A study on contact fatigue mechanisms

Abstract: Surfaces subjected to rolling and sliding contacts maysuffer from contact fatigue. This thesisdeals with solidmechanic aspects of contact fatigue including the descriptionand verificationof explaining mechanisms. The new mechanism forsurface initiated contact fatigue is basedon tensile surfacestresses from local asperity contacts. It is also realised thatsub-surfaceinitiated contact fatigue is the result of tensileresidual stresses that emanate from plasticdeformation belowthe surface. These mechanisms clearly show that contact fatiguecracksfollow the same rules as ordinary fatigue cracks inhardened steel. The thesis contains four papers that treat a new testprocedure named Standing ContactFatigue (SCF). The results ofthe test procedure have played an important role inthedevelopment and verification of the mechanisms for surfaceand sub-surface contact fatigue. The first part of the research work was experimental. Inthis part the SCF test properties wasdecided, crack resultsconfirmed and crack detection methods developed. Herecomparativestudies were performed using some differentmaterials and mechanical properties. It wasverified that SCFcould detect differences in contact fatigue resistance. Next a finite element model of the SCF test was evaluatedthrough the general-purposeprogram MARC. The model includedgraded material properties that originate from heattreatment.The residual surface deformation and surface compliance wereverified againstexperimental results. Crack initiation wasinvestigated in two ways. Firstly, the principalstresses atcritical locations were computed and plotted in a Haighdiagram. The diagramshowed that the cracks initiate in adirection perpendicular to the principal stress with thelargeststress range provided that the principal stress is tensilesometime during the load cycle. Secondly, some high cycle multiaxial fatigue criteria,including the Haigh principal stresscriterion, was evaluatedagainst the SCF crack initiation results. The surface cracklocation waspredicted by including statistical effects using aweakest-link criterion and a three-parameterWeibulldistribution. The SCF crack propagation was investigated by numericalevaluation ofJ1 andJ2integrals. The crack initiation and propagationphases were separated with a threshold criterionand a directioncriterion. It was found that during crack propagation bothsurface andsub-surface contact fatigue cracks follow thedirection with minimum mode II loading. Key words: contact fatigue mechanism; spall; spalling;surface crack; sub-surface crack;elasto-plastic indentation;contact compliance measurement; mixed-mode fatigue;fatiguecrack growth;J-integral; multiaxial fatigue; weakest-link.