Influence of the Initiation Length in Predictions of Life in Fretting Fatigue

Abstract: Two different phases are usually distinguished in the crack growth process: initiation and propagation. Within the models used in determining fatigue life, there are many that combine both phases, determining total life as the sum of the number of cycles spent in initiation, N_i, and propagation, N_p. In order to apply these models, it is necessary to define the crack length at which it is considered that initiation finishes and propagation begins: initiation length, a_i. This length is usually defined a priori based on the size of the smallest detectable crack, on the definition of failure in the S‐N curve, or by choosing the value that better fits the experimental results. The object of this paper is to analyse the influence of this initiation length over the estimated fatigue life in fretting fatigue. The model used calculates the initiation phase from an S‐N curve where the propagation cycles from the defined initiation length have been subtracted. This model is applied to a group of fretting fatigue tests with spherical contact.