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

Fretting damage consisting in cracking and wear generated by debris formation is induced by very small alternated displacements between contacting surfaces. It is often considered through the drastic drop in the fretting fatigue limit which is very detrimental for industrial applications. This paper focuses on the quantification of the fretting crack nucleation appearance by comparing fretting experiments obtained on well defined quenched 30NCD16 steel and a multiaxial fatigue approach. The plane/sphere configuration was studied under partial slip situations characterised by a central stick domain surrounded by an external sliding zone. Validated for classical fatigue conditions, the Dang Van's fatigue prediction is compared to fretting cracking mechanisms (10 6 cycles). The correlation is achieved according to some conditions: - the local friction coefficient operating in the annular partial slip contact has to be identified. It permits a more accurate estimation of the stress loading path evolution during the fretting cycle. - the loading states which are computed to determine the crack nucleation risk must be averaged on an elementary volume representative of the microstructure of the steel. It allows a convenient size effect consideration regarding the very small material volume stressed below the contact.