Initiation criteria in fretting fatigue with spherical contact

This paper analyses different methods used to estimate fatigue life adding initiation with propagation in fretting fatigue tests with spherical contact. In particular, the discussion will be centred on the initiation phase. Some of the methods presented define a crack length in advance, after which crack propagation is considered and calculated. Stresses close to the surface are then used to estimate initiation. The initiation life is estimated by using either the stresses averaged up to a certain depth or the stresses at a certain depth. Yet another method combines initiation with propagation, although in this case the initiation length is not a fixed quantity, it is obtained as a result of analysing each specific case. These methods are applied to a series of fretting fatigue tests with spherical contact, performed with 7075-T6 aluminium alloy and the results are discussed.