Issue 35

S. Blasón et alii, Frattura ed Integrità Strutturale, 35 (2016) 187-195; DOI: 10.3221/IGF-ESIS.35.22 192 This function can adopt, alternatively one of the following forms:       * * *  exp Qa T Z a T Ua       (6) where T, U, Q and ρ in former expression are constants to be determined by fitting the function Z. After proceeding in this way, it is possible to determine the reference crack growth curve a*-N* from which, in turn, any other crack growth curve can be obtained for the given initial crack size and stress range. Finally, the S-N field for crack propagation can be derived proving the relation existing between the stress based approach and that based on fracture mechanics on a probabilistic basis. Figure 5: a) Original and b) normalized crack growth rate [7]. The decision for selecting the initial crack size in order to proceed to the calculation of the fatigue life propagation can be adopted on the base of probabilistic considerations relative to the distribution of the cracks provided by the possible correspondence between the S-N curves and those referred to the crack growth a-N . Indeed, some questions remain unsolved, among them the following can be referred to: a) The experimental verification of the model, which is, at least partially, one of the contributions of this work, b) the interpretation of the S-N field as composed by the initiation and propagation lives, and c) the possibility of considering a fictitious microcrack size, which allows the correspondence between both fatigue lives, i.e. between the one determined from the S-N field and that resulting from the integration of the crack growth rate curve, to be determined. On its turn, the crack growth rate curve provides a direct relation between the initial crack size and its progression till failure. In this way,  K presents the advantage, or perhaps disadvantage, of coupling crack size a and stress range  as a unique parameter apparently related to the crack micromechanism. Nevertheless, the interpretation of the results is less apparent since the same  K can be arise from different combinations of crack size and stress range whereas notable differences are observed in the behavior according to the characteristics of the crack size (macrocracks and microcracks in the two variants, physical and microstructural): Therefore the interest consists in relating crack size and crack growth rate curve to the S-N field where both parameters are uncoupled. D ERIVATION OF THE PROPAGATION S-N CURVES FROM THE CRACK GROWTH CURVE n the development of this Section, the methodology proposed in [7] has been applied step by step. a- Selection of the normalizing variables First of all, the participating variables are normalized according with the values shown in Tab. 3. W [mm] KI C [MPa m 1/2 ] N 0 [cycles] 22 89.26 1000 Table 3: Values of the normalizing variables. I