PSI - Issue 26

Gonzalo M. Dominguez Almaraz et al. / Procedia Structural Integrity 26 (2020) 20–27 Almaraz et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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• Return to the first point for the new crack extension. Crack initiation is associated with two pre-corrosion pits; furthermore, at the front of the crack were inserted boundary elements delimiting the zone of crack propagation, as shown in Figure 8 (red dots). Evolution of SIF during crack propagation is illustrated in Figure 9, where L is the crack extension and D the diameter at the specimen neck section (D = 3 mm).

Figure 8. Two pre-corrosion pits taken as the crack initiation for determination of the SIF .

Figure 9. SIF evolution across the neck section of specimen, evaluated by the Stress Interpolation Method.

In Figure 9 are plotted the evolution of the SIF factor in mode I, evaluated by the Stress Interpolation Method. Finally, in Figure 10 show the facture surfaces of the two type of Inconel 718 specimens: with and without pre corrosion. For the first specimens, crack initiates frequently related to pre-corrosion pits along the neck section of specimen, as is illustrated in Figure 1o(a); whereas specimens without pre-corrosion, crack initiates more localized at the specimen neck section, where the higher stress is located, Figure 10(b).

3. Conclusions

The following conclusions can be draw from the present work: • Ultrasonic fatigue endurance results have been obtained on the superalloy Inconel 718, on pre-corroded specimens and non-pre-corroded specimens. • Pre-corroded specimens present a decrease on fatigue endurance with a factor of 10 approximately, compared with the non-pre-corroded specimens; particularly for the high applied load (746 MPa). • Numerical investigations confirm an important increase of stress concentration factors associated with pre-corrosion pits, which is at the origin of the drastic reduction in fatigue endurance; particularly for the high applied load.