Identification of the characteristic length scale for fatigue cracking in fretting contacts

Fretting damage consisting in cracking and wear generated by debris formati on is induced by very small alternated displacements between contacting sur faces. It is often considered through the drastic drop in the fretting fati gue limit which is very detrimental for industrial applications. This paper focuses on the quantification of the fretting crack nucleation a ppearance by comparing fretting experiments obtained on well defined quench ed 30NCD16 steel and a multiaxial fatigue approach. The plane/sphere config uration was studied under partial slip situations characterised by a centra l stick domain surrounded by an external sliding zone. Validated for classi cal fatigue conditions, the Dang Van's fatigue prediction is compared to fr etting cracking mechanisms (10(6) cycles). The correlation is achieved acco rding to some conditions: - the local friction coefficient operating in the annular partial slip cont act has to be identified. It permits a more accurate estimation of the stre ss loading path evolution during the fretting cycle. - the loading states which are computed to determine the crack nucleation r isk must be averaged on an elementary volume representative of the microstr ucture of the steel. It allows st convenient size effect consideration rega rding the very small material volume stressed below the contact.