PSI - Issue 24

Giulia Morettini et al. / Procedia Structural Integrity 24 (2019) 349–359 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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(b)

Fig. 3. S-N Curve of Horizontal specimens, (a) confidence bands using ASTM E739, (b) confidence and probability bends using ISO 12107

In Figure 2(b) and Figure 3(b), the mean curve is shown together with the bands defined by "simplified" ISO 12107 standard equation. These bands take into account the combination of 95% confidence and 90% survival probability. From them, it is possible to obtain the fatigue limit 1 0 % and 9 0 % corresponding to 10% and 90% of survival probability, and the scatter index T shown in Table 2. Table 2 also shows the values of slop B and intercept A of the mean Wöhler curves express in accordance with the ASTM E739 and the ISO 12107 for both tested cases. For completeness the values of the parameters a and the Basquin slope b are also reported according to the Basquin equation: = . Table 2. Statistical Parameter Build direction B A b a ̌ % % - [MPa] - [MPa] [cycles] [MPa] [MPa] [MPa] - Vertical -3.2836 12.735 -0.32836 7.5585 10 3 0.0483 100.86 91.10 82.29 1.23 Horizontal -3.6997 14.009 -0.36997 6.1168 10 3 0.1099 146.30 121.18 100.36 1.46 3.2. Fatigue surface investigation Figure 4 shows typical fracture surfaces for vertical and horizontal specimens. The whole fracture surface of vertical and horizontal is shown in Figures 4(a) and (b), respectively. A magnification of the edges from which fatigue is initiating is shown in Figures 4 (c) and (d) for both cases. The fracture surfaces display a clear difference between the two specimens; the vertical specimens have similar surface morphology on the different edges; i.e. no difference in surface roughness for the different surfaces in the specimen. The horizontal specimen, however, displays high surface roughness in the downward facing surface and lower roughness in the upward/sideways facing surfaces, Murakami (2002). For the vertical specimens, fatigue cracks were initiated mainly from a local surface defect with longer depth,