PSI - Issue 35

Martin Ferreira Fernandes et al. / Procedia Structural Integrity 35 (2022) 141–149 Martin Ferreira Fernandes et al. / Structural Integrity Procedia 00 (2021) 000 – 000

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(a)

(b)

Fig. 4. (a) Normality test and (b) regression plot of dwell-fatigue data.

3.3. SEM of fracture surfaces Figs. 5a and 5b show the fracture surfaces for fatigue and dwell-fatigue tests at a maximum stress level of 1000 MPa. Fig. 5a shows that the crack nucleation sites for fatigue tests occurred at the surface of the specimen, as indicated by an arrow. However, for dwell-fatigue tests, the crack nucleation did not occur on the surface. This indicates that the nucleation sites were multiple subsurface cracks for the dwell-fatigue tests due to the stress redistribution mechanisms observed for dwell-fatigue tests. The fracture surface was characteristic of a ductile fracture. Similarly, at a stress level of 975 MPa, the introduction of a dwell period changed the morphology of the fracture surface from typical fatigue (Fig. 6a) to ductile fracture (Fig. 6b). Figs. 7a and 7b show that the cracks nucleated at the surface for the fatigue (Fig. 7a) and dwell-fatigue (Fig. 7b) loadings at the lower stress level of 950 MPa.

Fig. 5. Fracture surfaces of (a) fatigue (12299 cycles) and (b) dwell-fatigue (1821 cycles) tests at 1000 MPa.