Similarities of stress concentrations in contact at round punches and fatigue at notches: implications to fretting fatigue crack initiation

A linear elastic model of the stress concentration due to contact between a rounded flat punch and a homogeneous substrate is presented, with the aim of investigating fretting fatigue crack initiation in contacting parts of v ibrating structures including turbine engines. The asymptotic forms for the stress fields in the vicinity of a rounded punch-on-flat substrate are der ived for both normal and tangential loading, using both analytical and fini te element methods. Under the action of the normal load, P, the ensuing con tact is of width 2b which includes an initial flat part of width 2a. The as ymptotic stress fields for the sharply rounded flat punch contact have cert ain similarities with the asymptotic stress fields around the tip of a blun t crack. The analysis showed that the maximum tensile stress, which occurs at the contact boundary due to tangential load Q, is proportional to a mode II stress intensity factor of a sharp punch divided by the square root of the additional contact length due to the roundness of the punch, Q/(root<(( b - a))over bar>root<(pi b)over bar>). The fretting fatigue crack initiatio n can then be investigated by relating the maximum tensile stress with the fatigue endurance stress. The result is analogous to that of Barsom and McN icol where the notched fatigue endurance stress was correlated with the str ess intensity factor and the square root of the notch-tip radius. The propo sed methodology establishes a 'notch analogue' by making a connection betwe en fretting fatigue at a rounded punch/flat contact and crack initiation at a notch tip and uses fracture mechanics concepts. Conditions of validity o f the present model are established both to avoid yielding and to account f or the finite thickness of the substrate. The predictions of the model are compared with fretting fatigue experiments on Ti-6Al-4V and shown to be in good agreement.