are given.The investigation of the effects of variousparameters on the fatigue life of the specimen arereported. The parameters chosen for the investigationwere the slip amplitude, the bulk stress and the normalload.Examination of the developing fretting damage wascarried out ty interrupting the tests at prescribedintervals. The subsequent observations made of thesurface and sue-surface damage are illustrated by opticaland scanning electron micrographs. Two distinct forms ofdamage are proposed, these have been termed type I andtype II fretting fatigue damage.Type I damage is considered to be by crack nucleationfrom the conjoint action of the bulk and surface stressfields. These cracks were found to nucleate in thedirection of maximum shear at the edge of the frettingcontact, and at the position where the alternating tensilestress was largest. The subsequent direction ofpropagation of these cracks, determines whether or notfatigue failure of the specimen will occur. The type Ifretting damage process, is noted to have been responsiblefor every case of catastrophic fatigue failure of the testspecimens.Type II fretting damage was found at the centre ofthe fretting contact area, where the hydrostatic stressand surface shear traction are maximum. The material inthis region was found to have undergone extensivemicrostructural alteration. White etching layers with ahardness in excess of 1300Hv (more than four times theoriginal 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 importantconsequences on the rate of wear, thus it is postulatedthat under certain conditions fretting wear and frettingfatigue may be linked ty the same mechanism, i.e. whiteetching layer formation. The white etching layers arealso discussed in terms of their potential for nucleatingfatigue cracks, this phenomenon is illustrated by aservice failure from a diesel engine.
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