13th International Conference on Fracture June 16–21, 2013,Beijing, China -6- shield completely the crack tip from fatigue damage [23]. Based on this concept, Donald proposed to calculate an effective stress intensity factor range as: ∆ࡷ/ࡼࡵ ൌ∆ࡷࢇ െ࣊ ൫ࡷ െࡷ൯ (2) This is one of several methods that have been used to estimate ∆Keff, although this method distinguishes over other because of his simplicity and because has provided successful correlation of the crack growth rate data for aluminum alloys. Fig. 6 shows the fatigue crack growth rate as a function of the effective stress intensity factor proposed by Donald. It can be observed that for our analyzed material the Donald`s effect does not provide a better correlation of the R-ratio effects than the traditional ∆Keff calculated by using the Eq. (1). Figure 5. Fatigue crack growth rate as a function of the effective stress intensity factor (a) (b) Figure 6. Fatigue crack growth rate as a function of the effective stress intensity factor proposed by Donald et al. (a) in the stable crack propagation region and (b) in the near threshold region. 4.3. Results using the Kujawski`s Parameter To explain the load ratio effects in fatigue crack growth, and because of the inconsistency in the measurement of crack closure and the difficulties to determine the fatigue damage associated to a crack partially open, Kujawski proposed a crack driving force parameter that is calculated by using
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