13th International Conference on Fracture June 16–21, 2013,Beijing, China -5- Figure 3. Fatigue crack growth rate vs stress intensity factor range at different load ratios (a) in the stable crack propagation region and (b) in the near threshold region. The influence of R values on near threshold crack growth rates is shown in Fig. 3(b). It can be seen how the Kth decreases with the increment in load ratio. The values of Kth are relatively high, especially at R = 0.1 and R = 0.5, in comparison with other austenitic stainless steel alloys or even other steel alloys. However the results are not surprising, according to the results found for other austenitic stainless steel that are shown in the table of Fig.3(b), where it can be observed that materials with martensitic transformation and/or in conditions of plane stress, produce high thresholds. 4.2. Results using the effective stress intensity factor The fatigue crack growth rate was plotted also by using the conventional Elber’s approach and defining the effective stress intensity range as: ∆ࡷࢋࢌࢌ ൌࡷࢇ࢞ െࡷ (1) Fig. 4 shows the obtained results. They show that in the stable region (far from thresholds) the crack closure is practically constant in the total range of crack propagation for the same test. It is only observes the same average value of So/Smax ≈ 0.35 for both cases at low load ratio (R = 0.1 and 0.3). The analysis was based in the work of J. Song et al [18], from which the offset criterion that better estimate load ratio effects in the Paris region is 4% (this analysis will be detailed in a paper that will be soon submitted for publication). Figure 4. Fatigue crack growth rate as a function of the effective stress intensity factor Fig. 5 shows the correlation obtained using Eq. (1) for the near threshold behavior. It shows that with the use of crack closure concepts the curves move closer each other but it is still not possible to join them in a unique curve. In this region of slow growth rate the relation of Kop/Kmax increase with decreasing the range of the stress intensity factor (approaching the threshold). 4.3. Results using the Donald`s effect The original concept of crack closure indicates that the crack cannot grow if the crack is not fully open. However Donald and coauthors have shown that the interference of crack faces does not
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