PSI - Issue 26

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

23

Almaraz et al. / Structural Integrity Procedia 00 (2019) 000 – 000

4

Figure 4. Specimens of Inconel 718, after the pre-corrosion process.

Ultrasonic fatigue results on pre-corroded and specimens without pre-corrosion are plotted and confronted on Figure 5. In literature have reported results on Inconel 718, obtained at room temperature and under the conventional and ultrasonic fatigue testing modalities  11-13  .

Figure 5. Ultrasonic fatigue results on pre-corroded and non-pre-corroded specimens.

Some tendencies can be observed from these results: for the high applied stress (746 MPa or the 90% of its elastic limit), the fatigue endurance for the specimens without pre-corrosion present fatigue life higher than the pre-corroded specimens, with a factor of 10; whereas in the low applied stress (510 MPa, the 60% of the elastic limit of this material), fatigue endurance is similar for the two type of specimens, with slightly higher values for specimens without pre-corrosion. Numerical simulations were carried out in order to reproduce the principal pits generated during the pre corrosion, in Figure 6a is illustrated the details of pre-corrosion on the neck section of specimen; whereas Figure 6b presents the results of numerical simulation reproducing the principal pits. Stress concentration factors associated with the pre-corrosion pits have been obtained numerically: the higher stress on specimens without pre-corrosion attains values of 667 MPa at the neck section, when its ends undergo tension displacement of 47  m; the stress attains 2750 MPa in the case of pre-corroded specimens under the same tension conditions, as shown in the numerical results of Figure 7. The stress concentration factor related to two close pits in transversal direction in regards the applied load, is close to 2750/667 = 4.12 approximately  14, 15  . The stress concentration associated with pre-corrosion pits is at the origin of important decrease on the ultrasonic fatigue endurance of this alloy. In order to evaluate the stress intensity factor (SIF) in mode I during the crack initiation and propagation, the Stress Interpolation Method was used taking the following considerations: